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| duration = 9 seconds | magnitude = 7.2 MW 〔 | intensity = IX (''Violent'') 〔 | PGA = 2.2''g'' (est) | depth = | location = 〔 | countries affected = North Coast (California) United States | tsunami = Yes | damage = $48.3–66 million 〔〔 | aftershocks = 6.5 MW April 26 at 0:41 6.6 MW April 26 at 4:18 | casualties = 356 injuries 〔〔 }} The 1992 Cape Mendocino earthquakes (or 1992 Petrolia earthquakes) occurred along the Lost Coast of Northern California on April 25 and 26. The three largest events were the M7.2 thrust mainshock that struck near the unincorporated community of Petrolia midday on April 25 and two primary strike-slip aftershocks measuring 6.5 and 6.6 that followed early the next morning. The sequence encompassed both interplate and intraplate activity that was associated with the Mendocino Triple Junction, a complex system of three major faults (including the Cascadia subduction zone, San Andreas Fault, and Mendocino Fracture Zone) that converge near Cape Mendocino. The total number of aftershocks that followed the events exceeded 2,000. The three shocks damaged and destroyed homes and businesses in Humboldt County and injured 356 people, but the single largest loss was due to a post-earthquake fire that consumed a business center in Scotia. Accelerometers that had been in place in the Cape Mendocino area since the late 1970s recorded the event and the readings were moderate to strong, with the exception of the instruments closest to the epicenter, which went off scale a few seconds into the recording. No surface ruptures were present in the epicentral area, but landslides closed roads and railroad tracks for at least a week while cleanup took place. Also discovered was about of coastal uplift near Cape Mendocino and Punta Gorda. As the largest earthquake in California since the 1989 Loma Prieta event several years earlier, the mainshock caused a non-destructive tsunami that quickly reached the coast, and eventually Alaska and Hawaii several hours later. The tsunami was significant not because of its run-up, but because of the speed with which it reached the coast and for how long the waves persisted. Other strong earthquakes have affected the same area, with some that were clearly associated with the (interplate) Mendocino Fracture Zone, and others (like the two shocks on April 26) were intraplate earthquakes that ruptured within the Gorda Plate, but events that are unequivocally associated with the Cascadia subduction zone are very infrequent. == Tectonic setting == The northernmost coastal area is one of California's most seismically active regions and, in a 50-year period, the area including the Mendocino Fracture Zone at the southern flank of the Gorda Plate generated about 25 percent of all seismic energy unleashed in the state. The Mendocino Triple Junction (strike-slip/strike-slip/trench) formed 29–30 mya at 31° N (west of present-day Baja California) when the Pacific-Farallon spreading center initially approached the subduction zone off the coast of western North America. Simultaneously, the Rivera Triple Junction shifted to the southeast to its current position at 23° N. Once the Pacific Plate and North American Plate connected the boundary became that of a transform fault (San Andreas) due to the northwestward motion of the Pacific Plate relative to the North American Plate. The San Andreas Fault continues to lengthen to the northwest and the southeast as the two triple junctions continue their transient motion. North of the Mendocino Triple Junction, the Gorda plate is subducting beneath the North American Plate at the Cascadia subduction zone, with a convergence rate of per year, but comparisons with other subduction zones have led to a belief that the convergence may be taking place aseismically. The distinct lack of interplate events there has generated contention regarding the zone's seismic hazard, though there are strong indications that substantial historic events have occurred in the Pacific Northwest. Submerged wetlands and raised marine terraces both illustrate the presence of past events, and radiocarbon dating of rock layers has revealed that three seismic events took place in the last 2,000 years, with the most recent event being the 1700 Cascadia earthquake. The Gorda Plate is undergoing a process of intraplate deformation and experiences large intraplate earthquakes that may be the result of north-south compression of the oceanic crust along the Mendocino Fracture Zone.〔〔 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「1992 Cape Mendocino earthquakes」の詳細全文を読む スポンサード リンク
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