Adjoint tomography of the southern California crust

Using an inversion strategy based on adjoint methods, we developed a three-dimensional seismological model of the southern California crust. The resulting model involved 16 tomographic iterations, which required 6800 wavefield simulations and a total of 0.8 million central processing unit hours. The new crustal model reveals strong heterogeneity, including local changes of +/-30% with respect to the initial three-dimensional model provided by the Southern California Earthquake Center. The model illuminates shallow features such as sedimentary basins and compositional contrasts across faults. It also reveals crustal features at depth that aid in the tectonic reconstruction of southern California, such as subduction-captured oceanic crustal fragments. The new model enables more realistic and accurate assessments of seismic hazard.

A Scattered-Wave Image of Subduction Beneath the Transverse Ranges

Over 5600 short-period recordings of teleseismic events were used to create deterministic maps of P-wave scatterers in the upper mantle beneath Southern California. Between depths of 50 and 200 kilometers, the southern flank of the slab subducting beneath the Transverse Ranges is marked by strong scattering. The marked scattering indicates that the edge of the slab is a sharp thermal boundary. Such a boundary could be produced by slab shearing or small-scale convection in the surrounding mantle. The northern limb of the slab is not a strong scatterer, consistent with thicker lithosphere north of the Transverse Ranges.

Small-Scale Convective Instability and Upper Mantle Viscosity Under California

Thermal calculations and convection analysis, constrained by seismic tomography results, suggest that a small-scale convective instability developed in the upper 200 kilometers of the mantle under California after the upwelling and cooling of asthenosphere into the slab window associated with the formation of the San Andreas transform boundary. The upper bound for the upper mantle viscosity in the slab window, 5 x 10(19) pascal seconds, is similar to independent estimates for the asthenosphere beneath young oceanic and tectonically active continental regions. These model calculations suggest that many tectonically active continental regions characterized by low upper mantle seismic velocities may be affected by time-dependent small-scale convection that can generate localized areas of uplift and subsidence.