Qualla: 1987 Superstition Hills Earthquakes

1987 Superstition Hills Earthquakes: Introduction

Thu, 22 Jan 2026 00:00:00 GMT

Photo credit Nzeemin, CC BY-SA 3.0. At 5:54 PM on November 24, 1987, the earth moved twice. First came the magnitude 6.2 Elmore Ranch earthquake, striking southwest of Westmorland in California's Imperial Valley. Twelve hours later, the magnitude 6.5 Superstition Hills mainshock hit along the San Jacinto Fault Zone. The Superstition Mountain seismic station recorded 0.9g horizontal acceleration, the largest and longest reading in its 55-year history. The ground did not stop moving for nearly a year, as aseismic slip continued along the Superstition Hills Fault, slowly increasing displacements long after the initial violence had passed. In the transition zone where the San Andreas Fault system meets the spreading Gulf of California, the Imperial Valley had once again demonstrated why it ranks among the most seismically active regions in North America.

Image: Nzeemin | CC BY-SA 3.0

1987 Superstition Hills Earthquakes: Where Continents Tear Apart

Thu, 22 Jan 2026 00:00:00 GMT

Photo credit Mikenorton, CC BY-SA 3.0. The Imperial Valley occupies a unique position in California's geology. Here, the San Andreas Fault and San Jacinto Fault Zone discontinue, marking a transition between continental transform faulting and oceanic rifting extending into the Gulf of California. The Brawley seismic z...

Image: Mikenorton | CC BY-SA 3.0

1987 Superstition Hills Earthquakes: The Double Strike

Thu, 22 Jan 2026 00:00:00 GMT

Photo credit United States Geological Survey, Public domain. The magnitude 6.2 foreshock occurred on a previously unmapped fault, later named the Elmore Ranch Fault Zone. It ruptured bilaterally, extending northeast toward the Brawley Seismic Zone and southwest to join the Superstition Hills Fault. Surface ruptures showed left-lateral slip...

Image: United States Geological Survey | Public domain

1987 Superstition Hills Earthquakes: A Year of Creeping

Thu, 22 Jan 2026 00:00:00 GMT

Photo credit United States Geological Survey, Public domain. The earthquakes did not end on November 24. Over 339 days, the Superstition Hills Fault continued producing aseismic slip, steadily increasing ground displacements. The Wienert Fault's southern portion generated vertical offsets that were actively growing when observed in 1988. T...

Image: United States Geological Survey | Public domain

1987 Superstition Hills Earthquakes: Damage Across the Border

Thu, 22 Jan 2026 00:00:00 GMT

Photo credit Thelmadatter, Public domain. The earthquakes struck a sparsely populated region, limiting casualties despite their magnitude. Two people died in Mexicali, Mexico, where 50 were injured and building damage was considerable. Another 44 people were injured in the Imperial Valley. In El Centro, Imperial, and Wes...

Image: Thelmadatter | Public domain

1987 Superstition Hills Earthquakes: Living on the Edge

Thu, 22 Jan 2026 00:00:00 GMT

Photo credit United States Geological Survey, Public domain. The Imperial Valley continues to experience regular seismic activity. The 1987 Superstition Hills earthquakes, while larger than the Whittier Narrows earthquake that struck Los Angeles the previous month, caused less damage precisely because they occurred in agricultural land rat...

Image: United States Geological Survey | Public domain