OSMS 94-24 


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A magnitude 5.0 (ML BRK; moment magnitude 5.0 BRK) earthquake occurred on December 20, 1994 at 2:27 am (PST), approximately 5 km west of Parkfield. The preliminary hypocenter is located at 35.86°N, 120.50°W with a depth near 6 km (USGS).

Table 1 summarizes the strong-motion data at 10 CSMIP stations, ranging in epicentral distance from 6 to 39 km. The locations of these stations and other CSMIP stations in the Parkfield area are shown on the map in the attached figure. The records from 7 ground-response stations are included in this early release.

The largest horizontal and vertical accelerations are 0.43 and 0.21 g, respectively. These accelerations were recorded at Fault Zone #7 located about 9 km from the epicenter. Interestingly, three nearby stations (Fault Zone #12, Gold Hill #1W and Turkey Flat #1) all recorded significantly smaller accelerations of 0.1 g or less. Recovery of data from the many other CSMIP stations in the Parkfield area continues.

Maximum accelerations from 3 structures are included in this early release. Peak structural accelerations of 0.09 and 0.20 g were recorded on the ground floor and roof of a 1-story woodframe school building in Parkfield. Peak accelerations of 0.01 and 0.02 g were recorded at the base and roof, respectively of a 1-story hospital in Templeton approximately 39 km SW of the epicenter. A peak acceleration of 0.05 g was recorded at a small concrete bridge abutment 24 km south of the epicenter.

Processed acceleration, velocity and displacement and response spectra (preliminary) from Fault Zone #7, Turkey Flat #1 and the 1-story School Building in Parkfield are included in this early release. The early processing and release of these data take advantage of new developments in instrumentation and communications. Files of the processed data from the 2 ground response stations can be obtained on the Internet.