Since no known faults had been identified in this area, no one expected this earthquake.
On May 2, 1983, an earthquake of magnitude 6.5 occurred about eight miles northeast of the town of Coalinga. It was a surprise quake since no known active faults had been identified within historical time anywhere near the epicenter. Subsequent investigations revealed that the quake was associated with a deep concealed fault that was twenty miles northeast of the San Andreas Fault. The area as a whole had recorded only low levels of seismicity within historical times. This earthquake caused an estimated $10 million in property damage and injured ninety-four people. Damage was greatest in Coalinga where the eight-block downtown commercial district was almost completely destroyed. Non reinforced brick walls did not stand up at all but newer buildings such as the Bank of America and the Guarantee Savings and Loan buildings were only slightly damaged.
Outside of Coalinga the worst damage was at Avenal, twenty miles southeast of the epicenter. Altogether 309 single-family homes and thirty-three apartment buildings were destroyed and 558 single-family homes, ninety-four mobile homes, and thirty-nine apartment buildings suffered major damage. There was minor damage to 811 single-family homes, twenty-two mobile homes, and seventy apartment buildings. All public utilities were damaged to some degree, delaying service for different lengths of time for all of them. There were thousands of rock falls and rockslides outwards from the epicenter as far as twenty miles northward, ten miles south, and fifteen miles to the southwest. The quake was felt from Los Angeles to north of Sacramento and from the coast to Nevada. Numerous aftershocks were recorded.
The town of Coalinga is almost completely surrounded by oil fields. The history of oil in this area dates from the late 1800s. By the 1990s, production was running at a rate of 14,000 barrels per day. Drilling continued to spread more widely in the Coalinga oil field as a whole despite the fact that oil prices in that far off time of 1905 was only fifteen cents a barrel. By 1920, production reached sixteen million barrels per day and almost immediately afterward a protracted strike by oil workers stopped production for most of the following years until World War II. Beginning in the early 1950s a new venture was launched in Coalinga, the injection of steam pressure into shallow tar sands, the same method that is today in use in Alberta, Canada, on a vast scale. For Coalinga the method proved to be economically viable and this approach along with others added up to a billion barrels of oil being produced before the earthquake of 1983 brought everything to a stop.
Damage to individual wells was seen in the representative case of Shell Oil. There, only twenty-six out of their more than nine hundred active wells were severely damaged. All electric power to the oilfields was cut when the earthquake struck. Several days were needed to repair downed lines and repair damaged transformers. Although no pumping units were heavily damaged, a large number were knocked out of alignment by as much as four inches. Most of Shell Oil’s pumping units were displaced. Oil tanks were damaged in various ways while pipelines fared quite well. The numerous leaks that developed were easily repaired. Buckling and cracking were common on roads and slumping frequently appeared on cut and-fill slopes and on berms.