1991 Sierra Madre earthquake
 Los Angeles
Palm Springs
Santa Barbara
 |
|
| Date | 28 June 1991 |
|---|---|
| Origin time | 14:43:55 UTC |
| Magnitude | 5.6 Mw |
| Depth | 6.2 mi (10 km) |
| Epicenter | 34°13′N 118°04′W / 34.22°N 118.07°WCoordinates: 34°13′N 118°04′W / 34.22°N 118.07°W |
| Type | Thrust |
| Areas affected |
Greater Los Angeles Area Southern California United States |
| Total damage | $33.5–40 million |
| Max. intensity | VII (Very strong) |
| Peak acceleration | .58g at Cogswell Dam |
| Landslides | Yes |
| Casualties | 2 deaths 100–107 injured |
The 1991 Sierra Madre earthquake occurred on June 28 at 07:43:55 local time with a moment magnitude of 5.6 and a maximum Mercalli intensity of VII (Very strong). The thrust earthquake resulted in two deaths, around 100 injuries, and damage estimated at $33.5–40 million. The event occurred beneath the San Gabriel Mountains on the Clamshell–Sawpit Fault, which is a part of the Sierra Madre–Cucamonga Fault System. Instruments captured the event at a number of strong motion stations in Southern California.
Due to its history of generating great earthquakes (1857, 1906) the focal point of earthquake hazard assessment in California has been the strike-slip San Andreas System of faults. However, a system of faults, including the Sierra Madre–Cucamonga Fault System that runs along the border of the San Gabriel and San Fernando Valleys, has also caused a series of moderate to large events in the Greater Los Angeles Area. The westernmost portion of this fault system ruptured in 1971 and has been identified as one of six major fault systems that pose a threat to the highly populated region.
Geodetic research has shown that crustal shortening is occurring in the region of the Transverse Ranges at a rate of 0.33 inches (8.5 mm) per year. Although a number of earthquakes have occurred in the Los Angeles area since 1971, the rate of moderate events since then may not be typical, and few large events have occurred since records have been kept. The rate of contraction and too few moderate events that have occurred indicated to researchers that a deficit of seismic energy release exists, and that larger, but less frequent events may also contribute to the release of strain accumulation.
To the west of the San Andreas Fault, the southern edge of the Transverse Ranges has been in a state of contraction during the Quaternary Period, with active reverse faulting that has uplifted San Gabriel Mountains. The west-striking (and range-bounding) Sierra Madre–Cucamonga Faults, as well as the San Cayetano Fault and Santa Susana Faults all exhibit a component of left-lateral strike-slip motion and share a similar trend.
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