Earthquake location in tectonic structures of the Alpine Chain: the case of the Constance Lake (Central Europe) seismic sequence

Abstract

A set of four magnitude Ml≥3.0 earthquakes including the magnitude Ml=3.7 mainshock of the seismic sequence hitting the Lake Constance, Southern Germany, area in July–August 2019 was studied by means of bulletin and waveform data collected from 86 seismic stations of the Central Europe-Alpine region. The frst single-event locations obtained using a uniform 1-D velocity model, and both fxed and free depths, showed residuals of the order of up±2.0 s, systematically afecting stations located in diferent areas of the study region. Namely, German stations to the northeast of the epicenters and French stations to the west exhibit negative residuals, while Italian stations located to the southeast are characterized by similarly large positive residuals. As a consequence, the epicentral coordinates were afected by a signifcant bias of the order of 4–5 km to the NNE. The locations were repeated applying a method that uses diferent velocity models for three groups of stations situated in diferent geological environments, obtaining more accurate locations. Moreover, the application of two methods of relative locations and joint hypocentral determination, without improving the absolute location of the master event, has shown that the sources of the four considered events are separated by distances of the order of one km both in horizontal coordinates and in depths. A particular attention has been paid to the geographical positions of the seismic stations used in the locations and their relationship with the known crustal features, such as the Moho depth and velocity anomalies in the studied region. Signifcant correlations between the observed travel time residuals and the crustal structure were obtained.