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Seismology & geology
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Seismic Moment Tensor Solutions from GeoNet Data to provide a Moment Magnitude Scale for New Zealand

Elizabeth de Joux Robertson - GNS Science (EQC funded project 06/522)

Technical Abstract

The aim of this project was to enable accurate and self-consistent earthquake magnitudes to be calculated routinely and in near real-time for larger New Zealand earthquakes. This would be done by using earthquake waveform data to obtain seimic moment tensors, and hence the earthquake’s moment magnitude, Mw. The earthquake’s focal mechanism, which shows the type of faulting (normal, reverse or strike-slip) and the orientation and dip of two possible faults, can also be obtained from the seismic moment tensor.

I used a well-established seismic moment tensor inversion method, the Time-Domain [seisnmic] Moment Tensor Inversion algorthm (TDNT_UNVC) and applied it to earthquale data recorded by the GeoNet project to generate seismic moment tensor solutions for New Zealand earthquakes. Some modifications to this software were made.

Seismic moment tensors were calculated for 24 New Zealand earthquakes from 2000 to  2005. The Global CMT project has calculated CMT solutions for 22 of these, and the Global CMT prokect solutions were compared to the solutions obtained in this project to test the accuracy of the solutions obtained using the TDMT_INVC code.

The moment magnitude values are close to the Global CMT values for all earthquakes. The focal mecganisms could only be determined for a few of the earthqyakes studied. The value calculated for the moment magnitude appears to be less sensitive to the velocity model and earthquake location (epicentre and depth) than the focal mechanism.

Distinguishing legitimate seismic signal from background seimic noise is likely to be the biggest problem in routine calculations and will limit routine application of the method to larger earthquakes (M>5).

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