Enhanced performance of ISC focal mechanism computations as a result of automatic first-motion polarity picking optimization

Abstract

The International Seismological Centre (ISC) routinely calculates and makes available automatic earthquake focal mechanisms by combining reported parametric data (first-motion polarities) available in the reviewed ISC Bulletin and auto-picked first-motion polarities obtained from waveform data using a broadband automatic picker. In order to further enhance the robustness of the auto-picked polarities, we set up an optimization strategy which is carried out using the neighbourhood algorithm on a 24-processor mini computer cluster. The aim is to minimize an objective misfit function which takes into account the data uncertainties and compares the first P-wave arrival times and polarities of a large dataset of nearly 18,000 manual picks and the associated auto-picked waveform phase arrivals. The optimization yielded an overall increase of matching auto-picks from 15 to 30% in comparison with the default setup of the automatic picker. We then applied the optimized automatic picker to a set of earthquakes from the reviewed ISC Bulletin where we could not obtain well-constrained mechanism solutions using its default setup. As a result of using the optimized picker, we obtained well-trusted mechanism solutions for 28% of these cases by increasing the number of first motion auto-picked polarities, and hence minimizing the station azimuthal gap in some cases, and/or correcting some of the erroneous auto-picked polarities where possible.