EARTHQUAKES STRIKE IMPERIAL VALLEY

IN "SUPERSTITION HILLS SEQUENCE"

November 23-24, 1987

A magnitude (M) 6.6 earthquake jolted th Imperial Valley on November 24, 1987 at 5:16 a.m., just 12 hours after a M 6.2 temblor struck the same area. The quake was the strongest in California since the Central Valley's M 6.7 Coalinga earthquake of May 2, 1983. The epicenter of the earthquake, like the M 6.2 quake that struck November 23, 1987, at 5:53 p.m., was in a lightly populated desert area located approximately 14 miles southwest of the small community of Westmorland in Imperial County.

The November 24 earthquake was on the northwest trending Superstition Hills fault, believed to be part of the San Jacinto fault system. The November 23 earthquake, centered about 6 miles northeast of the epicenter of the November 24 event, occurred on a northeast trending zone under the southern edge of the Salton Sea and perpendicular to the Superstition Hills fault. The November 23 and November 24 earthquakes are two separate shocks with different coordinates, both considered main shocks. California Institute of Technology seismologists called the two-day event, which also was marked by M 5.4 and M 5.5 aftershocks as well as more than 30 aftershocks above M 3.5. the "Superstition Hills Sequence."

More or less continuous surface fault rupture and afterslip were mapped by Division of Mines and Geology (DMG) field investigators along a 23-kilometer-long segment of the Superstition Hills fault. A maximum 65 centimeters of right-slip displacement was measured as of December 2, 1987. along with a few centimeters of vertical displacement. Up to 12 centimeters of left slip was documented along one of several northeast trending faults located north of the Superstition Hills fault and in the epicentral region of the November 23 earthquakes.

According to DMG geologist James E. Kahle, no rupture was observed along the Superstition Hills fault where it intersects Imler Road west of Westmorland by 2:30 a.m. on November 24. Kahle reported that an offset of 10-15 centimeters was present at 5:45 a.m. after the second event. By November 30, this offset had increased to 38.5 centimeters due to afterslip.

Strong motion data recorded by the California Strong Motion Instrumentation Program (CSMIP) during the earthquakes show that the duration of strong shaking recorded is unusually long for earthquakes of this magnitude. In addition, stations to the south of the epicenters had particularly strong shaking during the second earthquake.

The largest peak acceleration recorded on the ground was 0.36 gravity (g) at the El Centro-Imperial County Center CSMIP station during the second earthquake. The CSMIP station located at the El Centro-Highway 8/Meloland Overpass registered 0.24 g during the second event, the largest peak acceleration recorded on a structure. The bridge was also strongly shaken during the 1979 Imperial Valley earthquake.

At the Westmorland CSMIP station, located 16 kilometers and 22 kilometers east of both epicenters, respectively, peak acceleration of 0.18 gravity (g) was recorded during the first earthquake and 0.21 g during the second. The duration of strong shaking in the second event was more than 15 seconds. This station registered 0.50 g (0.80 g vertical), with a shaking duration of 8 seconds, during the 1981 5.7 M earthquake centered 7 kilometers north of the station.

The CSMIP station located at the El Centro-Highway 8/Meloland Overpass recorded 0.07 g on the structure during the first event and 0.24 g during the second. The bridge was also strongly shaken during the 1979 Imperial Valley earthquake.

State emergency officials reported that at least 94 people were injured in California and Mexico and some structural damage during the November 23-24 events. Damage in the epicentral area included cave-ins along the All-American Canal, which carries irrigation water from the Colorado River into the farmlands of Imperial Valley, and temporary closure of state highways 86 and 98. The distance of the earthquakes from populated areas kept damage and injuries down compared to the destruction caused by the M 5.9 Whittier Narrows earthquake that occurred on October 1, 1987, in heavily populated parts of the San Gabriel Valley just east of Los Angeles.

The Imperial Valley has historically been one of the most seismically active areas in California. There have been earthquakes of M 6 or greater in 1915, 1940, and 1979, with the strongest, a M 6.7 quake that killed seven people, occurring in 1940. The geological forces that caused the November 23-24, 1987, earthquakes are the same ones that pulled Baja California away from Mexico to form the Gulf of California.

An article by DMG geologists on surface faulting associated with the November 23-24, 1987 Superstition Hills earthquakes will be in the March 1988 CALIFORNIA GEOLOGY . . . Angela Blanchette, DMG.