Title: The strength of subduction megathrusts and implications to earthquake processes

Abstract:
The strength of subduction megathrusts and how it varies during slip are important to earthquake physics. Actual megathrusts feature multi-scale heterogeneity, but fundamental understanding can be gained by focusing on the big picture and representative end members. This seminar provides a big-picture sketch of what has been learned over the past three decades. An ideal creeping fault has only one strength, but an ideal stick-slip fault has three strengths: the true strength, apparent strength, and dynamic strength. At a regional scale, there are only two useful methods to determine megathrust strength: the study of force balance in which gravity provides a reference and the study of thermal fields in which frictional heat dissipation reflects fault strength. Global applications of these methods yield effective friction coefficients of megathrusts in the range 0.03 – 0.13, much lower than the value of about 0.4 predicted by Byerlee’s law with hydrostatic fluid pressure. Despite this remarkable weakness, stress drop in megathrust earthquakes generally is only 10 – 30% of the absolute fault strength. Seismogenesis depends not on the absolute strength but how the fault weakens or strengthens during slip. Thirty years ago, updip and downdip limits of the megathrust seismogenic zone were proposed on the basis of thermal controls on weakening or strengthening behaviour. Today, new understanding of fault mechanics and fault gouge petrology and rheology requires the development of a new conceptual framework. I will explain why and how the previously proposed limits should be changed to “soft barriers” to seismic slip.

Title: Coupling fault slip and pore pressure evolution in dynamic rupture and earthquake sequence models

Abstract:

Pore fluid pressure plays a crucial role in fluid-infiltrated fault strength evolution hence the source processes of earthquakes and episodic slow slip events. Pore pressure in the fault zone increases due to strong shear heating during rapid slip, leading to a thermal pressurization dynamic weakening effect which favors a larger extent of rupture propagation and higher amount of coseismic slip. On the other hand, accelerated slip rate within the fault zone of highly compacted granular materials can also lead to a dilatancy strengthening effect which temporally reduces pore pressure hence clamps the fault before pore pressure re-equilibrates with the ambient level. In this presentation, I will discuss numerical simulations that couple the pore fluid pressure and fault slip evolution in the framework of the laboratory-derived rate-state friction law, with applications to earthquake ruptures and slow slip sequences in subduction zones, oceanic transform faults, as well as the fluid-injection induced seismicity environments. In particular, strong dilatancy can effectively inhibit seismic slip in frictionally unstable (velocity-weakening) regions, resulting in aseismic slip transients which not only serve as a rupture barrier to magnitude 6 earthquakes but may also drive episodic seismic swarms as observed along the East Pacific Rise transform faults. Preliminary results from our dynamic rupture model, with application to the 2008 Wenchuan magnitude 7.9 earthquake, also indicate that 3D fault geometry has a first-order control of the general distribution of coseismic slip whereas thermal pressurization influences the quantitative comparison to the near-field peak ground motion and cumulative slip.