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The Ramapo Fault zone, spanning more than 185 miles (300 kilometers) in New York, New Jersey, and Pennsylvania, is a system of faults between the northern Appalachian Mountains and Piedmont areas to the east.〔(Earthquakes and the Ramapo Fault System in Southeastern New York State ). Earth Institute News Archive, Columbia University, 2004. Accessed October 24, 2009.〕 This fault is perhaps the best known fault zone in the Mid-Atlantic region, and some small earthquakes have been known to occur in its vicinity. Recently, public knowledge about the fault has increased, especially after the 1970s, when the fault's proximity to the Indian Point nuclear plant in New York was noticed. Some seismologists have argued that this fault has the potential to produce a major earthquake,〔Aggarwal, Y.P. and L.R. Sykes (1978), Earthquakes, faults and Nuclear Power Plants in Southern New York and Northern New Jersey, Science, 200, 425–429.〕〔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.〕 but earthquakes are scattered throughout this region, with no particular concentration of activity along the Ramapo fault (see Earthquake activity in the New York City area). Thus, others have argued that the Ramapo Fault has not been demonstrated to be any more active than any other fault zones in the greater New York City area.〔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.〕〔Kafka, A.L. M.A. Winslow, and N.L. Barstow (1989), Earthquake Activity in the Greater New York City Area: A Fault Finder's Guide, in Field Trip Guidebook (D. Weiss,Editor), 61st Annual Meeting, New York State Geological Association, 177-204.〕〔http://akafka.wordpress.com/faults-and-earthquakes-in-the-greater-new-york-city-area-reflections-at-the-intersection-of-science-media-and-the-public/ Faults and Earthquakes in the Greater NY City Area: Reflections at the Intersection of Science, the Media, and the Public〕 == 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 NYC area 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 NYC 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 U.S. are 100 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.〔http://gallery.usgs.gov/audios/?a=279 Why should people in the Eastern United States be concerned about earthquakes?〕 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. 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 NYC 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. The northeastern U.S. has many known faults, but numerous smaller or deeply buried faults probably remain undetected. Virtually all of the known faults have not been active for perhaps 90 million years or more. Also, the locations of the known faults are not well determined at earthquake depths. Accordingly, few earthquakes in the region can be unambiguously linked to known faults. Given the current geological and seismological data, it is difficult to determine if a known fault is still active today and could produce a modern earthquake. As in most other areas east of the Rocky Mountains, the best guide to earthquake hazard in the northeastern U.S. is probably the locations of past earthquakes themselves. The Ramapo Fault has been blamed for several past earthquakes, but the specific association of any significant earthquake with this fault has yet to be demonstrated.〔 A damaging earthquake affecting New York City in 1884 was incorrectly argued to be caused by the Ramapo fault, likely because it is the most prominent mapped fault in the greater New York City area. At the present, the relationship between faults and earthquakes in the New York City area is understood to be more complex than any simple association of a specific earthquake with a specific fault. 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「Ramapo Fault」の詳細全文を読む スポンサード リンク
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