Abstract
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 M W3.4–5.6 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.
Highlights
Accurate source parameters, including the location, mechanism, origin time, and magnitude of a seismic event, are important for responding to earthquake hazards, monitoring nuclear explosions, understanding tectonic processes, and using full-3D waveform tomography to improve the resolution of 3D Earth models.Most of the earlier earthquake location and source studies assume a one-dimensional (1D) velocity model and use traveltimes of seismic phases, usually the direct P and/or S wave, to locate the hypocenter of an event, and use phase polarities or a combination of phase polarities and ratios of the maximum P amplitude to the maximum S amplitude to determine the focal mechanism (e.g., Kisslinger 1980; Shen et al 1997; Hardebeck and Shearer 2002, 2003)
Since the initial locations of the events provided by global centroid moment tensor (GCMT) or the China Earthquake Networks Center Unified Catalog published by the China Earthquake Data Center (CEDC) should not be far from the true locations, we adapt a grid-search approach by comparing the objective function Eq (3) at the grid points around the initial locations to find the point with the best waveform fit as the event location
The event location is first determined by minimizing the cross-correlation traveltime difference between the synthetic and observed waveforms, further refined by grid search along the vertical direction with source mechanism inversion at each point
Summary
Accurate source parameters, including the location, mechanism, origin time, and magnitude of a seismic event, are important for responding to earthquake hazards, monitoring nuclear explosions, understanding tectonic processes, and using full-3D waveform tomography to improve the resolution of 3D Earth models. Hjorleifsdottir and Ekstrom (2010) estimated the effects of the 3D Earth model on the global centroid moment tensor (GCMT) solutions using synthetic data and found only small errors on average in the scalar moment and tensor elements, while the centroid depths and times were biased Another method to correct the 3D effect using 1D velocity models is the calibration event approach developed by Tan and Helmberger (2007) and Chu et al (2009), which utilizes a ground true event to derive various corrections for locating events. With no time shift between the synthetic and observed waveforms in the location step, we assume phase skipping is not an issue This approach is appropriate in places with well-determined 3D velocity models for the periods of the waves used in the source inversion. We describe the methodology, verification, and application in the Longmenshan fault (LMSF) zone
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