# An approach to jointly invert hypocenters and 1D velocity structure and its application to the Lushan earthquake series

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## Abstract

Earthquake location is essential when defining fault systems and other geological structures. Many methods have been developed to locate hypocenters within a 1D velocity model. In this study, a new approach, named MatLoc, has been developed which can simultaneously invert for the locations and origin times of the hypocenters and the velocity structure, from the arrival times of local earthquakes. Moreover, it can invert for layer boundary depths, such as Moho depths, which can be well constrained by the *Pm* and *Pn* phases. For this purpose, the package was developed to take into account reflected phases, e.g., the *Pm* phase. The speed of the inversion is acceptable due to the use of optimized matrix calculations. The package has been used to re-locate the Lushan earthquake series which occurred in Sichuan, China, from April 20 to April 22, 2013. The results obtained with the package show that the Lushan earthquake series defines the dip of the Guankou fault, on which most of the series occurred, to be 39° toward the NW. Further, the surface projection of the Lushan earthquake series is consistent with the regional tectonic strike which is about N45° E.

## Keywords

Earthquake re-location 1D velocity inversion Jacobi matrix Constrained LSQ optimization## Notes

### Acknowledgments

The project “Deep geological survey of the structural belt along the Longmenshan” directed by the Chinese Geological Survey sponsored this work. The Beijing Gangzhen Corp., Limited is thanked for providing the Chinese instruments for the fieldwork, while the Geophysical Instrument Pool of the Deutsches GeoForschungsZentrum Potsdam – GFZ is acknowledged for providing the German instruments. The Sichuan Province Land Resource Institute is thanked for providing support for the work as are the local people who guarded the instruments during the fieldwork. The routine “eqplot” from hypoDD was used with some small modifications to plot Figs. 2, 5, 6, 7, and 8. We also acknowledge the detailed suggestions of two reviewers for better presentation of this approach.

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