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Segunda-feira, 27.08.12

Sismo de 7.3 a 111km S de Puerto El Triunfo, El Salvador

Tectonic Summary

The August 27, 2012 M 7.3 interface between the Cocos and Caribbean plates. At the epicenter of this earthquake, the Cocos plate moves north-northeast with respect to the Caribbean plate at a velocity of approximately 73 mm/yr, and subducts beneath Central America at the Middle America Trench.
This plate boundary region has experienced over 20 earthquakes of magnitude 6 or greater over the past 40 years; three of these had magnitudes greater than 7. The largest of these were M 7.7 earthquakes in September 1992, approximately 140 km to the southeast of the August 26 earthquake, and another M7.7 earthquake in January 2001, 85 km to the north of this event. The 1992 earthquake spawned a large and damaging tsunami along the coastline of Central America, particularly in Nicaragua and Costa Rica, and resulted in over 100 fatalities. That earthquake has been labeled a tsunami earthquake, characterized by the unusually large tsunami it caused in relation to the size of the earthquake. The 2001 earthquake offshore of El Salvador resulted in over 850 fatalities, many of which were caused by large landslides in multiple El Salvador cities.

Seismotectonics of the Caribbean Region

Extensive diversity of tectonic regimes characterizes the perimeter of the Caribbean plate, involving no fewer than four major adjacent plates (North America, South America, Nazca, and Cocos). Inclined zones of deep earthquakes (Wadati-Benioff zones), deep ocean trenches, and arcs of volcanoes clearly indicate subduction of oceanic lithosphere along the Central American and Atlantic Ocean margins of the Caribbean plate, while shallow seismicity and focal mechanisms of major shocks in Guatemala, northern Venezuela, and the Cayman Ridge and Cayman Trench indicate transform fault and pull-apart basin tectonics.
Along the northern margin of the Caribbean plate, the North America plate moves west, relative to the Caribbean plate, at approximately 20 mm/yr, resulting in major transcurrent faults and troughs. Farther east, the North America plate subducts beneath the Caribbean plate resulting in surface expression of the deep Puerto Rico Trench and a zone of intermediate focus earthquakes in the subducted slab. The plate boundary curves around Puerto Rico and the northern Lesser Antilles where the plate motion vector of the Caribbean plate relative to the North and South America plates is less oblique, resulting in active island-arc tectonics. The North and South America plates subduct beneath the Caribbean plate along the Lesser Antilles Trench at rates of about 20 mm/yr; consequently, there are both intermediate focus earthquakes within the subducted South America plate and a chain of active volcanoes along the island arc.
The southern Caribbean plate boundary along with the South America plate strikes east-west across Trinidad and western Venezuela and is characterized by major strike-slip faults and shallow seismicity, resulting from relative plate motion of about 20 mm/yr. Further to the west, a broad zone of convergent deformation trends southwest across western Venezuela and central Columbia. Plate boundaries are not well defined across northern South America, but there is a transition from Caribbean/South America convergence in the east to Nazca/South America convergence in the west, described in more detail below. The transition zone is characterized by high seismic hazard.
The Nazca-Caribbean plate boundary offshore of Columbia is characterized by convergence (Nazca plate subducting under South America plate) at about 65 mm/yr. The 6 January 1906 Mw = 8.5 megathrust subduction earthquake occurred on a shallow-dipping interface of this plate boundary segment. Along the western coast of Central America, the Cocos plate subducts beneath the Caribbean plate at rates of 72???81 mm/yr, resulting in a relatively high seismic hazard and a chain of numerous active volcanoes; here intermediate-focus earthquakes occur within the subducted Cocos plate to depths of nearly 300 km.
The boundary between the Cocos plate and the Nazca plate is characterized by a series of north-south trending transform faults connected together by spreading centers. The largest and most seismically active of these transform boundaries is the Panama Fracture Zone. The Panama Fracture Zone terminates in the south theat the Galapagos rift zone and in the north at the Middle America trench where it forms the Cocos-Nazca-Caribbean triple junction. Earthquakes along the Panama Fracture Zone are generally shallow (0-70 km depth) and low- to intermediate-magnitude (<M7.2) and characteristically right-lateral strike-slip faulting earthquakes. Since 1900, the largest earthquake to occur along the Panama Fracture Zone was the July 26, 1962 M7.2 earthquake.
References for the Panama Fracture Zone:
Molnar, P., and Sykes, L. R., 1969, Tectonics of the Caribbean and Middle America Regions from Focal Mechanisms and Seismicity: Geological Society of America Bulletin, v. 80, p. 1639-1684.

Preferred Location Parameters

Magnitude7.3 MwwNot Specified
Location12.278°N, 88.528°W± 16.1 km
Depth20.3 km± 4.2 km
Number of Stations Used345
Number of Phases Used345
Minimum Distance130.2 km (1.17°)
Travel Time Residual1.21 sec
Azimuthal Gap66°
Review StatusREVIEWED
Event IDusc000c7yw

Fonte: USGS

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