Bulletin of the Seismological Society of America current issue

Ground Motion-Based Testing of Seismic Hazard Models in New Zealand [Article]

Stirling, M., Gerstenberger, M. — Tue, 27 Jul 2010 08:50:27 PDT

We develop a testing methodology for the New Zealand national probabilistic seismic hazard (PSH) model that builds on the groundwork of previous studies. Our fundamental approach is to test the full model, or in other words, the final output of the model (ground-motion exceedance for a given return period). Our results show that the PSH model is rejected as underpredicting the historical number of exceedances for specific peak ground acceleration (PGA) levels obtained directly from instrumental strong-motion data over the last 1–4 decades. However, when aftershock ground motions are removed from the strong-motion data, the model is not inconsistent with the observations. The implications for the PSH model are that the lack of aftershocks in the model led to initial model rejection and that the model may perform better for short (decadal) time periods if aftershocks are included in the PSH model. The results are different from those of earlier Modified Mercalli Intensity (MMI)-based studies that suggested the PSH model was predicting hazard slightly higher than that of the historical record. Our new test dataset has the advantage of using observed PGA rather than PGA inferred from MMI. Establishment of a protocol for formally testing future versions of the New Zealand PSH model within a testing center such as those using the Collaboratory for the Study of Earthquake Predictability protocol will require consideration of the fact that the tests are limited by the available datasets of strong earthquake shaking.

Effects of 3D Random Correlated Velocity Perturbations on Predicted Ground Motions [Article]

Hartzell, S., Harmsen, S., Frankel, A. — Tue, 27 Jul 2010 08:50:27 PDT

Three-dimensional, finite-difference simulations of a realistic finite-fault rupture on the southern Hayward fault are used to evaluate the effects of random, correlated velocity perturbations on predicted ground motions. Velocity perturbations are added to a three-dimensional (3D) regional seismic velocity model of the San Francisco Bay Area using a 3D von Karman random medium. Velocity correlation lengths of 5 and 10 km and standard deviations in the velocity of 5% and 10% are considered. The results show that significant deviations in predicted ground velocities are seen in the calculated frequency range (≤1 Hz) for standard deviations in velocity of 5% to 10%. These results have implications for the practical limits on the accuracy of scenario ground-motion calculations and on retrieval of source parameters using higher-frequency, strong-motion data.

Quantitative Comparison of Four Numerical Predictions of 3D Ground Motion in the Grenoble Valley, France [Article]

Tue, 27 Jul 2010 08:50:27 PDT

This article documents a comparative exercise for numerical simulation of ground motion, addressing the seismic response of the Grenoble site, a typical Alpine valley with complex 3D geometry and large velocity contrasts. Predictions up to 2 Hz were asked for four different structure wave-field configurations (point source and extended source, with and without surface topography). This effort is part of a larger exercise organized for the third international symposium on the effects of surface geology (ESG 2006), the complete results of which are reported elsewhere (Tsuno et al., 2009).

While initial, blind computations significantly differed from one another, a remarkable fit was obtained after correcting for some nonmethodological errors for four 3D methods: the arbitrary high-order derivative discontinuous Galerkin method (ADER-DGM), the velocity-stress finite-difference scheme on an arbitrary discontinuous staggered grid (FDM), and two implementations of the spectral-element method (SEM1 and SEM2). Their basic formulation is briefly recalled, and their implementation for the Grenoble Valley and the corresponding requirements in terms of computer resources are detailed.

Besides a visual inspection of PGV maps, more refined, quantitative comparisons based on time-frequency analysis greatly help in understanding the origin of differences, with a special emphasis on phase misfit. The match is found excellent below 1 Hz, and gradually deteriorates for increasing frequency, reflecting differences in meshing strategy, numerical dispersion, and implementation of damping properties.

While the numerical prediction of ground motion cannot yet be considered a mature, push-button approach, the good agreement reached by four participants indicates that, when used properly, numerical simulation is actually able to handle correctly wave radiation from extended sources in complex 3D media. The main recommendation to obtain reliable numerical predictions of earthquake ground motion is to use at least two different but comparably accurate methods, for instance the present formulations and implementations of the FDM, SEM, and ADER-DGM.

Numerical Analysis of Near-Field Asymmetric Vertical Motion [Article]

Tobita, T., Iai, S., Iwata, T. — Tue, 27 Jul 2010 08:50:27 PDT

During the 2008 Iwate–Miyagi Nairiku, Japan, earthquake (M_w 6.9), an unprecedented vertical surface acceleration of nearly four times gravity, was measured at the KiK-net, IWTH25 station located 3 km southwest of the epicenter. The station is equipped with three-component accelerometers, installed at both the free surface and the bottom of a 260-m borehole. The waveform of the vertical acceleration shows a clearly asymmetric form with large amplitude in the upward direction. Aoi et al. (2008) reported and qualitatively explained the mechanism of this phenomenon by the analogy of bouncing a piece of matter on a trampoline; thus, they called it the trampoline effect. To study this recently discovered nonlinear behavior of the surface ground motion, numerical analysis with a two-dimensional finite-element method has been conducted with parameters derived from the borehole data at the station. The analysis successfully simulates the asymmetric vertical motion. Results indicate that the asymmetric motion may be characterized by the existence of a lower bound of negative acceleration, which in most cases corresponds to the acceleration of gravity and high positive pulses caused by the compression stress of the disturbed surface ground material.

A Split-Step Algorithm for Effectively Suppressing the Numerical Dispersion for 3D Seismic Propagation Modeling [Article]

Yang, D., Wang, L. — Tue, 27 Jul 2010 08:50:27 PDT

In this article, we propose a 3D split-step algorithm (SSA) for seismic-wave simulation. We first transform the wave equations in 3D anisotropic media into a system of first-order partial differential equations with respect to time t. Then we use the multidimensional high-order interpolation method to approximate the high-order spatial derivatives, so that we obtain a system of semidiscrete ordinary differential equations (ODEs). Finally, the third-order implicit Adams method and truncated differentiator series method are applied to solve the semidiscrete ODEs. We provide the theoretical study on the properties of the 3D SSA, such as stability criteria, theoretical error, numerical error, numerical dispersion, and computational efficiency. We also compare some seismic modeling results of this method against those of some high-order finite-difference schemes. Theoretical analysis and numerical tests show that the 3D SSA is third-order accurate in time and fourth-order accurate in space. However, its computational costs and memory requirements are much less than those of the fourth-order Lax–Wendroff correction method and the fourth-order staggered-grid method. Using a multilayer elastic model with large velocity contrasts and free surface, we compare the result of the 3D SSA with that of the discrete-wavenumber method. We also present the synthetic seismograms in the 3D three-layer isotropic medium, the wave-field snapshots in the 3D two-layer medium, and the 3D transversely isotropic medium with a vertical symmetry axis. All these promising numerical results illustrate that the 3D SSA can suppress effectively the numerical dispersion caused by discretizing the wave equations when too few sampling points per minimum wavelength are used or when models have large velocity contrasts between adjacent layers, further resulting in both increasing the computational efficiency and saving the storage space when big grids are used.

Dynamic Rupture through a Branched Fault Configuration at Yucca Mountain, and Resulting Ground Motions [Article]

Templeton, E. L., Bhat, H. S., Dmowska, R., Rice, J. R. — Tue, 27 Jul 2010 08:50:27 PDT

We seek to characterize the likelihood of multiple fault activation along a branched normal-fault system during earthquake rupture using dynamic finite element analyses. This is motivated by the normal faults in the vicinity of Yucca Mountain, Nevada, a potential site for a high-level radioactive waste repository. The Solitario Canyon fault (SCF), a north–south trending fault located approximately 1 km west of the crest of Yucca Mountain, is the most active of these faults. Based on the results of previous branching work by Kame et al. (2003), branch activation in the hanging wall of a normal fault such as the SCF may be possible for fast ruptures propagating near the Rayleigh-wave speed at the branch junction. Dynamic branch activation along a splay of the SCF during a seismic event could have important effects on the rupture velocity and resulting ground motions at the proposed repository site. We consider elastic as well as a pressure-dependent elastic–plastic response of the off-fault material. We find that based on the regional stress state in the area, the only likely candidates for branch activation in the hanging wall of the SCF are more steeply westward dipping intrablock splay faults. We also find that the rupture velocity for an earthquake propagating updip along the SCF must reach supershear speeds in order for dynamic branch activation to occur. Branch activation can have significant effects on the ground motions at the proposed repository site, 1 km away from the SCF beneath the crest of Yucca Mountain, causing the repository site to experience a second peak in large vertical particle velocities. Elastic–plastic response near the branch junction reduces peak ground velocities and accelerations at the proposed repository site.

Stochastic Strong Ground Motion Simulation of the 12 November 1999 Duzce (Turkey) Earthquake Using a Dynamic Corner Frequency Approach [Article]

Ugurhan, B., Askan, A. — Tue, 27 Jul 2010 08:50:27 PDT

On 12 November 1999, only three months after the 17 August 1999 Kocaeli earthquake (M_w 7.4), an earthquake of M_w 7.1 occurred immediately to the east of the Kocaeli rupture in northwestern Turkey resulting in extensive structural damage in the city of Düzce and its surrounding area. It was reported to be a right-lateral strike slip event on the previously unbroken segment of the North Anatolian fault zone with a north-dipping fault plane. This paper presents stochastic finite-fault simulation of near-field ground motions from this earthquake at selected near-fault stations based on a dynamic corner frequency approach using the computer program EXSIM (Motazedian and Atkinson, 2005). The method requires region-specific source, path, and site characterizations as input model parameters. The source mechanism of the 1999 Düzce event and regional path effects are well constrained from previous studies of the earthquake. The local site effects at the selected stations are studied as a combination of the kappa operator and frequency-dependent soil amplification. The model parameters are validated against recordings and a stress-drop value of 100 bars is estimated for the 1999 Düzce earthquake. The validated model is then used to compute synthetic records around the fault. Distribution of peak ground-motion parameters is observed to be consistent with the building damage distribution in the near-fault region most affected by the seismic shaking. The attenuation of synthetic ground-motion parameters is compared with recent ground-motion prediction equations proposed for the region by Gülkan and Kalkan (2002), Ulusay et al. (2004), and Akkar and Bommer (2007), as well as two next generation attenuation models by Boore and Atkinson (2007) and Campbell and Bozorgnia (2007). Despite discrepancies at several stations, stochastic finite-fault modeling based on a dynamic corner frequency approach confirms to be a practical tool to reproduce the ground motions of large earthquakes.

Selection of Empirical Green's Functions by Waveform Similarity Analysis: An Approach to Predict Ground Motion in Areas with Saturated Records [Article]

Massa, M. — Tue, 27 Jul 2010 08:50:27 PDT

This work is focused on a procedure based on selecting suitable empirical Green’s functions (EGFs), able to predict ground motion for moderate earthquakes, in the case where no records are available due to saturation phenomena. The aim of the article is to generate synthetic seismograms for the 24 November 2004, M_L 5.2, Salò earthquake (northern Italy), an event capable of saturating all velocimetric stations installed within the first 100 km from the epicenter. The proposed approach uses a waveform similarity analysis, based on the normalized cross-correlation technique, and it is able to identify EGFs that represent doublet events of a target. The ground motion was finally simulated using the method proposed by Irikura (1986).

In this case, due to the saturation of near-source velocimetric instruments, the normalized cross-correlation matrix was calculated considering the first not saturated velocimetric station (ASO2, 108 km northeast to the epicenter), including the same selected portion of signals (on the base of signal-to-noise ratio) related both to the target and to 11 events, with M_L ranging from 2.2 to 3.0, that occurred in the same area. The similarity analysis, performed through the bridging technique, allows us to detect an M_L 2.9 aftershock characterized by a meaningful degree of similarity (70%) compared to the target. Given as a fact that if two events are similar for a far-field station they have to be similar also for a near-source one, it was in this way possible to use the selected aftershock (doublet), recorded in near source to reproduce the target for the saturated near-source stations. The results of the simulations were compared with ground-motion values predicted by empirical ground-motion prediction equations (GMPEs), calibrated using both Italian and European data.

Technically Induced Surface Wave Fields, Part I: Measured Attenuation and Theoretical Amplitude-Distance Laws [Article]

Auersch, L. — Tue, 27 Jul 2010 08:50:27 PDT

The attenuation of the amplitudes with distance of technically induced surface wave fields is analyzed in theory and experiments. Experimental results of technically induced ground vibration are presented and collected from literature, which show a power-low attenuation A~r^-q of amplitudes A with distance r and exponents q>0.5 higher than for elastic surface waves. Additional attenuation effects are analyzed theoretically. The most important effect is due to the material or scattering damping. Each frequency component is attenuated exponentially as A~exp(-kr), but for a broadband excitation, the sum of the exponential laws yields a power law with a higher exponent. Some more effects are discussed, for example the dispersion of the Rayleigh wave due to the layering of the soil, which yields a stronger attenuation A~r^-q-dq, including an additional exponent of dq=0.5 in case of an impulsive loading.

Technically Induced Surface Wave Fields, Part II: Measured and Calculated Admittance Spectra [Article]

Auersch, L. — Tue, 27 Jul 2010 08:50:27 PDT

Transfer admittance spectra of technically induced surface wave fields are analyzed in theory and experiments. Theoretical admittance spectra of layered soils are obtained by integration in wavenumber domain and compared with experimental admittances due to hammer or vibrator excitation. The admittance spectra are strongly influenced by the layering and damping of the soil. Deep stiff-soil layers yield a low-frequency cutoff, whereas a strong damping yields a high-frequency cutoff. A sharp cutoff in a narrow frequency band, which is measured at some sites, can be explained by a damping that increases with frequency, such as viscous material or scattering damping.

Multivariate Bayesian Regression Analysis Applied to Ground-Motion Prediction Equations, Part 1: Theory and Synthetic Example [Article]

Arroyo, D., Ordaz, M. — Tue, 27 Jul 2010 08:50:27 PDT

An application of a linear multivariate Bayesian regression model to compute pseudoacceleration (SA) ground-motion prediction equations (GMPEs) is presented. The model is able to include the correlation between observations for a given earthquake, the correlation between SA ordinates at different periods, and the correlation between regression coefficients of the ground-motion prediction model. We evaluate the advantages of the Bayesian approach over the traditional regression methods, and we discuss the differences between univariate and multivariate analyses. Because the application of the Bayesian method is in general complex and implies an increase in the numerical effort with respect to the traditional methods, our computer code to perform linear Bayesian analyses is freely available on request.

Multivariate Bayesian Regression Analysis Applied to Ground-Motion Prediction Equations, Part 2: Numerical Example with Actual Data [Article]

Arroyo, D., Ordaz, M. — Tue, 27 Jul 2010 08:50:27 PDT

An application of a linear multivariate Bayesian regression model, described in a companion article, to obtain a ground-motion prediction equation (GMPE) using a set of actual ground-motion records and a realistic functional form is presented. Based on seismological grounds and on an adopted functional form, we include a sound discussion about how the prior information required for the model can be defined. For the regression analyses we use two subsets of ground-motion records from the Next Generation of Ground-Motion Attenuation Models (NGA) database. We compare the results obtained with the Bayesian model with those obtained through the one-stage maximum-likelihood and the constrained maximum-likelihood methods. The advantages of the Bayesian approach over traditional regression techniques are discussed.

Issues in Choosing the References to Use for Spectral Ratios from Observations and Modeling at Cavola Landslide in Northern Italy [Article]

Bordoni, P., Di Giulio, G., Haines, A. J., Cara, F., Milana, G., Rovelli, A. — Tue, 27 Jul 2010 08:50:27 PDT

A reference site has to be free of amplification or deamplification effects, namely with no troughs and peaks in its Fourier amplitude spectrum. At the Cavola landslide we show that this spectrum is dependent on the direction of propagation of wavefronts for incidence angles in the range 30°–90°. Our study is based on comparison of spectral ratios from observations and 2D numerical simulations. We have modeled propagation in a 2D profile for SH and SV waves with several incidence angles in a range from 0° to ±90°, where 0° and 90° are, respectively, vertical and horizontal incidence, except that ±90° denotes Rayleigh waves in the P-SV. We discuss in detail the result for angles of incidence of 0°, ±20°, ±60°, ±90°. We have obtained observed horizontal-to-horizontal spectral ratios using three reference sites. Two of these have matching receivers in the model, located at the opposite ends of the 2D profile. Overall observations are matched best when the reference site is located on the same side of the landslide as the incoming wavefront. We also find general agreement of the observed H/H spectral ratios from earthquakes with H/V and H/H spectral ratios from noise, and the match between H/V values from noise and synthetic spectral ratios using an absolute, flat half-space reference is very good. On the other hand, 1D modeling performs poorly in comparison with 2D modeling in our case, for which the shape ratio h/D=0.2 is intermediate between primarily 1D and strongly 2D wave propagation according to the classification of Bard and Bouchon (1985).

Seismic Hazard Assessment in Terms of Macroseismic Intensity in Italy: A Critical Analysis from the Comparison of Different Computational Procedures [Article]

Gomez Capera, A. A., D'Amico, V., Meletti, C., Rovida, A., Albarello, D. — Tue, 27 Jul 2010 08:50:27 PDT

Two probabilistic seismic hazard (PSH) maps in terms of macroseismic intensity characterized by an exceedance probability of 10% for exposure time of 50 years are presented and compared. The first map adopts the standard Cornell–McGuire approach and follows the computational scheme developed for the reference Italian peak ground acceleration (PGA) hazard map (MPS04), while the second one is derived through an alternative methodology (referred to here as the site approach) that is based on statistical analysis of the site seismic history (i.e., macroseismic intensities documented for past earthquakes). Because the two procedures make a different use of available information, this comparison provides a new insight about the sensitivity of PSH estimates for the different possible methodological choices. In particular, it is shown that, though basic differences exist between the two adopted methodologies, relevant results appear consistent over most of Italy. However, at a significant number of investigated localities (Italian municipalities), PSH estimates provided by the site approach are larger than those derived from the standard technique. Thus, a detailed analysis has been carried out to evaluate the role played by different choices of computational models and input data. Among these, the use/nonuse of seismogenic zoning seems to act as the key element in determining the pattern of differences observed between the two PSH estimates.

Determination of Stress Parameters for Eight Well-Recorded Earthquakes in Eastern North America [Article]

Boore, D. M., Campbell, K. W., Atkinson, G. M. — Tue, 27 Jul 2010 08:50:27 PDT

We determined the stress parameter, , for the eight earthquakes studied by Atkinson and Boore (2006), using an updated dataset and a revised point-source stochastic model that captures the effect of a finite fault. We consider four geometrical-spreading functions, ranging from 1/R at all distances to two- or three-part functions. The values are sensitive to the rate of geometrical spreading at close distances, with 1/R^1.3 spreading implying much higher than models with 1/R spreading. The important difference in ground motions of most engineering concern, however, arises not from whether the geometrical spreading is 1/R^1.3 or 1/R at close distances, but from whether a region of flat or increasing geometrical spreading at intermediate distances is present, as long as is constrained by data that are largely at distances of 100 km–800 km. The simple 1/R model fits the sparse data for the eight events as well as do more complex models determined from larger datasets (where the larger datasets were used in our previous ground-motion prediction equations); this suggests that uncertainty in attenuation rates is an important component of epistemic uncertainty in ground-motion modeling. For the attenuation model used by Atkinson and Boore (2006), the average value of from the point-source model ranges from 180 bars to 250 bars, depending on whether or not the stress parameter from the 1988 Saguenay earthquake is included in the average. We also find that for a given earthquake is sensitive to its moment magnitude M, with a change of 0.1 magnitude units producing a factor of 1.3 change in the derived .

Seismicity of the New Madrid Seismic Zone Derived from a Deep-Seated Strike-Slip Fault [Article]

Tavakoli, B., Pezeshk, S., Cox, R. T. — Tue, 27 Jul 2010 08:50:27 PDT

A conceptual three-dimensional flower structure model of strike-slip faulting is proposed to explain the occurrence of earthquakes in the New Madrid seismic zone (NMSZ) and to illustrate the potential rupture faults for the 1811–1812 earthquake sequences. The proposed NMSZ model is based on elastic dislocation theory and concepts of material failure under a stress field. Using a conceptual model of a strike-slip subsidiary fault array, we identify tectonic features (geological structures) that are oriented properly relative to regional stresses and classify the regions where stresses might be expected to be amplified.

The brittle upper crust in the vicinity of the NMSZ is modeled as a uniform overburden with a horizontal-basal surface, which rests on a horizontal ductile lower crust that is cut by a vertical, northeast-striking right-lateral strike-slip shear zone. We acknowledge that many favorably oriented preexisting faults have been exploited as components of the flower structure. The brittle overburden material is subject to simple shearing stress parallel to the deep-seated lower crustal shear zone, and preexisting faults of the Reelfoot rift system give the upper crust a mechanical anisotropy (planes of weakness striking northeast) that is the correct orientation for development of P shear faults. The deep-seated fault movement deforms the overlying upper crust that controls the structural geometry, the modern seismicity, and the large earthquake sequences in the NMSZ.

The three-dimensional NMSZ model of faulting developed in this study shows that the Bootheel and Big Creek lineaments, inferred to be two subparallel P shear faults rooted in a deep-seated fault in the lower crust, are significant in shaping the geometry of the NMSZ. These series of faults produce a large-scale flower structure in cross section. The proposed NMSZ model uses the intersections of the deep-seated fault and the two subparallel P shear faults for the locations of the 1811 and 1812 earthquakes. The model gives rise to a predictable pattern of surface deformation that is in good agreement with the observed seismicity patterns in the region.

Short-Term Uplift Rates and Surface Deformation along the Reelfoot Fault, New Madrid Seismic Zone [Article]

Carlson, S. D., Guccione, M. J. — Tue, 27 Jul 2010 08:50:27 PDT

This study of variability in the amount and rate of deformation along the intraplate Reelfoot fault of the New Madrid seismic zone (NMSZ) utilizes diverse research methods, including geophysics, geomorphology, and fluvial sedimentology. The transpressional Reelfoot fault deforms late Holocene Mississippi River sediment into the Tiptonville dome and adjacent Reelfoot basin. Because Reelfoot Lake submerges a portion of the basin, subbottom acoustic profiling of the lake was used to quantify the relief on the uplift. At the natural levee of the abandoned Tiptonville meander, the most appropriate landform along the scarp for measuring cumulative structural relief, a maximum relative vertical uplift of approximately 11.3 m was measured. To identify spatial and temporal variability in surface deformation using geologic methods, we relate fluvial sedimentation to three documented earthquake events during the past 2300 yr (Tuttle et al., 2002, 2005), which is the approximate age of the oldest deformed floodplain sediment (Guccione et al., 2002). Radiocarbon dates from organic material in fluvial and lacustrine sediment indicate that most of the uplift occurred during the last two major seismic events. A minimum of 1.6–3.9 m of uplift was associated with the seismic episode at A.D. 1450±150 and a maximum of 5.9–8.2 m of uplift was associated with the 1812 episode. We estimate a mean short-term uplift rate of 1.2 cm/yr (0.4–2.1 cm/yr with 80% probability) or a mean short-term slip rate of 1.3 cm/yr (0.5–2.3 cm/yr with 80% probability). These values are approximately 2.5 times higher than the long-term rates based on the total amount of uplift averaged over the past 2300 yr (Mueller et al., 1999; Van Arsdale, 2000). Estimating deformation rates associated with individual earthquake recurrences provides information on fluctuating activity of the NMSZ.

Geologic Evidence for Surface Rupture Associated with the 1847 M 7.4 Zenkoji Earthquake at Dannohara, Nagano City, Japan [Article]

Sugito, N., Okada, A., Tsutsumi, H. — Tue, 27 Jul 2010 08:50:27 PDT

The 1847 M 7.4 Zenkoji, Japan, earthquake was caused by movement of the west-dipping, western margin reverse-fault zone of the Nagano basin (WFZNB) and produced surface ruptures reported in Japanese historical documents. Despite previous studies to assess fault activity of the WFZNB, our trench excavation at Dannohara, Nagano City, exposed near-surface fault structures associated with the Zenkoji earthquake for the first time and provided geologic evidence for three earlier paleoseismic events. Three core samples 10–15 m long defined a hanging-wall anticline beneath the trench. Although the primary fault was not observed, the revealed subsidiary features enabled us to discuss surface rupture morphology and its repetition patterns. We suggest that: (1) the east-dipping reverse faults exposed on the trench walls and the hanging-wall anticline are developed above a west-dipping blind reverse fault; (2) the east-dipping normal faults represent bending-moment faults at the crest of the hanging-wall anticline; (3) the exposed east-dipping faults slipped during the Zenkoji earthquake to produce elongated linear rise scarps described in historical documents; (4) the hanging-wall anticline grew during the earthquake to generate uplifted lands as reported in historical documents; and (5) faulting similar to that during the Zenkoji earthquake also occurred during the penultimate earthquake. We propose preliminary but better constraints on timing of the third and fourth latest faulting events, and a tentative and maximum recurrence interval of 800–1000 years for the WFZNB. To better understand reverse-fault rupture patterns for appropriate evaluation of seismic hazard, historical surface ruptures should be studied in more detail.

Slip Distribution of the 1952 Kamchatka Great Earthquake Based on Near-Field Tsunami Deposits and Historical Records [Article]

MacInnes, B. T., Weiss, R., Bourgeois, J., Pinegina, T. K. — Tue, 27 Jul 2010 08:50:27 PDT

We explore the magnitude and slip distribution of the 1952 Kamchatka earthquake (M_w 8.8–9.0) using constraints from the 1952 Kamchatka tsunami. Our new field data provide more comprehensive coverage of the near-field tsunami than had been available to date. We examine the effects of internal slip distribution within complex earthquake ruptures on near-field tsunami runup and evaluate some of the limitations of this approach. Our approach compares tsunami-deposit distribution with simulated runup from tsunamis generated by different configurations of seafloor deformation from hypothetical earthquakes resembling that of the 1952 Kamchatka earthquake. We identify areas of high slip because different distributions of seafloor deformation result in variations in tsunami runup in the near field. Mapped deposits and local observations of the 1952 Kamchatka tsunami indicate that near-field runup in central Kamchatka was consistently less than 10 m (averaging 6 m), while south Kamchatka to the northern Kuril Islands had more variability and higher average runup (8 m runup in South Kamchatka and 10 m runup in the northern Kuril Islands). Our simulations show that in order to produce the distribution of runup indicated by tsunami deposits and historical observations, the 1952 earthquake had regions of high slip off the coast of southern Kamchatka, and the location of high slip is shallower in the subduction zone than previously interpreted.

High-Resolution Seismic Reflection Imaging of Growth Folding and Shallow Faults beneath the Southern Puget Lowland, Washington State [Article]

Clement, C. R., Pratt, T. L., Holmes, M. L., Sherrod, B. L. — Tue, 27 Jul 2010 08:50:27 PDT

Marine seismic reflection data from southern Puget Sound, Washington, were collected to investigate the nature of shallow structures associated with the Tacoma fault zone and the Olympia structure. Growth folding and probable Holocene surface deformation were imaged within the Tacoma fault zone beneath Case and Carr Inlets. Shallow faults near potential field anomalies associated with the Olympia structure were imaged beneath Budd and Eld Inlets. Beneath Case Inlet, the Tacoma fault zone includes an ~350-m wide section of south-dipping strata forming the upper part of a fold (kink band) coincident with the southern edge of an uplifted shoreline terrace. An ~2 m change in the depth of the water bottom, onlapping postglacial sediments, and increasing stratal dips with increasing depth are consistent with late Pleistocene to Holocene postglacial growth folding above a blind fault. Geologic data across a topographic lineament on nearby land indicate recent uplift of late Holocene age. Profiles acquired in Carr Inlet 10 km to the east of Case Inlet showed late Pleistocene or Holocene faulting at one location with ~3 to 4 m of vertical displacement, south side up. North of this fault the data show several other disruptions and reflector terminations that could mark faults within the broad Tacoma fault zone. Seismic reflection profiles across part of the Olympia structure beneath southern Puget Sound show two apparent faults about 160 m apart having 1 to 2 m of displacement of subhorizontal bedding. Directly beneath one of these faults, a dipping reflector that may mark the base of a glacial channel shows the opposite sense of throw, suggesting strike-slip motion. Deeper seismic reflection profiles show disrupted strata beneath these faults but little apparent vertical offset, consistent with strike-slip faulting. These faults and folds indicate that the Tacoma fault and Olympia structure include active structures with probable postglacial motion.

A General Method to Estimate Earthquake Moment and Magnitude Using Regional Phase Amplitudes [Article]

Pasyanos, M. E. — Tue, 27 Jul 2010 08:50:27 PDT

This article presents a general method of estimating earthquake magnitude using regional phase amplitudes, called regional M₀ or regional M_w. Conceptually, this method uses an earthquake source model along with an attenuation model and geometrical spreading that accounts for the propagation to utilize regional phase amplitudes of any phase and frequency. Amplitudes are corrected to yield a source term from which one can estimate the seismic moment. Moment magnitudes can then be reliably determined with sets of observed phase amplitudes rather than predetermined ones and afterward averaged to robustly determine this parameter. We first examine in detail two events to demonstrate the methodology. We then look at various ensembles of phases and frequencies and compare results to existing regional methods. We find regional M₀ to be a stable estimator of earthquake size that has several advantages over other methods. Because of its versatility, it is applicable to many more events, particularly smaller events. We make moment estimates for earthquakes ranging from magnitude 2 to as large as 7. Even with diverse input amplitude sources, we find magnitude estimates with this method to be more robust than typical magnitudes and existing regional methods, and the magnitude estimates might be tuned further to improve upon them. The method yields a more meaningful quantity of seismic moment, which can be recast as M_w. Lastly, it is applied here to the Middle East region using an existing calibration model, but it would be easy to transport to any region with suitable attenuation calibration.

Changes of Reporting Rates in the Southern California Earthquake Catalog, Introduced by a New Definition of ML [Article]

Tormann, T., Wiemer, S., Hauksson, E. — Tue, 27 Jul 2010 08:50:27 PDT

Starting January 2008, local magnitudes M_L for southern California are determined by a new calibration that provides various improvements for determining M_L for small earthquakes. Magnitudes for the previous years are being recalculated and the catalog continuously updated, with the first year of overlapping data now being available. Recalibrating a magnitude scale can cause a break in homogeneity of reporting and often produces artifacts in the catalog statistics that can influence a wide range of seismicity studies. To search for such a break, we compare the old M_L and the new M_L catalogs for 2007. We find (1) the two magnitude values differ for 96% of the M_L events, and hand-determined magnitudes are also revised; (2) the magnitude differences are irregular from magnitude increases of up to 1.5 units to reductions by as much as 2.3 units, with an average change of -0.13 units; (3) the number of events above M 1.8 decreases by 32% for the new magnitude scale; (4) the completeness magnitude apparently drops by 0.3 units from 1.6 to 1.3; (5) the b-value reduces by approximately 0.2 units, dropping from 1.16 to 0.95; (6) the new magnitude calibration produces a more stable b-value estimate and can therefore be regarded as the better scaling.

We document selected examples of how the change in magnitude calibration may affect seismicity- and hazard-related analyses that are based on the Southern California Seismic Network (SCSN) catalog. Especially the change of the b-value from ~1.1 to ~0.9 has potentially major implications for hazard related applications.

Application of Gaussian-Beam Migration to Multiscale Imaging of the Lithosphere beneath the Hi-CLIMB Array in Tibet [Article]

Nowack, R. L., Chen, W.-P., Tseng, T.-L. — Tue, 27 Jul 2010 08:50:27 PDT

In this study, we apply Gaussian-beam (GB) migration of scattered teleseismic P waves to image the crust and upper mantle beneath Tibet using data from the Hi-CLIMB experiment. We use teleseismic P waves from three groups of earthquakes to the southeast, northeast, and northwest of the Hi-CLIMB array, each within a narrow range of azimuth and distance, to obtain stacked radial receiver functions, which we then use to image the lithosphere by GB migration. We produced images at several different frequency bands in order to constrain the multiscale scattering properties of the lithosphere. For each frequency band, three GB images are generated, each from earthquake sources in a distinct back-azimuth group. The three images are then stacked to form a composite image. The imaged Moho is generally strong and continuous under much of the Lhasa terrane in southern Tibet, corresponding to a well-defined Moho. A major disrupted zone in Moho scattering, extending over 200 km in length, is evident in the vicinity of the Bangong–Nujiang suture, where there is also increased crustal scattering. The disrupted zone marks the diffuse, subsurface join between stable portions of mantle lithosphere under southern and central Tibet, respectively. At the northern end of the profile in the Qiangtang terrane there is an increase in Moho reflectivity but at a shallower depth than under the Lhasa terrane. Comparable length scales of about 200 km between regions with disrupted and smooth-varying Moho suggest that the mechanically strong mantle lithosphere and the crust respond differently to collision, with the upper crust currently undergoing pervasive strain over the entire plateau.

How is Volcano Seismicity Different from Tectonic Seismicity? [Article]

Traversa, P., Grasso, J.-R. — Tue, 27 Jul 2010 08:50:27 PDT

We analyze the temporal patterns of volcano seismicity using the statistics of waiting times between subsequent earthquakes. We compare waiting time distributions of seismicity at Mt. Etna and Mt. Vesuvius volcanoes during (1) inter-eruption phases and (2) dyke propagations, with those of tectonic seismicity using the southern California catalog. For inter-eruption phases, no matter their duration, statistics of inter-event times are well approximated by the gamma distribution. This allows us to compute the proportion of background uncorrelated events (Molchan, 2005; Hainzl et al., 2006), which is recovered in the range 20%–40% for Vesuvius, three Etna inter-eruptive periods, and the southern California catalog. It argues for roughly 70% of earthquake activity to be cascades of aftershocks for both volcano inter-eruptive and tectonic seismicity. On the contrary, statistics of inter-event times recorded during both the 2001 and 2002 intrusive episodes at the Etna volcano reject the gamma distribution to describe the observations. These seismic crises are characterized by an average seismicity rate about 2 orders of magnitude larger than that of inter-eruptive periods. It suggests that the origin of the specificity of waiting time patterns during dyke injections is driven by the external forcing rate. Using the epydemic type aftershock sequences model simulations we explore the effect of seismicity rate increases on inter-event time distributions. Departures from the gamma law progressively emerges from both (1) an increase of the background seismicity rate and (2) a screening effect. It prevents us from quantifying the portion of uncorrelated seismicity within the considered catalog and from clearly quantifying the forcing rate that characterizes the volcano dynamics during dyke intrusions.

Shallow-Water Broadband OBS Seismology [Article]

Webb, S. C., Crawford, W. C. — Tue, 27 Jul 2010 08:50:27 PDT

The recent development of broadband ocean-bottom seismometers that can be deployed for more than a year has led to the construction of large ocean-bottom seismometer (OBS) fleets and to many successful experiments studying Earth structure and tectonics beneath the oceans. However, ocean surface waves raise noise levels at deep ocean-floor sites far above those at continental sites in the microseism band between 0.2 and 10 sec period, and currents and ocean waves raise noise levels at longer periods. Broadband OBSs are rarely deployed in shallow water because of a fear of loss due to bottom trawling and an expectation of very high noise levels from strong currents and the nearby ocean surface. However, these noise sources can be overcome such that shallow OBS deployments may provide noise levels that are comparable to deep-water sites at periods >10 sec and lower than deep-water sites at shorter periods. Burial of the instrument into the sediments can shield the seismometer from current noise, while the noise from deformation under wave loading can be removed using pressure gauge data. We predict the noise levels can be reduced to allow the detection of Rayleigh waves from 20 to 200 sec period with good signal-to-noise ratio (SNR) from teleseismic earthquakes as small as M_w 5. Short-period (<2 sec) noise levels will be 20–30 dB lower in shallow water than in deep water because short-period microseisms are greatly attenuated during propagation from deep to shallow water. Short-period (0.5–2 sec) teleseismic body waves should be detected with good SNR from events as small as M_w 4.5.

Relative Locations of the October 2006 and May 2009 DPRK Announced Nuclear Tests Using International Monitoring System Seismometer Arrays [Short Note]

Selby, N. D. — Tue, 27 Jul 2010 08:50:27 PDT

Waveform data recorded at seven of the International Monitoring System (IMS) seismometer arrays are used to measure the relative times of teleseismic P signals with three methods: analyst picks, and two methods that make full use of the capabilities of seismometer arrays, the cross correlation of array beams and the average of channel-by-channel cross correlations. These times are used to estimate the relative locations of the 9 October 2006 and 25 May 2009 Democratic People’s Republic of Korea announced underground nuclear tests. The 2009 test is found to have occurred about 1.8±0.8 km to the west and 0.4±0.6 km to the north of the 2006 test. The use of cross correlation reduces the standard deviation of the travel-time residuals from ~0.05 to ~0.01 sec, enabling the two epicenters to be statistically distinguished with high confidence. This result demonstrates the power of a small number of IMS seismometer arrays at teleseismic distances to detect and relatively locate small explosions with high precision.

Empirically Based Ground Truth Criteria for Seismic Events Recorded at Local Distances on Regional Networks with Application to Southern Africa [Short Note]

Boomer, K. B., Brazier, R. A., Nyblade, A. A. — Tue, 27 Jul 2010 08:50:27 PDT

We present a new approach to obtaining empirically based (EB) criteria for estimating the epicentral location accuracy (i.e., ground truth, GT) of seismic events recorded at local distances on a regional network. The approach has been developed using a jackknife resampling method applied to carefully picked Pg phase arrival times for GT reference events from several South African gold mines. The events were well recorded locally by Southern African Seismic Experiment (SASE) stations within the Archean Kaapvaal craton, an area of relatively simple crustal structure. The region-specific criteria obtained specify an EBGT3_95% level of epicentral accuracy if events are recorded on eight or more stations at distances less than the Pg/Pn crossover (215 km) when the stations have a primary azimuthal gap <202 degrees. In addition, when nine or more stations are used for event location and one of them is within 79 km of the event, we find that a focal depth accuracy of 4 km at the 95% confidence level can be obtained and that an accuracy of 6 km can be obtained if eight stations are used. This result illustrates that GT criteria commonly applied to global event catalogs can be relaxed if an accurate velocity model and carefully picked phase-arrival times are used for event locations. Consequently, it is likely that additional events can be added to GT compilations by developing EBGT criteria for other regional networks and using them to identify candidate GT events. For example, the EBGT criteria developed in this study, when applied to the SASE seismicity catalog, yields 10 new GT events.

A Reevaluation of the 1958 Fairweather, Alaska, Earthquake Sequence [Short Note]

Doser, D. I. — Tue, 27 Jul 2010 08:50:27 PDT

I have relocated aftershocks, performed waveform modeling, and modeled induced Coulomb failure stress to better determine rupture extent and controls on the rupture process of the 1958 Fairweather earthquake sequence in southeastern Alaska. I estimate a fault rupture length range from 260 km (source time function duration) to 370 km (aftershock zone length). I estimate an average slip of 3.5 m from waveform modeling, comparable to measured surface (2–3.5 m) slip. The largest pulses of moment release along the Fairweather fault zone may have occurred near Lituya Bay, where a huge landslide and subsequent water wave were produced, and near Dry Bay, the location of the mainshock’s intensity center. The region of high moment release near Lituya Bay correlates with a gravity high located northeast of the fault zone. Coulomb failure modeling suggests aftershocks were most likely to have occurred on northwest striking reverse faults. These new source parameters indicate the 1958 mainshock was comparable in size to the 2002 M 7.9 Denali fault earthquake and shared other similarities with the 2002 event.

Record-Breaking Earthquakes [Short Note]

Van Aalsburg, J., Newman, W. I., Turcotte, D. L., Rundle, J. B. — Tue, 27 Jul 2010 08:50:27 PDT

A record-breaking earthquake has a larger magnitude than any previous earthquake in the study region; a starting date and minimum magnitude must be specified. The first earthquake to satisfy this condition is, by definition, a record-breaking earthquake. The next record-breaking earthquake has a larger magnitude than the first and so forth. In this article we utilize the global Centroid Moment Tensor Project (CMT) catalog for the years 1977 to 2006. We consider earthquakes with moment magnitudes greater than 5.5. We determine the number of record-breaking earthquakes n_rb during 15 specified two-year intervals. We then average the n_rb at specified subintervals of time. We compare the results with the predictions for a random independent and identically distributed (i.i.d.) process. The expected number of record-breaking earthquakes n_rb in a specified period is independent of the statistical distribution of magnitudes. Good statistical agreement between the observations and the predictions is obtained. We carry out the same process for the magnitudes of the record-breaking earthquakes. We again compare the results with the predictions for a random (i.i.d.) process and find good agreement. For our analyses to be valid, it is necessary that the global earthquakes occur randomly, that is, they are not correlated. Thus, aftershocks and foreshocks will be sources of error. Studies of record-breaking temperatures have shown a sensitivity to global warming. A future direction for research is to apply the approach used here to regional earthquake catalogs. Aftershocks are expected to strongly influence the results.

Calibration of Acoustic Gauge in the Field Using Seismic Lg Phase and Coupled High-Frequency Local Infrasound [Short Note]

Kim, T. S., Hayward, C. T., Stump, B. W. — Tue, 27 Jul 2010 08:50:27 PDT

Local microphone field calibration can be performed using seismic signals recorded by a well calibrated seismometer. When the 29 May 2004 regional earthquake (M_w 5.1) occurred off the coast of Korea, the Pn, Pg, and Lg seismic phases produced high-frequency (1–4 Hz) local infrasound signals at the Chulwon Seismo-Acoustic Array (CHNAR), an array deployed in the Republic of Korea. Instrument-corrected waveforms of seismic signals are compared to those of local infrasound signals using time-varying coherence estimates to identify the time window and bandwidth for the calibration process. Based on this assessment, the first 6.4 sec of the Lg phase and the coupled infrasound signals from 1 to 4 Hz are used. Envelope functions for the instrument-corrected Lg phase and local infrasound signal are calculated and integrated in the time domain. The transfer function from ground velocity to atmospheric pressure perturbation is estimated in the time domain using the ratio of the integrated envelope functions. The observed transfer functions estimated in this way are compared to the theoretical transfer function based on a model relating ground velocity to pressure perturbation, which is dependent on the density of air and speed of sound at the surface. A maximum difference of 4 dB is apparent between observed transfer functions and theory for highly correlated seismic and coupled infrasound signals (>0.8). Six factors are considered as contributors to these differences, including the porous hose array attached to each acoustic sensor for noise reduction, variations in the density of air or speed of sound, local perturbations in air pressure, differences in ground velocity at each sensor, and differences in acoustic instrument sensitivity. Variations of the acoustic gauge’s sensitivities at CHNAR are identified using a laboratory calibration procedure and documented as the major contributor to the difference between observed and theoretical transfer function at CHNAR.

Transverse Tectonics in the Sikkim Himalaya: Evidence from Seismicity and Focal-Mechanism Data [Short Note]

Tue, 27 Jul 2010 08:50:27 PDT

In the present study, about 356 local earthquakes in the region of the Sikkim Himalaya have been accurately located and analyzed using 2181 P travel times and 2161 S travel times from a network of 11 broadband seismic stations operated by the National Geophysical Research Institute during January 2006 to November 2007. Further refinement of the hypocentral parameters using the hypoDD relocation program resulted in 198 well-constrained locations. Interestingly, this study reveals several characteristic features that distinguish Sikkim from the rest of the Himalaya. The seismicity distribution is found to be confined mostly between the main boundary thrust (MBT) and the main central thrust (MCT) but not quite associated with either. While the entire Himalayan front is generally characterized by shallow-angle thrust faulting, focal mechanisms in this region are predominantly of strike-slip type in conformity with a right-lateral strike-slip mechanism along the northwest-trending Tista and Gangtok lineaments. The P-axis trends of earthquake focal mechanisms are clearly oriented north-northwest, marking a clear transition from the ambient north-northeast trending direction of Indian plate motion with respect to the Eurasian plate all along the Himalayan front. Moderate-sized earthquakes occur down to 70 km depth in this region, compared to an average focal depth of 15–20 km in the rest of the Himalaya. Also, a high average crustal P velocity of 6.66 km/sec and a fairly low b value of 0.83±0.04 are obtained indicating the probability of occurrence of a higher magnitude earthquake in the future. A north–south section in the Sikkim region shows a relatively flat topography, unlike in the rest of the Himalayan mountain chain and suggestive of lower rates of convergence in the recent geologic past. It is proposed that crustal shortening in the Sikkim Himalaya has been substantially accommodated by transverse tectonics rather than underthrusting in recent times.

Ground-Motion Difference between Two Moderate-Size Intraplate Earthquakes in the United Kingdom [Short Note]

Ottemoller, L., Sargeant, S. — Tue, 27 Jul 2010 08:50:27 PDT

Two moderate-size earthquakes occurred in the United Kingdom, the first near Folkestone in 2007 with M_w 4.0 and the second near Market Rasen in 2008 with M_w 4.5. Both were strongly felt and caused some nonstructural damage. The earthquakes occurred at significantly different depths, the Folkestone earthquake at 5 km and the Market Rasen earthquake at 20 km. We determined the seismic moment and the stress drop of the two mainshocks, and two smaller earthquakes in the same locations, by modeling the source displacement spectra. We found stress drops of 30±34 bar and 344±136 bar for the Folkestone and Market Rasen mainshocks, respectively. This is a significant difference considering the earthquakes are only 275 km apart and both are of intraplate origin. We applied the stochastic ground-motion modeling technique and used the stress drop and seismic moment to compute vertical component peak ground acceleration. The modeled ground motions are consistent with the observations. We also computed vertical peak ground acceleration for a hypothetical M_w 6.0 high stress-drop (200 bar) earthquake and found that it would be 4.6 m/sec² at 20 km hypocentral distance.

Orientation-Independent, Nongeometric-Mean Measures of Seismic Intensity from Two Horizontal Components of Motion [Short Note]

Boore, D. M. — Tue, 27 Jul 2010 08:50:27 PDT

New measures of spectral intensity based on the horizontal components of ground shaking are introduced. These new measures are independent of the in situ orientation of the recordings and encompass the full range of spectral amplitudes over all possible rotation angles. Unlike previously introduced measures that are also orientation independent, no geometric means are used in the computation of the new measures. The new measures based on fiftieth percentile values of the response spectra show small but systematic increases (to a factor of about 1.07 at a 10 sec period) compared to the comparable geometric-mean measure.

Comment on "Review: Strong Ground Motions--Have We Seen the Worst?" by Fleur O. Strasser and Julian J. Bommer [Comment and Reply]

Castanos, H., Lomnitz, C. — Tue, 27 Jul 2010 08:50:27 PDT

Reply to "Comment on 'Review: Strong Ground Motions--Have We Seen the Worst?' by Fleur O. Strasser and Julian J. Bommer" by Heriberta Castanos and Cinna Lomnitz [Comment and Reply]

Strasser, F. O., Bommer, J. J. — Tue, 27 Jul 2010 08:50:27 PDT

Erratum to The Influence of Maximum Magnitude on Seismic-Hazard Estimates in the Central and Eastern United States [Erratum]

Mueller, C. S. — Tue, 27 Jul 2010 08:50:27 PDT