Alpine lithosphere slab rollback causing lower crustal seismicity in northern foreland

- Lower crustal seismicity in the foreland results from Alpine orogenic stress regime.
- In the Alpine subduction-collision orogeny stress is transferred from slab rollback.
- Stress in the northern Alpine foreland is reoriented after slab break-off.

Beneath the northern foreland of the Central Alps deep crustal earthquakes up to magnitude 4 regularly occur in the continental lithosphere. At 20 km to 30 km depths, where most of these earthquakes are located, temperatures above 450 °C are expected. This leads to a more ductile rheology of the lower continental crust. To better understand occurrence and underlying processes of these unusual earthquakes, we homogenize and improve hypocenter locations of events in the period 1984 to 2012 using a high-precision multi-phase earthquake location method in combination with a reliable three-dimensional P-wave velocity model of the crust and uppermost mantle. With the new approach, the average uncertainty in focal depth of well-locatable earthquakes is less than ±1 km. The homogeneously relocated hypocenters suggest a relatively uniform depth distribution throughout the lower crust. In agreement with previous studies, seismicity is entirely restricted to the crust and no evidence for seismicity in the mantle lithosphere beneath the northern Central Alpine foreland was found. The geographical distribution of lower crustal earthquakes in the foreland correlates remarkably well with the lateral extent of the European slab beneath the Central Alps where it is still attached to the European lithosphere. In addition, the directions of extensional axes derived from focal mechanisms of the deep crustal earthquakes are predominantly parallel to the Alpine front. This consistency of extensional axes can be interpreted as the result of the transferred buoyancy force of the lower crust in the subduction, transformed to a compressional force in the foreland perpendicular to the Alpine front. Existing 2-D thermo-mechanical models predict such viscous bending and stress transfer to the foreland. In our proposed model, the anomalously deep crustal seismicity is driven by stresses transferred into the foreland interrelated with the exhumation of the crust form the orogenic root caused by the rollback of the European lithosphere.