Thin creme brulee rheological structure for the Eastern California Shear Zone

Since the occurrence of the 1992 CE Mw 7.3 Landers and 1999 Mw 7.1 Hector Mine earthquakes in the Mojave Desert (California, USA), postseismic deformation following both earthquakes has been intensively studied, and models with a strong crust overlying a low-viscosity mantle asthenosphere have been favored. However, we recently found that the near-field postseismic transients after the two earthquakes have lasted longer than previously thought, which requires a revision of the postseismic modeling. Our new modeling results based on the revised postseismic transients show that: (1) the effective viscosity of the lower crust beneath the Mojave region at the decadal time scale is similar to 2 x 10(20) Pa.s (transient viscosity similar to 2 x 10(19) Pa.s), i.e., only similar to 5 times that of the underlying mantle asthenosphere, and (2) the transient viscosity of the upper mantle exhibits a time-dependent increase, providing fresh geodetic evidence for frequency-dependent rheology (e.g., Andrade or extended Burgers rheology). The inferred transient rheology for the first year agrees well with that obtained for the July 2019 Mw 6.4 and Mw 7.1 Ridgecrest earthquakes similar to 180 km north of the two Mojave events. Our modeling results support a thin creme brulee model for the Eastern California Shear Zone (part of the Pacific-North America plate boundary) in which both the lower crust and the upper mantle exhibit ductility at decadal time scales.

Automated summary

Research on postseismic deformation after the 1992 Landers and 1999 Hector Mine earthquakes in the Mojave Desert has shown that the near-field postseismic transients lasted longer than expected, leading to a revision of postseismic modeling. New models suggest that the lower crust in the Mojave region has a similar effective viscosity to the underlying mantle asthenosphere, and the transient viscosity of the upper mantle increases over time, supporting frequency-dependent rheology theories. These findings also align with rheological characteristics observed in the 2019 Ridgecrest earthquakes.