  Las Vegas Bakersfield Randsburg San Diego Palm Springs Santa Barbara  | |
| UTC time | 1991-06-28 14:43:55 |
|---|---|
| ISC event | 330437 |
| USGS-ANSS | ComCat |
| Local date | 28 June 1991 (1991-06-28) |
| Local time | 07:43:55 |
| Magnitude | 5.6Mw[1][2] |
| Depth | 6.2 mi (10 km)[1] |
| Epicenter | 34°13′N118°04′W / 34.22°N 118.07°W /34.22; -118.07[1] |
| Type | Thrust |
| Areas affected | Greater Los Angeles Area Southern California United States |
| Total damage | $33.5–40 million[2][3] |
| Max. intensity | MMI VII (Very strong)[3] |
| Peak acceleration | 0.58g atCogswell Dam[4] |
| Landslides | Yes |
| Casualties | 2 deaths[2] 100–107 injured[2][5] |
The1991 Sierra Madre earthquake occurred on June 28 at07:43:55 local time with amoment magnitude of 5.6 and a maximumMercalli intensity of VII (Very strong). Thethrust earthquake resulted in two deaths, around 100 injuries, and damage estimated at $33.5–40 million. The event occurred beneath theSan 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 inSouthern California.
Due to its history of generating great earthquakes (1857,1906) the focal point of earthquake hazard assessment in California has been the strike-slipSan Andreas System of faults. However, a system of faults, including the Sierra Madre–Cucamonga Fault System that runs along the border of theSan Gabriel andSan 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 in1971 and has been identified as one of six major fault systems that pose a threat to the highly populated region.[6]
Geodetic research has shown thatcrustal 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.[6]
To the west of the San Andreas Fault, the southern edge of theTransverse Ranges has been in a state of contraction during theQuaternary Period, with activereverse faulting that has uplifted San Gabriel Mountains. The west-striking (and range-bounding) Sierra Madre–Cucamonga Faults, as well as theSan Cayetano Fault and Santa Susana Faults all exhibit a component of left-lateralstrike-slip motion and share a similar trend.[7]
The Malibu Coast–Santa Monica Fault lies at the southern flank of theSanta Monica Mountains and the Raymond Fault runs to the south of theVerdugo Mountains and meets the San Gabriel Mountains atMonrovia. The Clamshell–Sawpit Fault lies beneath the San Gabriel Mountains inline with the Raymond Fault. The characteristics of the various faults are difficult to interpret, but they alldip steeply to the north with several thousand feet of displacement.[7]
The shock occurred on June 28, 1991, at 07:43:55 local time, and was the result ofthrust faulting on the Clamshell–Sawpit Fault at the southern boundary of the San Gabriel Mountains. At less than 0.39 in (1 cm), the total amount of uplift was very minimal, considering the 6 ft 7 in (2 m) of vertical displacement that was observed during the1971 San Fernando earthquake. The uplift initiated about two million years ago, and is a result of the convergence of thePacific andNorth American plates near the restraining bend of theSan Andreas Fault.[8]
The earthquake was felt fromSanta Barbara in the west, toPalm Springs in the east, and south to theMexico–United States border. It was centered about 7.5 miles (12.1 km) northeast ofSierra Madre. Hundreds of homes were damaged and at least three roads leading up toMount Wilson were blocked bylandslides. A woman was killed when a steel beam fell at theSanta Anita Park racetrack inArcadia and another woman died as a result of a heart attack inGlendale.[5]
Also in Arcadia, aMotel 6 was badly damaged. Other structural damage resulted from the shock. For example, bothMonrovia andPasadena each had more than a dozen buildings that were uninhabitable, but for the most part damage was limited to crackedplaster and broken windows. ThePasadena City Hall had moderate damage consisting of a cracked wall, dislodgedmarble barriers in the restrooms, and an offsetdome. In Sierra Madre, four apartment buildings and nine homes were also tagged as uninhabitable. Of the more than 100 injuries that were reported in Arcadia, Pasadena, Glendale, and Sierra Madre, most were cuts, bruises, and sprains, but some serious injuries also occurred.[5]
With an area of perceptibility of around 58,500 km2, the shock was felt fromSanta Barbara andBakersfield toSan Diego,Palm Springs, andLas Vegas. This is about half the felt area that was seen during the1987 Whittier Narrows earthquake, which was felt over an area of 110,000 km2. The maximum intensity observed in the urban areas that were affected was VII (Very strong), but the intensity in the epicentral area is unknown because of its remote location in the mountainousAngeles National Forest, though rock slides and landslides occurred there as a result of strong shaking. Intensity VI (Strong) effects were observed over 1,100 km2, fromAltadena andLa Verne along the base of the mountains, to East Los Angeles.[9]
TheCalifornia Strong Motion Instrumentation Program (CSMIP) obtained 120 records from 48 ground response stations, 38 buildings, sixdams, an airport control tower, and a power station. The individual stations that reported were as close as 2.5 miles (4 km) (Cogswell Dam) to as far as 50 miles (80 km) (Neenach), but most were within 40 km (25 mi). In downtown Pasadena, accelerations of 0.20g were measured at the bottom floor of three buildings, but the duration of strong motion was only about two seconds. At theAltadena station (8.1 mi (13 km) from the epicenter) 0.46g was recorded. The Cogswell Dam station recorded a peak horizontal acceleration of 0.58g, but no major damage occurred.[4]
Although the long term slip rate of the constituent faults that make up theSierra Madre Fault Zone are not precisely known, several studies have put forth scenarios for possible future activity. Two independent reports suggested that if the complete (62 mi (100 km)) fault system were to rupture, it could result in aMs 7.7 shock similar to the1952 Kern County earthquake. One researcher stated that the recurrence interval for such an event would be greater than 5,000 years. Another proposal was for one of the seven individual segments to rupture in a M6.4–6.6 event, comparable with the 1971 San Fernando earthquake. The recurrence interval for this type of event was placed at 200 years.[8]
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