Automatic shear wave splitting and time-varying anisotropy on Ruapehu volcano, New Zealand
Abstract
Paul Silver made major contributions to the field of seismic anisotropy and to the study of earthquake precursors. We present a method that combines these interests. Local earthquake data is becoming ever more voluminous and studies of time varying properties are best carried out with objective methods. We present an automatic shear wave splitting measurement tool tuned to local earthquakes, which can be run with the sole requirement that an S arrival time has been chosen. We apply the technique to three shear wave splitting data sets recorded on Ruapehu Volcano in New Zealand that have previously been manually evaluated and that were determined to have fast polarisations (Φ) that vary in time and with earthquake depth. The technique uses as its base the Silver & Chan eigenvalue minimisation technique, applied over multiple measurement windows. The best filter is chosen based on the signal to noise ratio and the dominant period of each waveform is used to choose minimum and maximum window lengths. Cluster analysis determines the best solution among all the windows, and a number of quality grading criteria are used to assess the results automatically. When the same filters are used for events determined to be high quality from manual studies, the automatic technique returns virtually identical results (Fig.). Scatter increases when the automatic technique chooses the best filter, but the average automatic results remain consistent with the manual results. However, when the automatic technique is used on data previously judged as poor quality, scatter increases again, and some stations yield distributions of Φ that include peaks that were not present in previously published results. Detailed examination suggests that there might have been some unconscious bias in the manual selection process which downgraded measurements that did not fit expectations. Nonetheless, results from the new objective analysis confirm changes in the average fast orientation of anisotropy between 1994 and 2002, and between shallow and deep events. Thus, the new technique is valuable for objective overall assessment of anisotropy and its variation in time. Comparison of automatic and manual Φ and time delay dt.
- Publication:
-
AGU Fall Meeting Abstracts
- Pub Date:
- December 2009
- Bibcode:
- 2009AGUFM.S13C..06T
- Keywords:
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- 7223 SEISMOLOGY / Earthquake interaction;
- forecasting;
- and prediction;
- 7280 SEISMOLOGY / Volcano seismology;
- 7290 SEISMOLOGY / Computational seismology;
- 8164 TECTONOPHYSICS / Stresses: crust and lithosphere