The earliest known earthquake in the U.S. state of California was documented in 1769 by the Spanish explorers and Catholic missionaries of the Portolá expedition as they traveled northward from San Diego along the Santa Ana River near the present site of Los Angeles. Ship captains and other explorers also documented earthquakes. As Spanish missions were constructed beginning in the late 18th century, earthquake records were kept. After the missions were secularized in 1834, records were sparse until the California Gold Rush in the 1840s. From 1850 to 2004, there was about one potentially damaging event per year on average, though many of these did not cause serious consequences or loss of life.[1][2]
Early developments at the Caltech lab included an earthquake observation network using their own custom-built short-period seismometers, the Richter magnitude scale, and the Modified Mercalli intensity scale (an updated version of the Mercalli intensity scale). In 1933, the Long Beach earthquake occurred in a populated area and damaged or destroyed many public school buildings in Long Beach and Los Angeles. Some decades later, the San Fernando earthquake affected the San Fernando Valley north of Los Angeles with heavy damage to several hospitals. In both cases, the perception of California policy makers changed, and state laws and building codes were modified (with much debate) to require commercial and residential properties to be built to withstand earthquakes. Higher standards were established for fire stations, hospitals, and schools, and construction of dwellings was also restricted near active faults.[4][5]
In the San Francisco Bay Area, the San Andreas system of faults spans offshore and into the East Bay area, with the bulk of the faults lying to the east of the main SAF. There is a 70% probability that one of these faults will generate a 6.7 Mw or greater earthquake before 2030, including the Hayward Fault Zone, which has gone beyond its average return period of 130 years (155 years ago as of August 2024). While the SAF is quiet north of San Francisco , the central SAF segment near San Juan Bautista is where aseismic creep was first studied, and to the south is where the recurring Parkfield earthquakes occur. The secondary faults lay to the west of the main SAF at the extreme southern portion, including the active and young San Jacinto Fault Zone, which may be taking over as the primary boundary south of Cajon Pass. A paleoseismic investigation using Lidar revealed that more than 16 feet (5 m) of slip has accumulated since the 1857 event on the southern SAF, which borders the Mojave Desert to the north and east of the Greater Los Angeles Area. Near the Transverse Ranges, reverse and thrust faults have produced damaging earthquakes in Santa Barbara and the San Fernando Valley.[6]
Stover & Coffman 1993 uses various seismic scales. Mla is a local magnitude that is equivalent to ML (Richter magnitude scale) and is used for events that occurred prior to the instrumental period. It is based on the area of perceptibility (as presented on isoseismal maps). Mw = moment magnitude scale and Ms = surface wave magnitude. The inclusion criteria for adding events are based on WikiProject Earthquakes' notability essay that was developed for stand alone articles. The principles described are also applicable to lists. In summary, only damaging, injurious, or deadly events should be recorded. k= thousand, M = million, bn = billion