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 G05018;

Modelling ground motion in the Hutt Valley, New Zealand

“…The focal mechanisms of the events (Table 1; Figure 3) were determined by the amplitude ratio method (Schwartz, 1995;Matcham et al, 2006), with the seismic data from New Zealand Standard Network and using the Wellington 1D velocity model (Robinson, 1986). (L16 and L17, flexible sediment sites).…”