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, 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 word tremor is also used for non-earthquake seismic rumbling.
The Modified Mercalli intensity scale, developed from Giuseppe Mercalli's Mercalli intensity scale of 1902, is a seismic intensity scale used for measuring the intensity of shaking produced by an earthquake. It measures the effects of an earthquake at a given location, distinguished from the earthquake's inherent force or strength as measured by seismic magnitude scales. While shaking is caused by the seismic energy released by an earthquake, earthquakes differ in how much of their energy is radiated as seismic waves. Deeper earthquakes also have less interaction with the surface, and their energy is spread out across a larger volume. Shaking intensity is localized, generally diminishing with distance from the earthquake's epicenter, but can be amplified in sedimentary basins and certain kinds of unconsolidated soils.
Seismic risk refers to the risk of damage from earthquake to a building, system, or other entity. Seismic risk has been defined, for most management purposes, as the potential economic, social and environmental consequences of hazardous events that may occur in a specified period of time. A building located in a region of high seismic hazard is at lower risk if it is built to sound seismic engineering principles. On the other hand, a building located in a region with a history of minor seismicity, in a brick building located on fill subject to liquefaction can be as high or higher risk.
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 Japan Meteorological Agency (JMA) Seismic Intensity Scale is a seismic intensity scale used in Japan to categorize the intensity of local ground shaking caused by earthquakes.
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.
Induced seismicity is typically earthquakes and tremors that are caused by human activity that alters the stresses and strains on Earth's crust. Most induced seismicity is of a low magnitude. A few sites regularly have larger quakes, such as The Geysers geothermal plant in California which averaged two M4 events and 15 M3 events every year from 2004 to 2009. The Human-Induced Earthquake Database (HiQuake) documents all reported cases of induced seismicity proposed on scientific grounds and is the most complete compilation of its kind.
A response spectrum is a plot of the peak or steady-state response of a series of oscillators of varying natural frequency, that are forced into motion by the same base vibration or shock. The resulting plot can then be used to pick off the response of any linear system, given its natural frequency of oscillation. One such use is in assessing the peak response of buildings to earthquakes. The science of strong ground motion may use some values from the ground response spectrum for correlation with seismic damage.
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.
Earthquake engineering is an interdisciplinary branch of engineering that designs and analyzes structures, such as buildings and bridges, with earthquakes in mind. Its overall goal is to make such structures more resistant to earthquakes. An earthquake engineer aims to construct structures that will not be damaged in minor shaking and will avoid serious damage or collapse in a major earthquake. A properly engineered structure does not necessarily have to be extremely strong or expensive. It has to be properly designed to withstand the seismic effects while sustaining an acceptable level of damage.
The Indian subcontinent has a history of devastating earthquakes. The major reason for the high frequency and intensity of the earthquakes is that the Indian plate is driving into Asia at a rate of approximately 47 mm/year. Geographical statistics of India show that almost 58% of the land is vulnerable to earthquakes. A World Bank and United Nations report shows estimates that around 200 million city dwellers in India will be exposed to storms and earthquakes by 2050. The latest version of seismic zoning map of India given in the earthquake resistant design code of India [IS 1893 2002] assigns four levels of seismicity for India in terms of zone factors. In other words, the earthquake zoning map of India divides India into 4 seismic zones unlike its previous version, which consisted of five or six zones for the country. According to the present zoning map, Zone 5 expects the highest level of seismicity whereas Zone 2 is associated with the lowest level of seismicity.
The 1993 Scotts Mills earthquake, also known as the "Spring break quake", occurred in the U.S. state of Oregon on March 25 at 5:34 AM Pacific Standard Time. With a moment magnitude of 5.6 and a maximum perceived intensity of VII on the Mercalli intensity scale, it was the largest earthquake in the Pacific Northwest since the Elk Lake and Goat Rocks earthquakes of 1981. Ground motion was widely felt in Oregon's Willamette Valley, the Portland metropolitan area, and as far north as the Puget Sound area near Seattle, Washington.
The 1999 Athens earthquake occurred on September 7 at near Mount Parnitha in Greece with a moment magnitude of 6.0 and a maximum Mercalli intensity of IX (Violent). The proximity to the Athens metropolitan area resulted in widespread structural damage, mainly to the nearby suburbs of Ano Liossia, Acharnes, Fyli, Thrakomakedones, Kifissia, Metamorfosi, Kamatero and Nea Philadelphia. More than 100 buildings across those areas collapsed trapping scores of victims under their rubble while dozens more were severely damaged. With damage estimated at $3–4.2 billion, 143 people were killed, and up to 1,600 were treated for injuries in Greece's deadliest natural disaster in almost half a century.
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 1986 Chalfant Valley earthquake struck southern Mono County near Bishop and Chalfant, California at 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.
Recent advances are improving the speed and accuracy of loss estimates immediately after earthquakes so that injured people may be rescued more efficiently. "Casualties" are defined as fatalities and injured people, which are due to damage to occupied buildings. After major and large earthquakes, rescue agencies and civil defense managers rapidly need quantitative estimates of the extent of the potential disaster, at a time when information from the affected area may not yet have reached the outside world. For the injured below the rubble every minute counts. To rapidly provide estimates of the extent of an earthquake disaster is much less of a problem in industrialized than in developing countries. This article focuses on how one can estimate earthquake losses in developing countries in real time.
Spectral acceleration (SA) is a unit measured in g that describes the maximum acceleration in an earthquake on an object – specifically a damped, harmonic oscillator moving in one physical dimension. This can be measured at different oscillation frequencies and with different degrees of damping, although 5% damping is commonly applied. The SA at different frequencies may be plotted to form a response spectrum.
The 1935 Helena earthquake occurred at in Montana, with an epicenter near Helena. It had a magnitude of 6.2 on the surface wave magnitude scale and a maximum perceived intensity of VIII (Severe) on the Mercalli intensity scale. The temblor on that date was the largest of a series of earthquakes that also included a large aftershock on October 31 of magnitude 6.0 and a maximum intensity of VIII. Two people died in the mainshock and two others died as a result of the October 31 aftershock. Property damage was over $4 million.
The 1980 Eureka earthquake occurred on November 8 at along the northern coastal area of California in the United States. With a moment magnitude of 7.3 and a maximum Mercalli intensity of VII, this strike-slip earthquake was the largest to occur in California in 28 years. Although damage was considered light, several loss estimates equaled or exceeded $2 million, and six injuries resulted when two vehicles came down with the partial collapse of a highway overpass on US 101 in Fields Landing. The north coast of California experiences frequent plate boundary earthquakes near the Mendocino Triple Junction and intraplate events also occur within the Gorda Plate.
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.
