Fault plane reflectivity along subduction thrusts: What does it really tell us?

Abstract:

At subduction zones, variations in fault zone seismic reflectivity provide key information about in situ physical properties. Commonly, negative polarity reflectors, which result from large impedance contrasts across the fault interface, are taken to indicate anomalously high porosity and pore fluid pressures. However, this idea has not been explored rigorously in the context of realistic sediment loading paths and consolidation behavior.

We use a forward model to quantify the effects of stress state and drainage conditions on fault zone reflection coefficient (R), focused on case studies from the Nankai and Costa Rican margins. We define constitutive relationships between: (1) stress state (mean and differential stress) and porosity (Φ); and (2) P-wave velocity (V_p) and Φ using data from triaxial laboratory consolidation experiments conducted on core samples. One key aspect of our approach is the consideration of differing stress states in the overriding wedge, which is likely to have experienced conditions at or near thrust failure, vs. the subducting sediment section, which likely undergoes uniaxial consolidation, at least in the near trench region.

For Costa Rica, the full range of pore pressure from hydrostatic to lithostatic yields negative values of R. Both the observation of decreased reflection amplitude beyond ~45 km from the trench and reported P-wave interval velocities in the subducting section are most consistent with modest overpressures (λ<~0.7). Decreased reflection amplitude landward of this point does not require the onset of drainage. Similarly, for Nankai, a wide range of pore fluid pressure conditions lead to negative values of R, because increased mean effective stress in the wedge leads to decreased Φ and higher V_p relative to the footwall. Positive polarity arises only if the underthrusting sediment transitions from a state of uniaxial compression to thrust failure, in combination with the onset of drainage - as might accompany décollement downstepping and duplexing. Our results show that fault reflectivity and its variation along-strike and downdip are unlikely to arise simply from variations in pore pressure. Future analyses should consider the role of stress state and sediment deformation properties in order to extract quantitative information from these signals.