Adjoint analysis of the source and path sensitivities of basin-guided waves

Steven M. Day, Daniel Roten, & Kim B. Olsen

Published May 2012, SCEC Contribution #1535

Simulations of earthquake rupture on the southern San Andreas fault (SAF) reveal large amplifications in the San Gabriel and Los Angeles basins (SGB and LAB) apparently associated with long-range path effects. Geometrically similar excitation patterns can be recognized repeatedly in different SAF simulations (e.g., Love wave-like energy with predominant period around 4 seconds, channeled southwestwardly from the SGB into LAB), yet the amplitudes with which these distinctive wave-field patterns are excited change, depending upon source details (slip distribution, direction and velocity of rupture). We describe a method for rapid calculation of the sensitivity of such predicted wave-field features to perturbations of the source kinematics, using a time-reversed (adjoint) wave-field simulation. The calculations are analogous to those done in adjoint tomography, and the same time-reversed calculation also yields path-sensitivity kernels that give further insight into the excitation mechanism. For rupture on the southernmost 300 km of SAF, LAB excitation is greatest for slip concentrated between the northern Coachella Valley and the Transverse Ranges, propagating to the NE, and with rupture velocities between 3250 and 3500 m/s along that fault segment. That is within or slightly above the velocity range (between Rayleigh and S velocities) that is energetically precluded in the limit of a sharp rupture front, highlighting the potential value of imposing physical constraints (such as from spontaneous rupture models) on source parameterizations. LAB excitation is weak for rupture to the SW and for ruptures in either direction located north of the Transverse ranges, while Ventura Basin is preferentially excited by NE ruptures situated north of the Transverse ranges. Path kernels show that LAB excitation is mediated by surface waves deflected by the velocity contrast along the southern margin of the Transverse Ranges, having most of their energy in basement rock until they impinge on the eastern edge of SGB, through which they are then funneled into LAB. Ventura Basin amplification is enhanced by a similar waveguide effect.

Day, S. M., Roten, D., & Olsen, K. B. (2012). Adjoint analysis of the source and path sensitivities of basin-guided waves. Geophysical Journal International, 189(2), 1103-1124. doi: 10.1111/j.1365-246X.2012.05416.x.