  El Centro  | |
| UTC time | 1915-06-23 03:59:00 |
|---|---|
| 1915-06-23 04:56:00 | |
| ISC event | n/a |
| USGS-ANSS | ComCat |
| ComCat | |
| Local date | June 22, 1915 (1915-06-22) |
| Local time | 19:59PST |
| 20:57 PST | |
| Duration | 11 seconds |
| Magnitude | 6.25Ms[1] |
| 6.25Ms[1] | |
| Epicenter | 32°48′N115°30′W / 32.8°N 115.5°W /32.8; -115.5 |
| Type | Strike-slip |
| Areas affected | United States, Mexico |
| Total damage | $900,000 |
| Max. intensity | MMI VIII (Severe)[1] |
| Foreshocks | 1 recorded |
| Aftershocks | until August 1915 |
| Casualties | 6 dead, several injured |
The1915 Imperial Valley earthquakes were two destructive shocks centered nearEl Centro, California on June 22. The earthquakes measuredMs 6.25 and occurred nearly one hour apart at 19:59 and 20:57 PST. Both shocks were assigned VIII (Severe) on theModified Mercalli intensity scale. Heavy damage occurred in the areas of Mexicali and El Centro, amounting to $900,000. At least six people were killed in the earthquakes.[2]
In November, the area was struck by another larger shock, measuringMw 7.0 centered inCerro Prieto,Baja California,Mexico.
TheSalton Trough is an active pull-apart basin forming due to offsets between the numerous strike-slip faults along its edges. It is a component of the much bigger San Andreas Fault System, joining theSan Andreas Fault with theImperial Fault Zone via theBrawley seismic zone. The San Andreas Fault is the mainplate boundary that defines the margin between thePacific andNorth American plates in California. However, the plate boundary is slightly more complex; rather than a single fault structure that makes up the boundary, the region is straddled and crisscrossed with numerous shorter faults to accommodate the movement of these two plates.[3]
The Imperial Fault Zone is a 69-km-long right-lateral strike-slip fault located near the cities ofBrawley,Imperial, El Centro,Calexico andMexicali. The fault is seismically active and was responsible for two damaging earthquakes in1940 and1979. In 1966, it produced a magnitude 3.6 earthquake that caused asurface rupture, making it the smallest earthquake associated with such a feature. Earthquakes on the Imperial Fault Zone have a recurrence interval of ~30 years, but larger events like the case of the 1940 Mw 6.9 earthquake occur every 700 years.[4]
The earthquakes may haveruptured the northern segment of the Imperial Fault Zone. No surface ruptures have been associated with the earthquakes, and very little is known about the seismic activity of the fault prior to 1940.[5]
Both shocks were equally severe; around the areas of El Centro and Calexico, buildings were heavily damaged, chimneys collapsed, and walls fell over. The first shock lasted 11 seconds and was strong enough to stop clocks and weaken many buildings in the Imperial Valley. Many people evacuated from their homes and other buildings to stay in the streets. When the second shock came, it caused unreinforcedmasonry structures to partially collapse, injuring some inhabitants.[1] People who reentered structurally weakened buildings after the first shock were injured during the second. In Mexicali,night entertainment was at its peak hour, so people continued to be entertained even after the first shock. The collapse of walls and falling debris killed six people when the second quake struck.[6]
The worst damage occurred at El Centro at an estimated $600,000. Serious damage was reported in Calexico,Heber, and Mexicali, but the economic losses were fewer because El Centro was a much bigger community. Nearly every brick building in the area were damaged, due to poor construction methods which could not withstand the earthquakes' intensity. The twomainshocks were felt as far asSan Bernardino andLos Angeles in the north, toParker andYuma, Arizona in the east, and south; inEnsenada, and likely beyond.[1]
Some fissures were observed in analluvium fan. Water irrigation installations had minor damage. Riverbanks of theAlamo andNew rivers slumped into the stream. At amarsh on the New River, cracks formed. Residents in Mexico reported plumes of steam emitted from a group ofmud volcanoes. Sounds of explosions were also heard. Manyaftershocks were felt; continuing into August 1915.[1]
The November 21 Mw 7.0 earthquake occurred inBaja California,[7] Mexico. The earthquake was assigned IX (Violent) on theModified Mercalli intensity scale.[7] It was associated with a rupture on theCerro Prieto Fault near avolcano of the same name. Widespread groundslumping,landslides, andliquefaction was reported around Cerro Prieto volcano, 20 km south of theepicenter. The rupture initiated on the fault and propagated bilaterally for a total length of 32 km.[8]
The earthquake caused little damage and no casualties. Plumes of steam up to 200 meters in height were reported in Laguna de los Volcanoes, an uninhabitated area. A massivefissure was reported on both sides of the New River for a length of 3 km. Two individuals at Laguna de los Volcanoes recounted that it was difficult to stand during the earthquake. Alevee was damaged.[1]
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