Fifth Internet Quick Report on

CISN Strong-Motion Data Recovered from the

M6.0 Parkfield Earthquake of September 28, 2004

Oct 7, 2004, 1800 PDT

The full set of records from the Parkfield array and nearby stations have been recovered and scaled. The peak acceleration values are available in the updated Internet Quick Report. In the Fourth Quick Report, the large amplitudes that occurred at the north end of the Parkfield area (though not at Parkfield itself) were reported. The remaining records of the array allow these early records to be seen in the larger context. The map (from a 1983 California Geology article describing the array) shows the locations of the stations and local geology. An interesting late record is from Slack Canyon, of the map to the northwest, along the fault. It has a peak acceleration of 0.35g, although it is 10 km NW of the end of the rupture, and has almost no high frequencies.

The IQR table also includes the records from an array of 11 GEOS high-resolution instruments installed by the USGS within the general area of the Parkfield array. The peak values and images of the records are included in the IQR. These instruments will have better resolution than possible from digitized film records. Detailed questions about the instruments and stations can be directed to R. Borcherdt.

A plot of PGA vs distance to the fault shows that the close-in data is both higher and lower than the BJF97 curve. At greater distances, the curve predicts stronger shaking than reflected by the observed data. This feature was also observed in the San Simeon data, and may lead to standard attenuation relationships incorporating a more rapid decay with distance in their formulation.

To assist in analysis of the data, the Text Table under the button at the upper left of the IQR table now includes closest distance to the fault (necessarily approximate, at this point), as well as the epicentral distance.

Digitization of records is underway. Processed data for the first five digitized analog records are being released today (Cholame 2W, Fault Zone 1, Fault Zone 8, Fault Zone 12 and Gold Hill 3W). The downloadable data files include the start time of the record (UTC). For these early releases, only the first 22-second section is used. Acceleration response spectra for some of these records are compared with the UBC curve in an Sa spectral plot. Records from digital instruments, mostly from more distant stations, will also be reviewed to increase their bandwidth, and their raw data will be released where data quality allows. The IQR table will be updated periodically as processed records and revised information are available.