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Although the eastern United States is not as seismically active as regions near plate boundaries, large and damaging earthquakes do occur there. Furthermore, when these rare eastern U.S. earthquakes occur, the areas affected by them are much larger than for western U.S. earthquakes of the same magnitude.〔http://gallery.usgs.gov/audios/?a=279 Why should people in the Eastern United States be concerned about earthquakes?〕 Thus, earthquakes represent at least a moderate hazard to East Coast cities, including New York City and adjacent areas of very high population density. As can be seen in the maps of earthquake activity in this region (shown in the figure on the right), seismicity is scattered throughout most of the New York metropolitan area, with some hint of a concentration of earthquakes in the area surrounding Manhattan Island. The largest known earthquake in this region occurred in 1884 and had a magnitude of approximately 5. For this earthquake, observations of fallen bricks and cracked plaster were reported from eastern Pennsylvania to central Connecticut, and the maximum intensity reported was at two sites in western Long Island (Jamaica, New York and Amityville, New York). Two other earthquakes of approximately magnitude 5 occurred in this region in 1737 and 1783.〔Kafka, A.L. E.A. Schlesinger-Miller, and N.L. Barstow (1985), Earthquake Activity in the Greater New York City Area: Magnitudes, Seismicity, and Geologic Structures, Bulletin of the Seismological Society of America, 75(1), 1285–1300.〕〔Sykes, L.R., J.G. Armbruster, W.Y. Kim, and L. Seeber (2008), Observations and Tectonic Setting of Historic and Instrumentally Located Earthquakes in the Greater New York City–Philadelphia Area, Bulletin of the Seismological Society of America, 98(4), 1696–1719.〕〔Rockwood, C.G., (1885), Notes on American Earthquakes, No. 14, American Journal of Science (Ser.3), 29, 425-432.〕 The figure on the right shows maps of the distribution of earthquakes of magnitude 3 and greater that occurred in this region from 1924 to 2010, along with locations of the larger earthquakes that occurred in 1737, 1783 and 1884. == Background == The New York City area is part of the geologically complex structure of the northern Appalachian Mountains. This complex structure was formed during the past half billion years when the Earth's crust underlying the Northern Appalachians was the site of two major geological episodes, each of which has left its imprint on the area's bedrock.〔(【引用サイトリンク】url=http://geomaps.wr.usgs.gov/parks/province/appalach.html )〕〔http://3dparks.wr.usgs.gov/nyc/highlands/highlands.html USGS, Geology of the NYC Region, The Highlands Province〕 Between about 450 million years ago and about 250 million years ago, the Northern Appalachian region was affected by a continental collision, in which the ancient African continent collided with the ancient North American continent to form the supercontinent Pangaea. Beginning about 200 million years ago, the present-day Atlantic ocean began to form as plate tectonic forces began to rift apart the continent of Pangaea. The last major episode of geological activity to affect the bedrock in the New York area occurred about 100 million years ago, during the Mesozoic era, when continental rifting that led to the opening of the present-day Atlantic Ocean formed the Hartford and Newark Mesozoic rift basins. Earthquake rates in the northeastern United States are about 50 to 200 times lower than in California, but the earthquakes that do occur in the northeastern U.S. are typically felt over a much broader region than earthquakes of the same magnitude in the western U.S.〔 This means the area of damage from an earthquake in the northeastern U.S. could be larger than the area of damage caused by an earthquake of the same magnitude in the western U.S.〔How Can Seismic Hazard Around the New Madrid Seismic Zone be Similar to That in California? (2004), Seismological Research Letters, 75(5), 575-586.〕 The cooler rocks in the northeastern U.S. contribute to the seismic energy propagating as much as ten times further than in the warmer rocks of California. A magnitude 4.0 eastern U.S. earthquake typically can be felt as far as 100 km (60 mi) from its epicenter, but it infrequently causes damage near its source. A magnitude 5.5 eastern U.S. earthquake, although uncommon, can be felt as far as 500 km (300 mi) from its epicenter, and can cause damage as far away as 40 km (25 mi) from its epicenter. Earthquakes stronger than about magnitude 5.0 generate ground motions that are strong enough to be damaging in the epicentral area. At well-studied plate boundaries like the San Andreas fault system in California, scientists can often make observations that allow them to identify the specific fault on which an earthquake took place. In contrast, east of the Rocky Mountains this is rarely the case.〔http://www2.bc.edu/~kafka/my_opinion.html Kafka, A.L. (2000), Public Misconceptions About Faults and Earthquakes in the United States: Is It Our Own Fault?, Seismological Research Letters, 71(3).〕 The New York City area is far from the boundaries of the North American plate, which are in the center of the Atlantic Ocean, in the Caribbean Sea, and along the west coast of North America. The seismicity of the northeastern U.S. is generally considered to be due to ancient zones of weakness that are being reactivated in the present-day stress field. In this model, pre-existing faults that were formed during ancient geological episodes persist in the intraplate crust, and the earthquakes occur when the present-day stress is released along these zones of weakness. The stress that causes the earthquakes is generally considered to be derived from present-day rifting at the Mid-Atlantic ridge. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Earthquake activity in the New York City area」の詳細全文を読む スポンサード リンク
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