Yuri Fialko is a Professor of Geophysics at the Institute of Geophysics and Planetary Physics. He got his PhD from Princeton University in 1998. Before joining SIO in 2000, he held an institutional postdoc position at California Institute of Technology. His research is focused on understanding physical and mechanical aspects of earthquakes, rock fracture, and magma migration in the Earth's crust. Prof. Fialko is combining observations from space-borne radar and global navigation satellites, numerical models, and laboratory experiments to investigate how the Earth’s crust responds to seismic, magmatic, and anthropogenic loading.
Strength of the upper brittle part of the Earth's lithosphere is one of the most debated and least constrained quantities in geophysics. Direct measurements of stresses acting at seismogenic depths are largely lacking. Seismic data (in particular, the earthquake focal mechanisms) have been used to infer orientations of the principal stress axes. I will discuss how the focal mechanism data can be combined with information from precise earthquake locations to place constraints not only on the orientation, but also on the magnitude of absolute stress at depth. The proposed method uses relative attitudes of conjugate faults to evaluate the amplitude and spatial heterogeneity of the deviatoric stress and frictional strength in the seismogenic zone. Relative fault orientations (dihedral angles) and sense of slip are determined using quasi-planar clusters of seismicity and their composite focal mechanisms. The observed distribution of dihedral angles between active conjugate faults in the area of Ridgecrest (California, USA) that hosted a sequence of strong earthquakes in 2019 suggests in situcoefficient of friction of 0.4-0.6, and depth-averaged shear stress of the order of 25-40 MPa, intermediate between predictions of the "strong" and “weak" fault theories.