Earthquakes occur by overcoming fault friction, and therefore quantifying the resistance of faults is central to earthquake physics. Both static and dynamic friction values are required, and the latter is especially difficult to determine on natural faults. Perhaps the only way to solve the problem is to drill into a fault after an extremely large earthquake. The Japan Trench Fast Drilling Project (JFAST) did just this after the 2011 Mw 9.1 Tohoku earthquake and showed that the shear strength during the earthquake was substantially below that predicted by Byerlee’s law, which was a touchstone of earthquake physics for a generation. The very low dynamic stress implies a complete reset of the earthquake cycle. The JFAST data also reopens the conundrum of why earthquakes have depth-independent stress drops that are smaller than would be anticipated if friction drops completely. We recreate the phenomenon in the laboratory by imaging ruptures in a transparent, analog model. Surprisingly, the laboratory stress drops are like the natural ones and independent of normal stress apparently due to self-organization of the stress field in a confined rupture.
Speaker: Emily Brodsky, UC Santa Cruz
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