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出典(authority):フリー百科事典『ウィキペディア(Wikipedia)』「2014/02/05 18:26:41」(JST)
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Hypocenter (Focus) and epicenter of an earthquake
The hypocenter (literally: 'below the center' from the Greek: ὑπόκεντρον) refers to the site of an earthquake or a nuclear explosion. In the former, it is a synonym of the focus;[1] in the latter, of ground zero.
Contents
- 1 Earthquakes
- 2 Air-burst explosions
- 3 See also
- 4 Notes
Earthquakes[edit]
An earthquake's hypocenter is the position where the strain energy stored in the rock is first released, marking the point where the fault begins to rupture.[1] This occurs directly beneath the epicenter, at a distance known as the focal or hypocentral depth.[1]
The focal depth can be calculated from measurements based on seismic wave phenomena. As with all wave phenomena in physics, there is uncertainty in such measurements that grows with the wavelength so the focal depth of the source of these long-wavelength (low frequency) waves is difficult to determine exactly. Very strong earthquakes radiate a large fraction of their released energy in seismic waves with very long wavelengths and therefore a stronger earthquake involves the release of energy from a larger mass of rock.
Computing the hypocenters of foreshocks, main shock, and aftershocks of earthquakes allows the three-dimensional plotting of the fault along which movement is occurring.[2] The expanding wavefront from the earthquake's rupture propagates at a speed of several kilometers per second, this seismic wave is what is measured at various surface points in order to geometrically determine an initial guess as to the hypocenter. The wave reaches each station based upon how far away it was from the hypocenter. A number of things need to be taken into account, most importantly variations in the waves speed based upon the materials that it is passing through.[3] With adjustments for velocity changes, the initial estimate of the hypocenter is made, then a series of linear equations is set up, one for each station. The equations express the difference between the observed arrival times and those calculated from the initial estimated hypocenter. These equations are solved by the method of least squares which minimizes the sum of the squares of the differences between the observed and calculated arrival times, and a new estimated hypocenter is computed. The system iterates until the location is pinpointed within the margin of error for the velocity computations;[3] this is known as linear regression.
Air-burst explosions[edit]
Monument at the atomic bomb hypocenter in Nagasaki
The term hypocenter also refers to the point on the Earth's surface directly below an atmospheric explosion. In principle, it applies to any such explosion but the term was not found to be necessary until the advent of massive nuclear explosions. In this context, the term 'ground zero' was synonymous with hypocenter, though the ground zero term is less precise, as it has been used ever more loosely.[citation needed]
See also[edit]
- Ground zero
- Hiroshima Peace Memorial - The closest structure to withstand the 1945 nuclear explosion, 150 metres (490 ft) from the hypocenter.
Notes[edit]
- ^ a b c The hypocenter is the point within the earth where an earthquake rupture starts. The epicenter is the point directly above it at the surface of the Earth. Also commonly termed the focus. "Earthquake Glossary - hypocenter". United States Geological Survey. Archived from the original on 15 March 2010.
- ^ Kennelly, Patrick J. and Stickney, Michael C. (2000). "Using GIS for Visualizing Earthquake Epicenters, Hypocenters, Faults and Lineaments in Montana". Digital Mapping Techniques '00 -- Workshop Proceedings. United States Geological Survey. USGS Open-File Report 00-325. Archived from the original on 23 March 2004.
- ^ a b "FAQs - Measuring Earthquakes: Q: How do seismologists locate an earthquake?". USGS Earthquake Hazrads Program. United States Geological Survey.
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English Journal
- Tomography of the subducting Pacific slab and the 2015 Bonin deepest earthquake (Mw 7.9).
- Zhao D1, Fujisawa M1, Toyokuni G1.
- Scientific reports.Sci Rep.2017 Mar 15;7:44487. doi: 10.1038/srep44487.
- PMID 28295018
- Groundwater helium anomaly reflects strain change during the 2016 Kumamoto earthquake in Southwest Japan.
- Sano Y1, Takahata N1, Kagoshima T1, Shibata T2, Onoue T3, Zhao D4.
- Scientific reports.Sci Rep.2016 Nov 29;6:37939. doi: 10.1038/srep37939.
- PMID 27897212
- The influence of follow-up on DS02 low-dose ranges with a significant excess relative risk of all solid cancer in the Japanese A-bomb survivors.
- Walsh L1, Schneider U2.
- Radiation and environmental biophysics.Radiat Environ Biophys.2016 Nov;55(4):509-515. Epub 2016 Sep 1.
- PMID 27586509
Japanese Journal
- 2012年に発生した北海道北部中川町付近の群発地震活動
- 一柳 昌義,高橋 浩晃,山口 照寛 [他],東 龍介,山田 卓司,大園 真子,眞城 亮成,笠原 稔,谷岡 勇市郎
- 北海道大学地球物理学研究報告 (78), 37-51, 2015-03
- … Hypocenters were calculated using the Double-Difference hypocenter determination procedure with a local one dimensional P-wave velocity structure. … Precise hypocenter data indicated that epicenters were distributed in very narrow area of 2 km×2 km with shallow depth from 4 km to 7 km. … An independent hypocenter cluster with shallower than 2 km was observed above the main activity area. …
- NAID 120005572453
- Matched Filter Methodによる群発地震解析の試み〜2013年飛騨山脈穂高岳付近の地震活動の例〜
- 大見 士朗
- 地震 第1輯 68(1), 1-15, 2015
- … In this analysis, MFM was implemented as an automatic hypocenter relocation system. …
- NAID 130005087197
- 断層極近傍のための理論地震動シミュレーション法を用いた断層表層領域破壊時の地震動推定
- 山田 雅行,羽田 浩二,今井 隆太,藤原 広行
- 日本地震工学会論文集 15(2), 2_77-2_90, 2015
- 断層極近傍における地震動予測を目的として、全無限一様弾性体のグリーン関数公式を用いた理論地震動の数値シミュレーション法を用いて、断層の極近傍において理論地震動シミュレーションを実施する際の積分誤差の評価を行った。その結果、観測点との距離Lに対して積分要素寸法が0.5×L以下となる必要があることを確認し、地表まで露頭した断層を考える場合の効率的な要素分割を提案した。その上で本論文の主題である、地震発 …
- NAID 130005073563
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