Focal Mechanism Determination Using High Frequency Waveform Matching and Its Application to Small Magnitude Induced Earthquakes

Abstract

We present a new method using high frequency full waveform information to determine the focal mechanisms of small, local earthquakes monitored by a sparse surface network. During the waveform inversion, we maximize both the phase and amplitude matching between the observed and modeled waveforms. In addition, we use the polarities of the first P-wave arrivals and the average S/P amplitude ratios to better constrain the matching. An objective function is constructed to include all four criteria. An optimized grid search method is used to search over all possible ranges of source parameters (strike, dip and rake). To speed up the algorithm, a library of Green’s functions is pre-calculated for each of the moment tensor components and possible earthquake locations. Optimizations in filtering and cross-correlation are performed to further speed the grid search algorithm. The new method is tested on a 5-station surface network used for monitoring induced seismicity at a petroleum field. The synthetic test showed that our method is robust and efficient to determine the focal mechanism when using only the vertical component of seismograms in the frequency range of 3 to 9 Hz. The application to dozens of induced seismic events showed satisfactory waveform matching between modeled and observed seismograms. The majority of the events have a strike direction parallel with the major NE-SW faults in the region. The normal faulting mechanism is dominant, which suggests the vertical stress is larger than the horizontal stress.