Improving Absolute Hypocenter Accuracy With 3D Pg and Sg Body-Wave Inversion Procedures and Application to Earthquakes in the Central Alps Region

Accuracy of hypocenter location, in particular focal depth, is a precondition for high-resolution seismotectonic analysis of natural and induced seismicity. For instance, linking seismicity with mapped fault segments requires hypocenter accuracy at the sub-kilometer scale. In this study, we demonstrate that inaccurate velocity models and improper phase selection can bias absolute hypocenter locations and location uncertainties, resulting in errors larger than the targeted accuracy. To avoid such bias in densely instrumented seismic networks, we propose a coupled hypocenter-velocity inversion procedure restricted to direct, first-arriving, mainly upper-crustal Pg and Sg phases. On the basis of synthetic tests and selected ground-truth events we demonstrate that a sub-kilometer hypocenter accuracy can be achieved by regional-scale, three-dimensional Pg and Sg velocity models combined with dynamic phase selection and a non-linear location algorithm. The tomographic inversion uses about 60,000 Pg and 30,000 Sg quality-checked phases of local earthquakes in the Central Alps region. The derived models image the V-P and V-S structure of the Central Alps upper crust at unprecedented resolution, including small-scale anomalies such as those caused by Subalpine Molasse units below the Alpine front. The relocation procedure is applied to more than 18,000 earthquakes and the relocated hypocenters reveal previously unrecognized seismogenic structures, for instance in the Swiss Molasse basin south of Bern. The M-L 4.6 Urnerboden earthquake of 2017 is used as an example to demonstrate how the derived 3D velocity structure and relocated hypocenters can be jointly interpreted to constrain the lithology hosting upper-crustal seismicity in the Central Alps. Plain Language Summary To better understand how mountain belts like the European Alps presently deform and what are the plate-tectonic forces driving this deformation requires accurate knowledge of the location of earthquakes within these continental collision zones. In this study, we achieve an accuracy of less than a kilometer for earthquake locations in Switzerland, based on detailed knowledge of the subsurface structure of the Earth's crust. We use three-dimensional tomographic imaging methods to improve subsurface structural models of the Central Alps. These models also provide new insights into the geological structure of this mountain range and in combination with the improved earthquake locations allow for detailed studies of present-day tectonic processes.

Automated summary

Accurate hypocenter location is essential for high-resolution seismotectonic analysis, and this study demonstrates the use of three-dimensional velocity models and dynamic phase selection to achieve sub-kilometer accuracy in earthquake locations in Switzerland. The relocation procedure reveals previously unrecognized seismogenic structures and provides important insights for further research on tectonic seismic activity in the Central Alps.