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.

Location and moment tensor inversion of small earthquakes using 3D Green＇s functions in models with rugged topography： application to the Longmenshan fault zone

With dense seismic arrays and advanced imaging methods, regional three-dimensional （3D） Earth models have become more accurate. It is now increasingly feasible and advantageous to use a 3D Earth model to better locate earthquakes and invert their source mechanisms by fitting synthetics to observed waveforms. In this study, we develop an approach to determine both the earthquake location and source mechanism from waveform information. The observed waveforms are filtered in different frequency bands and separated into windows for the individual phases. Instead of picking the arrival times, the traveltime differences are measured by cross-correlation between synthetic waveforms based on the 3D Earth model and observed waveforms. The earthquake location is determined by minimizing the cross-correlation traveltime differences. We then fix the horizontal location of the earthquake and perform a grid search in depth to determine the source mechanism at each point by fitting the synthetic and observed waveforms. This new method is verified by a synthetic test with noise added to the synthetic waveforms and a realistic station distribution. We apply this method to a series of Mw3.4-5.6 earthquakes in the Longmenshan fault （LMSF） zone, a region with rugged topography between the eastern margin of the Tibetan plateau and the western part of the Sichuan basin. The results show that our solutions result in improved waveform fits compared to the source parameters from the catalogs we used and the location can be better constrained than the amplitude-only approach. Furthermore, the source solutions with realistic topography provide a better fit to the observed waveforms than those without the topography, indicating the need to take the topography into account in regions with rugged topography.