- 2012-12-14 10:36:02 UTC
- 2012-12-14 02:36:02 UTC-08:00 at epicenter
- 2012-12-14 04:36:02 UTC-06:00 system time
Location: 31.213°N 119.560°W depth=11.3km (7.0mi)
- 263km (163mi) SSW of Avalon, California
- 269km (167mi) WSW of Rosarito, Mexico
- 276km (171mi) WSW of Imperial Beach, California
- 277km (172mi) SW of Coronado, California
- 746km (464mi) WSW of Phoenix, Arizona
The December 14, 2012 M 6.3 earthquake 250 km southwest of Avalon, California occurred as a result of shallow normal faulting within the oceanic lithosphere of the Pacific plate. This event is located some 400-450 km west-southwest of the plate boundary between the Pacific and North America plates – the San Andreas fault system in southern California – and is not associated with that fault system. Instead, this earthquake represents intraplate faulting along northeast-southwest trending normal faults within the crust of the Pacific plate, just to the west of California’s continental shelf. The causative fault is not known at this time. At the location of this event, the Pacific plate moves to the northwest with respect to the North America plate at a velocity of approximately 54 mm/yr.
Location of the earthquake off the California Coast
While the broad region surrounding the December 14, 2012 event experiences frequent earthquakes along the San Andreas and associated faults in southern California, the area offshore and within 250 km of this earthquake has not hosted any events greater than M 6 over the past 40 years. The largest nearby earthquake was a M 5.1 event 200 km to the northeast in June of 2004, 80 km west of the border between the US and Baja California. A M 3.3 earthquake struck approximately 35 km to the northeast in April 1981, representing the closest event in the USGS earthquake catalog.
Seismotectonics of Mexico
Located atop three of the large tectonic plates, Mexico is one of the world’s most seismologically active regions. The relative motion of these crustal plates causes frequent earthquakes and occasional volcanic eruptions. Most of the Mexican landmass is on the westward moving North American plate. The Pacific Ocean floor south of Mexico is being carried northeastward by the underlying Cocos plate. Because oceanic crust is relatively dense, when the Pacific Ocean floor encounters the lighter continental crust of the Mexican landmass, the ocean floor is subducted beneath the North American plate creating the deep Middle American trench along Mexico’s southern coast. Also as a result of this convergence, the westward moving Mexico landmass is slowed and crumpled creating the mountain ranges of southern Mexico and earthquakes near Mexico’s southern coast. As the oceanic crust is pulled downward, it melts; the molten material is then forced upward through weaknesses in the overlying continental crust. This process has created a region of volcanoes across south-central Mexico known as the Cordillera Neovolcánica.
The area west of the Gulf of California, including Mexico’s Baja California Peninsula, is moving northwestward with the Pacific plate at about 50 mm per year. Here, the Pacific and North American plates grind past each other creating strike-slip faulting, the southern extension of California’s San Andreas fault. In the past, this relative plate motion pulled Baja California away from the coast forming the Gulf of California and is the cause of earthquakes in the Gulf of California region today.
Mexico has a long history of destructive earthquakes and volcanic eruptions. In September 1985, a magnitude 8.1 earthquake killed more than 9,500 people in Mexico City. In southern Mexico, Volcán de Colima and El Chichón erupted in 2005 and 1982, respectively. Paricutín volcano, west of Mexico City, began venting smoke in a cornfield in 1943; a decade later this new volcano had grown to a height of 424 meters. Popocatépetl and Ixtaccíhuatl volcanos (“smoking mountain” and “white lady”, respectively), southeast of Mexico City, occasionally vent gas that can be clearly seen from the City, a reminder that volcanic activity is ongoing. In 1994 and 2000 Popocatépetl renewed its activity forcing the evacuation of nearby towns, causing seismologists and government officials to be concerned about the effect a large-scale eruption might have on the heavily populated region. Popocatépetl volcano last erupted in 2010.
California Earthquake History
The first strong earthquake listed in earthquake annals for California occurred in the Los Angeles region in 1769, probably near the San Andreas Fault. Four violent shocks were recorded by the Gaspar de Portola Expedition, in camp about 30 miles southeast of Los Angeles center. Most authorities speculate, even though the record is very incomplete, that this was a major earthquake.
Forty persons attending church at San Juan Capistrano on December 8, 1812, were killed by a strong earthquake that destroyed the church. Many mission buildings were severely damaged there and at San Gabriel. The shock probably centered on a submarine fault offshore.
A violent shock near Fort Tejon in January 1857 threw down buildings and large trees at the Fort. It was also severe in Los Angeles, San Francisco, and Sacramento. This earthquake has been compared to that of April 1906; both caused extensive displacement along the San Andreas Fault. One source notes, “The magnitude of the two events cannot have differed greatly.”
A strong earthquake occurred on the Hayward Fault, the principal active branch of the San Andreas in central California, in October 1868. Some 30 persons were killed in the region. Damage was severe at San Francisco; many buildings were wrecked at Hayward and San Leandro. Until 1906, this shock was often referred to as “the great earthquake.”
An earthquake in the Sierra – Nevada Fault system in March 1872, killed 27 people at Lone Pine and destroyed 52 of 59 adobe houses. Near Owens Lake, numerous depressions formed between cracks in the earth. One area 200 to 300 feet wide sank 20 to 30 feet; several long, narrow ponds formed. Thousands of aftershocks, some severe, appear to have occurred.
Nearly all brick structures were wrecked, and many frame buildings were damaged in Vacaville by an earthquake on April 19, 1892. Damage was similar at Winters and Dixon, two small towns nearby. Ground fissures were noted in the area. The shock centered north of Santa Rosa, in the Healdsburg Fault area.
On Christmas Day of 1899, six persons died and several were injured at Saboba, near San Jacinto, by a strong shock. At nearby Hemet, nearly all brick buildings were severely damaged, with only two chimneys remaining upright. This shock occurred on the San Jacinto Fault, and has been compared to the April 1918 (magnitude 6.8) shock in the same region.
Seven hundred persons died on April 18, 1906, in one of the greatest earthquakes ever to hit California. Damage was extensive in San Francisco, and was increased perhaps tenfold by raging fires. Total damage was estimated at over $500 million.
Two destructive shocks nearly one hour apart caused about $1 million property damage in southern Imperial Valley on June 22, 1915. Six persons were killed and several injured by the second quake at Mexicali, located just inside the Mexican border. Unstable banks of the New and Alamo Rivers caved in many places. Magnitude 6 1/4, both shocks.
A shock on the San Jacinto Fault in April 1918 caused heavy damage at San Jacinto and Hemet. Only one new concrete and one frame building remained standing in the business section of San Jacinto; property loss was about $200,000. The dry earth surface was broken up, as though by a harrow, in the San Jacinto Fault area southeast of Hemet. One auto was carried off the road by a slide; many area roads were blocked. Magnitude 6.8.
Santa Barbara sustained $8 million damage and 13 fatalities from an offshore shock in June 1925. The shock occurred in the Santa Barbara Channel, on an extension of the Mesa Fault or the Santa Ynez system. On State Street, the principal business thoroughfare, few buildings escaped damage; several collapsed. One on marshy ground withstood the shaking well, but its foundation sank 19 feet. The shock occurred at 6:42 a.m., before many people had reported for work and when streets were uncrowded, reducing death and injury. Magnitude 6.3.
The shock of November 1927 wrecked chimneys at Lompoc, shifted a house on its foundation, and caused heavy earth and rockslides on steep slopes. Water spurted from the ground in places; sand craters formed.
The Long Beach earthquake of March 1933 eliminated all doubts regarding the need for earthquake resistant design for structures in California. Forty million dollars property damage resulted; 115 lives were lost. The major damage occurred in the thickly settled district from Long Beach to the industrial section south of Los Angeles, where unfavorable geological conditions (made land, water-soaked alluvium) combined with much poor structural work to increase the damage. At Long Beach, buildings collapsed, tanks fell through roofs, and houses displaced on foundations. School buildings were among those structures most generally and severely damaged. The epicenter was offshore, southeast of Long Beach, on the Newport – Inglewood Fault. Magnitude 6.3.
Nine people were killed by the May 1940 Imperial Valley earthquake. At Imperial, 80 percent of the buildings were damaged to some degree. In the business district of Brawley, all structures were damaged, and about 50 percent had to be condemned. The shock caused 40 miles of surface faulting on the Imperial Fault, part of the San Andreas system in southern California. It was the first strong test of public schools designed to be earthquake-resistive after the 1933 Long Beach quake. Fifteen such public schools in the area had no apparent damage. Total damage has been estimated at about $6 million. Magnitude 7.1.
The towns of Tehachapi and Arvin were hit severely by the July 1952 Kern County earthquake. Twelve persons died, many were injured, and $60 million property damage was sustained. Damage to well designed structures was slight, but old and poorly built buildings were cracked, and many collapsed. Reinforced tunnels with walls 18 inches thick near Bealville were cracked, twisted, and caved in; rails were shifted and bent info S-shaped curves. Near Caliente, reinforced concrete railroad tunnels were demolished. Many aftershocks occurred, three over 6 on the Richter scale. One aftershock on August 22 (magnitude 5.8) centered near Bakersfield. It took two lives and caused extensive damage to many already weakened buildings. The Kern County earthquake, the largest with an epicenter in California since 1906, originated on the White Wolf Fault.
Abridged from Earthquake Information Bulletin, Volume 3, Number 2, March – April 1971, by Carl A. von Hake.
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