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The 2011 Virginia earthquake occurred on August 23 at 1:51:04 p.m. local time in the Piedmont region of the US state of Virginia. The epicenter, in Louisa County, was northwest of Richmond and south-southwest of the town of Mineral. It was an intraplate earthquake with a magnitude of 5.8 and a maximum perceived intensity of VII (very strong) on the Mercalli intensity scale. Several aftershocks, ranging up to 4.5 Mw in magnitude, occurred after the main tremor. The earthquake, along with a magnitude–5.8 quake on the New York–Ontario border in 1944, is the largest to have occurred in the U.S. east of the Rocky Mountains since an earthquake centered in Giles County in western Virginia occurred during 1897,〔(【引用サイトリンク】title=Largest Earthquakes by State: List of Earthquakes )〕 with an estimated magnitude of 5.8. The quake was felt across more than a dozen U.S. states and in several Canadian provinces, and was felt by more people than any other quake in U.S. history. No deaths and only minor injuries were reported. Minor damage to buildings was widespread and was estimated by one risk-modeling company at $200 million to $300 million, of which about $100 million was insured.〔Estimate by the Oakland, California-based catastrophe modeling and risk assessment company EQECAT, which estimated further that only 5 percent of East Coast property owners have earthquake coverage. 〕 The earthquake prompted research that revealed that the farthest landslide from the epicenter was , by far the greatest landslide distance recorded from any other earthquake of similar magnitude. Previous studies of worldwide earthquakes indicated that landslides occurred no farther than from the epicenter of a magnitude 5.8 earthquake. The Virginia earthquake study suggested that the added information about East Coast earthquakes may prompt a revision of equations that predict ground shaking. ==Geology== The earthquake occurred in the Virginia Seismic Zone, located in the Piedmont region. The Virginia Piedmont area was formed originally as part of a zone of repeated continental collision that created the ancestral Appalachian Mountains, a process that started during the Ordovician period with the Taconic orogeny and finished during the Carboniferous Period with the Alleghenian orogeny. The reverse faults formed during the various orogenies were partly reactivated as normal faults in extension during the Mesozoic Era as the supercontinent Pangaea broke apart. During the Cenozoic Era, compression from the opening and spreading of the Atlantic has caused some of these structures to be reactivated in a reverse sense. The earthquake's epicenter and most of the aftershocks lie between the surface traces of two structures, the Spotsylvania Fault, a southeast dipping zone of high ductile strain, and the Chopawamsic Fault, a thrust fault. The earthquake's focal mechanism shows reverse slip faulting on a north to northeast striking fault plane. The spatial distribution of aftershocks show that the causative fault dips to the southeast at 50–55°. There was no surface faulting associated with the earthquake. The size of the rupture is as yet uncalculated, but similar quakes have been caused by slippage along fault segments that are long.〔 After the earthquake, several websites speculated about whether hydraulic fracturing (fracking) for natural gas production could have caused or contributed to the quake. There were not any fracking operations in Virginia at the time of the quake. The nearest fracking was occurring in the Marcellus shale in West Virginia.〔 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「2011 Virginia earthquake」の詳細全文を読む スポンサード リンク
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