Regional Stress and Faulting Patterns in the Epicentral Area of the 2019 Ridgecrest (Eastern California) Earthquakes

Abstract:

The 2019 Ridgecrest earthquake sequence ruptured a network of
high-angle conjugate (left- and right-lateral) faults. The nearly
perpendicular fault orientations invoked a number of interpretations,
including near-zero coefficient of friction, ductile shear zones below
the seismogenic upper crust, dynamic process zones around propagating
rupture fronts, and tectonic rotation. I use a combination of seismic,
geodetic, and geologic observations to show that the pattern of
high-angle faulting is ubiquitous in the Ridgecrest area, and is best
explained by rotation due to simple shear since the inception of the
Eastern California Shear Zone (ECSZ). Analysis of catalogs of
precisely relocated seismicity using machine learning reveals two
predominant orientations of active strike-slip faults, one
corresponding to faults that likely formed at optimal angles to the
principal compression axis ~5 Ma ago (and rotated to the present-day
geometries, as predicted by models informed by the geodetically
measured secular deformation), and another corresponding to faults
that are at present optimally oriented with respect to the maximum
compressive stress. This, along with evidence for the trans-tensional
nature of the present-day stress field, allows one to put unique
constraints on the effective strength of the brittle upper crust
within a nascent plate boundary such as the ECSZ. Combining
information about a finite fault geometry provided by interrogation of
the precisely located earthquake catalogs with information provided by
the earthquake focal mechanism catalogs can improve the accuracy of
inversions for the state of stress in the seismogenic zone.