Imaging the Fine Structure of Earthquakes and Faults with High-Precision Aftershocks

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The geometry and structure of active faults, breaks in the Earth's crust along which movement takes place during an earthquake, are believed to play a controlling part in how earthquakes start, how big they become, and how they stop. Only where faults break the Earth's surface can we study the zones of active faulting directly; the rest, and often more complex part of the fault is buried deep underground. This project studies the fine-scale details of the structure and geometry of fault zones at crustal depths using aftershocks of larger earthquakes in California. Using highly accurate locations of aftershocks along faults that just broke in big earthquakes, the internal makeup of fault zones can be imaged at the meter scale. This will enable detailed studies of fault features such as the thickness of damage zones and irregularities in the fault surface, which are essential for understanding how faults work and which factors control the evolution of rupture.The characterization of three-dimensional fault zone structure and its evolution is essential for understanding how faults work and which factors control the evolution of rupture. This project studies the ultra-fine, three-dimensional structure of active faults in California, how it evolves with fault maturity, and how it ultimately governs the initiation, propagation, and termination of earthquakes. Geological studies of fault outcrops have provided a generalized and highly simplified, one-dimensional picture of faults in which the fault core, including the principal slip surface(s), is surrounded by a damage zone of fractured rock. In three dimensions faults appear to be much more complex and a complete description of them, even an idealized one, does not exist. This project?s main goal is a systematic and comparative analysis of over 150,000 high-precision aftershock locations and additional parameters associated with 78 large (M ≥ 5.5) strike-slip and thrust earthquakes in California that have the resolution power to image the internal structure and properties of fault zones at the scale of tens of meters or better. These data are used to investigate the evolution of fault zone structure with fault maturity, and to measure features of fault maturity such as the evolution of the smoothness of the fault and growth (or shrinkage) of the damage zone with fault displacement. We expect this study to recognize common factors in the fine structure of these faults and the earthquakes that occurred on them as well as how these features change with fault maturity. Furthermore, fault segmentation, jogs, and step overs will be characterized and their role in the nucleation and arrest of rupture as well as in fault growth and interaction investigated.