Second Internet Quick Report on

CISN Strong-Motion Data Recovered from the

M6.0 Parkfield Earthquake of September 28, 2004

Sept 29, 2004, 1600 PDT

Unusually strong shaking was recorded southeast of the epicenter of the 6.0 Mw Parkfield earthquake that occurred at 10:15 PDT on September 28th. Analysis by USGS and UC Berkeley (www.cisn.org/special/evt.04.09.28/) indicates that is the anticipated Parkfield earthquake, rupturing roughly the same segment as the 1966 event. The peak acceleration recorded is 0.85g, at Station Fault Zone 1, about 9 km southeast of the epicenter (CE36407.gif). Station Cholame 2W, slightly farther southeast (11 km) recorded 0.63g (CE36228.gif).

The high values southeast of the epicenter are in contrast with the lower values to the northwest of the epicenter: 0.16g basically over the rupture at Gold Hill 1W, 0.25g at Fault Zone 7 (7 km to the northwest), and 0.31g at Fault Zone 12 (10 km northwest of the epicenter). These stations are shown on the map of strong motion array at parkfield.jpg.

Rupture propagation from the epicenter to the northwest, indicated by the aftershocks, would predict just the opposite pattern - high amplitudes to the north, low amplitudes to the southeast. The high values southeast of the epicenter are clearly not consistent with that.

Station Cholame 2W is in almost the same location as the famous Station 2 which recorded about .6 g in the 1966 earthquake, 38 years ago. The acceleration observed at the west abutment of the nearby Cholame bridge (.87g) is quite consistent with the Cholame 2W record.

There are many additional strong motion stations in the area because of the Parkfield array. There are relatively few modern digital stations with communication capability, however. They are planned for upgrade as resources allow. As a result of the lack of data, early information may be significantly modified as more data is recovered in the field.

The CISN Internet Quick Report lists peak accelerations and distances for the first 29 stations recovered by the CGS/CSMIP and the USGS/NSMP programs of CISN. The first report on this event is available for reference.