Tectonics
The Coalinga earthquake was caused by an 0.5-meter uplift of the anticline ridge northeast of Coalinga, but surface faulting was not observed. Ground and aerial searches immediately after the earthquake revealed ground cracks and fissures within about 10 kilometers (6.2 mi) of the instrumental epicenter, none of which appeared to represent movement on deeply rooted fault structures. About five weeks later, on June 11, however, an aftershock caused surface faulting about 12 km (7.5 mi) northwest of Coalinga.
The Coalinga earthquake suggested to geologists that the State of California was in even worse seismological condition than had been thought. The pace of earthquake activity along the Pacific coast was identified as a relevant subject for further study; and the investigation of earthquakes stemming from unknown faults caused concern. California officials emphasized that the predicted great quake would do far more damage than this one and that if it struck in a densely populated area the damage would be incalculable.
Read more about this topic: 1983 Coalinga Earthquake