Walking the seismogenic zone: A field geology perspective on earthquakes

Abstract:

Why do we have an earthquake cycle? The simple answer is that faults lock up and accumulate strain until stressed to the point of failure, but this encapsulation neglects the rich slip behaviors that have been documented along faults in recent years. These behaviors can be described, but not explained, by simplified frictional or viscous (or hybrid) models which often overlap in their ability to reproduce strain events as observed from Earth’s surface. Explanations require understanding changes in transient conditions, like shear stress and pore pressure, and changes in the fault rocks themselves, such as in mineralogy or microstructure. There is no way to directly observe the rocks or the conditions at the depths that control earthquake behavior, but a bounty of ancient fault rocks are exposed at the Earth’s surface. Some exhumed faults are intact and preserved in the local tectonic context in which they evolved, and some fault rocks, often from deeper depths, became separated from the structures that formed them during their journey to the surface, for example, in a mélange. Exposed ancient faults contain some records of transient conditions, such as coseismic temperature spikes in narrow slip zones, and metasomatic aureoles recording the passage of reactive fluids. They also contain the only records of persistent changes in the rocks, such as mineral orientation patterns and anastomosing slip surface networks, which control deformation behavior in faults and shear zones. This library of Earth history is presently under-utilized as a powerful tool for understanding the seismic cycle, and more tools are needed to read these records and translate their messages to the greater earthquake science community.

I will describe both general and unique observations from a variety of faults, outline what I see as the cutting edge of earthquake geology, and suggest promising directions for future linkages between observational geology and other disciplines in earthquake science.