An earthquake is the shaking of the surface of the Earth resulting from a sudden release of energy in the Earth's lithosphere that creates seismic waves. Earthquakes can range in size from those that are so weak that they cannot be felt to those violent enough to propel objects and people into the air, and wreak destruction across entire cities. The seismicity, or seismic activity, of an area is the frequency, type, and size of earthquakes experienced over a period of time. The word tremor is also used for non-earthquake seismic rumbling.
In geology, a fault is a planar fracture or discontinuity in a volume of rock across which there has been significant displacement as a result of rock-mass movement. Large faults within the Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes.
A blind thrust earthquake occurs along a thrust fault that does not show signs on the Earth's surface, hence the designation "blind". Such faults, being invisible at the surface, have not been mapped by standard surface geological mapping. Sometimes they are discovered as a by-product of oil exploration seismology; in other cases their existence is not suspected.
The 1971 San Fernando earthquake occurred in the early morning of February 9 in the foothills of the San Gabriel Mountains in southern California. The unanticipated thrust earthquake had a magnitude of 6.5 on the scale, and a maximum Mercalli intensity of XI (Extreme). The event was one in a series that affected the Los Angeles area in the late 20th century. Damage was locally severe in the northern San Fernando Valley and surface faulting was extensive to the south of the epicenter in the mountains, as well as urban settings along city streets and neighborhoods. Uplift and other effects affected private homes and businesses.
The New Madrid Seismic Zone, sometimes called the New Madrid Fault Line, is a major seismic zone and a prolific source of intraplate earthquakes in the southern and midwestern United States, stretching to the southwest from New Madrid, Missouri.
An active fault is a fault that is likely to become the source of another earthquake sometime in the future. Geologists commonly consider faults to be active if there has been movement observed or evidence of seismic activity during the last 10,000 years.
The Hayward Fault Zone is a geologic fault zone capable of generating destructive earthquakes. This fault is about 74 mi (119 km) long, situated mainly along the western base of the hills on the east side of San Francisco Bay. It runs through densely populated areas, including Richmond, El Cerrito, Berkeley, Oakland, San Leandro, Castro Valley, Hayward, Union City, Fremont, and San Jose.
The California Geological Survey, previously known as the California Division of Mines and Geology, is the California state geologic agency.
The 1933 Long Beach earthquake took place on March 10 at 5:54 P.M. PST south of downtown Los Angeles. The epicenter was offshore, southeast of Long Beach, California, on the Newport–Inglewood Fault. The earthquake had a magnitude estimated at 6.4 , and a maximum Mercalli intensity of VIII (Severe). Damage to buildings was widespread throughout Southern California. 115 to 120 fatalities and an estimated forty million dollars' worth of property damage resulted. The majority of the fatalities resulted from people running out of buildings exposing themselves to the falling debris.
The Seismic Hazard Mapping Act was enacted by the California legislature in 1990 following the Loma Prieta earthquake of 1989. The Act requires the California State Geologist to create maps delineating zones where data suggest amplified ground shaking, liquefaction, or earthquake-induced landsliding may occur.
The Wasatch Fault is an active fault located primarily on the western edge of the Wasatch Mountains in the U.S. states of Utah and Idaho. The fault is about 240 miles long, stretching from southern Idaho, through northern Utah, before terminating in central Utah near the town of Fayette. The fault is made up of ten segments, five of which are considered active. On average the segments are approximately 25 miles long, each of which can independently produce earthquakes as powerful as local magnitude 7.5. The five active segments from north to south are called the Brigham City Fault Segment, the Weber Fault Segment, the Salt Lake City Fault Segment, the Provo Fault Segment and the Nephi Fault Segment.
The San Jacinto Fault Zone (SJFZ) is a major strike-slip fault zone that runs through San Bernardino, Riverside, San Diego, and Imperial Counties in Southern California. The SJFZ is a component of the larger San Andreas transform system and is considered to be the most seismically active fault zone in the area. Together they relieve the majority of the stress between the Pacific and North American tectonic plates.
The 1911 Kebin earthquake, or Chon-Kemin earthquake, struck Russian Turkestan on 3 January. Registering at a 7.7 magnitude, it killed 452 people, destroyed more than 770 buildings in Almaty, Kazakhstan, and resulted in 125 miles (201 km) of surface faulting in the valleys of Chon-Kemin, Chilik and Chon-Aksu.
The 1986 Chalfant Valley earthquake struck southern Mono County near Bishop and Chalfant, California at 07:42:28 Pacific Daylight Time on July 21. With a moment magnitude of 6.2 and a maximum Mercalli intensity of VI (Strong), the shock injured two people and caused property damage estimated at $2.7 million in the affected areas. There was a significant foreshock and aftershock sequence that included a few moderate events, and was the last in a series of three earthquakes that affected southern California and the northern Owens Valley in July 1986.
The Brawley Seismic Zone (BSZ), also known as the Brawley fault zone, is a predominantly extensional tectonic zone that connects the southern terminus of the San Andreas Fault with the Imperial Fault in Southern California. The BSZ is named for the nearby town of Brawley in Imperial County, California, and the seismicity there is characterized by earthquake swarms.
The Structural Engineers Association of Northern California (SEAONC) is a structural engineering association established in 1930. Its headquarters are in San Francisco, California. Initially a club for structural engineers to exchange technical information, it evolved into a professional organization advising on the development of building code requirements and California legislation related to earthquake hazard reduction such as the Field Act and Alquist Priolo Special Studies Zone Act.
The 2000 Iceland earthquakes struck southern Iceland on June 17 and 21. There were no fatalities but one person was injured and there was considerable damage to infrastructure. The two earthquakes were the first major seismic activity in Iceland for 88 years. The recorded magnitude of both of the main quakes was 6.5 .
The 1979 Imperial Valley earthquake occurred at 16:16 Pacific Daylight Time on 15 October just south of the Mexico–United States border. It affected Imperial Valley in Southern California and Mexicali Valley in northern Baja California. The earthquake had a relatively shallow hypocenter and caused property damage in the United States estimated at US$30 million. The irrigation systems in the Imperial Valley were badly affected, but no deaths occurred. It was the largest earthquake to occur in the contiguous United States since the 1971 San Fernando earthquake eight years earlier.
The 2015 Uniform California Earthquake Rupture Forecast, Version 3, or UCERF3, is the latest official earthquake rupture forecast (ERF) for the state of California, superseding UCERF2. It provides authoritative estimates of the likelihood and severity of potentially damaging earthquake ruptures in the long- and near-term. Combining this with ground motion models produces estimates of the severity of ground shaking that can be expected during a given period, and of the threat to the built environment. This information is used to inform engineering design and building codes, planning for disaster, and evaluating whether earthquake insurance premiums are sufficient for the prospective losses. A variety of hazard metrics can be calculated with UCERF3; a typical metric is the likelihood of a magnitude M 6.7 earthquake in the 30 years since 2014.
