Stress and Fault Strength
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Jeanne Hardebeck
USGS
- Date & Time
- Location
- Hybrid In-Person and Online seminar via Microsoft Teams
- Summary
Stresses in the Earth’s crust are the driving forces of earthquakes. Yet they are not known (or at least agreed upon) to an order of magnitude. A high-stress model, assuming laboratory-based fault friction and hydrostatic pore pressure, predicts shear stress on faults at seismogenic depths to be on the order of 100 MPa. Alternatively, in a low-stress model, shear stress is of similar magnitude to earthquake stress drops, on the order of 10 MPa. Direct measurements of stress in scientific boreholes are limited to the upper few kms in plate boundary regions and therefore do not resolve the problem of stress at seismogenic depths. I will show that indirect measurements of stress orientations from inverting earthquake focal mechanisms generally support the low-stress model. The high-stress model often accounts for the lack of observed frictional heating by asserting that major faults like the San Andreas have low frictional strength and are poorly oriented in the local stress field with little resolved shear stress. In contrast, focal mechanism stress inversions show that major faults are generally more favorably oriented, except for some creeping faults such as the creeping section of the San Andreas and weakly-coupled subduction zones. Additionally, temporal stress rotations due to earthquakes can be used to make quantitative estimates of the level of stress. Significant coseismic stress rotations of 10° or more are observed for many crustal and subduction zone earthquakes, implying low shear stress on the order of 10s of MPa.