Moho depth variation in southern California from teleseismic receiver functions

The number of broa.dband three-component seismic stations in southern California. has more tb an tripled recently. In th is study we use the teleseismic receiver function technique to determine the crustal thicknesses and Vp / Vs ratios for th ese stations and map out t he lateral variation of Moho dept h under southern California. It is shown t hat a. receiver function can provide a very good <'point" measurement of crustal thickness under a broadband s tation and is not sensitive to crus ta l P velocity. However , the crust al t hickness es t imated only from the delay t ime of the "Yioho P-to-S converted phase trades off strongly with the crustal VP/Vs ra.tio. The ambiguity can be reduced significantly by incorporating the later mult iple converted phases, namely, t he PpPs and PpSs+ PsP s. We propose a stacking algorith m which sums the amplitudes of receiver function a.t the predicted arrival times of these phases by different crustal thicknesses H and VP/Vs rat ios. This transforms the time domain receiver functions directly into the H -\/;,/'v's dom ain without need Lo identify these phases ;;i,nd to pick their arrival times. The best estimations of crustal thickness and \1v/Vs ratio a.re found when the three phases are stacked coherently. By stacking receiver functions from different. distances a nd directions, effects of lateral structu ral varia tion a.re su ppressed , and an average . crustal model is obtained . Applying this techniq ue to 84 digital broadband stations in sou thern California reveals that t he Moho depth is 29 km on average and varies from 21 to 37 km. Deeper Mohos are fo und under t he eas t ern Transverse R ange, t he P eninsular Range, and the Sierra Nevada Range. The centra i Transverse Range, however , docs not have a crustal root. Thin crust s exist in t he Inner California Borde rland (2122 km ) and the Salton Trough (22 km) . The Moho is relatively flat a t the average depth in the western and central Mojave Desert and becomes shallower to th e east u nder the Eas t ern California Shear Zone (ECSZ). Southern California c rust has an average l1~/V, ra tio of 1.78, with higher ratios of 1.8 to 1.85 in the mounta in ranges \Vith Mesozoic basement and lO\.ver ratios in the Mojave Block except for the ECSZ, where t he ratio increases .