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Anomalous Seismic Amplitudes Measured in the Los Angeles Basin Interpreted as a Basin-Edge Diffraction Catastrophe

¹ Department of Earth and Space Sciences and
Center for Embedded Networked Sensing
University of California, Los Angeles (UCLA)
Los Angeles, California 90095-1567

View larger version (102K): [in this window] [in a new window]   Figure 1. LABPSE seismic stations are red (noncaustic) and yellow (caustic) diamonds. Events used in this study are displayed as focal mechanisms. Those events that had no focal mechanism available are located within clusters of events with focal mechanisms. Event numbering goes as increasing back azimuth from station 11. LAB, Los Angeles basin; SB, Seal Beach; SGB, San Gabriel Basin; SGM, San Gabriel Mountains; WF, Whittier Fault.

View larger version (62K): [in this window] [in a new window]   Figure 2. The north–south component of events 17 and 18, which occurred in the same location within about 2 sec of each other. S.G Mts., San Gabriel Mountains; LA basin, Los Angeles.

View larger version (25K): [in this window] [in a new window]   Figure 3. Site Factors. The P-wave and S-wave site factors are determined from an inversion of equation (1). Coda site factors are determined from coda amplitudes.

View larger version (21K): [in this window] [in a new window]   Figure 4. Corner frequencies calculated for magnitudes 1 through 8 using the 2 model with 0 determined from our inversion.

View larger version (54K): [in this window] [in a new window]   Figure 5. The 1D S-wave velocity profile beneath each station taken from the SCEC Velocity Model (Magistrale et al., 2000) (solid line) and the linearized version used in the analytical solution for the ray tracing (dashed line). Stations are plotted left to right, top to bottom from north to south. The depth of the basin is taken to be the point where the slope changes on the dashed line. The P-wave velocity profiles are similar but not exactly proportional to S-wave velocity.

View larger version (134K): [in this window] [in a new window]   Figure 6. (a) The fits of the standard model (lines) to the data (dotted lines and *). The data consist of the average spectral amplitudes for the frequency band 1–5 Hz plotted for each of the 43 events. The vertical axes are spectral amplitudes and the horizontal axes are station numbers. = backazimuth. Station 1 is in the foothills of the San Gabriel Mountains, and station 18 is to the south at Seal Beach. Note that the standard model does not fit the group of arrivals at = 111–117°. (b) The fits of the standard model (lines) to the data (dotted lines and *). The data consist of the average spectral amplitudes for the frequency band 5–10 Hz plotted for each of the 43 events. The vertical axes are spectral amplitudes and the horizontal axes are station numbers. = backazimuth. Station 1 is in the foothills of the San Gabriel Mountains and station 18 is to the south at Seal Beach. Note that the standard model does not fit the group of arrivals at = 111–117°.

View larger version (83K): [in this window] [in a new window]   Figure 7. (a) The north–south component of event 22 normalized to the largest amplitude of all stations. Notice the dramatic difference between stations 10–12. (b) This figure shows the north–south component of event 14 normalized to the largest amplitude, in contrast to Figure 7a. Notice that station 11 does not exhibit abnormally large amplification compared with other stations that recorded event 22.

View larger version (130K): [in this window] [in a new window]   Figure 8. (a) Low-frequency (1–5 Hz) P-wave spectral amplitudes plotted in the format described in Figure 6. (b) High-frequency (5–10 Hz) P-wave spectral amplitudes plotted in the format described in Figure 6.

View larger version (19K): [in this window] [in a new window]   Figure 9. (upper curve) Averaged spectral ratios for the caustic events (17–22) measured at station 11 divided by average spectra over stations 1–6. The smooth curve is a fit of the caustic equation (10) to the data. The lower curve is the averaged spectral ratios for the noncaustic events. It has both lower amplitude and a flat variation that is consistent with the assumption that stations 1–6 are representative of noncaustic behavior.

View larger version (136K): [in this window] [in a new window]   Figure 10. (a) Low-frequency (1–5 Hz) S-wave spectral amplitudes plotted in the format described in Figure 6. Backazimuths between 120° and 124° (events 17–22) exhibit a spike at station 11. A caustic (equation 9) is included in the model for events 17–22. (b) High-frequency (5–10 Hz) S-wave spectral amplitudes plotted in the format described in Figure 6. Backazimuths between 120° and 124° (events 17–22) exhibit a spike at station 11. A caustic (equation 9) is included in the model for events 17–22.