Development of a new high resolution waveform migration location method and its applications to induced seismicity

Locating seismic events is a central task for earthquake monitoring.Compared to arrival-based location methods,waveformbased location methods do not require picking phase arrivals and are more suitable for locating seismic events with noisy waveforms.Among waveform-based location methods,one approach is to stack different attributes of P and S waveforms around arrival times corresponding to potential event locations and origin times,and the maximum stacking values are assumed to indicate the correct event location and origin time.In this study,to obtain a high-resolution location image,we improve the waveform-based location method by applying a hybrid multiplicative imaging condition to characteristic functions of seismic waveforms.In our new stacking method,stations are divided into groups;characteristic functions of seismic waveforms recorded at stations in the same group are summed,and then multiplied among groups.We find that this approach can largely eliminate the cumulative effects of noise in the summation process and thus improve the resolution of location images.We test the new method and compare it to three other stacking methods,using both synthetic and real datasets that are related to induced seismicity occurring in petroleum/gas production.The test results confirm that the new stacking method can provide higher-resolution location images than those derived from currently used methods.

Full moment tensor inversion constrained by doublecouple focal mechanism for induced seismicity

In this study,we propose a new method to determine full moment tensor solution for induced seismicity.This method generalizes the full waveform matching algorithm we have developed to determine the double-couple(DC)focal mechanism based on the neighbourhood algorithm.One major difference between the new method and the former one is that we adopt a new misfit function to constrain the candidate moment tensor solutions with respect to a reference DC solution in addition to other misfit terms characterizing the waveform matching.Through synthetic tests using a real passive seismic survey geometry,the results show the new constraint can help better recover the DC components of inverted moment tensors.We further investigate how errors in the velocity model and source location affect the moment tensor solution.The synthetic test results indicate that the constrained inversion is robust in recovering both the DC and non-DC components.We also test the proposed method on several real induced events in an oil/gas field in Oman using the same observation system as synthetic tests.While it is found that the full moment tensor solutions without using the DC constraints have much larger non-DC components than solutions with the DC constraints,both solutions are able to fit the observed waveforms at similar levels.The synthetic and real test results suggest the proposed DC constrained inversion method can reliably retrieve full moment tensor solutions for the induced seismicity.