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Probabilistic Ground-motion Assessment of Balanced Rocks in the Mojave Desert

William Lettis and Associates, Inc.
433 Park Point Drive, Suite 250
Golden, Colorado 80401 USA
oconnell{at}lettis.com
(D.R.H.O.)
laforge{at}lettis.com
(R.L.)

Pengcheng Liu

Bureau of Reclamation
P.O. Box 25007 86-68330
Denver, Colorado 80225-0007 USA
pliu{at}do.usbr.gov
(P.L.)

View larger version (77K): [in this window] [in a new window]   Figure 1. Plan view of shaded topography, the San Andreas fault (white line), and balanced rock sites in the Mojave Desert from Brune (1999) (white circles).

View larger version (21K): [in this window] [in a new window]   Figure 2. Balanced object fragility distribution used for the censoring simulations.

View larger version (14K): [in this window] [in a new window]   Figure 3. Balanced rock shear-velocity profiles modified from Abbott et al. (2001) for the Mojave Desert sites. Velocities are 30-m depth averages. Note the balanced rocks themselves are not located on 5-12 m thicknesses of grus; the profiles represent site velocities adjacent to the rock pedestals.

View larger version (160K): [in this window] [in a new window]   Figure 4. Shaded topography with the triangle enclosing the region of the Mojave Desert used to calculate background earthquake recurrence. Red circles are epicenters, with the smallest circle corresponding to M 3.5-4, and the largest circle to M 5.0-5.5. Seismicity from NCEDC (U.C. Berkeley)1930-April 2004. Constant light gray regions are areas below mean sea level.

View larger version (19K): [in this window] [in a new window]   Figure 5. Incremental recurrence estimates for the Mojave Desert background zone derived from the data in Figure 4.

View larger version (25K): [in this window] [in a new window]   Figure 6. Mean site-specific 1% in 100 years PHA results using Vs30=1250 m/s and only the San Andreas fault contribution are the diamonds connected by the solid line. The stars are the original quasi-static PHA estimates for the balanced rocks from Brune (1999), the crosses indicate the 29% increase to account for censoring bias, and the boxes indicate the estimates when censoring biases and dynamic toppling PHA biases from Anoonshehpoor et al. (2004) are included. The labeled dotted and dashed curves show the total PHA hazard from the San Andreas fault and background seismicity as smaller magnitudes are included in the background PHA hazard calculations.

View larger version (20K): [in this window] [in a new window]   Figure 7. Three scenario time-dependent balanced rock fragility curves. Solid line is linear, long-dashed curve is exponential, and short-dashed curve is double exponential. The double exponential curve spends nearly 80% of its lifetime at < 0.5 g and 54% of its lifetime at < 0.25 g (thin vertical dotted lines). In contrast, the exponential curve spends about 62% of its lifetime at < 0.5 g and only 35% of its lifetime at < 0.25 g (thick vertical dotted lines).

View larger version (24K): [in this window] [in a new window]   Figure 8. Plan view of receiver positions and surface velocities. The grey region is rock with a surface Vs=1250 m/s and the white region is grus with a surface Vs=300 m/s. Dots are receiver positions in 43 rings of 90 receivers at 4-degree spacing. A radius spacing of 5 m was used within 50 m of the rock-grus contact. Velocity-density relations are shown in Figure 9 and velocity model depth parameters in table 7 and figure 10.

View larger version (14K): [in this window] [in a new window]   Figure 9. Relationships between P-wave velocity and Vp/Vs and density in the 3D velocity model.

View larger version (26K): [in this window] [in a new window]   Figure 10. Mean spectral accelerations (5% damping) as a function of position relative to the rock/grus boundary for periods of 0.2 s (A) and 1.0 s (B) and corresponding ln() from the 90 receivers in each ring for periods of 0.2 s (C) and 1.0 s (D). Dotted vertical lines indicate position of rock/grus boundary. The dashed curves in (A) and (B) show the thicknesses of grus as a function of distance from the rock/grus boundary.

View larger version (20K): [in this window] [in a new window]   Figure 11. Mean response spectral acceleration ratios of 0.25 s response to 1.0 s response as a function of distance from the rock/grus boundary. Dotted vertical line indicates position of rock/grus boundary. Dotted lines indicate 25% and 50% amplification positions in the grus relative to rock sites 10 m from the rock/grus boundary.

View larger version (13K): [in this window] [in a new window]   Figure 12. Median fault-normal mean 5 %-damped acceleration response spectra from nine near-fault (0.6-3.5 km) M > 6 strike-slip earthquake recordings is the solid curve and the mean is the dotted curve. Shadded region is the period band where Abrahamson and Silva (1997), Boore et al. (1997), and Sadigh et al. (1997) predict the largest spectral acceleration will occur.