Fourth Internet Quick Report on

CISN Strong-Motion Data Recovered from the

M6.0 Parkfield Earthquake of September 28, 2004

Oct 2, 2004, 1630 PDT

Large amplitudes have been recovered at additional stations at the northeast end of the aftershock zone. In the Third Quick Report, the large amplitude of 1.3g at Fault Zone 14 was reported, and it was noted that this station had also had larger amplitudes than nearby stations in previous earthquakes.

Two additional records have been recovered from this area with amplitudes over 1 g. The map shows the locations of FZ 14 and other nearby stations. Station FZ 11, to the east of the fault, and about 3 km due east of Parkfield, has over 1 g amplitude motion in the accelerogram. Station FZ 16, also to the east of the fault but about 4 km northwest of Parkfield (and approximately across the fault from FZ 14 and FZ 15), has amplitudes over 1g . (The FZ 16 accelerogram shows that the traces are of high amplitude, >1g. In fact, the traces are crossing such that they can’t yet be untangled; one acceleration trace goes beyond the recording capacity of the film, at the top.)

Though these 3 stations in the vicinity of Parkfield all have acceleration over 1g, the recording in Parkfield is only about 0.3g, as shown in the FZ 12 record; the same peak acceleration is recorded in the nearby Parkfield elementary school.

All analog records from stations in the Parkfield area have been recovered. As they are scaled and digitized, they will be added to the expanding IQR table, which lists peak accelerations and epicentral distances for the 46 stations recovered by the CGS/CSMIP and the USGS/NSMP programs of CISN.

An early plot of peak horizontal acceleration vs fault distance is shown, with the curve predicted by BJF97; it will be revised as more data becomes available. To meet requests for raw data, the raw acceleration for the digital Cholame 5W record is also downloadable.