Epicenter
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The epicenter (/ˈɛpɪˌsɛntər/), epicentre, or epicentrum[1] in seismology is the point on the Earth's surface directly above a hypocenter or focus, the point where an earthquake or an underground explosion originates.
Determination
The primary purpose of a seismometer is to locate the initiating points of earthquake epicenters. The secondary purpose, of determining the 'size' or magnitude must be calculated after the precise location is known.[citation needed]
The earliest seismographs were designed to give a sense of the direction of the first motions from an earthquake. The Chinese frog seismograph[2] would have dropped its ball in the general compass direction of the earthquake, assuming a strong positive pulse. We now know that first motions can be in almost any direction depending on the type of initiating rupture (focal mechanism).[3]
The first refinement that allowed a more precise determination of the location was the use of a
From the first seismograms, as seen in the figure, it was noticed that the trace was divided into two major portions. The first seismic wave to arrive was the
One seismograph would give the distance, but that could be plotted as a circle, with an infinite number of possibilities. Two seismographs would give two intersecting circles, with two possible locations. Only with a third seismograph would there be a precise location.
Modern earthquake location still requires a minimum of three seismometers. Most likely, there are many, forming a seismic array. The emphasis is on precision since much can be learned about the
Surface damage
Before the instrumental period of earthquake observation, the epicenter was thought to be the location where the greatest damage occurred,
Epicentral distance
During an earthquake,
Once distances from the epicenter have been calculated from at least three seismographic measuring stations, the point can be located, using trilateration.
Epicentral distance is also used in calculating
Fault rupture
The point at which fault slipping begins is referred to as the focus of the earthquake.[8] The fault rupture begins at the focus and then expands along the fault surface. The rupture stops where the stresses become insufficient to continue breaking the fault (because the rocks are stronger) or where the rupture enters ductile material.[8] The magnitude of an earthquake is related to the total area of its fault rupture.[8] Most earthquakes are small, with rupture dimensions less than the depth of the focus so the rupture doesn't break the surface, but in high magnitude, destructive earthquakes, surface breaks are common.[8] Fault ruptures in large earthquakes can extend for more than 100 km (62 mi).[8] When a fault ruptures unilaterally (with the epicenter at or near the end of the fault break) the waves are stronger in one direction along the fault.[11]
Macroseismic epicenter
The macroseismic epicenter is the best estimate of the location of the epicenter derived without instrumental data. This may be estimated using intensity data, information about foreshocks and aftershocks, knowledge of local fault systems or extrapolations from data regarding similar earthquakes. For historical earthquakes that have not been instrumentally recorded, only a macroseismic epicenter can be given.[12]
Etymology
The word is derived from the
It is also used to mean "center of activity", as in "Travel is restricted in the Chinese province thought to be the epicentre of the SARS outbreak."
References
- ^ Oxford English Dictionary: "The point over the centre: applied in Seismol. to the outbreaking point of earthquake shocks."
- ^ "Chinese Seismograph". Archived from the original (jpg) on 2019-09-16. Retrieved 2023-08-11.
- ^ "USGS Earthquake Hazards Program". Archived from the original on 2005-12-22. Retrieved 2023-09-06.
- ^ "How Can I Locate the Earthquake Epicenter?".
- ^ "USGS Earthquake Hazards Program". Archived from the original on 2005-12-18. Retrieved 2023-09-06.
- ISBN 978-0-19-507827-5.
- ^ Fuis, Gary; Wald, Lisa. "Rupture in South-Central Alaska – The Denali Fault Earthquake of 2002". USGS. Retrieved 2008-04-20.
- ^ OCLC 798410008.
- ^ Tyler M. Schau (1991). "The Richter Scale (ML)". USGS. Archived from the original on 2016-04-25. Retrieved 2008-09-14.
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(help) - ^ William L. Ellsworth (1991). "Surface-Wave Magnitude (Ms) and Body-Wave Magnitude (mb)". USGS. Archived from the original on 2009-02-02. Retrieved 2008-09-14.
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(help) - ^ "What is Directivity?". earthquake.usgs.gov. Retrieved 2018-07-01.
- ISBN 9780124406520.
- ^ "epicenter". Merriam-Webster Online Dictionary. 2009. Retrieved 2009-10-19.
- ^ ἐπίκεντρος, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
- ^ ἐπί, Henry George Liddell, Robert Scott, A Greek-English Lexicon, on Perseus
- ^ epicentre, on Oxford Dictionaries
- ISBN 978-2-553-01021-7.
- ISBN 978-1-134-36915-7.
- ISBN 978-0-85265-138-4.
- ^ Safire, William (2001-05-06). "On Language". The New York Times Magazine. p. 22. Archived from the original on 2022-10-17. Retrieved 2022-10-17.
- ISBN 9780199888771.