Earthquake Location and Upper Mantle Structure From P Wave Polarization in French Polynesia and Australia: Preliminary Results

We report measurements of long-period P wave polarization in French Polynesia and in Australia. The 3D character of particle motion of P waves provides complementary and independent constraints on the upper mantle structure beneath a given station to that from shear-wave splitting. Analysis of the deviation of horizontal polarization and the vertical polarization angle as a function of event backazimuth are used to obtain information about: i) sensor misorientation, ii) seismic anisotropy, and iii) velocity heterogeneities. The measurements are realized with a method proposed by Schulte-Pelkum and others. Despite the availability of 15 years of data, the two permanent stations on Tahiti do not show any evidence of shear-wave splitting (Fontaine and others) whereas P wave polarization observations do. Using the latter technique the fast axis azimuth is oriented N118°E close to the direction of the present Pacific plate motion. At Rikitea, in the Gambier Islands, the fast axis azimuth is N109°E. This suggests that SKS waves sample either a complex upper mantle structure induced by the recent magmatism on Tahiti or a vertical mantle upwelling, while P waves identify an azimuthal anisotropy in the asthenosphere with a lateral offset, since the incidence angles are much larger than the SKS waves. Australia is ideally located for P wave polarization analysis due to the favourable distribution of seismicity around this continent. Moreover, some relatively dense networks of broadband seismometers were installed throughout the Australian region during the last decade that may give the possibility to discriminate between velocity perturbations and seismic anisotropy beneath some stations. In Tahiti, the tsunami warning centre uses only one seismic station in real-time to determine the earthquake location. The horizontal polarization of P waves deviates up to 10° depending on the backazimuth of the event. The introduction in the automatic earthquake location of a term for correction of the deviation of the direction of particle motion would improve the precision of the location and thus the accuracy of the tsunami warning system.