Magnitude 7.2 earthquake in Alta California
The 1812 Ventura earthquake (also known as the Santa Barbara earthquake) occurred on the morning of December 21 at 11:00 Pacific Standard Time (PST). The 7.2 earthquake was assigned a with a maximum Modified Mercalli intensity of X ("Extreme"). The seismic shaking was followed by a tsunami that considerably damaged places in present-day Santa Barbara and Ventura County, California, which was at the time a territory of the Spanish Empire. One person was killed during the earthquake while another from an aftershock. The earthquake occurred while the region was recovering from another event on December 8.
Geological setting.
The San Andreas is the "master fault" of California, it is the main plate boundary in the San Andreas Fault System, consisting of other major active faults. This zone of interaction between the Pacific and North American plates have resulted in hundreds of faults crisscrossing California. There are several strike-slip, reverse and thrust faults hidden under the Santa Barbara Channel that could produce devastating earthquakes and tsunamis, including a thrust fault that runs along the base of the Santa Monica Mountains. These faults are a result of the "Big Bend" in the San Andreas Fault when it meets with the Garlock Fault, exerting compression in the crust, forcing thrust and reverse faults to push crustal blocks upwards.
Earthquake origins.
Studies have placed the source of this earthquake on two faults. The location and source have not been agreed on, with some researchers providing the epicenter north of Wrightwood or in the Santa Barbara Channel. Toppozada and others in 1981 proposed a location southwest of the 1925 Santa Barbara earthquake. Details of a possible tsunami would suggest a thrust-mechanism for the earthquake; similar to the 1978 Santa Barbara earthquake ( ;M5) which had a left-lateral and north-dipping thrust mechanism. Coulomb stress scenarios were developed using models for the December 8 earthquake along the San Andreas Fault and indicate there was an increase in stress on faults which geometries may be the source of the December 21 shock. This indicate the latter shock was likely triggered by the December 8 earthquake.
San Andreas Fault.
Based on tree-ring sampling, forests may have suffered some type of trauma, and were experiencing slow growth in 1812. Missing rings and crowns were discovered at around the same time. These trees took several years or decades to fully recover and return to their normal stage. These affected trees were found only in close proximity to the San Andreas Fault; in an event of a strong earthquake, trees at a considerable distance should also display evidence of trauma. A plausible cause might be that during slippage, the roots of these trees were severed, thus reducing the intake of nutrients and water required for growth. The seismic shaking may also knock off branches and parts of the trees.
It is believed that the San Juan Capistrano earthquake on December 8 triggered a second rupture on the San Andreas Fault on December 21. The rupture on December 21 initiated in the south, and progressed northwards to Fort Tejon. Both events ruptured a total length of 170 km on the fault. The northern end of the rupture is still uncertain because of the overlapping southern rupture end of the 1857 Fort Tejon earthquake, 44 years later.
However, if the earthquake was sourced from the San Andreas Fault, the aftershocks should not be felt around the coastal regions of California. The documented aftershocks were felt locally, and there were reports of "odd disturbances" in the sea. Another challenge to the San Andreas hypothesis source is the lack of damage at Santa Barbara during the much more powerful 1857 earthquake. The section involved in the 1857 earthquake was closer to Santa Barbara and Ventura but were not felt severely like in the 1812 earthquake. Claims of a tsunami and odd disturbances in the sea, can be attributed to an earthquake-triggered landslide.
San Cayetano Fault.
Located northwest of Los Angeles, the long San Cayetano Fault is a north dipping reverse fault that extends from Ventura to the Sespe Mountains. The fault comprises two segments separated by a right stepover near Fillmore. The eastern segment, known as the Modelo lobe, is expressed at the surface along the southern margin of the mountains west of Fillmore. Its surface rupture diminishes to the east of Piru. The western segment's surface expression is located considrably above the mountain base because the segment lies atop the south-dipping Sisar Blind Thrust Fault and a north-dipping blind thrust. The compression rate along the Ventura area is estimated at per year; accommodated by the San Cayetano and Oak Ridge faults.
An epicenter proposed in the Santa Barbara Channel was based on reports of the large tsunami that flooded coastal communities. A study along the San Cayetano Fault in Ventura County revealed evidence of a fresh slip dating between 1660 and 1813 and has been interpreted as a powerful earthquake that was generated by the fault. Trenching by Dolan and Rockwell revealed slip of along the surface rupture of this earthquake. The event may correspond to the earthquake of December 21 or one occurring just before the historical period. Two large slip events were found at the trench dating back to the past 350 years along the fault. The magnitude of these events was estimated to be greater than 7.0 and likely ruptured the entire length of the fault.
Impact and aftermath.
At Mission La Purisima, padres, soldiers, and Native Indians ran out of the mission's buildings when a foreshock occurred. They were still outside when the mainshock struck 15 minutes later. Shaking from the mainshock was more intense. Bells of the mission church started ringing while its adobe walls began to fracture and collapse. At the mission, a large fissure carved through the slopes of a hill erupted mud and water. The foreshock likely prevented further deaths as it startled many people out of their buildings.
Damage was reported at Mission Santa Inés, Mission Santa Barbara, Santa Barbara Presidio, Mission San Buenaventura, and Mission San Fernando. One person was killed by a falling boulder at Agua Caliente. At the various missions, many houses, churches, chapels, and other structures totally collapsed. Three adobe buildings were destroyed by the advancing tsunami. At Mission San Buenaventura and Mission Santa Barbara, the destruction was great, and a native person died when a building collapsed on him during an aftershock.
Devastation from this earthquake was comparable to the 1906 San Francisco and 1857 Fort Tejon earthquakes, but over a much smaller area. Frightened Chumash people on Santa Cruz and Santa Rosa islands fled to the mainland in their canoes and relocated there. For fears of another tsunami and aftershocks persisting, the missions were abandoned until April 1813.
Tsunami.
Over of the Santa Barbara shore was affected by the tsunami. At Refugio, the tsunami run-up was about . Along the Santa Barbara and Ventura coast, the run-up height was about . The captain of the "Thomas Newland," a ship in Refugio Bay during the earthquake, watched as the sea receded and later returned in a wave that lifted the ship and dumped it at Refugio Canyon. The waves carried the ship back to sea. Run-up heights of were recorded at Gaviota, although eyewitness said the tsunami may have been up to high. The tsunami was also recorded at San Francisco and a ~ run-up may have occurred at Kona, Hawaii.
References.
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