Seismic source tracking with six degree-of-freedom ground motion observations

Back azimuth information can be determined from combined measurements of rotations and translations at a single site. Such six degree-of-freedom (6-DoF) measurements are reasonably stable in delivering similar information compared to a small-scale array of three-component seismometers. Here we investigate whether a 6-DoF approach is applicable to tracking seismic sources. While common approaches determining the timing and location of energy sources generating seismic waves rely on the information of P-waves, here we use S-waves. We track back azimuths of directly arriving SH-waves in the 2-D case, P-converted SV-waves, direct SV- and direct SH-waves in the 3-D case. For data analysis, we compare a cross-correlation approach using a grid-search optimization algorithm with a polarization analysis method. We successfully recover the rupture path and rupture velocity with only one station, under the assumption of an approximately known fault location. Using more than one station, rupture imaging in space and time is possible without a priori assumptions. We discuss the effects of rupture directivity, supershear rupture velocity, source-receiver geometry, wavefield interference, and noise. We verify our approach with the analysis of moving traffic noise sources using 6-DoF observations. The collocated classic seismometer and newly-built ring laser gyroscope ROMY near Munich, Germany, allow us to record high-fidelity, broadband 6-DoF (particle velocity and rotational rate) ground motions. We successfully track vehicles and estimate their speed while traveling along a nearby highway using the estimated BAz as a function of time of a single station observation.