Characteristics of Deep-Focus Earthquakes in the Northwestern Pacific
Wu, Li-Ru
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https://hdl.handle.net/2142/86554
Description
Title
Characteristics of Deep-Focus Earthquakes in the Northwestern Pacific
Author(s)
Wu, Li-Ru
Issue Date
1999
Doctoral Committee Chair(s)
Chen, Wang-Ping
Department of Study
Geology
Discipline
Geology
Degree Granting Institution
University of Illinois at Urbana-Champaign
Degree Name
Ph.D.
Degree Level
Dissertation
Keyword(s)
Geology
Language
eng
Abstract
Characteristics of two important deep earthquake sequences are studied: (i) the September 29, 1973 Japan Sea earthquake (Mw 7.8, depth 575 km)---the largest deep earthquake in the entire western Pacific, and (ii) the August 23, 1995 Mariana earthquake sequence ( Mw 7.0, depth 595 km), noted for its anomalously high aftershock activity. Rupture properties of the 1973 Japan event are precisely determined through the inversion of body waveforms. This event is shown to have occurred along a sub-horizontal fault, over a dimension of 55 +/- 10 km by 25 +/- 10 km in the north-south and in the east-west direction, respectively. When combined with a precisely determined geometry of the slab, the rupture has a small extent of only 15 +/- 6 km across the slab's thickness. This configuration of the seismogenic zone removes the problem of fitting a large vertical fault in a shallowly dipping slab faced by previous researchers. Unlike some large deep earthquakes in South America, the entire seismogenic zone can in fact be placed within the coldest core of the slab. The distribution of slip during this earthquake can be explained by a circular rupture, propagating at a normal speed of 3--4 km/s. These findings are then discussed in the context of ruptures during other large deep earthquakes. Seismic moment released by the aftershocks during the 1995 Mariana sequence was found to amount up to 17% of the mainshock's moment. Such a high aftershock productivity is extremely unusual even for shallow earthquakes. Based on the inversion of broadband P and SH waveforms, however, source mechanisms of this sequence show no unusual features. By comparing the productivity of aftershocks among the 33 largest deep earthquakes on a global scale, it is proposed that anomalously high level of aftershock activity in the Mariana is linked to the unique configuration of the slab near the base of the transition zone. Overall, it is established that deep earthquakes have definitive characteristics in rupture dimensions and in aftershock productivity. These characteristics, in turn, form a basis for constraining physical mechanisms of deep earthquakes.
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