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1981 Westmorland earthquake

From Wikipedia, the free encyclopedia

1981 Westmorland earthquake
UTC time1981-04-26 12:09:31
ISC event628772
Local dateApril 26, 1981 (1981-04-26)
Local time12:09 UTC[1]
Magnitude5.9 Mw(ISC)
Depth10 km (6.2 mi)[1]
Epicenter33°01′N 115°41′W / 33.02°N 115.68°W / 33.02; -115.68[1]
Areas affectedSouthern California
(United States)
Total damageUS$1–3 million[3]
Max. intensityVII (Very strong)[3]

The 1981 Westmorland earthquake occurred at 05:31 Pacific Daylight Time (12:31 UTC) on April 26. The moderate strike-slip shock took place in the Imperial Valley of Southern California, just north of the Mexico–United States border in the United States. No injuries or deaths occurred, but damage was estimated at $1–3 million. With a Mercalli intensity of VII (Very strong), this was one of fifteen intensity VII or greater shocks in the Imperial Valley that were observed in the 20th century up until April 1981. The region experiences large stand-alone events and earthquake swarms due to its position in an area of complex conditions where faulting transitions from strike-slip movement to the north and divergence to the south.


The Imperial Valley of Southern California is very active seismically and has been subject to numerous destructive earthquakes since communities began to form in this desert area in the late 19th and early 20th centuries. Records of events prior to this period do not exist due to a lack of settlers in the area. Fifteen shocks of intensity VII or higher were recorded prior to April 1981 and beginning with the intensity VIII shock of April 18, 1906, which was a triggered event following the 1906 San Francisco earthquake.[4][5]

Tectonic setting

The Gulf of California Rift Zone underlies the Gulf of California from the mouth of the Colorado River in Baja California and stretching to the southeast. It contains numerous right-lateral transform faults and rift zones that make up the PacificNorth American plate boundary. The southern portion has matured into seafloor spreading, and at the northern end of the province is the Salton Trough, which is considered to be an onshore spreading center. The rift zone is covered by thick layers of sediment from the Colorado River drainage basin.[6]


The Global Centroid Moment Tensor Database shows a NNW strike on a nearly vertical fault, with a rake angle that is within 25 degrees of being pure strike-slip.[2]


The maximum observed intensity for the event was VII (Very strong).[3]

See also


  1. ^ a b c ISC (2015), ISC-GEM Global Instrumental Earthquake Catalogue (1900–2009), Version 4.0, International Seismological Centre
  2. ^ a b USGS (September 4, 2009), PAGER-CAT Earthquake Catalog, Version 2008_06.1, United States Geological Survey
  3. ^ a b c Stover, C.W.; Coffman, J.L. (1993). Seismicity of the United States, 1568–1989 (Revised). U.S. Geological Survey professional paper 1527. United States Government Printing Office. pp. 96, 168.
  4. ^ Barnhard, Thenhaus & Algermissen 1982, pp. 1, 1A, 1B
  5. ^ Meltzner, A. J.; Wald, D. J. (2003), "Aftershocks and Triggered Events of the Great 1906 California Earthquake", Bulletin of the Seismological Society of America, 93 (5): 2, 160, 2, 164, 2, 166, 2, 169, 2, 170, doi:10.1785/0120020033
  6. ^ Han, Liang; Hole, John A.; Stock, Joann M.; Fuis, Gary S.; Kell, Annie; Driscoll, Neal W.; Kent, Graham M.; Harding, Alistair J.; Rymer, Michael J.; González-Fernández, Antonio; Lázaro-Mancilla, Octavio (2016), "Continental rupture and the creation of new crust in the Salton Trough rift, Southern California and northern Mexico: Results from the Salton Seismic Imaging Project", Journal of Geophysical Research: Solid Earth, 121 (10): 7471, 7472, doi:10.1002/2016JB013139


Further reading

This page was last edited on 27 November 2020, at 01:01
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