Focal Mechanism Studies Research Articles

Methods of focal mechanism studies with application to two atlantic earthquakes

A comparative study is made of various techniques to investigate earthquake focal mechanisms, including both direction of displacements and amplitudes or amplitude ratios. Both body waves (P and S) and surface waves are used in this comparison. The methods are applied to two earthquakes on the Mid-Atlantic Ridge, using records at numerous stations around the world. Only incomplete comparisons of different techniques for mechanism studies have been published earlier. The earthquakes studied are from a region, for which very little is known in terms of focal mechanism. The different methods show generally a good agreement in the cases studied, although several complications are encountered, especially double P onsets.

The use of transverse waves in focal mechanism studies

A literature review is given of the use of transverse seismic waves, principally S, to determine focal mechanisms. The problem has been attacked both from the theoretical and the observational side as well as by means of laboratory model studies. The difficulties, as for example questions of uniqueness of solutions, in interpretation of S-wave records are particularly emphasized. Among the different methods described, the direction of first motion and the polarization of transverse waves are the most significant. Various amplitude ratios may give additional valuable information. A number of different theoretical source models are discussed in relation to observational data.

The S wave project for focal mechanism studies earthquakes of 1962

Abstract The Department of Geophysics of Saint Louis University has instituted a routine program for the determination of the focal mechanism of the larger earthquakes of each year using methods developed for the use of S waves in focal mechanism studies. Suites of records from selected stations are assembled from the WWSS microfilm file for each earthquake of interest. A combination of P-wave first motion and S-wave polarization data is then used to determine graphically the mechanism of the earthquakes. Thirty-six earthquakes of 1962 were selected for study. The focal mechanism solutions are presented for twenty-three of these shocks. There is evidence of patterns characteristic of the focal mechanism of earthquakes occurring in Kamchatka, the Aleutian Islands and South America. A complete presentation of all the data and of all the solutions is available in a more lengthy report.

A comparison of multiple solutions of focal mechanisms

Abstract The occurrence of focal mechanism solutions of the same earthquake by different authors is examined. Of 42 instances of such multiple solutions for earthquakes which occurred around the borders of the Pacific Ocean, it is found that of 84 possible pairs of multiple solutions agreement is excellent in 8 cases, good in 13, fair in 19, poor in 19, and no agreement in 25. The implication for focal mechanism studies is illustrated by examining the focal mechanism solutions of deep-focus earthquakes under the Sea of Japan.

A comparison of some S-wave studies of earthquake mechanisms

Abstract Graphical and analytical techniques for using S-waves in focal mechanism studies are compared. In previous applications the analytical technique has shown little or no agreement with the results of fault-plane solutions from P-waves, whereas for other groups of earthquakes the graphical methods have shown good agreement between the S-waves and the P-wave solutions. It is shown that the graphical and analytical techniques are identical in principle and that when the graphical methods are applied to the same three earthquakes to which the analytical technique had been applied the identical results are obtained. Closer examination of the graphical presentation of the data, however, shows that the disagreement between the S-waves and the fault plane solutions from P is largely apparent. The discrepancy follows upon the peculiar scatter in the S-wave data and the chance occurrence of observations of S at stations located along closely parallel planes of polarization of S. Once this is understood, it is seen that the direction of polarization of S-waves is in substantial agreement with the methods of analysis of focal mechanisms from P-waves, and that the data are consistent with a simple dipole as the point model of the earthquake focus.