Arrival-time data for local earthquakes were used to study lateral velocity variations and error statistics for southern California. Crustal P velocities averaged over the major geomorphic provinces vary by only a few per cent. Regions of relatively high velocity include the Peninsular ranges, the Imperial valley, the Pomona basin, and the Colorado desert. The Santa Monica, southern Sierra, San Bernardino, and San Jacinto mountains are relatively slow, as is the offshore area. Near-surface station delays, which are unknown to within several tenths of a second, are a major source of uncertainty in the regional velocities.
Arrival-time errors include two types: (1) small errors caused by imperfect timing and unmodeled velocity variations; and (2) larger errors apparently caused by improper identification of arrivals and by blunders. We found no reliable way to identify the latter errors unless they result in large arrival-time residuals. For a strongly edited catalog, the variance of arrival-time residuals appears to increase linearly with epicentral distance, as predicted for wave propagation in a randomly heterogeneous medium. The variance of residuals is strongly station dependent. Residuals are nearly Gaussian within the 70 per cent confidence interval, but not outside this interval. The 95 per cent confidence limit occurs at about three standard deviations.