Left: Rapidly spreading Pacific-Antarctic ridge with earthquakes concentrated along transform faults. Arrows show the direction of plate motion. Figure 12.17 Locations of earthquakes of magnitude 4 and greater from 1990 to 2010 along two mid-ocean ridges. This triggers earthquakes many kilometres away from the ridge. As spreading proceeds and crust moves away from the ridge, the bend is relaxed, and the crust stretches and breaks. Along rapidly spreading ridges, new ocean crust is bent upward into wide, high ridges. As a result, most of the earthquakes (white dots) are located along transform faults, where rocks are cooler and stronger. Rapid spreading causes rocks near the axis of the spreading centre to be hot and weak. The Pacific-Antarctic Ridge (left) is spreading relatively rapidly at 42 to 94 mm/year, depending on the location along the ridge. The locations of earthquakes along mid-ocean ridges, and the mechanisms for causing them, depend on how rapidly the mid-ocean ridges are spreading. Mid-ocean ridge divergent plate margins are offset by numerous transform faults (Figure 12.17). If deformation is permanent, then removing the stress does not result in the rocks snapping back to their original shape. Earthquakes at Divergent and Transform Plate BoundariesĮarthquakes along divergent and transform plate margins are shallow (usually less than 30 km deep) because below those depths, rock is too hot and weak to avoid being permanently deformed by the stresses in those settings. Locations include the Great Rift Valley area of Africa, the Lake Baikal area of Russia, and Tibet. Some are related to the buildup of stress due to continental rifting or the transfer of stress from other regions, and some are not well understood. Click the image for terms of use.Įarthquakes are also relatively common at a few locations away from plate boundaries. Source: Lisa Christiansen, Caltech Tectonics Observatory (2008) view source. Wide bands of scattered earthquakes mark continent-continent convergent margins (e.g., between the Indian and Eurasian plates), or continental rift zones (e.g., in eastern Africa). Wider bands with earthquakes at a range of depths are subduction zones. Narrow bands with shallow earthquakes (marked in red) indicate transform boundaries or mid-ocean ridge divergent boundaries. Bands of earthquakes mark tectonic plates. Figure 12.16 Earthquakes greater than magnitude 5, from 2000 to 2008. Wide swaths of scattered earthquakes may also correspond to continental rift zones, such as in eastern Africa. Wide swaths of scattered earthquakes may correspond to continent-continent collision zones, such as between the Eurasian plate and the African, Arabian, and Indian plates to the south. Bands of earthquakes are wider along subduction zones because they take place throughout the subducting slab that extends beneath the opposing plate. Subduction zones have earthquakes at a range of depths, including some more than 700 km deep. Mid-ocean ridges and transform margins have shallow earthquakes (usually less than 30 km deep), in narrow bands close to plate margins. The depths of earthquakes, and the width of the band, depend on the type of plate boundary. Bands of earthquakes trace out plate boundaries (coloured dots, Figure 12.16).
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