HR: 1340h
AN: T53A-1405    [Abstracts]
TI: Seismicity of the Shallow Thrust Zone and Celestial and Big Blue Serpentinite Seamounts in the Northern Mariana Arc From Ocean Bottom Seismographs
AU: * Emry, E L
EM: ericae@seismo.wustl.edu
AF: Department of Earth and Planetary Sciences, Washington University in St. Louis, Campus Box 1169, 1 Brookings Drive, St. Louis, MO 63130-4899 United States
AU: Wiens, D A
T53A-1405 AF: Department of Earth and Planetary Sciences, Washington University in St. Louis, Campus Box 1169, 1 Brookings Drive, St. Louis, MO 63130-4899 United States
AU: Shore, P J
T53A-1405 AF: Department of Earth and Planetary Sciences, Washington University in St. Louis, Campus Box 1169, 1 Brookings Drive, St. Louis, MO 63130-4899 United States
AU: Shiobara, H
T53A-1405 AF: Earthquake Research Institute, University of Tokyo, 1-1-1 Yayoi, Bunkyo-ku, Tokyo, 113-0032 Japan
AU: Sugioka, H
T53A-1405 AF: IFREE, JAMSTEC, 2-15 Natsushima-Cho, Yokosuka, 237-0061 Japan
AB: The Northern Mariana forearc is the only arc with large serpentinite seamounts and is also unusual in that it lacks large (Mw > 7) shallow thrust zone earthquakes. During June, 2003 to May, 2004, three ocean bottom seismometers (OBS) were located in triangular arrays around both Big Blue and Celestial seamounts. These ocean bottom seismographs were used along with approximately forty other OBSs and twenty land stations deployed throughout the rest of the Mariana arc to study seismicity associated with the seamounts as well as the seismicity of the shallow thrust zone and subducting plate beneath the seamounts. All six of the OBSs at the seamounts operated successfully during the first 50 days of the deployment, providing high resolution earthquake locations during this time period. The results indicate a lack of significant seismicity within or directly beneath the seamounts, and suggest that the seamounts are located above a relatively aseismic portion of the subduction zone. A few well-located events in the vicinity of the seamounts suggest that the shallow thrust zone is located at a depth of 15-25 km beneath the seamounts. Patches of extremely high seismicity in the shallow thrust zone are located to the west of both seamounts at depths of 30-40 km. The patches are approximately 20 km in width and suggest extremely heterogeneous faulting properties along the shallow thrust zone. Another zone of earthquakes at depths of about 60 km initiate immediately to the west of the seamounts and extend towards the west, representing faulting within the down-going plate. This zone of seismicity appears to be continuous with the lower zone of the Mariana double seismic zone. 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 results 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: 7240 Subduction zones (1207, 1219, 1240)
SC: Tectonophysics [T]
MN: Fall Meeting 2005