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

世界上有200万至1500万人口的城市中，有一半以上面临7级或以上地震的威胁。最近在海地、日本和尼泊尔发生的灾难性事件突出表明，发达国家和发展中国家都面临着高风险。对地球科学家来说，预测地震仍然是一个遥远的目标。其中一个原因是控制地质断层强度和行为的物理条件和过程是未知的。这些参数不能用地震学或地球物理方法测量。我的长期研究计划旨在通过检查岩石记录中的证据来调查断层强度和断层在地震中破裂的趋势。通过使用“化石地震”作为今天发生的地震的代表，我可以直接观察、描述和测量地震发生的断层的性质。突出的未知数包括：断层的绝对强度，强度非均质性的程度，以及主办地震的断层的性质如何控制破裂的特征。

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.