Differences in Vp and Vs at a seismogenic subduction interface: application to fluid pressure estimation by AVO analysis with Nankai seismic profile
Abstract
Fluid pressure along subduction plate boundaries is thought to play a role in seismogenesis and tsunami genesis because it is strongly related to physical properties of faults. The fluid pressure at on-set of "seismogenic zone" (in transition zone) along plate boundary can be a clue to understand the mechanisms of rupture propagation to shallow outer wedge due to fluid pressure. Many studies have been done to estimate fluid pressure along subduction interface combining physical properties of sediments and seismic profiles. Those studies focused on shallower part of decollement or megaspray fault, not deeper seismogenic subduction interface. In this study, we have conducted Vp and Vs measurements for hanging wall and footwall of a fossil subduction interface with pseudotachylyte located at the northern edge of the Mugi mélange, Cretaceous Shiamnto Belt, Shikoku, SW Japan. Differences in the velocities between hanging wall and footwall were applied to estimate fluid pressure at the seismogenic fault by Amplitude Variation with Offset (AVO) analysis on Nankai seismic profiles. Mugi mélange is composed of tectonic mélanges comprised mainly by shale matrices and sandstone blocks. Temperature and pressure are ~180°C and ~135MPa, respectively, on the basis of fluid inclusion analysis. A fossil seismogenic fault with pseudotachylyte is observed at the northern most boundary of Mugi mélange, juxtaposing with a coherent unit including mainly sandstones in the north. We have collected 5 sandstones from hanging wall, 4 mudstones from footwall; total number of samples is 14. Samples were formed into cylindrical shape; 1.5 inches in diameter and ~6cm long. Direction of measurements is perpendicular to foliations. We controlled both fluid and confining pressures using two pumps. Effective pressure in each experiment ranges form 5MPa to 65MPa with 5 MPa intervals. We used 500kHz of S-wave transducer (PZT) as a source-receiver pair. Because it also generates weak p-wave, Vp and Vs are obtained from the single measurement. Vp and Vs of sandstones increase exponentially with effective pressure from about 4500m/s to about 5000m/s and from about 2500m/s to about 3000m/s, respectively. Vp and Vs of mudstones also increase exponentially from about 4100m/s to about 4500m/s and from about 1900m/s to about 2200m/s, respectively. We used a seismic profile of Nankai trough off Muroto (Park et al., 2001) for AVO analysis. The reflector of decollement from 40-45km landward from trench axis was taken for the analysis where is at the transition zone with about 4.5km in depth corresponding to about 64MPa of effective pressure under hydrostatic condition. Because sandstone in the hanging wall is lack of mineral veins and Mugi mélanges in footwall includes much amount of mineral veins along minor faults, we assumed hydrostatic conditions for hanging wall and gave variations in fluid pressure only in footwall. Combining the velocities obtained from this study and AVO analysis on the Moroto profile, about 17MPa of effective pressure was estimated as a maximum effective pressure. The result indicates that abnormal fluid pressure ratio is about 0.74 along decollement at the depth.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2011
- Bibcode:
- 2011AGUFM.T14B..08H
- Keywords:
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- 5102 PHYSICAL PROPERTIES OF ROCKS / Acoustic properties;
- 7240 SEISMOLOGY / Subduction zones;
- 8045 STRUCTURAL GEOLOGY / Role of fluids;
- 8170 TECTONOPHYSICS / Subduction zone processes