Oral Presentation

Natural faults have ubiquitous geometrical irregularities that produce high off-fault stresses, which generate rock damage (reduced elastic moduli) that modify properties of earthquake ruptures and seismic radiation, which modify in a feedback loop properties of fault zones and future earthquakes. Important features during single ruptures include: (i) Radiation of high frequency waves with significant isotropic component from zones sustaining rock damage, leading to strong changes of normal stress, pulse-type ruptures, reduction of frictional heat, and modification of P & S radiation patterns and energy ratio. (ii) The altered wavefields propagate (with attenuation) to the far field and affect seismic motion at large distances. (iii) Interactions of propagating ruptures with waves generated at edges of low velocity zones modify the rupture type, velocity, and extent.

Statistical results on evolving earthquakes and faults point to three general dynamic regimes. (i) Fault systems with a broad range of heterogeneities generate power law event statistics, spatio-temporal clustering of moderate to large events, and accelerated seismic release before large events. (ii) Relatively simple structures produce characteristic earthquake statistics and quasi-periodic large events without accelerated release. (iii) For a range of conditions, the evolving structures and response switch back and forth on a long timescale between the forgoing two dynamic regimes.

The temperature, fluid content, and thickness of sedimentary cover influence the seismic coupling and properties of event sequences. (i) Relatively cold regions with dense crystalline rocks have high seismic coupling, few foreshocks, and burst-type aftershocks dominated by a few vigorous generations. (ii) Relatively hot/high-fluid regions and thick sedimentary basins have low seismic coupling, increased foreshocks activity, and swarm-type sequences with many mild aftershock generations. (iii) Large mainshocks produce transient deepening of the brittle-ductile transition with early aftershocks in the nominally viscous lower crust.
Continuing research on coupled evolution of earthquakes, faults, and seismic motion has high potential for providing new insights and improved ability to forecast earthquakes and ground motion. Dense arrays of sensors crossing large fault sections will provide fundamental information for developing and testing next generation models of earthquakes, faults, and seismic ground motion.