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Estabrook, C.H., 1992

The rupture process of large earthquakes along convergent margins: Case studies from Alaska and Armenia

Bibliographic Reference

Estabrook, C.H., 1992, The rupture process of large earthquakes along convergent margins: Case studies from Alaska and Armenia: New York City, New York, Columbia University, Ph.D. dissertation, 261 p., illust., maps.


This thesis focuses on the varied nature of earthquake ruptures in regions of convergent tectonics. Several modern and historical earthquakes along the Alaska-Aleutian arc and in Armenia are investigated using body and surface waves. The 1979 St. Elias, Alaska earthquake is analyzed because of unresolved questions about its depth, focal mechanism, seismic moment and location in a seismic gap. Source mechanisms and locations of the two distinct mainshock subevents and aftershock locations define a shallow dipping surface at the eastern edge of the Pacific plate. The amount of strike-slip motion increases with time in the mainshock implying that a change from thrusting to strike-slip faulting occurs along the plate interface. Aftershock relocation suggests that the main plate boundary may be offset as a result of collision and subduction of the Yakutat terrane. Seismic waves from the 1988 Armenia earthquake are best fit with a source model that includes three subevents. Rupture apparently initiated as a shallow reverse fault at a point of maximum bending on a right-lateral strike-slip fault, and then extended bilaterally towards the southeast and west. The largest events along the Alaska Peninsula and Shumagin Islands segments of the Aleutian arc since 1917 are studied to understand the likely modes of rupture in the next sequence of large and great events and to delineate the plate interface geometry. Results from body and surface wave modeling suggest that the 1917 earthquake was smaller than previously thought and was restricted to the eastern-most portion of the gap. Seismic moment and gross rupture characteristics of the great 1938 earthquake along the Alaska Peninsula are estimated using teleseismic body and surface waves. Two sources, one near the mainshock epicenter, and the second, about 170 km to the northeast, best describe moment release during the mainshock. Rupture into the Shumagin region is not supported by surface wave data. Subduction of seamounts may account for nearly identical moments, focal mechanisms and depths of five magnitude 7 events along the Alaska Peninsula. The nearly uniform dip of the plate interface between the Alaska Peninsula and Shumagin segments implies that segmentation is not controlled by the plate interface orientation. Stress indicators collected for continental Alaska, the Aleutian Islands and surrounding areas display a direction of maximum horizontal compression that is generally consistent with Pacific/North America plate interaction along the southern margin of Alaska, with effects reaching into continental Alaska.

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