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1987 Superstition Hills earthquakes

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Coordinates: 33°05′24″N 115°47′31″W / 33.090°N 115.792°W / 33.090; -115.792
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1987 Superstition Hills earthquakes
1987 Superstition Hills earthquakes is located in California
Los Angeles
Los Angeles
Las Vegas
Las Vegas
San Diego
San Diego
1987 Superstition Hills earthquakes
UTC time1987-11-24 01:54:14
 1987-11-24 13:15:56
ISC event453653
 453722
USGS-ANSSComCat
 ComCat
Local dateNovember 23, 1987
 November 24, 1987
Local time17:54 PST
 05:15 PST
Magnitude6.0 Mw
 6.5 Mw
Depth2.3 km (1.4 mi)
 7.5 km (4.7 mi)
Epicenter33°05′24″N 115°47′31″W / 33.090°N 115.792°W / 33.090; -115.792
TypeStrike-slip
Areas affectedCalifornia and Mexico
Total damageUS$3 million
Max. intensityMMI VI (Strong)

MMI VII (Very strong)
Casualties2 dead, 94 injured

The 1987 Superstition Hills earthquake affected the Imperial Valley of California and Baja California on November 24, nearly 12 hours after a Mw 6.0 foreshock, called the Elmore Ranch earthquake. The mainshock, measuring Mw 6.5, struck at 17:54 PST, along the Superstition Hills segment of the San Jacinto Fault Zone. The earthquake generated a 27 km (17 mi) surface rupture with additional postseismic slip continuing for a year.

The earthquake caused damage amounting to US$3 million. Damage was considerable in Mexicali, were two people were killed and 50 people sustained injuries. Another 44 people were injured in the Imperial Valley. Buildings El Centro, Imperial, and Westmorland sustained collapsed chimneys, ruptured pipes and shattered windows. The rebuilt Imperial County services building, which replaced an originally five-story structure, remained structurally intact. Damage to canal infrastructure cost over US$600 thousand.

Geology

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The Imperial Valley is a seismically active area in California where the San Andreas Fault and San Jacinto Fault Zone (SJFZ) discontinues. This region represents a transition zone between continental transform faulting and oceanic rifting in the Gulf of California. South of the San Andreas Fault and between the Imperial Fault is the Brawley Seismic Zone—the northernmost ridge feature in the ridge–transform boundary. Faults in the Brawley Seismic Zone typically trend northeast, perpendicular to the plate boundary faults. Labeled "cross-faults", these are oriented perpendicular to the direction of rifting and were previously interpreted to be normal faults. However, recent seismic activity show that these faults produce left-lateral strike-slip motion.[1]

The SJFZ is a 244 km (152 mi)-long right-lateral structure that runs parallel to the San Andreas Fault.[2] The SJFZ is southern California's most seismically active fault zone. It was responsible for a large surface-rupturing earthquake in 1968. The fault zone is highly segmented, consisting of seven individually named segments.[3] Despite being part of one fault system, these segments are assigned names such as the Coyote Creek, Superstition Hills, and Superstition Mountain faults. The Superstition Hills Fault (SHF) is a vertically-dipping, 38 km (24 mi)-long section.[4] It displayed motion in 1951, when a ML 5.6 earthquake produced a small surface rupture. Triggered slip also occurred during earthquakes in 1968 and 1979.[5] The SHF was not known to be capable of produing large earthquakes prior to 1987 although it may have been the source of the April 19, 1906 and June 23, 1915 earthquakes. However, other faults were also candidates for producing these earthquakes.[6]

Earthquakes

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Elmore Ranch earthquake

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The Mw  6.0 foreshock struck at a depth of 2.3 km (1.4 mi), at 17:54 PST.[7] The epicenter was located in a sparsely populated area 22.5 km (14.0 mi) southwest of Westmorland.[8] It occurred on a previously unmapped fault, later called the Elmore Ranch Fault Zone (ERFZ).[5] This event was associated with aftershocks that implied rupture occurred along a northeast striking fault based on its distributions. The rupture propagated bilaterally for 20–25 km (12–16 mi); northeast towards the Brawley Seismic Zone and southwest, where it joined the SHF. The surface rupture was only observed for half the rupture length. It was associated with left-lateral slip and had a maximum offset of nearly 20 cm (7.9 in). Between 2.8 cm (1.1 in) and 4.8 cm (1.9 in) of slip was measured on the Lone Tree, Kane Springs, and Eastern Kane Springs segments of the ERFZ. The main ERFZ strand had a measured displacement of 13 cm (5.1 in).[1]

Superstition Hills earthquake

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About 12 hours later, a Mw  6.5 occurred at 7.5 km (4.7 mi) depth[9] with an epicenter at the northwestern termination of the SHF, where it intersects the ERFZ. It was followed by an aftershock zone roughly parallel to the SHF formed, although situated several kilometers southwest of the main rupture trace.[1] The shock also produced a 27 km (17 mi) surface rupture running southeastward from the epicenter.[10][11] Right-lateral offsets were observed along two sections of the SHF and on 4 km (2.5 mi) of the Wienert Fault.[10]

Over the next 339 days, the SHF continued to produce aseismic slip, increasing the ground displacements.[12] By August 1988, up to 80.2 cm (31.6 in) of offset was measured along the SHF, with much of the slip occurring within the first five days of the mainshock.[13] At at the time of observation in 1988, the Wienert Fault's southern portion produced vertical offsets of 25 cm (9.8 in) and was actively increasing.[10]

The mainshock also triggered slip on the central section of the Coyote Creek Fault, north of the SHF. The 3 km (1.9 mi) rupture comprised two semi-continuous segments. It had a maximum right-lateral slip of 1.5 cm (0.59 in), and minor vertical offsets were also recorded. Installed creepmeters indicated several millimeters of slip occurred on the San Andreas Fault, but was insufficient to create a surface rupture.[14] One of these instrument recorded 0.69–0.76 cm (0.27–0.30 in) of slip.[13] Triggered slip was also measured on the Imperial Fault.[14] Movements on these faults may have initiated less than three minutes after the two large earthquakes. The Imperial Fault also produced motion about three hours following the mainshock.[13]

Ground motion

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Main article: Strong ground motion
A USGS ShakeMap for the November 24 mainshock

Strong motion network was established by the United States Coast and Geodetic Survey in 1932 to record strong-motion data across the western United States. In the Imperial Valley, it involved installating an accelerograph at El Centro which would record important data for the earthquakes of 1940 and 1987. By 1979, the network expanded to 30 stations in the valley, and nearly 700 strong motion accelerograms were gathered from earthquakes by 1987.[15]

The 1987 events produced 65 recordings from 40 stations across the valley. Horizontal peak ground accelerations (pga) during the foreshock exceeded 0.1 g at six stations. The nearest station at Superstition Mountain was 13 km (8.1 mi) away; recording a pga of 0.13 g. The highest pga was 0.22 g, measured at Calipatria, 26 km (16 mi) away.[15]

Twenty-five stations recorded a maximum horizontal pga exceeding 0.1 g within 60 km (37 mi) of the mainshock's epicenter. These stations recorded a pga of at least 0.3 g. The pga at the Superstition Mountain station was the largest and longest ever observed in its 55-year records; 0.9 g and 0.7 g in the horizontal and 0.6 g in the vertical directions.[15] The site also recorded 16 seconds of intense ground motion.[16]

Damage from the foreshock in Calipatria, El Centro, Heber and Westmorland corresponded to VI (Strong) on the Modified Mercalli intensity scale (MMI).[17] The mainshock had a maximum MMI of VI–VII (Strong–Very strong) at El Centro and Westmorland. Intensity VI was felt at Brawley, Calexico, Calipatria, Heber, Holtville, Imperial and Seeley. It was widely felt across southern California. Shaking was also felt over 300 km (190 mi) away in Las Vegas, Nevada and Tempe, Arizona, and over 100 km (62 mi) away in Mexico's Tijuana and Ensenada.[12][18]

Impact

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Although both earthquakes were larger than the Whittier Narrows earthquake of October, damage was moderate due to their location in the sparsely populated valley,[19] estimated at US$3 million.[12] At least 94 people were injured; 50 in Mexicali and 44 in Imperial County. In Mexicali, a two people died after an oncoming vehicle crashed into them while evacuating a factory. Some people had broken bones, sprains, cuts and some heart attacks. Thirty-five buildings were damaged including three hospitals and the city hall. Some 650 people were evacuated from these buildings. An Immigration and Naturalization Service station between the city and Calexico experienced heavy interior damage.[20] Fires also broke out in other buildings.[21]

The large foreshock cut power in some parts of Niland, but the town was undamaged. In a Westmorland grocery store, merchandaise fell and a window cracked. Damage to telephone lines temporarily disrupted communication services in the valley. At the San Onofre Nuclear Plant, there was an "unusual event" declaration. A runway at the Naval Air Facility El Centro was closed for inspections, which later revealed small cracks.[19][20]

Collapsed chimneys, severed pipes, shattered windows, and highways were offset in the towns of El Centro, Imperial, and Westmorland. Liquefaction damage rendered a bridge across the New River a complete loss. Near Westmorland, operations at the Desert Test Range Control Center ceased due to damaged a damaged water tank and equipment.[12] Several buildings in Calexico's business district sustained damage, and a brick wall toppled onto some cars.[20] The two-story Imperial County services building, a steel frame structure, only received nonstructural damage. County buildings in El Centro were structurally resilient but had material debris.[16] The Southern California Irrigation District estimated the damage to its facilities at US$600–750 thousand. The foreshock caused minor buckling of the canal's concrete reinforce while the mainshock collapsed many of the concrete.[12] Concrete channels and pipe siphons had significant damage. Cracking was also extensive along the aqueduct.[16]

Response

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The mainshock immediately disrupted power to 65,000 homes and businesses, but it was restored within 20 minutes. Telecommunications, power and gas services were interrupted throughout Westmorland, El Centro, Calexico and Mexicali. Despite the damage to canal facilities, officials said it did not affect the flow of water. In El Centro, detainees of the Immigration and Naturalization Service were relocated to the courtyard due to damage at the station. Law enforcement officers were on high alert for looters in Calexico. California 86 between Westmorland and El Centro was damaged and closed off. A buckled road near the junction with Interstate 8 also forced California 98 to close. Aircraft from the United States Customs Service and Mexico were flown to inspect the damage.[20]

In Orange County, residents stocked up on their earthquake preparedness kits. One store also saw a rise in sales for bottled water, water purification devices and propane tanks. In La Habra, four households called for a home inspection. The California Department of Transportation dispatched 150 personnel to inspect freeways in the county for damage. The San Diego Fire-Rescue Department reported small cracks in the walls and ceilings at five stations.[20]

See also

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References

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  1. ^ a b c Hudnut, K.W.; Seeber, L.; Rockwell, T.K.; Goodmacher, J.; Klinger, R.; Lindvall, S.; McElwain, R. (1989). "Surface ruptures on cross-faults in the 24 November 1987 Superstition Hills, California, earthquake sequence" (PDF). Bulletin of the Seismological Society of America. 79 (2): 282–296. doi:10.1785/BSSA0790020282 (inactive 2024-09-11). Archived (PDF) from the original on 2024-10-10. Retrieved 2024-10-09.{{cite journal}}: CS1 maint: DOI inactive as of September 2024 (link)
  2. ^ Singer, Eugene (1998), Geology of California's Imperial Valley: A Monograph by Eugene Singer, archived from the original on 24 March 2011, retrieved 13 December 2020
  3. ^ Dorsey, Rebecca J. (2002). "Stratigraphic record of Pleistocene initiation and slip on the Coyote Creek fault, lower Coyote Creek, southern California". Contributions to Crustal Evolution of the Southwestern United States. Vol. 365. Geological Society of America. pp. 251–269. doi:10.1130/0-8137-2365-5.251. ISBN 9780813723655. Archived from the original on 2022-10-14. Retrieved 2022-08-25.
  4. ^ Treiman, J.A.; Lundberg, M.; compilers (1999). "Fault number 125d, San Jacinto fault, Superstition Hills section, in Quaternary fault and fold database of the United States". United States Geological Survey. Archived from the original on 25 August 2022. Retrieved 19 August 2022.
  5. ^ a b Magistrale, Harold; Jones, Lucile; Kanamori, Hiroo (April 1989). "The Superstition Hills, California, earthquakes of 24 November 1987". Bulletin of the Seismological Society of America. 79 (2): 239–251. Bibcode:1989BuSSA..79..239M. doi:10.1785/BSSA0790020239. ISSN 0037-1106. Archived from the original on 27 July 2024. Retrieved 9 October 2024.
  6. ^ Hudnut, K. W.; Sieh, K. E. (1989). "Behavior of the Superstition Hills fault during the past 330 years". 79 (2): 304–329. doi:10.1785/BSSA0790020304. Archived from the original on 2024-10-08. Retrieved 2024-10-08.
  7. ^ International Seismological Centre, Bulletin of the ISC, Thatcham, United Kingdom [Event 453653 ].
  8. ^ Blanchette, Angela (1988). "Earthquakes Strike Imperial Valley in "Superstition Hills Sequence"". California Geology. 41 (2). Archived from the original on 17 March 2004.
  9. ^ International Seismological Centre, Bulletin of the ISC, Thatcham, United Kingdom [Event 453722 ].
  10. ^ a b c Sharp, R. V.; Budding, K. E.; Boatwright, J.; Ader, M. J.; Bonilla, M. G.; Clark, M. M.; Fumal, T. E.; Harms, K. K.; Lienkaemper, J. J.; Morton, D. M.; O'Neill, B. J.; Ostergren, C. L.; Ponti, D. J.; Rymer, M. J.; Saxton, J. L.; Sims, J. D. (1989). "Surface faulting along the Superstition Hills fault zone and nearby faults associated with the earthquakes of 24 November 1987". Bulletin of the Seismological Society of America. 79 (2): 252–281. Bibcode:1989BuSSA..79..252S. doi:10.1785/BSSA0790020252.
  11. ^ Hudnut, K.W.; Seeber, L.; Pacheco, J. (1989). "Cross-fault triggering in the November 1987 Superstition Hills Earthquake Sequence, southern California" (PDF). Geophysical Research Letters. 16 (2): 199–202. Bibcode:1989GeoRL..16..199H. doi:10.1029/GL016i002p00199. Archived (PDF) from the original on 2024-10-10. Retrieved 2024-10-08.
  12. ^ a b c d e Stover, Carl W.; Coffman, Jerry L. (1993). Seismicity of the United States, 1568-1989 (Revised) (1527 ed.). United States Government Printing Office, Washington: U.S. Geological Survey Professional Paper. doi:10.3133/pp1527. Archived from the original on 2022-08-06. Retrieved 2022-08-25.
  13. ^ a b c McGill, Sally F.; Allen, Clarence R.; Hudnut, Kenneth W.; Johnson, David C.; Miller, Wayne F.; Sieh, Kerry E. (1989). "Slip on the Superstition Hills fault and on nearby faults associated with the 24 November 1987 Elmore Ranch and Superstition Hills earthquakes, southern California". Bulletin of the Seismological Society of America. 79 (2): 362–375. doi:10.1785/BSSA0790020362. Archived from the original on 3 December 2023.
  14. ^ a b Hudnut, K. W.; Clark, M. M. (1989). "New slip along parts of the 1968 Coyote Creek fault rupture, California". Bulletin of the Seismological Society of America. 79 (2): 451–465. doi:10.1785/BSSA0790020451 (inactive 2024-09-11).{{cite journal}}: CS1 maint: DOI inactive as of September 2024 (link)
  15. ^ a b c Porcella, R.L.; Etheredge, E.; Maley, R.; Switzer, J. (1987), Strong-motion data from the Superstition Hills earthquakes of 0154 and 1315 [GMT], November 24, 1987 (PDF), Open-File Report 87-672, United States Geological Survey, pp. 2–6, archived (PDF) from the original on August 25, 2022, retrieved August 25, 2022
  16. ^ a b c "Superstition Hills Earthquakes–November 23 and 24, 1987 Imperial County, California" (PDF). Earthquake Engineering Research Institute. 1988. Archived (PDF) from the original on 8 October 2024. Retrieved 8 October 2024.
  17. ^ ANSS. "M 6.2 - 17km WNW of Westmorland, CA". Comprehensive Catalog. U.S. Geological Survey.
  18. ^ ANSS. "M 6.6 - 22km W of Westmorland, CA". Comprehensive Catalog. U.S. Geological Survey.
  19. ^ a b Feldman, Paul; Reza, H.G. (24 November 1987). "Imperial Valley Rocked by 6.2 Quake; Damage Minor". Los Angeles Times. Archived from the original on 2020-11-29. Retrieved November 21, 2020.
  20. ^ a b c d e Reich, Kenneth; Reza, H. G. (25 November 1987). "6.3 Quake Injures 94 Along Mexican Border: Imperial Valley Temblor, 2nd in 12 Hours, Damages Buildings in Mexicali, El Centro". Los Angeles Times. Archived from the original on 2024-10-08. Retrieved 2024-10-08.
  21. ^ Armstrong, Scott (25 November 1987). "California gets more reminders of quake potential. Experts say temblors that hit the lower Imperial Valley and Mexicali, Mexico don't mean more are likely to follow soon". The Christian Science Monitor. Archived from the original on 26 August 2022. Retrieved 26 August 2022.
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