SEISMOTECTONICS, SEISMICITY, AND STRESS STATE OF THE RIDGECREST-COSO REGION FROM THE 1930S THROUGH 2019

Decadal scale variations in the seismicity rate in the northern part of the Eastern California Shear Zone, called the Ridgecrest-Coso region, included seismic quiescence from the 1930s to the early 1980s, followed by increased seismicity through to the 2019 M_w6.4 and M_w7.1 Ridgecrest sequence. This sequence exhibited complex rupture patterns on almost orthogonal faults and triggered aftershocks over an area of ~90 km by ~5 to ~10 km wide, which is only a fraction of the previously seismically active area. During the last 40 years, the seismicity has been predominantly strike-slip faulting extending north from the Garlock fault, along the Little Lake and Airport Lake fault zones, and approaching the southernmost Owens Valley fault to the north. The Coso Range forms an extensional step over between these two strike-slip fault systems. This evolution of a plate boundary zone is driven by the northwestward motion of the Sierra Nevada, and crustal extension along the southwestern edge of the Basin and Range Province. Stress inversion of focal mechanisms showed that the new stress state rotates across the step over with the Coso Range and adjacent areas to the north have s₁trending ~N17°E while along the M_w7.1 mainshock rupture the trend is ~N6°E. The friction angles as measured between fault strikes and the s₁trends correspond to a frictional coefficient of 0.75, suggesting average fault strength. In comparison, the mature Garlock fault has a smaller frictional coefficient of 0.28, similar to weak faults like the San Andreas fault. Thus it appears that the heterogeneously spatially distributed but strong Ridgecrest Coso faults that accommodate seismicity at seemingly random places and times within the region, are in the process of self-organizing and forming a major through going plate boundary segment.