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

Abstract

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 VP and VS 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 ML 4.6 Urnerboden earthquake of 2017 is used as an example to demonstrate how the derived 3-D velocity structure and relocated hypocenters can be jointly interpreted to constrain the lithology hosting upper-crustal seismicity in the Central Alps.