Present-day deformation in the Upper Rhine Graben from GNSS data

The Upper Rhine Graben (URG) undergoes continuous microseismicity recorded by seismic monitoring networks and moderate-magnitude earthquakes, such as in the zone north of the Alpine front, which includes the Jura thrust front, the Vosges, the Black Forest, the Swabian Jura and the Alsace plain. The surface velocity field is a good indication of the occurrence and location of strain gradients likely associated with seismogenic structures. To explore that possibility, we use long time-series of displacements measured from 2002 to 2018 with the dense GURN network (GNSS URG Network). The processing based on double differences of phase measurements offers an up-to-date surface velocity field of the URG and surrounding areas. The surface velocity field relative to the Eurasia reference frame (ITR2014) points out some areas where coherent movements are present for groups of stations. The heterogeneous velocity field across the Rhine Graben (Vosges, URG, Black Forest and Swabian Jura) displays velocity values below 0.2 mm yr−1. We compute a deformation field from the GNSS velocities to link the deformation to the spatial distribution of earthquakes in the Rhine Graben. The earthquakes are concentrated south of an east–west line passing through Strasbourg to the Alpine front. Instrumental and historical seismicity presents spatial similarities in the Vosges and Swabian Jura. Considering the consistent northward movement observed in the Alpine front, we suggest that the strong seismicity in the south of the URG is caused by the shortening of the Alps. North of the URG, the seismicity is more dispersed where the velocity field does not show coherent movements.