During recent years the accuracy in determining the arrival times of earthquake waves in southern and central California has been notably improved, so that more detailed results concerning the structure of the earth's crust in California are to be expected from the study of such records. Fifty shocks were selected, and their epicenters, depths of foci, origin times, and magnitudes determined and discussed. All known corrections have been applied; in particular, the effect of the altitude of some of the stations is beyond the probable error of the readings. With this improved material it is possible not only to investigate more accurately than heretofore the wave velocities in the various layers, but also differences in structure, especially local effects near the stations.

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