Experimental Constraints on Heterogeneous Mechanical Properties of Rocks and Faults at Subduction Zones and Numerical Investigations of Their Roles in Megathrust Earthquake Characteristics

Abstract

Subduction plate boundary faults are responsible for devastating earthquakes and tsunami. Over the past several decades, multiple offshore scientific drilling projects have advanced our understanding of fault and rock properties of subduction zones, and numerical simulations have unraveled earthquake processes at various scales. However, we have not constrained enough static and dynamic behaviors of the plate boundary fault.

I center my research on mechanical properties and constitutive bahaviors of rocks and faults in shallow seismogenic portions at subduction zone boundaries and their effects on rupture characteristics of megathrust earthquakes. Chapter 2 integrates results of rock deformation experiments and critical state soil mechanics. A suite of triaxial deformation experiments is conducted at different pressure conditions. We find that rocks at 2200 meters below seafloor reside in the brittle regime and that inner prism rocks become more brittle as depth increases.

Chapter 3 obtains the shear stress along the plate boundary fault of the Nankai Trough. State parameters required in the Coulomb wedge model are obtained based on the experimentally based relation between p-wave velocity and mechanical properties. The shear stress along the plate boundary fault of a heterogenous wedge is larger than that of a homogeneous wedge. We also find that the inner wedge hosts a larger sliding coefficient of friction than the outer wedge.

Chapter 4 quantifies roles of heterogeneous wall rock properties and fault frictional properties in rupture characteristics of subduction zone earthquakes using dynamic rupture modeling. We find that depth-varying fault friction dominates high-frequency depletion at shallow depth, while shallow low-velocity layers in the upper plate enhance high-frequency radiation. Slip may concentrate near the trench if the shallow portion is largely velocity weakening. Otherwise, the largest slip occurs at depth. Both velocity strengthening friction and low velocity upper-plate layers contribute to slow rupture near the trench.

The dissertation advances our knowledge of mechanical properties within an accretionary prism. This dissertation also quantifies the effects of wall rock properties and fault friction on rupture characteristics of megathrust earthquakes. These key findings can be readily applied to real-world subduction zones for hazard mitigation and reduction.