Notable Earthquake of the Week

North Palm Springs, California, M5.8 Earthquake  

In the early morning hours of July 8, 1986, a moderately strong earthquake struck in the Northern Coachella Valley. The temblor registered M5.8 and occurred at 2:21 a.m. 6 miles northwest of North Palm Springs in the Mission wash area between the Mission Creek and Banning faults.

At least 29 people were injured and significant damage occurred to at least 51 buildings in the Palm Springs and Morongo Valley areas. Landslides and rock falls happened in the mountains. The total damage cost from the quake was on the order of $4 million.

Analysis showed the focal point was at 11.3 km beneath the surface and the fault motion was nearly pure right-lateral strike-slip along a 65° north dipping fault plane. A discontinuous zone of surface cracks and churned earth was observed along the Banning fault between Cottonwood Canyon to Devers Hill occurred over a zone less than 10 meters wide over a distance of 7 km.

Where the zone crossed Highway 62, cracks showed an average of 1.8 cm (3/4 in.) right-lateral movement, with maximum offsets as large as 9.7 cm (4 in.). Detailed analysis suggested the cracks were probably induced subsidence fractures along the fault and not tectonic fault ruptures. A robust aftershock sequence lasted for years and included as many as 13 M4's.

Tectonic Implications:

(1) It was the first M5+ temblor in the Coachella Valley in historic times and the first earthquake in this region of California since the Desert Hot Springs M6.5 earthquake in 1948. That temblor actually occurred well within the Little San Bernardino Mountains.

(2) Since the epicenter was located between the Mission Creek and Banning faults, scientists had speculated which fault the quake had occurred along. The aftershock pattern presented the answer. When plotted in cross section, the aftershocks outlined a north dipping rupture plane, which essentially implicated the Banning fault as the source fault. This was important development because it confirmed the fault changes orientation from vertical near Indio to north dipping as it bends westward through the San Gorgonio Pass. It implied the "Big Bend" in the San Andreas fault causes the Banning fault to lean over and actually become a thrust fault, lifting the San Bernardino Mountains up and over the San Jacinto block to the south.

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