Source mechanism determination using broadband data in the New Zealand seismic environment

Abstract

Data from the permanent broadband seismometer SNZO, and other temporary broadband deployments are used to test the operation of the regional, full waveform inversion source mechanism determination technique of Dreger (1995) in New Zealand. An amplitude ratio technique similar to the AMPRAT technique (Robinson and Webb, 1996), which is widely used in New Zealand, is also tested with the same data. Tests using synthetic data are carried out and indicate that the waveform inversion will not be 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 is expected to further improve the results. To test the applicability of these test results to actual studies, all regional earthquakes with published moment tensors since the installation of SNZO are investigated using the waveform inversion technique. Where possible, additional data from temporary broadband deployments are added to the data from SNZO. The results of these tests support the synthetic results. While 33% of the events return source mechanisms consistent with the published solutions, this is too infrequent for it to be possible to reliably use the technique with data from only one station. Addition of further stations improves the results, and the technique is considered reliable with data from more than one station. The same events are tested using an amplitude ratio technique, and similar results are found. Surprisingly, the amplitude ratio technique is found to be more reliable than the waveform inversion when only one station is used, with 58% of the events returning source mechanisms consistent with the published solutions. This is still not sufficiently reliable to guarantee good results. Addition of further stations again improves the reliability of the results. In addition to the tests of the techniques, new source mechanisms are presented for the 18th June 1994 Arthur's Pass and 24th November 1995 Cass earthquakes, and evidence is presented for a preferential amplification of low frequency seismic waves travelling southwest - north-east within New Zealand. This is thought to be caused by the high quality factor core of the subducted Pacific Plate.