HR: 1340h
AN: T53A-1114    [Abstracts]
TI: Observations of Shallow Thrust Seismicity in the Northern Mariana Subduction Zone From Broadband Ocean-bottom and Land Seismometers
AU: * Emry, E L
EM: ericae@seismo.wustl.edu
AF: Department of Earth and Planetary Sciences, Washington University in St. Louis, 1 Brookings Drive, St. Louis, MO 63130-4899, United States
AU: Wiens, D A
EM: doug@seismo.wustl.edu
AF: Department of Earth and Planetary Sciences, Washington University in St. Louis, 1 Brookings Drive, St. Louis, MO 63130-4899, United States
AU: Shore, P J
EM: patrick@seismo.wustl.edu
AF: Department of Earth and Planetary Sciences, Washington University in St. Louis, 1 Brookings Drive, St. Louis, MO 63130-4899, United States
AU: Shiobara, H
EM: shio@eri.u-tokyo.ac.jp
AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113- 0032, Japan
AU: Sugioka, H
EM: hikari@jamstec.go.jp
AF: IREE, JAMSTEC, 2-15 Natsushima-Cho, Yokosuka, 237-0061, Japan
AB: The Mariana arc is considered the type `aseismic' or `decoupled' subduction zone, since the concept was first proposed by Uyeda and Kanamori [1979]. The historical seismic record going back to about 110 years shows that the largest events in the Northern and Central Mariana were Ms 7.4 and 7.1 events in 1902 and 1934 respectively, but the locations and depths are not well constrained and it is thus unclear whether they represent shallow thrust faulting events. Recent large (Mw > 7.0) earthquakes are confined to the southernmost arc. Thus the largest confirmed shallow thrust faulting event along a 1000 km section of the Central and Northern Mariana is Mw 6.4, a remarkable seismic deficit considering the convergence rate of 4 cm/yr. We study the seismicity of the Mariana forearc using records from the 2003-2004 Mariana Subduction Factory Imaging Experiment to explore the mode of seismic release on the shallow thrust zone. The deployment of 20 broadband land stations and 58 semi-broadband ocean-bottom seismometers (OBS) ran for 11 months. Locations obtained using the genloc algorithm within the Antelope software package and further refined using a local velocity model from Takahashi, et al [2007] indicate sparse seismicity within regions immediately beneath the Big Blue and Celestial serpentinite seamounts, but do show dense clusters of seismicity at depths of 25-50 km immediately to the west of these seamounts. Focal mechanisms of the largest events occurring in these clusters were studied using a regional waveform inversion method and suggest the events occur along the shallow thrust zone. A preliminary comparison of microearthquake seismicity rates to the CMT record shows a disparity between occurrence of teleseismic events and smaller events recorded by our deployment, suggesting a small b-value along the shallower part of the thrust zone and a larger b-value along the deeper part. Observations of low microseismicity at shallow depths (0-25 km) and prolific microseismicity at deeper depths have been interpreted as evidence of a locked shallow subduction interface for several other subduction zones such as Costa Rica, where it is consistent with geodetic data. However, it is not clear whether a similar interpretation is feasible in the Mariana Islands considering the lack of historical seismicity.
DE: 7215 Earthquake source observations (1240)
DE: 7230 Seismicity and tectonics (1207, 1217, 1240, 1242)
DE: 7240 Subduction zones (1207, 1219, 1240)
DE: 8123 Dynamics: seismotectonics
DE: 8170 Subduction zone processes (1031, 3060, 3613, 8413)
SC: Tectonophysics [T]
MN: 2007 Fall Meeting