Second Internet Quick Report

CISN Strong-Motion Data from the

M6.5 San Simeon Earthquake of December 22, 2003

Update of December 24, 2003

The magnitude 6.5 (USGS/UC Berkeley) earthquake that occurred at 11:15 PST on December 22 was located in an area of relatively few strong motion stations. The epicenter was 11 km NE of San Simeon (110 km SE of Monterey), at a depth of about 8 km. The first Quick Report gives more background on the stations and earthquake.

Most of the modern, digital stations were at a distance of 60 km or more from the epicenter of this earthquake. A few stations are closer, and these records have bee recovered. Some of these stations are the early film recorders, and the films have been developed and scaled. The Internet Quick Report, at www.cisn-edc.org, lists the 58 records recovered so far and the peak values and distances for these records. For the film records, only a peak acceleration is listed, since these need to be digitized before velocities and displacement will be known.

The largest shaking observed was at a CGS-instrumented 1-story hospital in Templeton, about 38 km from the epicenter (though much closer to the projected southern end of the rupture). The hospital recorded about 50%g horizontal motion and 30%g vertical motion at the ground level. The 5% acceleration response spectra of the recorded ground-level motion are plotted in the linked figure. For comparison, the UBC design spectrum is also shown. A peak value of 128%g was recorded at the roof of the hospital (1-story, wood-frame construction). The ground station nearby was temporarily offline due to construction near the hospital, but the ground level of the hospital is a good indication of the shaking in the vicinity, because the hospital is small and light. Templeton is about 10 km from Paso Robles, where significant damage occurred.

The closest station was near Cambria, 12 km form the epicenter, near a Caltrans bridge on Hwy 1. Though the station is much closer, it only recorded 18%g. Thus the early data shows apparent strong directivity in the direction of the rupture propagation (toward Templeton). The pattern is reminiscent of the 1984 Morgan Hill earthquake, which also showed strong apparent directivity toward the Coyote Dam.

The nest closest station is at San Antonio Dam, 22 km NE of the epicenter (on the opposite side of the fault from Cambria). The dam (earthfill) is lightly instrumented, and only has instruments on the crest and toe. The crest instruments recorded about 22%g. It is expected that the structural response of the dam itself affected the motion, but it is likely that the crest motion differs by perhaps 10% or less from the shaking in the area. (We have included a scan of the film record under the “view” button for this record.) An instrument at the toe has not yet been recovered because of logistics problems, and will be recovered later. These records will be digitized soon.

An early record recovered was from the San Luis Obispo, about 61 km to the south of the epicenter, where 17%g was recorded at a USGS station downtown.

Some of the next closest records are from the Parkfield area. Although the records are much simpler than the San Antonio Dam records, they have a peak value slightly higher (23%g at Cholame 12W), but it is an isolated excursion on the record.

There are more stations in the area of the type requiring onsite record recovery. As more records from the CGS and USGS are recovered, the data will be available in future revisions of the Internet Quick Report at the CISN Engineering Data Center.