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Instrumental study of the Manix Earthquakes

CALIFORNIA INSTITUTE OF TECHNOLOGY, PASADENA, CALIFORNIA

Abstract

Readings from seismograms of thirty-eight shocks occurring on the Mojave Desert near Manix, California, are tabulated, plotted, and discussed. For a large proportion of the shocks (Groups A and D), the most clearly identified seismic waves have travel times as follows (times in seconds, in kilometers):

D is calculated from assuming a depth of 16 km.

For the remaining shocks (Groups B and C), the constant terms in the equations for Pn, Pm, Py are increased to 6.0, 4.5, and 2.2, respectively; the other equations are unchanged, but D is calculated for a depth of 10 km.

The Group A shocks are assigned to four subgroups. Epicenters are as follows:

Epicenters A1 to B follow an alignment striking about N 30° W, crossing the Manix fault at a large angle. No shocks are located elsewhere along the Manix fault, but the smaller shocks cannot be placed accurately.

The principal earthquake, of magnitude 6.2, is assigned to the A2 epicenter, with origin time 07:58:05.6 P.S.T. (15:58:05.6 G.C.T.), April 10, 1947. This agrees well with the times recorded at distant stations. A catalogue is given listing all subsequent shocks of magnitude 3.0 or greater in the area to the end of April, 1950, with assignment to one of the groups when possible.

Recorded initial compressions and dilatations at the several stations are equally consistent with right-hand strike-slip on a hypothetical fault following the line of located epicenters, or with left-hand strike-slip on the Manix fault; the latter displacement was actually found. It is suggested that both occurred.

Representative seismograms are reproduced.

Footnotes

^* Manuscript received for publication June 23, 1950.

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