ON THE BEHAVIOR OF FAULTS AND FAULT-SYSTEMS: NEW INSIGHTS FROM EARTHQUAKE GEOLOGY (Invited Presentation)

The mechanical controls on earthquake recurrence in time and space remain poorly understood. Indeed, recent work suggests that the occurrence of individual ruptures may be governed by fundamentally chaotic underlying mechanisms. But geologic data, particularly an ever-increasing number of incremental fault slip rates, are beginning to reveal basic constraints on both the behavior of individual faults and entire fault systems, thus providing insights into the possible mechanics controlling these behaviors. For example, comparison of the incremental slip-rate behavior of isolated faults with that of faults embedded within complex, integrated systems of mechanically complementary faults reveals that the steadiness of fault slip is largely controlled by the degree of structural complexity of the surrounding plate-boundary fault system. Extending this further, comparison of incremental slip-rate data with geodetically constrained slip-deficit rate estimates reveals that elastic strain accumulation rates remain constant on isolated faults but vary through time on faults within complex fault networks. These data demonstrate that wherever a mechanically complementary alternative fault exists, the system will “trade-off” slip between the various faults that comprise the system. This is illustrated well by the first comprehensive analysis of an entire plate-boundary fault system, specifically the Marlborough fault system in New Zealand, which reveals that, while the overall plate-boundary system-level rate remains constant, the slip rates of individual faults vary, with alternating periods of fast and slow slip. Moreover, where data from strike-slip faults are sufficiently detailed, all of the studied faults exhibit periods of fast slip that accommodate ~20-30 m of fault slip, suggesting that the mechanical controls on fault slip operate at these displacement scales. A comprehensive review of possible mechanisms in the brittle-ductile transition region that might control these behaviors reveals a small subset of possible mechanisms, providing additional focus to better understand these mechanisms and the roles they play in controlling patterns of fault displacement through time. Taken together, these promising observations point to the need to acquire many more such data to further narrow the range of possible controls on fault behavior.