Girish Mahajan

1990 Upland earthquake

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Date  February 28, 1990
Magnitude  5.7 MW
Fault  San Jose Fault
Origin time  23:43:37 UTC
Depth  10 km (6.2 mi)
Type  Strike-slip
1990 Upland earthquake

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. A large number of strong motion instruments captured the event, with an unexpectedly-high value seen on water tank near the epicentral area.

Contents

Tectonic setting

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.

Earthquake

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.

Strong motion

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 .1g at distances of up to 30 km (19 mi), but some closer stations had significantly higher responses.

Damage

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.

References

1990 Upland earthquake Wikipedia


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