Earthquakes off the coast of northern California*

Abstract

Summary The epicenters as located for the earthquakes of July 6, 1934, January 2, 1935, and June 3, 1936, indicate that earthquakes centering off the coast of Humboldt County do not originate on a linear extension of the San Andreas fault in Mendocino County. Figure 1 shows the epicenters of these shocks as located by two methods, and also the epicenter of the earthquake of January 31, 1922, as located by Professor Macelwane. On the basis of least square residuals alone, the epicenters located by assuming a straight-line travel-time curve for Pn to Δ=18° are better than those located by the Jeffreys-Bullen curves. But for the shock of June 3, 1935, the epicenter located by the first method is inconsistent with the S-P interval at Ferndale, whereas that obtained by the second method is consistent. For the other two shocks the epicenters located by the two methods are little different in position. Whereas the July 6 shock shows a definite Sn travel-time curve with an intercept approximately equal to that of the Pn curve, the other two earthquakes show a marked wave preceding Sn and almost masking it at the coastal stations south of the epicenter. The travel-time curve of this wave has an intercept about 15 seconds below that of Pn. This wave is longitudinal and has a period of 4.35 km/sec. It is suggested that it is P in the sedimentary layer, although objections to such an interpretation are cited. The travel-time curve of P for the July 6 shock between 26° and 40° indicates that it is composed of two nearly straight branches rather than the more smooth curve of Jeffreys and Bullen.