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Mori, J.J., 1984

Localized stress drops within the rupture areas of great thrust earthquakes: Importance to strong ground-motion

Bibliographic Reference

Mori, J.J., 1984, Localized stress drops within the rupture areas of great thrust earthquakes: Importance to strong ground-motion: New York City, New York, Columbia University, Teachers College, Ph.D. dissertation, 148 p.


This thesis investigates the range of stress drops that occur over relatively small areas with dimensions of a few hundred meters to a few kilometers in the subduction zone regions of Alaska and Japan. Using short-period teleseismic data and regionally recorded strong-motion data, stress drops of moderate earthquakes were calculated. Also sources of high-frequency radiation were identified within three great subduction zone earthquakes and associated with localized areas of high stress drop. A variety of estimates, including the static, dynamic and arms) stress drops were used, although the different formulations can yield different values depending on the complexity of the event. The large range of values from a few bars to over several kilobars, is interpreted as reflecting the differences in material strengths that fail in earthquakes and differences in dynamic friction during the rupture process. Compared to Western U.S. data, the accelerations data of subduction zones events appear to scatter over a much larger range, which may be attributed to a greater range of stress drops. Particularly important are the very high stress drops in the kilobar range observed for both moderate events and subevent within larger earthquakes. Stress drops of this magnitude may commonly occur over small areas during the rupture of great earthquakes. Information about the stress drop on this scale is crucial for estimating strong ground-motions, since the accelerations that are radiated in the frequency range that will cause damage to man-made structures will scale proportionately to the magnitude of this stress drop. Analysis of the 1968 Tokachi-Oki earthquake (Mw = 8.2) reveals two areas of very high localized stress drop in the kilobar range, which provides an explanation for the high acceleration levels associated with the earthquake. A surface wave inversion to determine the distribution of moment release on the fault plane of the Tokachi-Oki event showed that large amounts of 10 to 25 sec energy were released in the same areas as the high stress drop subevents.

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