We performed Jackknife tests on the four events with significant isotropic components that had four or more stations in their solution (Events 97.11.22a, 97.11.22b, 97.11.22d, and 97.11.30) to determine the likelihood of non-DC events incorrectly being identified as DC events. For all station combinations of three or more, we determined if the volumetric component was significant at or above the 95% significance level by using the F test statistic. All 52 combinations of four or more stations recovered the statistically significant isotropic component. For solutions with three stations, six iterations out of 60 failed to recover the isotropic component. It is reasonable to assume that significant isotropic components can be recovered with as few as three, but preferably with at least four, stations in the solution.
We also investigated the possibility of obtaining a spurious isotropic component due to poor data coverage. For this test, we took three high quality DC solutions (Events 97.12.31, 98.06.09, and 98.07.15) and performed Jackknife tests to see if any combination of three or more stations would result in a statistically significant isotropic component. Of 75 three station solutions, one returned a false positive. Of 65 four station solutions, three incorrectly determined that the event had a significant isotropic component. Five and six station solutions did not return false positives. Thus, we feel confident that the isotropic components of our non-DC events with at least five stations in their inversion are not due to poor data coverage. This test, however, casts a small amount of doubt as to the validity of non-DC Event 93.08.11 which has only three stations in its solution.
Berkeley Seismological Laboratory
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