Earthquake source mechanism analysis for events between 1992 and 1997 using sparse New Zealand broadband data

Abstract

Data from the permanent broadband seismic station SNZO and temporary broadband deployments have been used to test the operation of the regional, full waveform inversion source mechanism determination technique of Dreger and Helmberger (1991) in New Zealand. An amplitude ratio technique similar to the AMPRAT technique, which is widely used in New Zealand, was also tested with the same data. Tests using synthetic data showed that the waveform inversion is not reliable if data from only one station are available, but that the reliability of the technique improves when a second station is added, and the reliability is directly proportional to the azimuthal separation of the stations. Addition of a third station further improves the results. To test the applicability of these synthetic results, regional earthquakes with moment tensors published between 1992 and 1997, and which were recorded at SNZO, were investigated using the waveform inversion technique. Where possible, additional data from temporary broadband deployments were also included in the inversion. The results of these tests on the 11 events support the synthetic results. While two of the events returned source mechanisms consistent with the published solutions, there was no known way to separate the consistent from the inconsistent mechanisms, making it impossible to use the technique reliably with data from only one station. When further stations were available for addition to the inversion, the results were improved. However, there were insufficient events recorded at multiple stations to reach any firm conclusions. The same events were analysed using an amplitude ratio technique plus first motion polarities from the New Zealand short period network, and similar results were found. The amplitude ratio technique was more reliable than the waveform inversion when only one station (plus first motions) was used, with six of the events returning source mechanisms consistent with the published solutions. This was not sufficiently reliable to guarantee good results. Addition of further stations again improved the reliability of the results. In addition to the tests of the techniques, new source mechanisms are presented for the 1994 June 18 Arthur's Pass and 1995 November 24 Cass earthquakes.