Seismicity of the northern Upper Rhine Graben — Constraints on the present-day stress field from focal mechanisms

Abstract

We present new results for the microseismic activity in the northern Upper Rhine Graben by analyzing seismogram recordings from two temporary networks of up to 13 broad-band stations in combination with data from permanent stations. Previous studies have mainly focused on the southern and intermediate sections of the Upper Rhine Graben, where the station coverage by regional networks is more dense. Between October 2010 and September 2013 we recorded 56 earthquakes that occurred within the immediate vicinity of the northern Upper Rhine Graben with local magnitudes ranging from 0.5 to 3.2. The majority of the hypocenters are located beneath the eastern graben shoulder and in the graben itself, while activity observed along the western graben shoulder is lower. A further active region is located along the southern rim of the Taunus Mountains to the northwest of the study area. The seismicity extends to a depth of 25km; the hypocentral depth distribution exhibits a pronounced peak of seismic energy release in the depth range between 12 and 18km. Fault plane solutions for a total of 58 earthquakes (including data from previous recordings) were derived from P-polarities and SV/P amplitude ratios. This new data set more than quadruples the number of available data for regional stress field analysis. The majority of the focal mechanisms show predominantly strike-slip movements; only a few events are of reverse or normal-faulting type. T-axis trends are oriented NE–SW. The calculated focal mechanisms presented in this study confirm that the area is in transtension, however, in contrast to previous studies, a stronger strike-slip component is identified. We also derived the azimuths of the principal stress axes by inverting the fault plane solutions and calculated the direction of the maximum horizontal stress, which is mainly oriented N135°E.