Historical Earthquake Sequences

Abstract

A rudimentary seismic network has existed in the United States since the mid 1920' s. Many important earthquakes in mid-size range (5 < M < 6), have been recorded by the relatively low gain instruments from this network, Galitzin's and Wood-Anderson torsions. Because the networks were so sparse, they have not been effectively used. In this report, we have examined records collected form the 1946 Walker Pass and 1947 Manix events and conducted a pilot study into how these records can be used in conjunction with modern data to understand the characteristics of these historic events. One well-proven method for assessing old events is to compare existing historic recording with observations from modern events (calibration event). The 1962 Walker Pass event (M_L= 4.9) was studied in detail for this purpose and observations at Pasadena (PAS) and Florisent (FLO), Missouri compared with the 1946 mainshock and foreshock (M_L = 5.2). This comparison yields a M_o = 1.2 x 10²⁵ for the main event with the foreshock estimate lower by about a factor of 3 to 5. The mechanisms do not seem to be the same but all three events are relatively deep. A comparison of these events with the Manix event recorded at these two stations as well as at Weston, MA indicate that the Manix event is considerably larger. A detailed modeling effort was conducted on the (PAS) local strong motion recordings of the events separately and in conjunction with the bodywaves recorded at FLO. Considerable effort was devoted to calibrating the upper-mantle model needed in generating the synthetics at FLO. Our best estimate for the Walker Pass mainshock is that it had a strike, dip, and rake of (0°, 40°, 70°) with a moment of 1.5 x 10²⁵ ergs, and occurred at a depth of 20 km. Results for the Manix event proved similar to those reported by Doser (1990); essentially a strike-slip event with (65°, 85°, 8), a moment of 3.5 x 10²⁵ ergs and a depth of 6 km.

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