Improvement of Earthquake Epicentral Locations Using T-Phases: Testing by Comparison With Surface Wave Relative Event Locations

摘要

A deployment of 51 ocean-bottom seismometers (OBS) on the seafloor spanning 800 km across the East Pacific Rise provides a unique opportunity to test the robustness of epicentral location techniques using T-phases. A standard technique for locating events with T-phases is to pick the arrival the time of peak energy, then proceed as if it were an unscattered phase originating at the epicenter. Such an approach has been shown to have no apparent bias in epicentral location. Comparison of waveforms at nearby stations, however, shows that peak energy arrival time can shift to different parts of the wavetrain due to incoherent interference between waves excited or scattered from different locations, even for stations only a few kilometers apart, forcing operator identification of particular features in the waveform. At greater sensor separations, such identification cannot be performed with confidence. We show that a 75% reduction in variance relative to picks of peak arrival times can be achieved by fitting an assumed functional shape to the entire envelope of the T-phase. Since most of the variation in the envelope is caused by scattering and interference of the waves, 'noise' is proportional to signal and is log-normal. Best results are obtained by fitting the log of the envelope, which transforms the noise into a nearly constant, Gaussian distributed background and de-emphasizes individual peaks. By fitting the entire long wavetrain of the T-phase, excitation by individual bathymetric features is also de-emphasized. We test the stability of this approach for events of greatly different size using a mainshock/aftershock sequence of earthquakes at the northern end of the Easter microplate. In addition, for the larger earthquakes, we can compare relative event locations with those determined by cross-correlating waveforms of Rayleigh and Love surface waves recorded teleseismically. The T-phases from the OBSs are supplemented by T- phases recorded at GSN station RPN. Rela.