First Internet Quick Report on

CISN Strong-Motion Data Recovered from the

M6.0 Parkfield Earthquake of September 28, 2004

Sept 28, 2004, 1830 PDT
 

A magnitude 6.0 Mw (USGS/UC Berkeley) earthquake occurred at 10:15 PDT this morning in the Parkfield area. The hypocenter was 11 miles SSE of Parkfield, at a depth of 8 km. Early analysis by USGS and UC Berkeley indicates the event had a strike slip mechanism and most likely occurred on the San Andreas fault. Further information is available at www.cisn.org/special/evt.04.09.28/, which states that this event is the anticipated Parkfield earthquake, rupturing roughly the same segment as the 1966 event.

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

The early CISN Internet Quick Report, at www.quake.ca.gov/cisn-edc/IQR/Parkfield_28Sep2004/iqr_dist.htm lists peak accelerations and distances for the first 17 stations recovered by the CGS/CSMIP and the USGS/NSMP programs of CISN. At this point the largest recorded ground shaking, 0.25g, is from the Cholame 5W station, about 13 km from the epicenter. The closest station with data so far is one of the few instrumented structures in the area, the Caltrans bridge on Hwy 46, 11 km south of the epicenter and about 500 feet west of the San Andreas fault. The shaking at the west abutment was 0.67g, and the shaking on the deck near the east abutment was 1.05 g (see www.quake.ca.gov/cisn-edc/IQR/Parkfield_28Sep2004/ce36668.gif). It is expected that this peak reflects expansion joint interaction (banging at the east abutment) during the earthquake shaking. Caltrans is currently performing field inspection of bridges in the area.

Note that the record at the bridge shows interesting motion on the longitudinal component (approximately transverse to the fault) which may reflect a near field pulse (52 cm/sec velocity, 11 cm peak displacement). Ground stations in the area, from which field data is being recovered, will provide more information on this motion.

Data is being recovered in the field from stations to the north of the epicenter. Since it appears that the event ruptured from the epicenter toward the North, we may expect that the shaking at these stations will be higher. As these records are recovered, the data will be available in future revisions of the Internet Quick Report at the CISN Engineering Data Center.