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We determined a detailed three-dimensional seismic velocity structure of the crust and mantle to a depth of 800 km beneath southern California using local and teleseismic data simultaneously. We used 131372 P-wave arrival times from 6437 local earthquakes and 12134 travel time residuals from 193 teleseismic events recorded by 251 seismic stations of the Caltech-US Geological Survey Southern California Seismic Network. Compared with previous local and teleseismic studies, the station coverage is considerably improved, and the number of data used is greatly increased in the present study. This, together with the local and teleseismic joint inversion approach, results in a unified three-dimensional (3-D) P-wave velocity model of the crust and mantle in southern California with a higher resolution than the previous models. The result obtained shows very heterogeneous structures in the crust and upper mantle in southern California. Shallow crustal structures correlate well with surface geological features. Sedimentary basins such as the Los Angeles Basin, Ventura Basin and Santa Maria Basin are imaged well as low velocities, and batholiths such as the Peninsular Ranges and San Gabriel Mountains are imaged as high velocities. The velocity in the crust is generally low in the Mojave Desert. The crustal velocities change abruptly across major faults such as the San Andreas, San Jacinto, Elsinore and Garlock faults. In the upper mantle, three major anomalies are found, beneath the Transverse Ranges, Salton Trough and Southern Sierra Nevada. The Transverse Ranges feature appears as a curtain-like, east-trending and north-dipping high-velocity anomaly with a thickness of approximately 50 km, and extends most deeply at its eastern end to about 250 km. The Salton Trough feature is composed of low velocities which trend southeast and extend to approximately 200 km depth. The Southern Sierra Nevada feature is a slab-like high-velocity anomaly which extends to about 250 km. The mantle velocity is generally low in the upper 100-200 km beneath the volcanic areas in southern California. The seismic velocity anomalies in the mantle beneath southern California are considered to result from small-scale mantle convection.