AMPRAT and MECHTOOL : programs for determining focal mechanisms of local earthquakes

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Robinson, R.; Webb, T.H. 1996 AMPRAT and MECHTOOL : programs for determining focal mechanisms of local earthquakes . Lower Hutt: Institute of Geological & Nuclear Sciences Institute of Geological & Nuclear Sciences science report 96/07 24 p.

Abstract: A common problem in determining the focal mechanism of local earthquakes from first motion observations is that there are not enough data to usefully restrain the solution. P to S amplitude ratios can theoretically reduce the range of possible solutions. However, amplitude ratios have a rather checkered history, largely because ratios calculated using simple ray theory assume that the user can reliably identify on the seismograms the various phases (e.g., P, Pn, P*, PG, S, Sn, S*, SG, etc) and also that various internal reflections are not important. Detailed waveform modelling is another possibility but requires very accurate locations and velocity model. Also, the theoretical waveforms generated by layered models often don't look much like the real thing if the structure is actually more complex. This is particularly true at higher frequencies. Low pass filtering helps, but only for larger events. Computing theoretical waveforms for complex 3-D velocity models is computationally impractical. Schwartz (1995) introduced the idea of using the envelope of complete theoretical seismograms rather than details of the waveform. This helps by reducing the dependence on fine details and precise timing, while retaining the complete seismogram approach (all phases are included). Experience indicates that low-pass filtered seismograms (4.0 Hz) are useful for events down to about magnitude 2.0. The programs ''AMPRAT'' and ''MECHTOOL'' have been developed to apply this approach to the large number of short-period digital seismograms available in New Zealand, particularly from aftershock sequences and other temporary deployments. In addition, they can provide an easy to use interface for determining mechanisms from first motion data only. The purpose of this report is to give detailed instructions on how to use the programs, plus some background information on how they work. (auth)