Bulletin of the Seismological Society of America current issue

[Articles] Inhibition of Very Strong Ground Motion in Response Spectral Attenuation Models and Effects of Site Class and Tectonic Category

Zhao, J. X., Rhoades, D. A., McVerry, G. H., Somerville, P. G. — 2009-06-07

In current ground-motion models, the uncertainty in predicted ground motion is usually modeled with a lognormal distribution. One consequence of this is that predicted ground motions do not have an upper limit. In reality, however, there probably exist physical conditions that limit the ground motion. Applying the usual uncertainty distribution in probabilistic seismic hazard analysis may lead to ground-motion estimates that are unrealistically large, especially at the low annual probabilities considered for important structures, such as dams or nuclear reactors. A recently proposed statistical procedure to compare the actual and expected numbers of predicted spectral accelerations exceeding a given value gives clear results when applied to a ground-motion model developed for Japan from a very large strong-motion data set. It shows that, for increasingly large spectral accelerations, the actual number of exceedances becomes progressively less than the expected number of exceedances. The pattern of this discrepancy depends on the site class and the earthquake tectonic category. These results suggest that assuming a normal distribution for the prediction errors of an attenuation model (empirical ground-motion prediction equation) is likely to result in overestimation of the extreme values of spectral accelerations.

[Articles] Making the Most of Available Site Information for Empirical Ground-Motion Prediction

2009-06-07

This article proposes a new framework for the inclusion of site effects in empirical ground-motion prediction equations (GMPEs) by characterizing stations through their one-quarter wavelength velocities and assessed confidence limits. The approach is demonstrated for 14 stations of the French accelerometric network (Réseau Accélérométrique Permanent). This method can make use of all the available information about a given site, for example, the surface geology, the soil profile, standard penetration test measurements, near-surface velocity estimated from the topographic slope, depth to bedrock, and crustal structure. These data help to constrain the velocity profile down to a few kilometers. Based on a statistical study of 858 real profiles from three different regions (Japan, western North America, and France) physically realistic profiles are generated that comply with the information available for each site.

In order to evaluate the confidence limits for the shear-wave velocity profiles and derived site amplifications for each station, a stochastic method is adopted: several thousand profiles are randomly generated based on parameters derived in the statistical study and the constraints available for each station. Then, the one-quarter wavelength assumption is used to estimate the amplification for each station. It is found that a good knowledge of near-surface attenuation (i.e., or Q) is mandatory for obtaining precise amplification estimates at high frequencies. Nevertheless, the proposed scheme highlights the important differences in the uncertainties of the site amplifications, depending on the information available for a given station. We suggest that these results could, therefore, be used when developing GMPEs by weighting records from each station depending on the variability in the computed one-quarter wavelength velocities.

This approach relies on the assumption that local site effects are only one-dimensional, which is far from true, especially in sedimentary basins. However, most GMPEs only model one-dimensional site effects, so this is not an issue specific to this study. Finally, a way to improve this technique is to use earthquakes or noise recorded at the stations to further constrain the shear-wave velocity profiles and to consequently derive more accurate one-quarter wavelength velocities.

[Articles] Numerical Study of Ground-Motion Differences between Buried-Rupturing and Surface-Rupturing Earthquakes

Pitarka, A., Dalguer, L. A., Day, S. M., Somerville, P. G., Dan, K. — 2009-06-07

Recent ground-motion observations suggest that surface-rupturing earthquakes generate weaker near-fault ground motion than buried earthquakes. This difference is significant in the period range of 0.3–3 sec. Contributing factors to this phenomenon may include the effect of fault zone weakness at shallow depth on rupture dynamics and rupture directivity during earthquakes.

We present results from numerical experiments of spontaneous dynamic rupture and near-source ground-motion simulations of surface rupturing and buried earthquakes and discuss mechanisms for the observed ground-motion differences. The surface-rupturing earthquake is modeled with a shallow zone of 5 km thickness containing areas of negative stress drop (within the framework of the slip-weakening friction model) and lower rigidity. Surface-rupturing models with this weak zone generate lower amplitude ground velocity than do models without this modification.

Observed ground-motion differences between surface and buried events are qualitatively reproduced by imposing higher stress drop in the buried earthquakes than in the surface earthquakes, combined with introducing a deeper rupture initiation for buried rupture, enhancing upward rupture-directivity effects for the latter events. In the context of our simplified model parameterization, then, the observed differences in ground motion could arise from combined effects of relative weakness of the shallow layer of faults, the relatively larger stress drops of buried ruptures, and a tendency of near-fault sites to record strong upward directivity from buried ruptures.

[Articles] Near-Fault Strong Ground Motions Recorded during the Morelia Normal-Fault Earthquakes of May 2006 in Mexicali Valley, B. C., Mexico

Munguia, L., Glowacka, E., Suarez-Vidal, F., Lira-Herrera, H., Sarychikhina, O. — 2009-06-07

By the end of May 2006, a series of earthquakes had occurred in the Mexicali Valley, Baja California, México. This activity, which included an M_w 5.4 event, developed in the vicinity of the Cerro Prieto geothermal field. The earthquakes occurred on the Morelia fault, one of the east-dipping normal faults in the Mexicali Valley. Cracks and fissures visible on the ground after the stronger earthquake defined two parallel fault segments of 4 and 2 km length, the shorter segment being 1 km to the east of the other one. Down-dip displacements of up to 25–30 cm were measured at some places along this surface break zone.

The ten larger events of the series were recorded by strong-motion stations that operate in the epicenter area, so that accurate hypocenter locations were obtained on the basis of P-wave arrival times from the strong-motion recordings. The estimated shallow depths, combined with the earthquake magnitude, explain the strong shaking felt by residents and some damage observed on the Cerro Prieto geothermal plant.

The M_w 5.4 event produced peak ground accelerations that go from 0.002g at CUC (on rock) to 0.5g at GEO (on sediments), at 13 and 1.7 km from the epicenter, respectively. The station GEO recorded closer to the epicenters and on the hanging-wall side of the fault. Static ground displacements and a predominance of the strike-normal over the strike-parallel velocity components were determined from the acceleration records of this station. These and other ground-motion characteristics are also seen on pseudovelocity and absolute acceleration response spectra calculated from data of the larger event. Altogether, the observed ground-motion characteristics provide useful insights into the levels of ground shaking that near-fault structures in the Mexicali Valley should be designed to withstand.

[Articles] Predicted Ground Motions for Great Interface Earthquakes in the Cascadia Subduction Zone

Atkinson, G. M., Macias, M. — 2009-06-07

Ground motions for earthquakes of moment magnitude (M) 7.5–9.0 on the Cascadia subduction zone interface are simulated based on a stochastic finite-fault model and used to estimate average response spectra for firm-site conditions in Vancouver, Victoria, and Seattle. We also express the response spectra as ground-motion prediction equations (GMPEs) for Cascadia events. The simulations are calibrated by modeling the wealth of ground-motion data from the M 8.1 Tokachi-Oki earthquake sequence of Japan. Adjustments to the calibrated model are made to consider average source, attenuation, and site parameters for the Cascadia region.

We perform best estimate simulations for a preferred set of input parameters. Typical results suggest mean values of 5%-damped pseudoacceleration in the range from about 100 to 200 cm/sec², at frequencies from 1 to 4 Hz, for firm-ground conditions in Vancouver, Victoria, and Seattle. Uncertainty in stress drop causes uncertainty in simulated response spectra of about ±50%. Uncertainties in the attenuation model produce even larger uncertainties in response spectral amplitudes—a factor of about 2 at 100 km, becoming even larger at greater distances. It is thus important to establish the regional attenuation model for ground-motion simulations. Furthermore, combining data from regions with different attenuation characteristics—in particular Japan and Mexico—into a global subduction zone database for development of global empirical GMPEs may not be a sound practice.

Time histories of acceleration for the stochastically simulated motions are provided for reference sites in Vancouver, Victoria, and Seattle. An alternative set of motions, based on lightly modifying real recordings from the Tokachi-Oki earthquake to match expected conditions for Cascadia cities, are also provided. These alternative records have similar spectral content to the simulated motions but contain additional complexity and more realistic phasing. The provision of alternative record sets allows users to conduct studies to determine the importance of these effects for structural response.

[Articles] Sedimentary Basin Effects in Seattle, Washington: Ground-Motion Observations and 3D Simulations

Frankel, A., Stephenson, W., Carver, D. — 2009-06-07

Seismograms of local earthquakes recorded in Seattle exhibit surface waves in the Seattle basin and basin-edge focusing of S waves. Spectral ratios of S waves and later arrivals at 1 Hz for stiff-soil sites in the Seattle basin show a dependence on the direction to the earthquake, with earthquakes to the south and southwest producing higher average amplification. Earthquakes to the southwest typically produce larger basin surface waves relative to S waves than earthquakes to the north and northwest, probably because of the velocity contrast across the Seattle fault along the southern margin of the Seattle basin. S to P conversions are observed for some events and are likely converted at the bottom of the Seattle basin. We model five earthquakes, including the M 6.8 Nisqually earthquake, using 3D finite-difference simulations accurate up to 1 Hz. The simulations reproduce the observed dependence of amplification on the direction to the earthquake. The simulations generally match the timing and character of basin surface waves observed for many events. The 3D simulation for the Nisqually earthquake produces focusing of S waves along the southern margin of the Seattle basin near the area in west Seattle that experienced increased chimney damage from the earthquake, similar to the results of the higher-frequency 2D simulation reported by Stephenson et al. (2006). Waveforms from the 3D simulations show reasonable agreement with the data at low frequencies (0.2–0.4 Hz) for the Nisqually earthquake and an M 4.8 deep earthquake west of Seattle.

[Articles] Evidence of Two-Dimensional Amplification Effects in an Alluvial Valley (Valnerina, Italy) from Velocimetric Records and Numerical Models

Lenti, L., Martino, S., Paciello, A., Scarascia Mugnozza, G. — 2009-06-07

The results of field surveys, velocimetric records, and numerical models are reported with the goal of analyzing the local seismic response in a section of the Nera River alluvial valley in central Italy. Alluvial deposit geometry and dynamic properties were defined by geological and geophysical investigations. 2D amplification effects were pointed out by horizontal-to-vertical spectral ratios (HVSRs) as well as by horizontal-to-horizontal spectral ratios (HHSRs) to a reference station; HVSRs were derived from both noise and weak-motion records, while HHSRs were only computed from weak motions. Where travertine deposits are interlayered within the alluvia, 2D amplification effects are shown only by HHSRs. The observed amplifications: (1) consist of peaks of the amplification functions A(f) in the range 1–10 Hz, (2) generally correspond to two frequency ranges whose values mainly depend on the valley shaped ratio and on the local heterogenities of the alluvial deposits, and (3) can be regarded as due to a 1D resonance combined with lateral wave propagation. Numerical models via the finite difference method were performed by the Italian National Institute for Geophysics and Volcanology Web Interface for Seismological Application and by the Fast Lagrangian Analysis of Continua codes. The obtained results: (1) show a good agreement with HHSRs, (2) prove that some 2D amplification effects are constrained by both the shape ratio of the valley and the heterogeneity of the alluvial deposits, (3) reveal focalization effects within the alluvial deposits strictly related to the shape of the valley, and (4) allow the comparison of the A(f) functions in both linear and nonlinear dynamic behavior and the computation of the shear strains due to ground motion within the alluvial deposits.

[Articles] Determination of Shallow Shear-Wave Attenuation in the Mississippi Embayment Using Vertical Seismic Profiling Data

Ge, J., Pujol, J., Pezeshk, S., Stovall, S. — 2009-06-07

We used vertical seismic profiling (VSP) data collected in four shallow boreholes (about 40 to 60 m deep) to study the shear-wave attenuation in the Mississippi embayment in southwestern Tennessee. The source was an air-powered hammer that produces repeatable SH waves that were recorded by monitor geophones deployed on the surface very close to the source. The spectral ratio method was used to estimate the shear-wave quality factor (Q_S). The method assumes that the amplitudes of the seismic waves decay exponentially in the frequency domain. The spectral ratio was computed using a VSP trace at a certain depth and the corresponding monitor trace. Using the source monitor trace as reference eliminates possible artifacts that may be introduced by changes in the source-ground coupling. The slope of a least-square line fitted to the logarithm of the spectral ratio versus frequency gives the attenuation coefficient (z) for that depth. Then a straight line is fitted to (z) as a function of z, and the slope of this line is used to estimate an average value of Q_S. For the four sites, Shelby Farms, Covington, Brownsville, and Jackson, the Q_S values are 25.3±2.6, 18.6±3.4, 18.4±7.6, and 16.4±2.6, respectively. To assess the reliability of these results, synthetic VSP data were generated from the first cycles of the monitor traces recorded in the four sites and these Q_S values. Except for Brownsville, good matches with correlation coefficients equal to 96%–99% were obtained between the synthetic VSPs and the actual VSPs, indicating that the Q_S values we obtained are reliable. In addition, these values are also consistent with recent results obtained by others using seismic data generated by small earthquakes recorded by the Japanese strong-motion network Kiban-Kyoshin (KiK-net).

[Articles] In Situ Measurements of Nonlinear and Nonequilibrium Dynamics in Shallow, Unconsolidated Sediments

Lawrence, Z., Bodin, P., Langston, C. A. — 2009-06-07

We present results and in situ measurements from a field experiment in which a large seismic shaker truck is used to induce nonlinear and nonequilibrium dynamics in shallow, unconsolidated sediments. An array of accelerometers was deployed adjacent to the shaker truck to record strong ground motions exceeding 1 g. We determined high-strain Rayleigh-wave dispersion across the array in the band from 5 to 60 Hz. Rayleigh-wave phase velocities at frequencies above ~10 Hz are wave amplitude-dependent and a function of the driving-force amplitude; as driving force was increased phase velocity decreased, consistent with nonlinear dynamics. We demonstrate the existence of a temporary, nonequilibrium state occurring in the near-surface soils during and after the induced nonlinear behavior. Nonlinear conditioning is demonstrated by measuring changes in Rayleigh-wave phase velocity for input signals with the same applied driving-force amplitude. A logarithmic slow dynamic recovery process is observed by analyzing the temporal variation in velocity of the noise field produced by the shaker truck when sitting idle. Measurements from weak-motion seismic surveys taken before and after shaking show compelling evidence that induced nonlinear behavior in the shallow sediments is reversible. We demonstrate that an active source, field-based approach has the potential to expand our knowledge of how sediments respond to strong ground motions, provide additional insight into the poorly understood slow dynamic recovery process, and possibly even lead to a new, site-specific, and noninvasive technique for characterizing the nonlinear properties of sedimentary deposits.

[Articles] On Baseline Corrections and Uncertainty in Response Spectrafor Baseline Variations Commonly Encounteredin Digital Accelerograph Records

Akkar, S., Boore, D. M. — 2009-06-07

Most digital accelerograph recordings are plagued by long-period drifts, best seen in the velocity and displacement time series obtained from integration of the acceleration time series. These drifts often result in velocity values that are nonzero near the end of the record. This is clearly unphysical and can lead to inaccurate estimates of peak ground displacement and long-period spectral response. The source of the long-period noise seems to be variations in the acceleration baseline in many cases. These variations could be due to true ground motion (tilting and rotation, as well as local permanent ground deformation), instrumental effects, or analog-to-digital conversion. Very often the trends in velocity are well approximated by a linear trend after the strong shaking subsides. The linearity of the trend in velocity implies that no variations in the baseline could have occurred after the onset of linearity in the velocity time series. This observation, combined with the lack of any trends in the pre-event motion, allows us to compute the time interval in which any baseline variations could occur. We then use several models of the variations in a Monte Carlo procedure to derive a suite of baseline-corrected accelerations for each noise model using records from the 1999 Chi–Chi earthquake and several earthquakes in Turkey. Comparisons of the mean values of the peak ground displacements, spectral displacements, and residual displacements computed from these corrected accelerations for the different noise models can be used as a guide to the accuracy of the baseline corrections. For many of the records considered here the mean values are similar for each noise model, giving confidence in the estimation of the mean values. The dispersion of the ground-motion measures increases with period and is noise-model dependent. The dispersion of inelastic spectra is greater than the elastic spectra at short periods but approaches that of the elastic spectra at longer periods. The elastic spectra from the most basic processing, in which only the pre-event mean is removed from the acceleration time series, do not diverge from the baseline-corrected spectra until periods of 10–20 sec or more for the records studied here, implying that for many engineering purposes elastic spectra can be used from records with no baseline correction or filtering.

[Articles] Regions of Reduced Static Stress Drop near Fault Tips for Large Strike-Slip Earthquakes

Lovely, P. J., Pollard, D. D., Mutlu, O. — 2009-06-07

We present a case study of slip distributions for the 1992 Landers (M_w 7.3) and the 1999 Hector Mine (M_w 7.1) earthquakes in California’s Mojave Desert. Slip distributions, as determined from geophysical inversion of geodetic, strong ground motion, and teleseismic data, are complex, heterogeneous, and often exhibit linearly tapering or concave-upward patterns toward segment tips, distinctly different from the elliptical slip distributions characteristic of uniform stress drop. Mechanical interaction of discontinuous fault segments fully explains the reduced slip near the southern termination of the Camp Rock/Emerson fault segment of the Landers rupture; however, numerical models demonstrate that such interactions are insufficient to explain slip distributions observed at other segment terminations. Numerical models demonstrate that long (5–25 km) zones of reduced stress drop in the vicinity of some rupture segment terminations can explain the slip distributions for these large earthquakes. Zones of reduced stress drop are implemented as regions of increased Coulomb strength. Slip distributions are improved 30%–70% relative to models with uniform stress drop. Regions of reduced stress drop appear to play a relatively greater role near segment tips at which rupture terminates than near segment tips at which rupture jumps to a nearby fault segment. Similar results are obtained implementing discrete stepwise and spatially linear reductions of stress drop. Plausible mechanical explanations for such zones of reduced stress drop include heterogeneous fault strength or friction, spatial or temporal changes in pore pressure, geometric complexity of the fault surface, heterogeneity of normal tractions resolved on the fault surface, inelastic deformation, and dynamic rupture effects.

[Articles] Source Scaling and Site Effects at Vesuvius Volcano

Galluzzo, D., Del Pezzo, E., La Rocca, M., Castellano, M., Bianco, F. — 2009-06-07

The site-corrected source scaling pattern is estimated for local earthquakes (0.9≤M_D≤3.6) at Mt. Vesuvius. The dataset comprises 35 low-to-moderate local earthquakes recorded by 14 three-component seismic stations during 1993, 1996, and 1999.

Site-transfer functions in the frequency range 1 Hz–25 Hz are estimated from the spectra of S waves and coda waves and from the horizontal-to-vertical (H/V) spectral ratios. We applied the direct spectral ratios method to S waves, considering as a reference the average spectrum and the inversion method to S waves and coda waves. The site amplification on the coda waves was also compared with that evaluated using the wavelet transform. The standard deviation associated with the experimental results is computed for all of the used methods.

Results indicate a general agreement among the methods, and the site-transfer functions show interesting features. The highest amplifications are found for frequencies lower than 12 Hz for sites located at lower altitude. The methods based on coda waves show highest amplification with respect to the methods based on S waves for most of the sites located in the summit part of the volcano. This can be a phenomenon of coda localization, which consists in the trapping inside the upper part of the volcano of scattered waves. The H/V spectral ratios do not show total agreement with the other methods, mostly for the sites located in the summit part of the volcano. The discrepancies among the results obtained in this work are also due to the different normalization applied in the methods of analysis.

Generalized inversion method allowed us to estimate the source scaling of the site-corrected source seismic spectrum for the investigated area. The source scaling obtained in terms of seismic moment and source radii shows that the seismicity of Mt. Vesuvius is characterized by stress drop as low as a few bars (10 bars) except for the event of ( ). The scaling pattern shows an apparent linear relationship between source size and seismic moment (for M_D≤3.3) but the statistical test shows that the linear trend has low reliability.

[Articles] Dynamic Driving of Small Shallow Events during Strong Motion

Fischer, A. D., Sammis, C. G. — 2009-06-07

High-pass filtering (>20 Hz) of acceleration records from the 1999 Chi-Chi, Taiwan, and 2004 Parkfield, California, earthquakes reveals a series of bursts that occur only during strong shaking. Initially interpreted as originating from asperity failure on the Chelungpu fault, bursts observed during the Chi-Chi earthquake were subsequently determined to be a local effect within about 1 km of the seismic stations. Similar bursts were observed at the U.S. Geological Survey Parkfield seismic array during the Parkfield earthquake and were constrained to originate less than 20 m from the instruments. Such small shallow events cannot result from the triggered release of stored elastic energy because rate-and-state friction rules out stick-slip instability on such small, shallow patches. Our hypothesis is that the bursts are not triggered but are driven by simultaneous shear and tensile stresses near the surface during the strong motion. At 2 Hz, SV- to P-wave mode conversion at the free surface produces tensile stresses to depths of 70 m. Where standard triggering releases stored elastic energy and adds to the incident wave field, this new driving mechanism takes energy out of the 2 Hz strong motion and reradiates it at high frequencies. It is thus an attenuation mechanism that we estimate can contribute 3% to the net attenuation in the very shallow crust.

[Articles] Structural Geometry and Slip of the Palos Verdes Fault, Southern California: Implications for Earthquake Hazards

Brankman, C. M., Shaw, J. H. — 2009-06-07

The Palos Verdes fault (PVF) is an active structure in southern California comprised of several segments that together form a complex fault system. The fault has been active since the Miocene and is a major regional seismic source. Using marine petroleum industry seismic data, we define the geometry of the fault and offsets of Tertiary stratigraphic units that constrain the along strike segmentation and slip rates on the fault system. In San Pedro Bay, patterns of deformed stratigraphic units suggest a deep fault geometry that dips moderately to the southwest and was initially formed as a Miocene normal fault. Plio–Pleistocene transpression resulted in the reactivation of this normal fault and its subsequent inversion with oblique, right-lateral reverse displacement. To the south, the Lasuen Knoll segment dips steeply to the northeast and is not a reactivated Miocene-age structure but rather formed in middle Pliocene time. Plio–Pleistocene transpression has linked these and other segments together to form the currently active PVF system. The resulting complex fault geometry and slip patterns suggest that the PVF is a highly segmented fault, which may rupture in moderate-size earthquakes that involve individual segments or large events that are able to propagate across significant geometric discontinuities. Estimates of moment magnitudes based on empirical relationships with rupture (fault) area indicate the possibility of M_w 6.6–6.9 earthquakes for single-segment ruptures and M_w 7.1–7.3 multisegment ruptures, with recurrence intervals ranging from 181 to 534 yr. Given the high inferred slip rates on the fault and its proximity to the urban population, these events pose significant earthquake hazards to metropolitan Los Angeles.

[Articles] The 1911 M~6.6 Calaveras Earthquake: Source Parameters and the Role of Static, Viscoelastic, and Dynamic Coulomb Stress Changes Imparted by the 1906 San Francisco Earthquake

Doser, D. I., Olsen, K. B., Pollitz, F. F., Stein, R. S., Toda, S. — 2009-06-07

The occurrence of a right-lateral strike-slip earthquake in 1911 is inconsistent with the calculated static stress decrease imparted by the 1906 rupture at that location on the Calaveras fault, and 5 yr of calculated post-1906 viscoelastic rebound does little to reload the fault. We have used all available first-motion, body-wave, and surface-wave data to explore possible focal mechanisms for the 1911 earthquake. We find that the event was most likely a right-lateral strike-slip event on the Calaveras fault, larger than, but otherwise resembling, the 1984 M_w 6.1 Morgan Hill earthquake in roughly the same location. Unfortunately, we could recover no unambiguous surface fault offset or geodetic strain data to corroborate the seismic analysis despite an exhaustive archival search. We calculated the static and dynamic Coulomb stress changes for three 1906 source models to understand stress transfer to the 1911 site. In contrast to the static stress shadow, the peak dynamic Coulomb stress imparted by the 1906 rupture promoted failure at the site of the 1911 earthquake by 1.4–5.8 bar. Perhaps because the sample is small and the aftershocks are poorly located, we find no correlation of 1906 aftershock frequency or magnitude with the peak dynamic stress, although all aftershocks sustained a calculated dynamic stress of ≥3 bar. Just 20 km to the south of the 1911 epicenter, we find that surface creep of the Calaveras fault at Hollister paused for ~17 yr after 1906, about the expected delay for the calculated static stress drop imparted by the 1906 earthquake when San Andreas fault postseismic creep and viscoelastic relaxation are included. Thus, the 1911 earthquake may have been promoted by the transient dynamic stresses, while Calaveras fault creep 20 km to the south appears to have been inhibited by the static stress changes.

[Articles] A Viscoelastic Earthquake Simulator with Application to the San Francisco Bay Region

Pollitz, F. F. — 2009-06-07

Earthquake simulation on synthetic fault networks carries great potential for characterizing the statistical patterns of earthquake occurrence. I present an earthquake simulator based on elastic dislocation theory. It accounts for the effects of interseismic tectonic loading, static stress steps at the time of earthquakes, and postearthquake stress readjustment through viscoelastic relaxation of the lower crust and mantle. Earthquake rupture initiation and termination are determined with a Coulomb failure stress criterion and the static cascade model. The simulator is applied to interacting multifault systems: one, a synthetic two-fault network, and the other, a fault network representative of the San Francisco Bay region. The faults are discretized both along strike and along dip and can accommodate both strike slip and dip slip. Stress and seismicity functions are evaluated over 30,000 yr trial time periods, resulting in a detailed statistical characterization of the fault systems. Seismicity functions such as the coefficient of variation and a- and b-values exhibit systematic patterns with respect to simple model parameters. This suggests that reliable estimation of the controlling parameters of an earthquake simulator is a prerequisite to the interpretation of its output in terms of seismic hazard.

[Articles] Statistics of Earthquake Stress Drops on a Heterogeneous Fault in an Elastic Half-Space

Bailey, I. W., Ben-Zion, Y. — 2009-06-07

We investigate properties of earthquake stress drops in simulations of evolving seismicity and stress field on a heterogeneous fault. The model consists of an inherently discrete strike-slip fault surrounded by a 3D elastic half-space. We consider various spatial distributions of frictional properties and analyze results generated by 150–300 model years. In all cases, the self-organized heterogeneous initial stress distributions at the times of earthquake failure lead to stress drops that are systematically lower than those predicted for a homogeneous process. In particular, the large system-sized events have stress drops that are consistently ~25% of predictions based on the average fault strength. The type and amount of assumed spatial heterogeneity on the fault affect the stress-drop statistics of small earthquakes (M_L<5) more than those of the larger events. This produces a decrease in the range of stress drops as the earthquake magnitudes increase. The results can resolve the discrepancy between traditional estimates of stress drops and seismological observations. The general tendency for low stress drops of large events provides a rationale for reducing the statistical estimates of potential ground motion associated with large earthquakes.

[Articles] Earthquake Stress Drops and Inferred Fault Strength on the Hayward Fault, East San Francisco Bay, California

Hardebeck, J. L., Aron, A. — 2009-06-07

We study variations in earthquake stress drop with respect to depth, faulting regime, creeping versus locked fault behavior, and wall-rock geology. We use the P-wave displacement spectra from borehole seismic recordings of M 1.0–4.2 earthquakes in the east San Francisco Bay to estimate stress drop using a stack-and-invert empirical Green’s function method. The median stress drop is 8.7 MPa, and most stress drops are in the range between 0.4 and 130 MPa. An apparent correlation between stress drop and magnitude is entirely an artifact of the limited frequency band of 4–55 Hz. There is a trend of increasing stress drop with depth, with a median stress drop of ~5 MPa for 1–7 km depth, ~10 MPa for 7–13 km depth, and ~50 MPa deeper than 13 km. We use S/P amplitude ratios measured from the borehole records to better constrain the first-motion focal mechanisms. High stress drops are observed for a deep cluster of thrust-faulting earthquakes. The correlation of stress drops with depth and faulting regime implies that stress drop is related to the applied shear stress. We compare the spatial distribution of stress drops on the Hayward fault to a model of creeping versus locked behavior of the fault and find that high stress drops are concentrated around the major locked patch near Oakland. This also suggests a connection between stress drop and applied shear stress, as the locked patch may experience higher applied shear stress as a result of the difference in cumulative slip or the presence of higher-strength material. The stress drops do not directly correlate with the strength of the proposed wall-rock geology at depth, suggesting that the relationship between fault strength and the strength of the wall rock is complex.

[Articles] Differential Energy Radiation from Two Earthquakes in Japan with Identical Mw: The Kyushu 1996 and Tottori 2000 Earthquakes

Choy, G. L., Boatwright, J. — 2009-06-07

We examine two closely located earthquakes in Japan that had identical moment magnitudes M_w but significantly different energy magnitudes M_e. We use teleseismic data from the Global Seismograph Network and strong-motion data from the National Research Institute for Earth Science and Disaster Prevention’s K-Net to analyze the 19 October 1996 Kyushu earthquake (M_w 6.7, M_e 6.6) and the 6 October 2000 Tottori earthquake (M_w 6.7, M_e 7.4). To obtain regional estimates of radiated energy E_S we apply a spectral technique to regional (<200 km) waveforms that are dominated by S and Lg waves. For the thrust-fault Kyushu earthquake, we estimate an average regional attenuation Q(f)=230f^0.65. For the strike-slip Tottori earthquake, the average regional attenuation is Q(f)=180f^0.6. These attenuation functions are similar to those derived from studies of both California and Japan earthquakes. The regional estimate of E_S for the Kyushu earthquake, 3.8x10¹⁴ J, is significantly smaller than that for the Tottori earthquake, E_S 1.3x10¹⁵ J. These estimates correspond well with the teleseismic estimates of 3.9x10¹⁴ J and 1.8x10¹⁵ J, respectively. The apparent stress (_a=µE_S/M₀, with µ equal to rigidity) for the Kyushu earthquake is 4 times smaller than the apparent stress for the Tottori earthquake. In terms of the fault maturity model, the significantly greater release of energy by the strike-slip Tottori earthquake can be related to strong deformation in an immature intraplate setting. The relatively lower energy release of the thrust-fault Kyushu earthquake can be related to rupture on mature faults at a subduction environment. The consistence between teleseismic and regional estimates of E_S is particularly significant as teleseismic data for computing E_S are routinely available for all large earthquakes whereas often there are no near-field data.

[Articles] A Method for Rapid Determination of Moment Magnitude Mw for Moderate to Large Earthquakes from the Near-Field Spectra of Strong-Motion Records (MWSYNTH)

Delouis, B., Charlety, J., Vallee, M. — 2009-06-07

Seismic moment and the corresponding moment magnitude M_w are classically obtained from the spectrum of far-field body waves. Near-field records are generally not used for that purpose, particularly in the case of large earthquakes because different types of wave arrive simultaneously, preventing the definition of a simple relation between the seismic moment and the spectrum. We developed an original method to determine M_w from the displacement spectra of near-field records. The spectral amplitude at low frequency obtained from the real records is compared to that of synthetic records computed using kinematic rupture models scaled with M_w. Synthetic records are computed and averaged for various fault orientations and for epicentral distances ranging from 1 to 100 km. The initial portion of the spectrum affected by baseline shift in the acceleration records is automatically identified and removed by high-pass filtering using a cutoff frequency adapted to each station. The synthetic spectral values as a function of moment magnitude, epicentral distance, and filtering are computed only once and stored in tables. The spectral amplitudes of the real records are simply interpolated in the tables of synthetic data, allowing a fast determination of M_w. The method has been validated using 22 shallow earthquakes (depth<50 km) with magnitude ranging from 3.9 to 7.7. We show that a window of 80 sec of signal after the earthquake origin time provides robust values of M_w for the whole magnitude range considered here. Shorter time windows may be used but with M_w underestimated for large events. The method is well suited for near real-time fast determination of M_w.

[Articles] Comparison of Magnitude Estimates for New Zealand Earthquakes: Moment Magnitude, Local Magnitude, and Teleseismic Body-Wave Magnitude

Ristau, J. — 2009-06-07

New Zealand is one of the more seismically active countries in the world, with more than 15,000 earthquakes located each year. Routine moment tensor analysis of regional seismic data for earthquakes with moment magnitude M_w>~3.5–4.0 has recently been implemented in New Zealand. Nearly 330 regional moment tensor (RMT) solutions have been calculated for earthquakes in the New Zealand region dating back to late 2003. This complements local magnitude (M_L), the primary magnitude calculated by GeoNet in New Zealand. The RMT catalog, along with 155 Global Centroid Moment Tensor (Global CMT) catalog solutions, is used to compare M_w with M_L for New Zealand earthquakes. In addition to M_w and M_L, there are more than 330 teleseismic body-wave magnitude (m_b) values available from the United States Geological Survey and International Seismological Center catalogs for events that also have an M_w. These are used to examine the relationship between M_w, M_L, and m_b for New Zealand earthquakes. There is a clear distinction in the relationship of M_L to M_w for shallow focus (≤33 km depth) and deep focus (>33 km depth) earthquakes. Shallow focus earthquakes show M_L to be fairly consistent with M_w, particularly for events with M_w>~4.5 and with . Deep focus earthquakes have M_L consistently larger than M_w (more than a full magnitude unit for some events) with . M_w and m_b are in fairly good agreement regardless of the depth, whereas m_b estimates are consistently smaller than M_L for deep events. This suggests that M_L is overestimated for deep focus events and that the large M_Ls are not the result of physical properties of the source.

[Articles] Source Parameters for the 28 April 2007 Mw 4.0 Earthquake in Folkestone, United Kingdom

Ottemoller, L., Baptie, B., Smith, N. J. P. — 2009-06-07

A moderate size earthquake (M_L 4.3, M_w 4.0) occurred in southeastern England on 28 April 2007. The earthquake was of some significance as it caused damage in the town of Folkestone and produced the largest peak horizontal ground acceleration (PGA, 0.1g) measured in the United Kingdom to date. It was followed by 12 aftershocks between M_L 0.8 and 1.7. The earthquake was the first of this size recorded by a significant number of newly installed broadband stations in the United Kingdom. The hypocenter of the event was at a depth of about 5 km beneath Folkestone, with an error ellipse indicating horizontal errors in a location of about 5 km. The depth was well constrained using a number of techniques, of which local travel-time inversion and teleseismic depth phase modelling are most reliable. A stress drop of 28.6 bars and a source radius of 0.5 km were determined from the analysis of displacement source spectra. We derived a near-surface attenuation factor =0.02 from the aftershock data that were used in the spectral analysis of the mainshock. Applying the horizontal to vertical (H/V) spectral ratio technique to microtremor data recorded at a station 2 km from the epicenter revealed site amplification at frequencies of 0.4 and 3.9 Hz. This amplification is likely to have contributed to the mainshock PGA of 0.1g measured at the same site. Similar site conditions may have been responsible for the damage in parts of Folkestone. The moment tensor computed from regional broadband data showed a strike-slip mechanism with a normal component and either right-lateral movement on a west-southwest–east-northeast-striking or left-lateral movement on a north-northwest–south-southeast-striking nodal plane. The north-northwest–south-southeast-striking nodal plane matches the trend of the main faults affecting the Kent coalfield and also possibly the Variscan front. It is thus possible that the causative fault was associated with the Variscan front, a major structural boundary at the northern limit of late Carboniferous folding and thrusting.

[Articles] First Application of the New IASPEI Teleseismic Magnitude Standards to Data of the China National Seismographic Network

Bormann, P., Liu, R., Xu, Z., Ren, K., Zhang, L., Wendt, S. — 2009-06-07

This article presents the results of the application of the new measurement standards for teleseismic magnitudes. They have been applied in parallel with traditional Chinese magnitude procedures to more than 14,000 digital broadband-velocity seismograms of 531 earthquakes in the magnitude range 4–9. The records were made between 2001 and 2007 at stations of the China National Seismographic Network (CNSN) and analyzed at the China Earthquake Network Center (CENC). The regression relations between different types of standard magnitudes, traditional Chinese magnitudes, and the respective values of m_b, M_S, and M_e published by the National Earthquake Information Center (NEIC), as well as Global Centroid Moment Tensor (GCMT) moment magnitudes M_w, are presented. The new broadband body-wave magnitude m_B(BB) is measured at dominant periods in the range 0.2<T<30 sec that on average increase exponentially with magnitude, as expected by seismic scaling laws. The broadband surface-wave magnitude M_S(BB) measures the maximum Rayleigh-wave amplitudes in a wide range of periods (3≤T<60 sec) and epicentral distances (2°≤≤160°), as originally proposed for the current International Association of Seismology and Physics of the Earth’s Interior (IASPEI) standard calibration function for M_S determination. The article analyzes and disproves with rich data several widespread prejudices or misunderstandings concerning the applicability, stability, and accuracy of magnitudes based on readings from unfiltered broadband records in general and the IASPEI M_S formula in particular. It is shown that the use of surface-wave amplitudes in a broad period range removes most of the systematic distance dependence that is observed when the IASPEI formula is restricted to periods around 20 sec. In the distance range between 2° and 103°, we have investigated the trend between M_S(BB) and M_S(20), which is the common surface-wave magnitude determined by the NEIC for periods between 18 and 22 sec and at teleseismic distances only. The two magnitudes agree rather well and have comparable measurement errors, even when M_S(BB) is measured at regional distances of less than 20°. On average, however, M_S(BB) is slightly larger than M_S(20) at distances less than 45° with a small and consistent distance-dependent trend in the residuals M_S(BB)-M_S(20) of -0.0029 magnitude units per degree in both the regional and teleseismic distance range. The analyzed regression relations further reveal the following: (1) the new standard for m_b delays saturation and yields about 0.5 units larger m_b values for great earthquakes; (2) broadband m_B(BB) moves the saturation limit for body-wave magnitudes further up to about 8.3. This makes m_B(BB) a good candidate for providing rapid magnitude estimates of strong earthquakes well ahead of M_w; (3) standard M_S(20) scales perfectly with the M_S published by the NEIC; (4) M_S(BB) reduces the underestimation of surface-wave magnitude by M_S(20) for weaker earthquakes and those recorded at regional distances for which the maximum surface-wave amplitudes occur mostly at periods well below 16 sec; and (5) new m_B(BB) and M_S(BB), that is, the body-wave and surface-wave magnitudes directly measured on velocity-broadband records, reproduce well the classical Gutenberg–Richter relation between m_B and M_S, which—together with the relation between seismic energy E_S and m_B—formed the basis for deriving the modern moment- and energy-magnitude scales M_w and M_e.

[Articles] Source Phenomenology Experiments with Borehole Explosions of Special Design in Israel

Gitterman, Y. — 2009-06-07

Two source phenomenology explosion experiments—decoupling and depth-of-burial (DOB)—were conducted by the Geophysical Institute of Israel (GII) in July 2006 and January 2007 in Negev, Israel. A special blast design and technology were utilized to form large 1240 kg (decoupling) and 4200 kg (DOB) near-spherical Ammonium Nitrate and fuel oil charges in cavities at different depths (26–63 m), in marls and phosphates, created beforehand by small shots in boreholes.

The main goal of the experiments was investigation of empirical relationships between the contained spherical source and energy/magnitude parameters of different seismic phases at near-source and regional distances.

Small decoupling factors for the heavily overdriven tests were estimated as 2.8 and 6.4 for charge/volume ratios 250 and 70 (kg/m³), respectively. Very high signal frequencies (40–90 Hz) accompanied by the highest peak accelerations were observed at near-source distances for the deep (63 m) decoupled explosion. A clear magnitude/energy reduction with increasing depth of contained sources was obtained at regional distances (similar to the 1997 Balapan DOB experiment), complemented by near-source recordings of higher frequencies and larger amplitude/energy for deeper charges. Observations of the Negev DOB experiment are remarkably consistent with the Mueller and Murphy (1971) model predictions of source spectra features. The same conversion point of spectra dominance at f_c~10 Hz for different depth shots, predicted by the model, was observed over a broad distance range (0.2–230 km).

The novel aspects of the experiments include (1) investigating seismic coupling in a source medium (soft sediments) that was not previously studied to a large extent, (2) using a homogeneous source medium for all the tests, (3) using nearly spherical explosive sources as opposed to the long cylindrical charges used in most previous large-scale high explosives tests, and (4) full containment of all explosions.

The experiments, designed to simulate nuclear sources, provide data and knowledge for improvement of nuclear test monitoring for low-yield explosions.

[Articles] Automatic S-Wave Picker for Local Earthquake Tomography

Diehl, T., Deichmann, N., Kissling, E., Husen, S. — 2009-06-07

High-resolution seismic tomography at local and regional scales requires large and consistent sets of arrival-time data. Algorithms combining accurate picking with an automated quality classification can be used for repicking waveforms and compiling large arrival-time data sets suitable for tomographic inversion. S-wave velocities represent a key parameter for petrological interpretation, improved hypocenter determination, as well as for seismic hazard models. In our approach, we combine three commonly used phase detection and picking methods in a robust S-wave picking procedure. Information from the different techniques provides an in situ estimate of timing uncertainty and of the reliability of the automatic phase identification. Automatic picks are compared against manually picked reference picks of selected earthquakes in the Alpine region. The average accuracy of automatic picks and their classification is comparable with the reference picks, although a higher number of picks is downgraded to lower quality classes by the automatic picker. In the production-mode, we apply the picker to a data set of 552 earthquakes in the Alps recorded at epicentral distances ≤150 km. The resulting data set includes about 2500 S phases with an upper error bound of 0.27 sec.

[Articles] Determination of Seismogenic Structures in Southeastern Sicily (Italy) by High-Precision Relative Relocation of Microearthquakes

Brancato, A., Hole, J. A., Gresta, S., Beale, J. N. — 2009-06-07

In December 1990, southeastern Sicily was struck by a moderate earthquake (M_L 5.4). The event and the subsequent aftershocks represent renewed activity for the Malta Escarpment fault system (MEFS) three centuries after the most recent of several large destructive historical earthquakes. An 11 yr period of quiescence was interrupted with a minor swarm in 2002 (M_max 3.6). These sequences and regional seismicity were relocated through 3D tomography. The resulting velocity model indicates a velocity contrast in the vicinity of MEFS. High-precision relative relocations were derived by the cross-spectral method for 18 events from the 1990 sequence and 10 earthquakes from the 2002 swarm. These swarms were aligned along fault segments striking east-northeast–west-southwest, conjugate to the north-northwest–south-southeast orientation of the MEFS. We conclude that the MEFS is fragmented in segments with different seismic behaviors, and that great stress accumulation along the main segment of the MEFS is less likely.

[Articles] Teleseismic Receiver Function and Surface-Wave Study of Velocity Structure beneath the Yanqing-Huailai Basin Northwest of Beijing

Zhou, R.-M., Stump, B. W., Herrmann, R. B., Yang, Z.-X., Chen, Y.-T. — 2009-06-07

Shear-wave velocities beneath the Yanqing-Huailai Basin, 90–140 km northwest of Beijing, are estimated from the joint inversion of surface-wave phase velocities and teleseismic receiver functions. The data set is from a temporary broadband seismic network supported by the Program for Array Seismic Studies of the Continental Lithosphere (PASSCAL) in the basin and includes 34 teleseismic events from 2003 to 2005.

Receiver functions from the teleseismic events are similar for the stations around the Yanqing-Huailai Basin and exhibit little variation with azimuth. The velocity models constrained by receiver functions and surface-wave dispersion curves are also similar. The resulting models reflect the low-velocity basin sediments to 2 km followed by a positive velocity gradient to 15 km with shear-wave velocity increasing from 2.0 to 3.55 km/sec. Evidence of a midcrust low-velocity layer starts at 15 km with a shear velocity decrease to 3.3 km/sec that extends to approximately 25 km. The total crustal thickness is 38–42 km with a smooth Moho transition to an upper-mantle shear velocity of 4.3 km/sec. The low-velocity zone is consistent with recent extension, geothermal activity, and earthquake locations above this depth.

The average shear velocity model for the basin has similarities to other regional and global models but provides more detailed structure in the uppermost and lower portions of the crust. The new model includes the effect of the sediments in the basin, the low-velocity layer, and the gradual Moho transition. Predicted P- and S-travel times are 1–3.5 sec slower than the previous models at regional distances.

[Articles] P- and S-Wave Receiver Function Images of Crustal Imbrication beneath the Cheyenne Belt in Southeast Wyoming

Hansen, S., Dueker, K. — 2009-06-07

We extend the receiver function deconvolution methodology of Bostock (2004) to S-wave receiver functions and develop a method of source function spectrum estimation to constrain the crustal structure across the Archean–Proterozoic Cheyenne belt suture in southeast Wyoming using data from a dense deployment of seismic stations. S-wave receiver functions are particularly useful because free-surface reverberations do not contaminated the direct S_dp arrivals, and the S-wave receiver function image is able to validate our P-wave receiver function image. P- and S-wave receiver function images and a teleseismic P-wave tomogram find a structure consistent with the imbrication of Proterozoic lower crust across the Cheyenne belt. Both P and S-wave receiver function images delineate a double Moho north of the Cheyenne belt: the Archean Moho is imaged at 41–43 km depth with a deeper velocity step at 60–62 km depth. South of the Cheyenne belt, the P-wave receiver function image finds the Proterozoic Moho dipping ~7° northwest consistent with observed back-azimuth dependent P_ms amplitudes. Given the lateral continuity with the northwest dipping Proterozoic Moho, the deeper velocity step of the double Moho is interpreted as the imbricated Proterozoic Moho. Modeling of P-wave receiver function amplitudes suggests a 6.4–7.4 km/sec velocity step across the shallower Archean Moho and a 7.4–7.9 km/sec velocity step across the deeper imbricated Proterozoic Moho. We speculate that imbrication of the Proterozoic lower crust was contemporaneous with the 1.76 Ga uplift and deformation of the 50 km-wide Palmer Canyon block immediately north of the Cheyenne belt exposed in the Laramie Mountains.

[Articles] 3D Scattering Image of Mt. Vesuvius

Tramelli, A., Del Pezzo, E., Fehler, M. C. — 2009-06-07

In this article we apply a passive scattering-imaging method, derived from the method developed by Nishigami (1991) to data from the coda of the local volcano-tectonic (VT) earthquakes of Mt. Vesuvius. This method provides the space distribution of the strong scatterers together with a rough estimate of their strength. In the development of our method we use a realistic raytracing calculated with a ray-bending approach in the 3D velocity model of Mt. Vesuvius structure obtained with travel-time inversion. The inversion procedure adopted for the scattering imaging in the present study is based on the conjugate gradient method (CGM). The volume under study is divided into cubic cells with different dimensions in a multiscale approach. We obtain the best resolution (900 m cubic cell size) in the central part of the volume under study (roughly in a radius of 4 km centered in the crater) within a depth of 5 km. We analyzed the coda signals after filtering in two frequency bands, the first centered at 12 Hz and the second at 18 Hz, where most of the seismic energy is concentrated. Results show that most of the strong scatterers are located in the depth range between the surface and 3000 m below the sea level, in correspondence with the crater axis where most of the seismicity occurs. Part of the scatterers are located in the zones characterized by the maximum velocity contrasts.

[Articles] Deconvolution of Three-Component Teleseismic Data from Southern Tibet Using the SVA Technique

Dasgupta, S., Nowack, R. L., Mitra, S. — 2009-06-07

In this study, the SV autocorrelation (SVA) technique of Dasgupta and Nowack (2006) is tested using three-component P-wave data from selected INDEPTH II and CDSN stations in southern Tibet. The SVA technique involves constructing an estimate of the source and distant Earth wavelet from the autocorrelation of the SV component and using this to deconvolve the data. The deconvolved vertical components are then used to infer the crustal P-velocity structure. Initial models for the inversion of the deconvolved vertical components were based on the S-velocity results obtained from the inversions of radial receiver functions by Mitra et al. (2005). The Moho depths obtained from inverting the vertical components deepen from the south to the north in the study area and are comparable to the depths obtained by Mitra et al. (2005) to the north and similar but somewhat shallower to the south. The crustal V_P/V_S ratios are obtained from the inverted V_S models from Mitra et al. (2005) and the inferred V_P speeds obtained from this study. Except for station BB18 with a somewhat higher V_P/V_S ratio, the other stations to the north in the Tethyan Himalayas and the southern Lhasa terrane have V_P/V_S ratios between 1.75 and 1.80 with a mean of 1.77 (a Poisson’s ratio of 0.265). The range of values could result from noise in the data as well as from lateral heterogeneity in the region with different piercing points at depth for the Ps and PpPp phases.

[Articles] Localized Boundary Integral Equation-Discrete Wavenumber Method for Simulating Wave Propagation in Irregular Multiple Layers, Part I: Theory

Zhou, H., Chen, X. — 2009-06-07

We present a new method of synthesizing seismograms for irregular multilayered problems. It is an extension of the local (lo) boundary integral equation (BIE) discrete wavenumber method (DWM) topography problem. Following similar procedures as those developed in solving the P-SV waves of topography problems, we first provide the formulation of Bouchon and Campillo’s BIE–DWM (Bouchon, 1985; Campillo and Bouchon, 1985) for the multilayered problem. By orthogonally decomposing the forces on irregular and flat parts of each interface and applying a discrete Fourier transform (DFT) we derive their relation. Finally, considering the continuity of displacement and traction on each interface, we get a linear equation only involving the unknown forces on irregular parts of interfaces and discuss its solution. In this algorithm the dimension of the lineal equation is decided by the sampling number on irregular parts of interfaces. Therefore, its computation efficiency increases dramatically, particularly for the problem in which the corrugated part of the layer is highly localized.

[Articles] Localized Boundary Integral Equation-Discrete Wavenumber Method for Simulating Wave Propagation in Irregular Multiple Layers, Part II: Validation and Application

Zhou, H., Chen, X. — 2009-06-07

In the present study, the localized boundary integral equation–discrete wavenumber method (loBIE–DWM) obtained in a companion article (Zhou and Chen, 2009) is validated. For SH waves our results are tested by comparing them with Cao’s (Cao, 2003), while for P-SV waves they are compared with Kawase’s (Kawase and Aki, 1989). The good agreement demonstrated that the method has the same accuracy as other methods. We subsequently used this method to synthesize wave propagation in two examples and discuss the effects of irregular interfaces. Finally, we synthesized the velocity records at the Baijiatuan station due to the Zhangbei earthquake to demonstrate the ability of the loBIE–DWM to handle the problem with the long epicenter distance. In order to help understand the formulation of the loBIE–DWM, the numerical implementation and efficiency analysis for a simple example are given.

[Short Notes] Is There a Basis for Preferring Characteristic Earthquakes over a Gutenberg-Richter Distribution in Probabilistic Earthquake Forecasting?

Parsons, T., Geist, E. L. — 2009-06-07

The idea that faults rupture in repeated, characteristic earthquakes is central to most probabilistic earthquake forecasts. The concept is elegant in its simplicity, and if the same event has repeated itself multiple times in the past, we might anticipate the next. In practice however, assembling a fault-segmented characteristic earthquake rupture model can grow into a complex task laden with unquantified uncertainty. We weigh the evidence that supports characteristic earthquakes against a potentially simpler model made from extrapolation of a Gutenberg–Richter magnitude-frequency law to individual fault zones. We find that the Gutenberg–Richter model satisfies key data constraints used for earthquake forecasting equally well as a characteristic model. Therefore, judicious use of instrumental and historical earthquake catalogs enables large-earthquake-rate calculations with quantifiable uncertainty that should get at least equal weighting in probabilistic forecasting.

[Short Notes] The Seismicity along the Dead Sea Fault during the Last 60,000 Years

2009-06-07

Evidence for unchanging slip rate and a Gutenberg–Richter relation for earthquake distribution along the Dead Sea fault during the past 60,000 yr are presented. The evidence comes from three different segments, approximately 100 km apart, and from three different timescales: prehistoric–paleoseismic, historical, and modern (instrumental) records. The paleoseismic data are based on two different methods. In the southern Arava Valley and the northern Jordan Valley segments, the amount of normal displacement along several faults is used, while in the Dead Sea basin the appearance of brecciated beds, which are considered as seismites, is used. We found that for the southern Arava Valley segment a constant dip-slip rate of 0.5 mm/yr can explain the cumulative normal slip during the past 45,000 yr. This suggests that normal faulting is only ~10% of the total left-lateral strike-slip motion. We also found that for all three segments, the paleoseismic and historical records of strong earthquakes lie on the linear extrapolation of the frequency–magnitude relation of the instrumental record. The calculated b-values for all three segments are between 0.85 and 1, similar to other major strike-slip faults in the world. It is concluded that the Gutenberg–Richter distribution is a stable mode in the tectonic setting of the Dead Sea fault during the past 60,000 yr.

[Short Notes] Arrival-Time Order Location Revisited

Rosenberger, A. — 2009-06-07

A fast method to estimate an earthquake epicentral region from the very first P-wave arrivals is relevant in the context of early warning and rapid response systems. Arrival-time order location (AOL) uses the sequence in which an array of sensors detects an event to constrain the epicentral region based on simple geometrical relations. No velocity model is needed. This article recasts AOL using the theory of spatial tessellations and describes a solution based on higher order Voronoi diagrams. The implementation can take advantage of modern computational geometry algorithms. The required calculations are fast and are suitable for a real-time system.

[Short Notes] Brittle Creep Damage as the Seismic Signature of Dyke Propagations within Basaltic Volcanoes

Traversa, P., Grasso, J.-R. — 2009-06-07

Contemporary to nine dyke intrusions on Piton de la Fournaise, Etna, and Miyakejima volcanoes, we recover stationary seismicity rate and energy release over time, whether the dyke reaches the surface or not. This generic seismicity pattern for the dyke propagation of low viscosity magma argues for the fluid driven crack propagation to be a scale independent stationary process. This prevents any prediction of the time to eruption during the dyke propagation phase using seismicity rate alone. The seismic signature of the volcano deformation triggered by dyke injections corresponds to brittle creep damage in a strain driven setting. Whether mechanical properties of host rock structure or geometrical effects influence this generic stationary response is not resolved by the seismic data. Because a few, if any, aftershocks are resolved contemporary to dyke intrusion, the seismicity is purely driven by the dyke dynamics, that is, a proxy for the dyke volumetric growth.

[Short Notes] Source Parameters for Moderate Earthquakes in the Zagros Mountains with Implications for the Depth Extent of Seismicity

Adams, A., Brazier, R., Nyblade, A., Rodgers, A., Al-Amri, A. — 2009-06-07

Six earthquakes within the Zagros Mountains with magnitudes between 4.9 and 5.7 have been studied to determine their source parameters. These events were selected for study because they were reported in open catalogs to have lower crustal or upper mantle source depths and because they occurred within an area of the Zagros Mountains where crustal velocity structure has been constrained by previous studies. Moment tensor inversion of regional broadband waveforms has been combined with forward modeling of depth phases on short-period teleseismic waveforms to constrain source depths and moment tensors. Our results show that all six events nucleated within the upper crust (<11 km depth) and have thrust mechanisms. This finding supports other studies that call into question the existence of lower crustal or mantle events beneath the Zagros Mountains.

[Errata] Erratum to Dynamic Stresses, Coulomb Failure, and Remote Triggering

Hill, D. P. — 2009-06-07