An earthquake – also called a quake, tremor, or temblor – is the shaking of the Earth's surface resulting from a sudden release of energy in the lithosphere that creates seismic waves. Earthquakes can range in intensity, from those so weak they cannot be felt, to those violent enough to propel objects and people into the air, damage critical infrastructure, 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. The seismicity at a particular location in the Earth is the average rate of seismic energy release per unit volume.
The Modified Mercalli intensity scale measures the effects of an earthquake at a given location. This is in contrast with the seismic magnitude usually reported for an earthquake.
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.
The New Madrid Seismic Zone (NMSZ), 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.
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.
In seismology, strong ground motion is the strong earthquake shaking that occurs close to a causative fault. The strength of the shaking involved in strong ground motion usually overwhelms a seismometer, forcing the use of accelerographs for recording. The science of strong ground motion also deals with the variations of fault rupture, both in total displacement, energy released, and rupture velocity.
Seismic magnitude scales are used to describe the overall strength or "size" of an earthquake. These are distinguished from seismic intensity scales that categorize the intensity or severity of ground shaking (quaking) caused by an earthquake at a given location. Magnitudes are usually determined from measurements of an earthquake's seismic waves as recorded on a seismogram. Magnitude scales vary based on what aspect of the seismic waves are measured and how they are measured. Different magnitude scales are necessary because of differences in earthquakes, the information available, and the purposes for which the magnitudes are used.
The Virginia Seismic Zone in the U.S. state of Virginia covers about 8,000 square kilometers (3,100 sq mi) in the Piedmont province. Earthquakes in the state are irregular and rarely reach over 4.5 in magnitude.
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.
The 1965 Puget Sound earthquake occurred at 08:28 AM PDT on April 29 within the Puget Sound region of Washington state. It had a magnitude of 6.7 on the moment magnitude scale and a maximum perceived intensity of VIII (Severe) on the Mercalli intensity scale. It caused the deaths of seven people and about $12.5–28 million in damage. There were no recorded aftershocks.
The United States Geological Survey (USGS), founded as the Geological Survey, is an agency of the United States government whose work spans the disciplines of biology, geography, geology, and hydrology. The agency was founded on March 3, 1879, to study the landscape of the United States, its natural resources, and the natural hazards that threaten it. The agency also makes maps of extraterrestrial planets and moons based on data from U.S. space probes.
Prompt Assessment of Global Earthquakes for Response (PAGER) is a monitoring system for earthquakes. The service is operated by the U.S. Geological Survey, from its office in Golden, Colorado.
The 1957 San Francisco earthquake occurred on March 22 at 11:44:22 local time with a moment magnitude of 5.7 and a maximum Mercalli Intensity of VII. It was located just off the San Francisco Peninsula near the San Andreas Fault and was felt in a limited portion of Northern and Central California. There was a non-destructive foreshock and aftershock sequence that lasted for several months. With financial losses of around US$1 million, damage was considered minimal, with one death and forty injuries.
Seismic intensity scales categorize the intensity or severity of ground shaking (quaking) at a given location, such as resulting from an earthquake. They are distinguished from seismic magnitude scales, which measure the magnitude or overall strength of an earthquake, which may, or perhaps may not, cause perceptible shaking.
The Advanced National Seismic System (ANSS) is a collaboration of the U.S. Geological Survey (USGS) and regional, state, and academic partners that collects and analyzes data on significant earthquakes to provide near real-time information to emergency responders and officials, the news media, and the public. Such information is used to anticipate the likely severity and extent of damage, and to guide decisions on the responses needed.
The Lompoc earthquake of 1927 occurred at 5:49 a.m. Pacific Standard Time (PST), on November 4 with an epicenter off the coast of Lompoc, Santa Barbara County in Southern California. It is one of the largest earthquakes to have occurred off the coast of California, measuring a surface-wave magnitude of 7.3. The earthquake may have originated along the Hosgri Fault, an entirely offshore structure. Shaking from the earthquake and an unusually large tsunami caused some damage to communities near the earthquake. Due to its location and the area being sparsely populated at the time, there were no human fatalities reported. It is the only California-generated tsunami recorded in Hawaii.
An earthquake struck near the Hawaiian island of Lanai on February 19, at 10:11 pm HST with an estimated magnitude of 7.5 on the moment magnitude scale (Mw ). It remains one of the largest seismic event to hit the Hawaiian Islands since the 1868 Ka‘ū earthquake, with its effects being felt throughout the entire archipelago. It caused severe damage on the islands of Lanai, Molokai and Maui. A tsunami may have been generated however there are speculations if it had actually happened. Despite the size of the quake and the extent of damage, there were no deaths.
On December 20, 2022, a magnitude 6.4 earthquake struck Ferndale, California in Humboldt County, United States at 10:34:25 UTC, or 2:34 a.m. PST.
David Jay Wald is a seismologist with the U.S. Geological Survey (USGS) at the National Earthquake Information Center (NEIC) in Golden, Colorado. He is an affiliated faculty member at the Colorado School of Mines, and served as the editor-in-chief of the Earthquake Engineering Research Institute's (EERI) journal, Earthquake Spectra, from 2018–2022. He also served on the Southern California Earthquake Center Science Planning Committee, 2014–2020.
Kate E. Allstadt is a geologist and seismologist employed by the U.S. Geological Survey (USGS) who works out of Golden, Colorado. She is a self-described "present-day geologist" for her interest in connections between geology of the Pacific Northwest and the people in its local communities. She is a published expert on the 2014 Oso landslide.
