Structural and rheological implications of lower-crustal earthquakes below northern Switzerland

Abstract A detailed study of the recent seismicity in the northern Alpine foreland of Switzerland reveals that earthquakes with magnitudes between 0.9 and 4.2 occur not only in the upper part of the crust, but, contrary to observations in most other intracontinental settings, seismicity extends down to depths of about 30 km in the lower crust. Observed P-wave velocities in the focal region of even the deepest earthquakes are less than 6.3 km s −1 . The average value of Poisson's ratio for the entire focal-depth range (6–30 km) lies between 0.23 and 0.24. Focal mechanisms are mostly a combination of strike-slip and normal faults, with consistent orientations of P- and T-axes, but without any systematic dependence on focal depth. From geological observations, regional strain rates are of the order of 10 −13 s −1 . Two-dimensional temperature modelling, indicates that temperatures in the lower-crustal seismogenic zone are well above 450°C. Under the assumption of a dry environment, the observed P-wave velocities and Poisson's ratios imply either a granitic or quartz-rich gneissic composition or possibly a gabbroic (quartz-free) granulite. The deformation would be governed either by the rheology of quartz or of plagioclase. The onset of ductility observed in laboratory studies lies around 300°C for quartz and around 450°C for plagioclase. Thus, conditions in the lower crust beneath northern Switzerland appear to be incompatible with the occurrence of either brittle failure or plastic instabilities, as is implied by the existence of earthquakes. Alternatively, lower-crustal earthquakes could be a manifestation of the presence of fluids in the lower crust. Fluids, under near-lithostatic pressure, either evolved in situ from metamorphic reactions or from infiltration from the mantle, would tend to decrease the effective stress on pre-existing faults and thus allow brittle friction failure to occur at depths where, under dry conditions or low fluid pressures, only ductile deformation would be expected.