TriNet Strong-Motion Data from the M 7.1 Hector Mine, California, Earthquake of 16 October 1999

The M_w 7.1 Hector Mine earthquake of October 16, 1999 was recorded by more than 300 stations of TriNet, which is administered cooperatively by the California Division of Mines and Geology's California Strong Motion Instrumentation Program (CDMG/CSMIP), California Institute of Technology, and the U.S. Geological Survey (USGS). The earthquake occurred in a remote part of the Mojave Desert, approximately 190 km northeast of downtown Los Angeles, and there were no strong-motion stations close to the surface rupture. The nearest station, Hector, is about 27 km north of the epicenter; it recorded a peak horizontal ground acceleration of 0.33g. The two next closest stations, Amboy and Joshua Tree, are to the east and south, both at epicentral distances of about 50 km; each recorded peak ground accelerations of about 0.2g. The new digital instruments installed for the TriNet project recorded a large set of reliable data at epicentral distances up to 275 km. These data can significantly improve empirical peak ground motion attenuation relationships, which are usually developed for distances only up to 100 km (Boore et al., 1993, 1997) because adequate data have not been available at greater distances. Hector Mine peak ground motions demonstrate reasonable agreement with empirical attenuation relationships for acceleration. In contrast, higher than expected ground velocities and displacements were recorded at epicentral distances of about 150 to 220 km, especially in the Los Angeles sedimentary basin, where anomalously high-amplitude displacements with periods of 6 to 7 sec were recorded in Los Angeles, Long Beach, and other areas. These long-period surface- or basin-generated waves can have significant effects on large structures. The M_w 7.3 Landers earthquake of 1992 similarly produced strong, long-period waves in the basin. The peak ground motions produced by the Landers earthquake were on average 1.6 times higher than for the Hector Mine earthquake in the Los Angeles area. Ground-motion data recorded by digital instruments were uniformly processed in the frequency band 0.067 to 46 Hz (0.022–15 sec). The processed data set includes records from 213 ground-response stations. In an effort to make strong-motion data available quickly to the engineering and scientific communities, important records from this event were made available by file transfer protocol (ftp) beginning the day of the earthquake

A Study of Possible Ground-Motion Amplification at the Coyote Lake Dam, California

Abstract The abutment site at the Coyote Lake Dam recorded an unusually large peak acceleration of 1.29g during the 1984 Morgan Hill earthquake. Following this earthquake another strong-motion station was installed about 700 m downstream from the abutment station. We study all events (seven) recorded on these stations, using ratios of peak accelerations, spectral ratios, and particle motion polarization (using holograms) to investigate the relative ground motion at the two sites. We find that in all but one case the motion at the abutment site is larger than the downstream site over a broad frequency band. The polarizations are similar for the two sites for a given event, but can vary from one event to another. This suggests that the dam itself is not strongly influencing the records. Although we can be sure that the relative motion is usually larger at the abutment site, we cannot conclude that there is anomalous site amplification at the abutment site. The downstream site could have lowerthan-usual near-surface amplifications. On the other hand, the geology near the abutment site is extremely complex and includes fault slivers, with rapid lateral changes in materials and presumably seismic velocities. For this reason alone, the abutment site should not be considered a normal free-field site

Preliminary Report on the 22 December 2003, M 6.5 San Simeon, California Earthquake

The Mw 6.5 San Simeon earthquake struck the central California coast on 22 December 2003 at 19:15:56 UTC (11:15:56 am local time.) The epicenter was located 11 km northeast of the town of San Simeon, and 39 km west-northwest of Paso Robles (Figure 1), as reported by the California Integrated Seismic Network (CISN, the California region of the Advanced National Seismic System [ANSS]). The mainshock nucleated at 35.702°N, 121.108°W and a depth of 7.1 km, and the rupture propagated unilaterally to the southeast. The strong directivity of the rupture resulted in a concentration of damage and aftershock..