HR: 08:30h
AN: T31A-03    [Abstracts]
TI: Seismogenic characteristics of the Mariana shallow thrust zone: What controls plate coupling?
AU: * Wiens, D A
EM: doug@wustl.edu
AF: Dept. of Earth and Planetary Sciences, Washington University, 1 Brookings Dr., St. Louis, MO 63130, United States
AU: Emry, E
EM: ericae@seismo.wustl.edu
AF: Dept. of Earth and Planetary Sciences, Washington University, 1 Brookings Dr., St. Louis, MO 63130, United States
AU: Shiobara, H
EM: shio@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, Tokyo, Japan
AU: Sugioka, H
EM: hikari@jamstec.go.jp
AF: IFREE, JAMSTEC, Yokosuka, JPN ,
AB: The Mariana arc has been the type example of an "aseismic" or "decoupled" subduction zone since the concept was first proposed by Uyeda and Kanamori [1979]. Most past studies of the seismogenic zone characteristics have focused on "coupled" subduction zones that show large thrust earthquakes. Comparative studies of "decoupled" zones are essential for understanding the factors controlling the occurrence of large earthquakes. We use land and ocean bottom seismograph records from the 2003-2004 Mariana Subduction Factory Imaging Experiment to study the seismicity of the Mariana forearc. This 11 month experiment consisted of 20 broadband stations deployed on islands and 58 semi-broadband ocean bottom seismographs (OBS), with 14 OBSs located in the forearc. Events were detected and arrival times picked with Antelope package and relocated with a hypocentroidal decomposition relative location method. Overall, we have located ~ 500 events in the Mariana forearc between 16-19 N with mb 2 - 5. We determined the focal mechanisms of the largest events using a regional waveform inversion method. We have recently also detected episodic tremor that may be similar to that found at Cascadia, Japan, and several other subduction zones. The results indicate a lack of significant seismicity in the outer forearc beneath Big Blue and Celestial serpentinite seamounts. A few well-located events suggest that the shallow thrust zone is located at a depth of 15-25 km beneath the seamounts. Patches of high seismicity in the shallow thrust zone are located to the west of the seamounts at depths of 30-40 km. The patches are approximately 20 km in diameter and suggest heterogeneous faulting properties along the shallow thrust zone. Another zone of earthquakes at depths of about 60 km initiate beneath the seamounts and extend towards the west, representing faulting within the subducting plate. These events indicate that the lower zone of the Mariana double seismic zone initiates in the forearc and extends continuously to intermediate depths. We also detect episodic tremor that may be radiating from the downgoing plate interface deeper than the zones of high seismicity. We suggest that the relatively aseismic character of the outer forearc of the Mariana islands results from large-scale serpentinization of the outermost mantle wedge, as serpentinite is characterized by stable sliding rather then stick-slip behavior. In addition, the lack of large subduction zone thrust earthquakes may also result from the extreme heterogeneity of the shallow thrust interface, as revealed by the patches of high seismicity and aseismicity at depths of 30-40 km.
DE: 1207 Transient deformation (6924, 7230, 7240)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8118 Dynamics and mechanics of faulting (8004)
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Joint Assembly