HR: 1340h
AN: S53A-0184    [Abstracts]
TI: Outer Rise Stresses of the Tonga Subduction Zone Inferred from Earthquakes and 2D Models of Flexure
AU: * Shiro, B R
EM: shiro@wustl.edu
AF: Washington University, Campus Box 1169, One Brookings Drive, St. Louis, MO 63130 United States
AU: Wiens, D A
EM: doug@wustl.edu
AF: Washington University, Campus Box 1169, One Brookings Drive, St. Louis, MO 63130 United States
AU: Phillips, R J
EM: phillips@wustl.edu
AF: Washington University, Campus Box 1169, One Brookings Drive, St. Louis, MO 63130 United States
AB: We investigated outer rise intraplate stresses at the Tonga Trench using both observed earthquakes and flexure models. The depths and focal mechanisms for 21 intraplate outer rise earthquakes spanning 1987-2003 were determined using teleseismic P and SH body waveform inversion, and these were combined with 14 other prior earthquakes whose source parameters had been similarly determined. The 35 earthquakes were co-located using a joint-hypocenter decomposition technique in order to obtain precise relative locations with uncertainties generally less than 3 km. A well-defined pattern emerged with distinct groups of 13 shallow (10-26 km) tensional and 15 deeper (30-49 km) compressional events within an approximately 40 km thick seismic lithosphere. The approximately 10 km gap between these two groups provides an estimate of the lithosphere's elastic core thickness and constrains the depth of the neutral surface to about 28 km. Additionally, 3 deep (35-60 km) and shallow (14-30 km) tensional and compressional earthquakes occur greater than 75 and 100 km downdip and seaward of this zone, respectively. These imply that the plate is under inplane compression before it enters the zone of maximum bending and that it is fully under tension past it. We also discuss the temporal and spatial outer rise earthquake patterns and show that compressional and tensional events cluster into regions along the trench and tend not to occur in the same area at the same time. Flexure models of the Pacific Plate were computed using a 2D finite difference technique with a multilayered brittle-plastic rheology and plate cooling models defined by either constant- or depth-dependent thermodynamic parameters. In addition to modeling a general Tonga slab with age 90 Ma, we also separately investigated the areas north and south of the Capricorn Seamount. The parameters modeled included the bending moment, interplate coupling force, inplane force, and strain rate. The best fitting models to bathymetry and free air gravity profiles accurately predict the observed location of the neutral surface and seismic stress patterns. We find that the compressional seismicity observed in the lower lithosphere occurs within the deep plastic portion of the plate, suggesting either that the semi-brittle region extends quite deep within oceanic lithosphere or that some type of ductile faulting is taking place. The best fitting models require a compressional inplane force, which implies that slab pull is not a dominant force in this section of the subducting plate.
DE: 7230 Seismicity and seismotectonics
DE: 8120 Dynamics of lithosphere and mantle--general
DE: 8159 Rheology--crust and lithosphere
DE: 8164 Stresses--crust and lithosphere
DE: 7215 Earthquake parameters
SC: Seismology [S]
MN: 2004 AGU Fall Meeting