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OSMS 83-04

"Strong Motion Data From the Coalinga, California Earthquakes and Aftershocks"

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

Shakal, A.F. and J.T. Ragsdale (1983). Strong Motion Data From the Coalinga, California Earthquakes and Aftershocks. The 1983 Coalinga, California Earthquakes, CDMG Special Publication 66.

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Strong-motion records from the Coalinga aftershocks indicate that accelerations in the city of Coalinga are commonly three times higher than at a rock site 3 km west of the city. The high levels of damage in Coalinga during the mainshock are probably due in part to this effect. The alluvium underlying the Coalinga valley appears to be the reason for the increased accelerations relative to the rock site at the valley edge, observed in all but one of the six large (5 - 6 ML) aftershocks. No strong-motion records were obtained in Coalinga during the mainshock but accelerations as high as 70% in the aftershocks imply that equivalent or larger accelerations probably occurred during the mainshock.

The set of free-field strong-motion records obtained during the Coalinga earthquake is the largest ever obtained from a single event. The data set is important for studies of ground motion attenuation and local variations of ground motion. Proximate stations in the Parkfield strong-motion array recorded significantly different acceleration levels though sited on the same geologic formation.

Significant records were also obtained from the Coalinga aftershocks. The 5.1 ML event of 9 May was very well recorded, with some 26 strong-motion, records obtained in the epicentral area by various agencies. Deployment of co-located analog and digital accelerographs for instrument-evaluation purposes yielded several accelerogram pairs and triplets with peak accelerations of 10 – 70%g.