[S01-1] Open session I

[S01-1-06] Towards routine determinations of earthquake focal mechanisms obtained from P-wave first motion polarities

^○Konstantinos Lentas, Dmitry Storchak (International Seismological Centre)

We investigate the possibility to calculate earthquake focal mechanisms in a routine and systematic way based on P-wave first motion polarities. We use any available parametric data in the ISC database. We also use ISC auto-picked polarities from waveform data up to teleseismic epicentral distances (up to 90 degrees) for stations in the international registry that are not reported to the ISC. A bi-product of our study can potentially be an additional set of picked polarities and direct P-wave teleseismic arrival times, and/or the identification of stations associated with polarity reversals. We use the HASH algorithm for the determination of the earthquake mechanisms as it provides an elegant way to take into account uncertainties in earthquake location and Earth's structure. We slightly modified the algorithm to deal with a wide range of epicentral distances and to take into account the ellipsoids that define the ISC location errors. We first carry out benchmark tests for a set of ISC reviewed earthquakes (mb > 4.5). We use the HASH classification to define the mechanism quality and we find that the mechanisms of quality A, B, and C with an azimuthal gap up to 90 degrees are well compared to our benchmark mechanisms. Nevertheless, the majority of our mechanisms fall into class D as a result of limited polarity data from stations in local/regional epicentral distances. Specifically, we compute the minimum rotation angle between our mechanisms and the benchmarks and we find 24% of D cases with a rotation angle up to 35 degrees and azimuthal gap up to 120 degrees. We finally specify selection criteria with respect to the stations' distribution, misfit and nodal plane uncertainties in order to separate the trusted mechanisms from the rest.