1812 Ventura earthquake

see also: Thrust tectonics and Transverse Range

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.[3] 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.

The first earthquake may have been a foreshock, it occurred at around 10:00 to 10:15 a.m. At Mission La Purisima, residents; padres, soldiers, and Indians ran out of buildings and never returned.[4] Fifteen minutes later, the foreshock was accompanied by the mainshock. This time, the shaking intensity was much more violent. Bells of the mission church started ringing and the adobe walls began to fracture and collapse.

The captain of the Thomas Newland, a ship that was in Refugio Bay, watched as the sea receded, and later returned in a wave that lifted the ship and dumped it at Refugio Canyon, before taking it back out to sea.[5][6] A run-up height of 3–4 meters was recorded at Gaviota, although eyewitness said the tsunami may have been up to 15.2 meters (50 feet).[7][8] The tsunami was also recorded at San Francisco.

Ruins of the chapel at Mission La Purisima.

Bells of the mission church started ringing and the adobe walls began to fracture and collapse. Damage was reported at Mission Santa Ines, Mission Santa Barbara, Santa Barbara Presidio, Mission San Buenaventura (Ventura), and Mission San Fernando as well.[9] One person was killed by a falling boulder at Agua Caliente. At the various missions, many houses, churches, chapels, and other structures totally collapsed.[10][11] Three adobe buildings were destroyed by the advancing tsunami. At Mission San Buenaventura and Mission Santa Barbara, the destruction was great, and a native man died when a building collapsed on him because of an aftershock. At Mission La Purisima, a large fissure carved through the slopes of a hill erupted mud and water.[10]

The low death toll for this large earthquake was attributed to a foreshock 15 minutes before the mainshock which had driven many people out of buildings that would collapse later.[12]

Devastation from this earthquake was comparable to the 1906 San Francisco and 1857 Fort Tejon earthquake, 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.[13][14] For fear of another tsunami and aftershocks persisting, the missions were abandoned until April of 1813.[6]

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.

San Andreas Fault

Based on tree-ring sampling, forests were found to have suffered some form of trauma and was experiencing a slow growth rate dating back to 1812. They were found to have been missing rings and crowns.[15] The trees took several years or decades to fully recover and return to their normal stage. Those affected trees however were found to be in close proximity to the San Andreas Fault, and not anywhere else; in an event of a strong quake, trees at a considerable distance should show signs 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 violent shaking might also knock branches and parts off the trees off.[15]

The San Andreas Fault in California.

It is believed that the San Juan Capistrano earthquake on December 8 triggered the second rupture on December 21.[16] The earthquake on December 21 began from the south and broke northwards to Fort Tejon. Both events ruptured a total length of 170 km on the fault.[16] 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.[16][17]

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 source is the lack of damage at Santa Barbara during the much more powerful 1857 earthquake. The section involved in the 1857 quake was closer to Santa Barbara and Ventura but were not felt severely like in the 1812 earthquake.[15][17]

As for claims of a tsunami and odd disturbances in the sea, a quake triggered landslide may have generated the large waves.[6][18]

San Cayetano Fault

The San Cayetano Fault in Southern California

The San Cayetano Fault is a 40 km long, north dipping reverse fault that extends from Ventura to the Sespe Mountains.

The Santa Barbara Channel epicenter is based on reports of the large tsunami that flooded the coastal communities.[15] Trenching done in Ventura County along the San Cayetano Fault 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.[19] 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 at greater than M_w 7.0 and likely ruptured the entire length of the fault.[20]

Santa Barbara would later be damaged by another earthquake in 1925, that earthquake triggered a moderate, non-destructive tsunami. That earthquake may have occurred along the Mesa Fault or the Santa Ynez Fault System.

• List of earthquakes in California
• List of earthquakes in the United States
• 1927 Lompoc earthquake