Internet Quick Report on

CISN Strong-Motion Data Recovered from the

M6.5 San Simeon Area Earthquake of December 22, 2003

December 22, 2003

A magnitude 6.5 (USGS/UC Berkeley) earthquake occurred at 11:15 PST this morning in a sparsely populated area in central coastal California. The epicenter was 11 km NE of San Simeon (and 110 km SE of Monterey), at a depth of about 8 km. Early analysis by USGS and UC Berkeley indicates that it had a thrust (reverse faulting) mechanism.

There are relatively few strong motion stations in the area of this earthquake. Although the number of stations was increased in southern California under TriNet, areas in northern California, like this area, have far fewer stations. However more stations are planned for central and northern California under the CISN project, and this event highlights the need.

Most of the modern, digital stations are at a distance of 60 km or more from the epicenter. The Internet Quick Report, at <>, lists the 41 records recovered so far and the peak values and distances for the records. The largest shaking observed was at a CGS-instrumented small 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. A peak value of 128%g was recorded at the roof of the hospital (1-story, wood-frame construction). A nearby ground station was temporarily removed due to construction work at the hospital, but the ground level of the hospital is a good indication of the shaking in the vicinity. Templeton is only about 10 km from Paso Robles, which the early reports indicate had some significant damage.

The next largest shaking was at San Luis Obispo, where 17%g was recorded at a USGS station downtown (note that these stations are widely spaced.)

There are more stations in the area, some close in, but most are of the early film type or other types requiring onsite record recovery. As these records are recovered, the data will be available on future revisions of the Internet Quick Report at the CISN Engineering Data Center.