Seismic Tomography of Southern California based upon Adjoint Methods

Speaker: Jeroen Tromp

Adjoint tomography utilizes 3D simulations of seismic wave propagation in conjunction with a tomographic technique based on adjoint methods. We begin with an initial 3D model of shear and compressional wavespeeds for southern California provided by the Southern California Earthquake Center (SCEC; model CVM-H), extending to a depth of 60 km. We use the spectral-element method to simulate 140 good-quality local earthquakes, each recorded by as many as 160 stations. We compute misfits between observed and synthetic seismograms by using an automated time-window selection algorithm that picks any time window within which the data and 3D synthetics are reasonably similar (e.g., P, S, Love, and Rayleigh waves).

Within each time window we measure a frequency-dependent traveltime anomaly. For each record with a measurement, we compute an adjoint source that is used to create an adjoint wavefield. The interaction between the adjoint wavefield and the regular wavefield forms the gradient of the misfit function for one event. These gradients are combined using a source subspace projection method to compute a model update. This process is iterated using a conjugate gradient method.

We are presently on the tenth iteration of a southern California crust and upper mantle model. Thus far we have applied changes in excess of ±20% from the initial 3D model. With each iteration, the changes in wavespeeds have improved the data fit, and we are able to include additional seismograms whose fits to the data for previous model iterations were too poor for selection. In general, the tomographic results compare well with surface geology, the most striking features being the low wavespeeds of the southern San Joaquin basin associated with the Big Bend in the San Andreas Fault, the high wavespeeds and depth extent of the Santa Monica Mountains, the low wavespeeds in the Coast Ranges, the low avespeeds in the eastern Mojave shear zone, and the sharp contrast at the eastern front of the Sierra Nevada due to volcanism in the Coso Junction area. The new model is able to capture 6~s surface wave propagation along paths between Parkfield and the Salton Sea. Having applied relatively large-scale, large-amplitude changes, we can now exploit more of the shorter-period measurements to resolve and reveal km-scale features in the southern California crust.