3D Lithospheric-scale thermal model of central and southern California

The relationship between the long-term strength of the lithosphere and seismic hazard has remained a fundamental, yet open question in geosciences. The lithosphere's long-term rheology controls its deformation patterns, playing a crucial role in understanding the spatial and temporal distribution of seismicity in a given region. One of the primary factors influencing the rheological state of the lithosphere is its thermal regime, which is strongly affected by the heterogeneous properties of both the crust and the lithospheric mantle, as well as by the three-dimensional interactions between deeper and shallower domains.

To explore how long-term off-fault rheology influences the spatial distribution of seismicity, we leverage extensive geophysical data from Central and Southern California, a region where the San Andreas Fault represents a significant seismic hazard. Previous thermal models of the area have not converged on a consistent thermal structure for the lithosphere, resulting in uncertainties in the rheological models based on them.

Our 3D thermal model is built using a data-integrative approach that incorporates recent tomographic models and a detailed, heterogeneous crustal architecture drawn from prior community efforts. Furthermore, our model fits the general pattern of observed surface heat flow in the region.  The lower boundary condition in our 3D model -temperature at 70 km depth - is based on an integrated geophysical – petrological inversion within a self-consistent thermodynamic formalism of Rayleigh and Love surface-wave dispersion curves (0.5 x 0.5 degree lateral resolution), supplemented by other geophysical data and models: satellite data, surface heat flow and average temperature, topography, Moho depth, P-wave seismic crustal velocities, and sedimentary thickness.

Notably, our model is consistent with major regional tectonic features, such as the fossil Monterey microplate slab, which is responsible for the well-known high-velocity Isabella Anomaly. We discuss the implications of this anomaly, focusing on the dehydration of the slab and its potential role in seismogenesis, especially in the creeping section of the San Andreas Fault near Parkfield.