The weakest link: new methods to measure the strength of geological faults

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More than half of the world’s cities with 2-15 million people are at risk from earthquakes of magnitude seven or greater. Recent devastating events in Haiti, Japan, and Nepal have highlighted that both developed and developing countries are exposed to high risks. Predicting earthquakes remains a distant goal for Earth scientists. One of the reasons is that the physical conditions and processes that control the strength and behavior of geological faults are unknown. These parameters cannot be measured with seismological or geophysical methods. My long-term research program aims to investigate fault strength and the tendency of faults to rupture in earthquakes by examining evidence from the rock record. By using ‘fossil earthquakes’ as representative of earthquakes that happen today, I can directly observe, characterize and measure the properties of faults where earthquakes initiated. Outstanding unknowns include: the absolute strength of faults, the degree of strength heterogeneity, and how the properties of the faults that host earthquakes control the characteristics of the ruptures. Over the next five years, my core research questions will be: 1) How strong is a fault? 2) How important are the geometric boundary conditions in controlling fault behavior? I will examine these questions by integrating digital field data with numerical and laboratory experiments. These methods are novel, and I will build on my previous experience with digital mapping technologies to develop a new experimental capability by utilizing 3-D printing. Results arising from this multidisciplinary approach are potentially transformative. Three testable hypotheses will form the focus of two PhD, one Masters and five undergraduate research projects: • The geometry of fault surfaces controls the strength and stability of faults; • The characteristics of earthquakes depend on fault geometry; • The strength of faults is recorded by structures preserved in deformed rocks. The results will be the first step towards my career goal of developing new methodologies for investigating rock mechanics in the field and ultimately bridging the divide between structural geology and seismology. The results will place constraints on the magnitudes of stresses that drive fault motions, which will address a long-standing problem in earthquake seismology and geodynamics. Additionally, they will establish for the first time how the geometry of faults influences fault stability, a key parameter in understanding the distribution of earthquakes globally. By addressing the fundamental controls on fault slip, this research will have significant national and international applicability to earthquake and tsunami hazard appreciation and risk mitigation.