iPlex Lunch fall-2017

The relationship between the internal structure of fault zones and the earthquake source is poorly understood. To investigate possible connections, I examine three active earthquake sequences in southern California and Japan (2010 Mw 7.3 El Mayor-Cucapah, 2016 Mw 5.2 Borrego Springs, and 2016 Mw 6.2 Tottori) that exhibit complex spatiotemporal patterns. A template matching algorithm is used to construct expanded high-resolution seismicity catalogs to characterize the geometry of the fault systems at depth. The precisely located seismicity delineates highly-complex active fault structures consisting of orthogonal or subparallel faults with frequent segmentation and branching. Slip models are derived for the Borrego Springs and Tottori mainshocks, along with the seismic energy, stress drop, and radiation efficiency. These events occurred in complex immature fault zones and are found to have very low radiation efficiency values; this suggests that most of the available strain energy was dissipated as off-fault brittle fracturing. More than 155,000 aftershocks of El Mayor-Cucapah in the Yuha Desert and postseismic geodetic strain reveal two distinct processes driving the evolution of the sequence. Early aftershocks spread away from the mainshock rupture terminus with the logarithm of time, and were likely driven by afterslip. A second wave of aftershocks swept back across the region with the square-root of time and swarm-like characteristics, and were likely driven by fluid diffusion. The discussed studies provide links between properties of fault zone structures, earthquake source processes, and driving mechanisms behind aftershock production.