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 intensity, 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 seismic activity of an area is the frequency, type, and size of earthquakes experienced over a particular time period. The seismicity at a particular location in the Earth is the average rate of seismic energy release per unit volume. 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 movements. Large faults within Earth's crust result from the action of plate tectonic forces, with the largest forming the boundaries between the plates, such as the megathrust faults of subduction zones or transform faults. Energy release associated with rapid movement on active faults is the cause of most earthquakes. Faults may also displace slowly, by aseismic creep.
A seismic hazard is the probability that an earthquake will occur in a given geographic area, within a given window of time, and with ground motion intensity exceeding a given threshold. With a hazard thus estimated, risk can be assessed and included in such areas as building codes for standard buildings, designing larger buildings and infrastructure projects, land use planning and determining insurance rates. The seismic hazard studies also may generate two standard measures of anticipated ground motion, both confusingly abbreviated MCE; the simpler probabilistic Maximum Considered Earthquake, used in standard building codes, and the more detailed and deterministic Maximum Credible Earthquake incorporated in the design of larger buildings and civil infrastructure like dams or bridges. It is important to clarify which MCE is being discussed.
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 6.6 on the scale, and a maximum Mercalli intensity of XI (Extreme). The event was one in a series that affected Los Angeles county 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.
Engineering geology is the application of geology to engineering study for the purpose of assuring that the geological factors regarding the location, design, construction, operation and maintenance of engineering works are recognized and accounted for. Engineering geologists provide geological and geotechnical recommendations, analysis, and design associated with human development and various types of structures. The realm of the engineering geologist is essentially in the area of earth-structure interactions, or investigation of how the earth or earth processes impact human made structures and human activities.
Peak ground acceleration (PGA) is equal to the maximum ground acceleration that occurred during earthquake shaking at a location. PGA is equal to the amplitude of the largest absolute acceleration recorded on an accelerogram at a site during a particular earthquake. Earthquake shaking generally occurs in all three directions. Therefore, PGA is often split into the horizontal and vertical components. Horizontal PGAs are generally larger than those in the vertical direction but this is not always true, especially close to large earthquakes. PGA is an important parameter for earthquake engineering, The design basis earthquake ground motion (DBEGM) is often defined in terms of PGA.
The Alquist-Priolo Earthquake Fault Zoning Act was signed into California law on December 22, 1972, to mitigate the hazard of surface faulting to structures for human occupancy.
The California Geological Survey, previously known as the California Division of Mines and Geology, is the California state geologic agency.
The Utah Geological Survey is based in Salt Lake City, Utah, United States. It also has an office in Cedar City, Utah.
The California Department of Conservation is a department within the government of California, belonging to the California Natural Resources Agency. With a team of scientists, engineers, environmental experts, and other specialists, the Department of Conservation administers a variety of programs vital to California's public safety, environment and economy. The department's mission is to manage California's working lands. It regulates oil, natural gas and geothermal wells; studies and maps earthquakes and other geologic phenomena; maps and classifies areas containing mineral deposits; ensures reclamation of land used for mining; and administers agricultural and open-space land conservation programs. A division within the department dedicated to encouraging beverage container recycling has been moved into the newly created Department of Resources Recovery and Recycling (CalRecycle). Despite the similar name, the Department of Conservation should not be confused with the California Conservation Corps, another department within the Natural Resources Agency, which provides work experience for young adults. The Department of Conservation often collaborates with its federal equivalents, such as the U.S. Geological Survey.
The California Earthquake Prediction Evaluation Council (CEPEC) is a committee of earthquake experts that reviews potentially credible earthquake predictions and forecasts. Its purpose is to advise the Governor of California via the California Office of Emergency Services (CalOES).
The Natural Hazards Disclosure Act, under Sec. 1103 of the California Civil Code, states that real estate seller and brokers are legally required to disclose if the property being sold lies within one or more state or locally mapped hazard areas. The law specifies that the six (6) required hazards be disclosed on a statutory form called the Natural Hazard Disclosure Statement (NHDS).
The geology of Alabama is marked by abundant geologic resources and a variety of geologic structures from folded mountains in the north to sandy beaches along the Gulf of Mexico coast. Alabama spans three continental geologic provinces as defined by the United States Geological Survey, the Atlantic Plain, Appalachian Highlands, and Interior Plains. The Geological Survey of Alabama breaks these provinces down into more specific physiographic provinces.
Seismic microzonation is defined as the process of subdividing a potential seismic or earthquake prone area into zones with respect to some geological and geophysical characteristics of the sites such as ground shaking, liquefaction susceptibility, landslide and rock fall hazard, earthquake-related flooding, so that seismic hazards at different locations within the area can correctly be identified. Microzonation provides the basis for site-specific risk analysis, which can assist in the mitigation of earthquake damage. In most general terms, seismic microzonation is the process of estimating the response of soil layers under earthquake excitations and thus the variation of earthquake characteristics on the ground surface.
Landslides in California occur mainly due to intense rainfall but occasionally are triggered by earthquakes. Landslides are common in Southern California, the San Francisco Bay Area and other parts of Northern California, and the Sierra Nevada. Although they most often are reported when they impact residential developments, landslides also damage roads, railroads, pipelines, electrical lines, and other infrastructure throughout the state, and occur in unoccupied parts of the state.
Diablo Canyon (Nuclear) Power Plant, located in San Luis Obispo County California, was originally designed to withstand a 6.75 magnitude earthquake from four faults, including the nearby San Andreas and Hosgri faults, but was later upgraded to withstand a 7.5 magnitude quake. It has redundant seismic monitoring and a safety system designed to shut it down promptly in the event of significant ground motion.
The Oklahoma earthquake swarms are an ongoing series of human activity-induced earthquakes affecting central Oklahoma, southern Kansas, northern Texas since 2009. Beginning in 2009, the frequency of earthquakes in the U.S. state of Oklahoma rapidly increased from an average of fewer than two 3.0+ magnitude earthquakes per year since 1978 to hundreds each year in the 2014–17 period. Thousands of earthquakes have occurred in Oklahoma and surrounding areas in southern Kansas and North Texas since 2009. Scientific studies attribute the rise in earthquakes to the disposal of wastewater produced during oil extraction that has been injected more deeply into the ground.
