Physically-Based Fault Zone Constitutive Responses and Consequences for Earthquake Dynamics

Harvard University, Cambridge MA

Investigators

Abstract

Physically based descriptions of fault constitutive response are being used to address key problems in the dynamics of earthquakes. There are two main topics: (1) In current attempts to devise models of crustal earthquake sequences, using experimentally motivated temperature variation (hence depth variation) of rate and state constitutive parameters, it has been noticed that important new features emerge as the state-evolution slip distance L is decreased towards values in the laboratory range. These are the emergence of a population of small events that is clustered towards the base of the seismogenic zone, and the effect of the resulting heterogeneous residual stress patterns from those events on the earliest phases of seismic radiation in large events. This process seems promising to explain the initially hesitant radiation in many large events, known as the "seismic nucleation phase". To fit such calculations on present computers, L must be made much larger than laboratory values, which are of order of magnitude 10 microns, since the required numerical grid size scales with a large factor (of order 10^5) times L. Yet interesting behavior is emerging as L is reduced towards those values. This project addresses the small L range by a combination of new numerical studies, coordinated with asymptotic analysis of simplified models, as L is decreased in size, to provide a basis for interpretation and extrapolation. (2) Thermal weakening effects are thought to occur during rapid slip in major earthquakes, causing the effective friction coefficient to diminish from lab-like values, present when a propagating rupture front first reaches a point on a fault, to much lower values when rapid and large slip occurs. This problem is being addressed by building on initial studies that focus separately on the earliest phases of sliding, before melting occurs, and on the mature stage of active pseudotachylyte development. Flash heating at asperity contacts is expected to be the primary thermal weakening process when slip rates are high (> 1 m/s) but total slip is still small, in a sense that can be quantified. A preliminary analysis captures some features of available experiments. Some ideas are being developed on how to address the much larger slip range when partial melting occurs. These include a view, supported by pseudotachylyte observations, that a granular fault gouge becomes liquefied through development of small amounts of partial melt, and that a self-regulated process of velocity-weakening character develops as this highly pressurized phase permeates into the adjoining fault walls. These concepts are being developed for purposes of integrating them with numerical simulations and theory on how the mode of rupture depends on constitutive response, to examine consequences for rupture dynamics. That will contribute to understanding how major fault systems can operate at realistically low overall driving stresses, even when the stress needed locally to initiate slip is much larger, and to quantifying the minimum average stress level for which a rupture, once initiated at a location of locally high shear stress or low effective normal stress, can propagate over large distances.