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Shallow subduction zone earthquakes and their tsunamigenic potential

Polet, J. and Kanamori, Hiroo (2000) Shallow subduction zone earthquakes and their tsunamigenic potential. Geophysical Journal International, 142 (3). pp. 684-702. ISSN 0956-540X. doi:10.1046/j.1365-246x.2000.00205.x.

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We have examined the source spectra of all shallow subduction zone earthquakes from 1992 to 1996 with moment magnitude 7.0 or greater, as well as some other interesting events, in the period range 1-20 s, by computing moment rate functions of teleseismic P waves. After comparing the source spectral characteristics of ‘tsunami earthquakes’ (earthquakes that are followed by tsunamis greater than would be expected from their moment magnitude) with regular events, we identified a subclass of this group: ‘slow tsunami earthquakes’. This subclass consists of the 1992 Nicaragua, the 1994 Java and the February 1996 Peru earthquakes. We found that these events have an anomalously low energy release in the 1-20 s frequency band with respect to their moment magnitude, although their spectral drop-off is comparable to those of the other earthquakes. From an investigation of the centroid and body wave locations, it appears that most earthquakes in this study conformed to a simple model in which the earthquake nucleates in a zone of compacted and dehydrated sediments and ruptures up-dip until the stable sliding friction regime of unconsolidated sediments stops the propagation. Sediment-starved trenches, e.g. near Jalisco, can produce very shallow slip, because the fault material supports unstable sliding. The slow tsunami earthquakes also ruptured up-dip; however, their centroid is located unusually close to the trench axis. The subduction zones in which these events occurred all have a small accretionary prism and a thin layer of subducting sediment. Ocean surveys show that in these regions the ocean floor close to the trench is highly faulted. We suggest that the horst-and-graben structure of a rough subducting oceanic plate will cause contact zones with the overriding plate, making shallow earthquake nucleation and up-dip propagation to the ocean floor possible. The rupture partly propagates in sediments, making the earthquake source process slow. Two factors have to be considered in the high tsunami-generating potential of these events. First, the slip propagates to shallow depths in low-rigidity material, causing great deformation and displacement of a large volume of water. Second, the measured seismic moment may not represent the true earthquake displacement, because the elastic constants of the source region are not taken into account in the standard CMT determination.

Item Type:Article
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URLURL TypeDescription DOIArticle
Kanamori, Hiroo0000-0001-8219-9428
Additional Information:© 2000 RAS. Accepted 2000 March 3. Received 2000 March 3; in original form 1999 August 16. We would like to thank Dr Emil Okal, an anonymous reviewer and Dr Steven Ward for their helpful comments and suggestions for improvements. This research was partially supported by US Geological Survey grant 99HQGR0035 and NSF grant EAR-9909352. Contribution #8667, Division of Geological and Planetary Sciences, California Institute of Technology.
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Subject Keywords:earthquake; subduction; tsunami
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Caltech Division of Geological and Planetary Sciences8667
Issue or Number:3
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Official Citation:J. Polet and H. Kanamori Shallow subduction zone earthquakes and their tsunamigenic potential Geophys. J. Int. (2000) 142 (3): 684-702 doi:10.1046/j.1365-246x.2000.00205.x
Usage Policy:No commercial reproduction, distribution, display or performance rights in this work are provided.
ID Code:51535
Deposited By: Tony Diaz
Deposited On:11 Nov 2014 00:15
Last Modified:10 Nov 2021 19:12

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