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Comparison of Seismic and Geodetic Scalar Moment Rates across the Basin and Range Province

¹ Seismological Laboratory and Department of Geological Sciences
University of Nevada
Reno, Nevada 89557
pancha{at}seismo.unr.edu
jga{at}unr.edu
 (A.P., J.G.A.)

² Nevada Bureau of Mines and Geology
Mail Stop 178
University of Nevada
Reno, Nevada 89557-0088
kreemer{at}unr.edu
 (C.K.)

View larger version (138K): [in this window] [in a new window]   Figure 1. Map of the western United States, showing topography and earthquakes with M 4.8 (blue circles with radius proportional to magnitude). The study area, outlined with a bold polygon, encloses all major earthquakes that can be associated with deformation of the Basin and Range province.

View larger version (14K): [in this window] [in a new window]   Figure 2. Coda magnitude Md versus moment magnitude Mw for 113 earthquakes from 1990 to 2000 located within the western Great Basin. Moments were determined by Ichinose (2003). On average, Md – Mw = 0.10 ± 0.25.

View larger version (23K): [in this window] [in a new window]   Figure 3. Average rate of earthquakes in four magnitude ranges, as a function of catalog duration, where a y-year average is based on the most recent y years of the catalog ending 31 December 1999. Magnitude ranges are: (a) 4.8 M 5.4; (b) 5.5 M 5.9; (c) 6.0 M 6.9; (d) M 7.0. The duration of catalog completeness is estimated from the point at which the rate of earthquakes falls off (shown by arrows). Horizontal dotted lines indicate the upper and lower range of the earthquake rates. Completeness durations determined from these figures are applied to smaller magnitude intervals to generate the occurrence rates shown in Figure 4.

View larger version (17K): [in this window] [in a new window]   Figure 4. b-value curves for the study region. (a) Discrete occurrence rates, where n is the discrete number of earthquakes in the magnitude range M ±0.25. Error bars show the uncertainty range determined using the method illustrated in Figure 3. A different estimate for the uncertainty in the number of events per year can be inferred from the plot because earthquake occurrences are approximately a Poisson process (the approximation is better for M > 6), and for a Poisson process the variance is equal to the mean. (b) Cumulative earthquake occurrence rates, where N is the total number of earthquake events of magnitude M or greater.

View larger version (31K): [in this window] [in a new window]   Figure 5. Map of study showing the location of regional domains through which the profiles shown in Figures 6, 7, and 8 are taken.

View larger version (31K): [in this window] [in a new window]   Figure 6. The magnitude of the velocity field determined from inversion of geodetic data (see text) as a function of XSW, the perpendicular distance from the southwestern boundary of the study region. The profiles are located along the upper half of domain A and the centers of domains B, C, and D (Fig. 5). Modeled velocities are shown (solid circles) along with one-standard-deviation error bars. GPS data used to derive the model are also shown (open circles) as well as their one-standard-deviation uncertainty limits.

View larger version (25K): [in this window] [in a new window]   Figure 7. Profiles through domains A (a), B (b), C (c), and D (d) (Fig. 5), along the western edge of the province. Each domain extends 300 km inward from the edge of the study region. For each domain, the top plot shows the cumulative number of earthquake events within the domain and located at distances greater than XSW from the southwest boundary. The center plot shows the magnitude of velocity from Figure 6. The bottom plot shows cumulative seismic moment release of all events within the domain located at a distance greater than XSW from the southwestern boundary of the study region (Fig. 5). The thin line in Figure 7b gives the cumulative moment release with the MW 7.58 Owens Valley event removed. Right axes of each graph show normalized values.

View larger version (21K): [in this window] [in a new window]   Figure 8. Profiles through the eastern domain (Fig. 5) (a) Cumulative number of earthquake events within the domain, (b) the magnitude of velocity determined from inversion of geodetic data through the center of the domain, and (c) cumulative seismic-moment release within the domain, as a function of the east–west distance. Modeled velocities are shown (filled circles) along with one-standard-deviation error bars. Data used to derive the model are also shown (open circles) as well as their one-standard-deviation error limits. Scales along the ordinate axes are normalized by the largest values in the total study region for the latitude range of this domain.

View larger version (15K): [in this window] [in a new window]   Figure 9. Plot of cumulative seismic-moment release with time over the study region, based on preferred moment estimates for each earthquake. Lines show possible release rates: r1 gives an average rate since 1857 (7.64 x 1025 dyne cm/yr); r2 gives an average rate since 1871 (8.52 x 1025 dyne cm/yr); r3 gives the slip-predictable bound (6.15 x 1025 dyne cm/yr); and r4 gives the time-predictable bound (8.84 x 1025 dyne cm/yr). The time- and slip-predictable models of moment release usually apply to a single fault, and extending the concepts to a region with multiple faults does not have the physical relationship to stress and friction as in the model by Shimazaki and Nakata (1980).

View larger version (22K): [in this window] [in a new window]   Figure 10. Distribution of the average seismic moment rate since 1857 (r1) and 1871 (r2) as well as the slip (r3) and time-predictable (r4) bounds of the seismic-moment rates determined from 50,000 Monte Carlo simulations. The distribution designated as r5, not shown on Figure 9, is derived from a least-squares fit to the cumulative moments, with each year contributing one data point. The bin width is 0.1 x 1025 dyne cm/yr.

View larger version (19K): [in this window] [in a new window]   Figure 11. Depth distribution of earthquakes within Nevada (triangles) and Utah (circles).

View larger version (17K): [in this window] [in a new window]   Figure 12. Comparison of the range of moment rates determined from the historical seismicity with those determined from geodesy and geology (Table 6). Both the extreme values (thin line) and the most likely bounds (thick line) on the seismicity rate are shown.

View larger version (15K): [in this window] [in a new window]   Figure 13. Cumulative earthquake occurrence rates [N(m)] for the three functional forms of the Gutenburg-Richter curves of Anderson and Luco (1983), using b = 1.00 Figure 4b. These models depend on (1) N1 = 10a–bM H (Mmax – M) the rate of occurrence at a reference magnitude, (2) n2 = –dN2/dM = 10a–bM H(Mmax = M) a b-value, and (3) n3 = –dN3/dM = (10a–bM – 10a–bMmax) H(Mmax = M) the maximum magnitude, Mmax. The plots have been normalized using a moment-release rate of 11.3 x 1025 dyne cm/yr corresponding to the most likely maximum estimate determined from historical seismicity. To match the moment release rate Mmax is set to 7.58, the largest event in the catalog for N1, Mmax = 8 for n2, and Mmax = 8.2 for n3.