1992 Cape Mendocino earthquakes
 San Francisco
Eureka
Monterey
 |
|
| Date | April 25, 1992 |
|---|---|
| Origin time | 11:06 PDT |
| Duration | 9 seconds |
| Magnitude | 7.2 MW |
| Depth | 10.5 km (7 mi) |
| Epicenter | 40°20′N 124°14′W / 40.33°N 124.23°WCoordinates: 40°20′N 124°14′W / 40.33°N 124.23°W |
| Areas affected |
North Coast (California) United States |
| Total damage | $48.3–75 million |
| Max. intensity | IX (Violent) |
| Peak acceleration | 2.2g (est) |
| Tsunami | Yes |
| Aftershocks | 6.5 MW April 26 at 0:41 6.6 MW April 26 at 4:18 |
| Casualties | 98–356 injuries |
The 1992 Cape Mendocino earthquakes (or 1992 Petrolia earthquakes) occurred along the Lost Coast of Northern California on April 25 and 26. The three largest events were the M7.2 thrust mainshock that struck near the unincorporated community of Petrolia midday on April 25 and two primary strike-slip aftershocks measuring 6.5 and 6.6 that followed early the next morning. The sequence encompassed both interplate and intraplate activity that was associated with the Mendocino Triple Junction, a complex system of three major faults (including the Cascadia subduction zone, San Andreas Fault, and Mendocino Fracture Zone) that converge near Cape Mendocino. The total number of aftershocks that followed the events exceeded 2,000.
The three shocks damaged and destroyed homes and businesses in Humboldt County and injured up to 356 people, but the single largest loss was due to a post-earthquake fire that consumed a business center in Scotia. Accelerometers that had been in place in the Cape Mendocino area since the late 1970s recorded the event and the readings were moderate to strong, with the exception of the instruments closest to the epicenter, which went off scale a few seconds into the recording. No surface ruptures were present in the epicentral area, but landslides closed roads and railroad tracks for at least a week while cleanup took place. Also discovered was about 1 m (3 ft 3 in) of coastal uplift near Cape Mendocino and Punta Gorda.
As the largest earthquake in California since the 1989 Loma Prieta event several years earlier, the mainshock caused a non-destructive tsunami that quickly reached the coast, and eventually Alaska and Hawaii several hours later. The tsunami was significant not because of its run-up, but because of the speed with which it reached the coast and for how long the waves persisted. Other strong earthquakes have affected the same area, with some that were clearly associated with the (interplate) Mendocino Fracture Zone, and others (like the two shocks on April 26) were intraplate earthquakes that ruptured within the Gorda Plate, but events that are unequivocally associated with the Cascadia subduction zone are very infrequent.
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