Progress report, seismological laboratory, California Institute of Technology, for 1951

These observations continue under the supervision of C. F. Richter. Activity of local shocks was low during 1951. The swarm type of activity which recommenced in northeastern Imperial Valley in July, 1950, has continued. A shock of magnitude 5.6 originated near Calipatria on January 23–24, 1951. This generated an air wave recorded at Pasadena [Benioff, Ewing, and Press, 1951]. On December 5, 1951, there was another shock that was felt widely in the same area; the instrumental magnitude did not exceed 4.5. Several other minor groups of these shocks were recorded.