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in both the subducting slab (foot wall) and the overriding plate (hanging wall). Many of Japan’s historic, destructive earthquakes, including the 1891 Nobi and 1995 Hyogo-ken Nanbu earthquakes, were caused by shallow faults within the overriding plate. In regions of oblique convergence, relative plate motion tends to separate into two parts, a subduction component taken up by reverse dip-slip faulting perpendicular to the trench and a lateral component taken up as strike-slip faulting parallel to the trench in the overriding plate (61); examples of such strike-slip structures include the Great Sumatran fault in Indonesia, the Philippine fault, and the Median Tectonic Line of Japan. Earthquakes that occur seaward of the trench are caused by flexing of the downgoing slab. These events may accommodate either stretching or contraction of the downgoing slab along either normal or reverse faults. Two of the largest historical events in this setting—the 1977 Sumba, Indonesia (M 8.3) and the 1933 Sanriku, Japan (M 8.4) earthquakes—were caused by normal faulting of the bending subducting plate.
Continental convergence zones have generated some of history’s most destructive earthquakes. They are common throughout the broad Alpine-Himalayan belt that resulted from the closure of the ancient Tethys Ocean (Figure 3.18), but are also found behind ocean-continent subduction zones, such as the foreland fold-and-thrust belts in the eastern foothills of the Andes. The style of deformation ranges from block motions along large reverse faults that penetrate deep into the crystalline basement, as in the Pampean Ranges in northwestern Argentina and the Zagros of Iran and
FIGURE 3.18 Four giant earthquakes—1897, 1905, 1934, and 1950—have resulted from slip on shallow thrust faults along the southern front of the Himalaya in India and Nepal. Although many active faults reach the surface in this 2000-kilometer-long region of continental convergence, the ruptures of these devastating earthquakes did not break to the surface. SOURCE: G. LeBlanc and F. Anglin, Induced seismicity at the Manic 3 Reservoir, Quebec, Bull. Seis. Soc. Am., 68, 1469-1485, 1978. Copyright Seismological Society of America.