OROVILLE EARTHQUAKE

At about 1:20 P.M. on 1 August a magnitude 6.1 earthquake shook the Sacramento Valley. The earthquake, centered south of Oroville, was preceded by a magnitude 5.0 foreshock at about 9:30 A.M.

After the 5.0 foreshock the California Division of Mines and Geology (CDMG) installed two strong motion accelerographs in the area to study aftershocks, and a geologist was sent into the area to look for surface effects. The seismologists were in the process of installing the first instrument when the main shock occurred, and the instrument was running by 2:00 P.M. The second instrument was in operation by 4:00 P.M. The seismologists and geologists then conducted preliminary surveys for damage and surface effects south and southeast of Oroville.

Meanwhile, in the Sacramento District the staff was planning operations to be conducted in the Oroville area the next day (Saturday, 2 August). Under recommendations adopted by the now defunct Governor's Earthquake Council, CDMG is responsible for establishing a clearinghouse after a major earthquake in order to coordinate the activities of various investigators. Consequently, a geologist and seismologist set up the clearinghouse early Saturday morning in the California Division of Forestry's Oroville office. Throughout the day the clearinghouse staff maintained contact with field investigators and CDMG's Sacramento District Office.

CDMG also sent two seismologists and a technician out Saturday to install three more strong motion accelerographs in the epicenter area, and a geologist to look for surface faulting. By late Saturday many instances of ground failure due to the shaking had been observed, but none of these were attributable to surface faulting like that which accompanied the 1971 San Fernando earthquake.

As of late Thursday (7 August), no surface faulting had been found. However, CDMG plans to have people in the field for the next several days to thoroughly cover the area where there may be surface faulting.

Earthquakes of this magnitude are not unusual in California. One such event occurs, on the average, every 3 to 4 years in the State. However, such events are relatively rare in the foothills of the Sierra Nevada. It has been suggested that the Oroville earthquake might have been, triggered by water loads due to Lake Oroville. The lake was first filled in 1968, and the newly applied load should have been greatest then. At present, the weight of the water seems unlikely to have been the cause of the recent earthquake.

Probably the most important lesson to be learned from this earthquake is that magnitude 6 or greater earthquakes could occur anywhere in the foothills of the Sierra Nevada, even though the overall seismicity of the area is low. This fact must be taken into consideration when designing critical facilities such as dams, power plants, hospitals, and high-rise buildings.

Don A. Rodgers