1979_Imperial_Valley_earthquake

The 1979 Imperial Valley earthquake occurred at 16:16 Pacific Daylight Time (23:16 UTC) on 15 October just south of the Mexico–United States border. It affected Imperial Valley in Southern California and Mexicali Valley in northern Baja California. The earthquake had a relatively shallow hypocenter and caused property damage in the United States estimated at US$30 million. The irrigation systems in the Imperial Valley were badly affected, but no deaths occurred. It was the largest earthquake to occur in the contiguous United States since the 1971 San Fernando earthquake eight years earlier.
The earthquake was 6.5 on the Mw scale, with a maximum perceived intensity of IX (Violent) on the Mercalli intensity scale. However, most of the intensity measurements were consistent with an overall maximum intensity of VII (Very strong), and only the damage to a single structure, the Imperial County Services building in El Centro, was judged to be of intensity IX. Several comprehensive studies on the total structural failure of this building were conducted with a focus on how the building responded to the earthquake's vibration. It was one of the first heavily instrumented office buildings to be severely damaged by seismic forces.
The Imperial Valley is surrounded by a number of interconnected fault systems and is vulnerable to both moderate and strong earthquakes as well as earthquake swarms. The area was equipped with an array of strong motion seismographs for analyzing the fault mechanisms of nearby earthquakes and seismic characteristics of the sediments in the valley. The earthquake was significant in the scientific community for studies of both fault mechanics and repeat events. Four of the region's known strike-slip faults and one additional newly discovered normal fault all broke the surface during the earthquake.