OSMS 83-06

"Coalinga Earthquake Recorded Ground Motion"

by J.T. Ragsdale and A.F. Shakal

Ragsdale, J.T. and A.F. Shakal (1983). Coalinga Earthquake Recorded Ground Motion. Proceedings Structural Engineers Association of California 52nd Annual Meeting, Coronado, September 29 - October 1, 1983.

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The origin of the Coalinga earthquake of May 2, 1983 (ML = 6.7) was approximately 13 km northeast of the city of Coalinga at a depth of about 10 km (Fig. 1). No surface rupture was observed. This earthquake was the largest to occur in California since the San Fernando earthquake of 1971 and it caused major damage to old buildings in downtown Coalinga and to old homes elsewhere in the city. During the three months following the mainshock six aftershocks of magnitude 5 and greater occurred.

Permanent strong-motion stations operated by the California Division of Mines and Geology (CDMG) in the region surrounding Coalinga are shown in Figs.1 and 2. Fig. 2 shows the detailed station locations and local geology of the strong-motion array near Parkfield described by McJunkin and Shakal, (1983). The USGS maintains a permanent station at the Pleasant Valley Pumping Plant (Fig. 3). Temporary CDMG stations were placed in the epicentral area after the mainshock to augment permanent stations as shown in Figure 3.

Nearly 100 strong-motion instruments were triggered by the Coalinga earthquake mainshock and the films collected represent the largest number of records from a single event since the San Fernando earthquake of 1971. Most of the records were obtained from ground level triaxial accelerographs. Sixty of the records were from CDMG stations and the coordinate locations, accelerations, and reproductions of the records are included in a report by Shakal and McJunkin (1983). Records from the USGS stations are similarly reported by Maley et al ., (1983).