Third Internet Quick Report Update

CISN Strong-Motion Data from the

M6.5 San Simeon Earthquake of December 22, 2003

Update of December 31, 2003

The magnitude 6.5 earthquake of 11:15 PST on December 22 was recorded by CGS and USGS instruments out to distances of over 300 km. The epicenter was 11 km NE of San Simeon (110 km SE of Monterey). The First and Second Quick Reports give more background on the stations and earthquake.

Records from the many analog film stations in the area have now mostly been recovered and developed. The Internet Quick Report, at www.cisn-edc.org, lists the 83 records recovered so far and their peak values and distances. For film records, only a peak acceleration is listed.

Improvements of data at the cisn-edc for this event include:

A quick comparison of the peak acceleration data for this event with that predicted by a standard relationship is useful. Figure 1 shows a plot of peak acceleration vs distance (log-log) for the 81 records obtained so far. The distances range from 12 km, for the Cambria station, to many stations at distances of over 250 kilometers. For reference, the Boore-Joyner-Fumal (BJF97, Boore et al., 1997) attenuation relationship is shown. (Coefficients for a reverse fault and an average shallow Vs of 700 m/sec were used; the thin line indicates distances beyond the suggested limit of the authors, 80 km). The data shows reasonable agreement with BJF97 in its applicable range. Beyond that, higher attenuation with distance than predicted by the extrapolated BJF97 curve is indicated. These new data, and other recent data from digital instruments, allow extending the existing relationships to greater distances.

The point above the curve at about 40 km is Templeton, which had 0.48g, the largest value recorded in the earthquake; lying above the curve is consistent with directivity-increased shaking in the rupture direction. The two closest stations, Cambria and San Antonio Dam, both plot below the curve, consistent with directivity-reduced values in the direction away from the rupture.