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Geophysikalisches Institut, Universitaet Karlsruhe, Hertzstrasse 16, D-7500 Karlsruhe 21, FRG
The intraplate seismicity of the Hohenzollern Graben in southwest Germany is concentrated within a small region on the northern end of long shear lineaments extending from the Alps into their northern foreland. Strong shocks with magnitudes not more than 5.6 started in 1911 and repeat about every 30 years with a tendency to migrate northward and at the same time to become shallower. Recent seismic reflection and refraction work have suggested that the Hohenzollern Graben is located in the southeastern border region of an anomalous low velocity, probably high temperature body, near the Urach geothermal anomaly. It is also suggested that the top of the lower crust dips sharply from this body to the southwest. The hypocentres of the deepest earthquakes become shallower as the lineaments approach and traverse the Hohenzollern Graben in the direction towards the Urach body. From all these observations taken together, it appears that the release of seismic energy is concentrated into the area of the Hohenzollern Graben by an extensive lower crustal heterogeneity. The stress concentration in the brittle upper crust is induced by the thickening of the lower crust with its ductile behaviour. The viscosity of the lower crust in the Urach body has suffered further reduction by increased temperatures. Future research has to clarify the dominant cause of the stress concentration: is it the heterogeneity in material or the heterogeneity in thermal properties?
