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Surface- and body-wave spectra of CANNIKIN and shallow Aleutian earthquakes

DIVISION OF SEISMOLOGY EARTH PHYSICS BRANCH DEPARTMENT OF ENERGY, MINES AND RESOURCES, OTTAWA, ONTARIO, KIA OE4, Canada

Abstract

Rayleigh waves and P codas of CANNIKIN and shallow Aleutian earthquakes are studied in order to determine the parameters most influential in shaping the respective teleseismic amplitude spectra and in governing the surface (M_S) and bodywave (m_b) magnitudes. For surface-wave generation, the dipole strength (P_o) of the underground nuclear explosion, and the seismic moment (M_o) of the earthquake are the important parameters. The M_S for these events are comparable when M_o is approximately 3 to 4 times greater than P_o. Theoretical considerations of bodywave spectra indicate that the important parameters are the permanent offset in the reduced displacement potential [()] of explosions and the product of area of faulting (S) and average dislocation () of earthquakes. These source parameters are related by P_o = 4( + 2µ) () and M_o = µ S where and µ are the elastic Lamé parameters. An additional parameter, namely, the peak frequency (f_p) in the explosion P-coda spectrum and the corner frequency (f_c) in the earthquake spectrum, is of paramount importance in determining the respective m_b. For the events considered, f_p is from 5 to 10 times greater than f_c and this is manifest in the corresponding m_b, where the m_b for CANNIKIN is at least a unit greater than that of the earthquakes. The linear dimensions of the respective sources appear to exert a major influence upon f_c and f_p.

The large scatter between theoretical and actual m_b values, unfortunately, precludes a firm confirmation of the above theoretical considerations.

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