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1990 Upland earthquake

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1990-02-28 23:43:36
380064
ComCat
February 28, 1990
.mw-parser-output .tooltip-dotted{border-bottom:1px dotted;cursor:help}23:43:37 UTC
5.7 Mw
10 km (6.2 mi)
.mw-parser-output .geo-default,.mw-parser-output .geo-dms,.mw-parser-output .geo-dec{display:inline}.mw-parser-output .geo-nondefault,.mw-parser-output .geo-multi-punct,.mw-parser-output .geo-inline-hidden{display:none}.mw-parser-output .longitude,.mw-parser-output .latitude{white-space:nowrap}34°10′N 117°44′W / 34.16°N 117.73°W / 34.16; -117.73
San Jose Fault
Strike-slip
Greater Los Angeles Area Southern California United States
$12.7 million
MMI VII (Very strong)
1.05 g
30 injured

The 1990 Upland earthquake occurred at 15:43:37 local time on February 28 with a moment magnitude of 5.7 and a maximum Mercalli Intensity of VII (Very strong). This left-lateral strike-slip earthquake occurred west of the San Andreas Fault System and injured thirty people, with total losses of $12.7 million. Many strong motion instruments captured the event, with an unexpectedly high value seen on water tank near the epicentral area.

In the Transverse Ranges west of the right-lateral San Andreas Fault System, some faults exhibit left-lateral slip, including some at the boundary of the Los Angeles Basin and the Transverse Ranges.

While several examinations of earthquake focal mechanisms in the Upland area showed both thrust and strike-slip faulting on opposite ends of the San Jose Fault, the mechanism of the 1990 event showed pure left-lateral motion on a steeply-dipping (70°) and northeast-striking fault. Field investigations revealed that the bilateral rupture did not reach the surface.

The shock was felt from Santa Barbara in the west and to Las Vegas in the east. It was also felt in Ensenada, Baja California and it triggered 82 strong motion instruments at distances of 3–76 kilometers (1.9–47.2 mi). Typical ground accelerations were around 0.1 g at distances of up to 30 km (19 mi), but some closer stations had significantly higher responses.

Rockslides left some roads closed in the San Gabriel Mountains and Pomona city hall was heavily damaged. In Claremont some buildings were damaged, but only one building in Upland's historic downtown area was tagged for non-occupation. Those that were injured suffered cuts and bruises (mainly from falling objects) but two men were injured as a result of a landslide.

List of earthquakes in 1990
List of earthquakes in California
List of earthquakes in the United States
M5.5 – 6km NNE of Claremont, CA – United States Geological Survey
The International Seismological Centre has a bibliography and/or authoritative data for this event.

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