W-phase Source Inversion Using High-rate Regional GPS Data for Large Earthquakes.

Abstract:

W-phase moment tensor inversions have been proven to be a reliable method for rapid characterization for large earthquakes. W-phase is a long period seismic (100s-1000s) wave that arrives between P and S waves and can be synthesized by normal mode summation. For global purposes it has been used at USGS, PTWC and IPGS. These implementations provide moment tensor solutions within 30-60 min after the origin time of moderate and large worldwide earthquakes. W-phase inversion has been successfully implemented at the Chilean National Seismological Center (CSN) for regional distances (5º-12º) obtaining the first solution ~6 minutes after the earthquake. However until now it has been used only with broadband instruments, which saturate in the near field. Therefore, we use near field records from high-rate regional GPS data for some large earthquakes that have occurred in the past five years and with relatively dense azimuthal and station density coverage.Originally the inversion takes the time interval between Tp and Tp + 15 *delta (distance from the epicenter in degrees). In the near field W-phase doesn’t develop as well as in the intermediate or far field, therefore we increased the time window for these events. Here we tried different time windows to find the most accurate result for each earthquake and to reduce time response for tsunami early warning purposes. We took near field GPS for the following earthquakes: The 2010 Mw 8.8 Maule Earthquake, the 2011 Mw 9.1 Tohoku Earthquake, The 2014 Mw 8.2 Iquique Earthquake, and The 2014 Mw 7.8 Iquique Aftershock. The solutions for the examples tested here are potentially available 5 min after the origin time. The calculated magnitude for each earthquake is: Mw 8.9 for the Maule earthquake, Mw 9.1 for the Tohoku earthquake, Mw 7.9 for the Iquique earthquake, and Mw 7.8 for the Iquique aftershock. The mechanisms, as expected, are thrust with some variations with respect to the WCMT from National Earthquake Information Center. The results for each earthquake for strike, dip and rake are: 11, 12, 65; 209, 9.3 113; 358, 47, 104 and 308, 9, 40, respectively. Such variations are likely due to local structure mismodeling, limited number of stations used and azimuthal coverage. With more GPS data available and the possibility of including real time processing solutions will have significant improvements.