Complexity Breeds Complexity: Heterogeneous Stress State Around the Southern Big Bend of the Southern San Andreas Fault, California

Session: Crustal Stress and Strain and Implications for Fault Interaction and Slip
Type: Oral
Date: 4/28/2020
Time: 11:00 AM
Room: 240
Description: 

Regions of complex 3D faulting, such as southern California, show spatial heterogeneity of crustal stress state inferred from focal mechanisms. While planar segments of the central San Andreas fault can produce relatively homogeneous interseismic loading of dextral shear, the stress state within the southern Big Bend shows 3D variability of interseismic loading that corresponds with complex active fault configuration. Mechanical models show that lateral variation in deep creep rate along the northern San Jacinto fault associated with transfer of strain to the San Andreas fault can account for normal slip focal mechanisms within the San Bernardino basin. These enigmatic focal mechanisms occur predominantly below 10 km, consistent with deep interseismic creep. Within the San Gorgonio Pass, reverse focal mechanisms are consistent with contraction between the southern and northern branches of the southern San Andreas fault. Because the crustal stress state is sensitive to changes in active fault geometry, we can use focal mechanism stress inversions to differentiate amongst interseismic stress states predicted by mechanical models of plausible fault configurations. Furthermore, we can estimate the cumulative present-day fault tractions from interseismic stressing rates using the time since last earthquake and co-seismic models of the impact of recent nearby earthquakes. The cumulative tractions along the faults differ significantly from the resolved tractions from regional loading in that the largest cumulative dextral traction is located within the restraining bend comprising the pass, which has not had recent earthquakes, rather than outside of the bend, which is more preferentially oriented under tectonic loading. Prescribing these tractions to the faults within a 3D mechanical model allows us to estimate the present-day off-fault crustal stresses. We compare these stresses to both focal mechanism slip sense and inferred stress drops. Regions of high compressive stress in the model correspond to regions of high (>20 MPa Goebel et al., 2015) observed stress drop.

Presenting Author: Michele L. Cooke

Authors