Spatial Distribution of Pore Pressure and Effective Stress at the Western Nankai Trough Plate Interface Based on 3D Seismic Reflection Data
Abstract
Pore fluid pressure modulates stress on faults and is a key parameter governing faulting mechanics, including seismic and tsunamigenic slip on subduction zone megathrusts, yet has been notoriously difficult to measure directly. However, seismic velocity can serve as a proxy for porosity, which in turn can be used to estimate pore pressure and vertical effective stress in clastic sediments undergoing consolidation. 3D pre-stack depth migration (PSDM) provides the best available measure of interval velocity from seismic reflection data. We use PSDM velocity extracted along a surface immediately beneath the decollement in a an 8 x 25 km portion of a 3D survey of the accretionary wedge at the Nankai Trough subduction zone off Shikoku Island, Japan. Using a transform locally calibrated by ODP boreholes, we first compute porosity at the base of the plate boundary decollement and the adjacent underlying sediments from the PSDM velocity to 20 km down-dip. We next calculate pore fluid pressure and effective vertical (fault-normal) stress for the top of the underthrust sediment section using a compaction disequilibrium approach and porosity data from drilling. Because the pore fluid pressure at the fault interface is likely controlled by that of the top of the underthrust sediment section, this calculation results in a quantitative profile of effective stress and pore pressure at the plate interface. Analysis of the uncertainties in the successive transforms suggests that the derived pressure and stress have a precision of approximately ± 10%. Results show that the fault and immediately adjacent footwall are nearly undrained as sediments subduct, and that the fault slips under a resolved shear stress of no more than ~ 5 MPa, or only about half the value expected if excess pore pressure played no role. Spatial heterogeneity in effective stress within the survey volume implies areas of relatively high and low strength.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2006
- Bibcode:
- 2006AGUFM.T12C..06T
- Keywords:
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- 8045 Role of fluids;
- 8104 Continental margins: convergent;
- 8118 Dynamics and mechanics of faulting (8004);
- 8163 Rheology and friction of fault zones (8034);
- 8170 Subduction zone processes (1031;
- 3060;
- 3613;
- 8413)