Focal mechanism determination using high-frequency waveform matching and its application to small magnitude induced earthquakes

Abstract

S U M M A R YWe 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 modelled 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 five-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 3-9 Hz. The application to dozens of induced seismic events showed satisfactory waveform matching between modelled 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.

Microseismic joint location and anisotropic velocity inversion for hydraulic fracturing in a tight Bakken reservoir

“…Except for a few events, the crosscorrelation coefficients (normalized to unity) for event pairs are greater than 0.7, indicating that the hypocenters and the source properties are similar. The reoccurrence of events in the same location is well-known in earthquake seismology and usually implies reactivation of a fault (e.g., Li et al, 2011). Thus, the close clustering of these 29 big events from different stages at approximately 150 m above the treatment well suggests that a natural fracture swarm or a fault zone is being repeatedly activated due to injection of pressurized fluids during some of the completions.…”

Focal mechanism determination of induced microearthquakes in an oil field using full waveforms from shallow and deep seismic networks

“…A detailed description of the method can be found in Li et al (2011). Here the method is briefly explained.…”

“…In Li et al (2011), we tested the robustness of the method on the surface network. To account for the uncertainty of the 1-D velocity model, a 5% random perturbation was applied.…”

“…Previously, we tested our newly developed focal mechanism determination method on induced microearthquakes monitored by a five-station surface network at an oil field in Oman (Li et al, 2011). The field, operated by Petroleum Development Oman (PDO), was discovered in 1962 and put into production in 1969.…”

“…To retrieve reliable solutions, we developed a method to use high frequency, full waveform information (both P and S) to determine the focal mechanisms of small earthquakes (Li et al, 2011). Using the known velocity model (one-dimensional layered model in this study), we calculate the Green's functions for all moment tensor components of the source at each location (hypocenter) and then the synthetic seismograms by convolving them with the source time function.…”