Earthquakes and Temperatures in the Lower Crust Below the Northern Alpine Foreland of Switzerland
- Robert F. Mereu,
- Stephan Mueller and
- David M. Fountain
Published Online: 9 APR 2013
Copyright 1989 by the American Geophysical Union
Properties and Processes of Earth's Lower Crust
How to Cite
Deichmann, N. and Rybach, L. (2013) Earthquakes and Temperatures in the Lower Crust Below the Northern Alpine Foreland of Switzerland, in Properties and Processes of Earth's Lower Crust (eds R. F. Mereu, S. Mueller and D. M. Fountain), American Geophysical Union, Washington, D. C.. doi: 10.1029/GM051p0197
- Published Online: 9 APR 2013
- Published Print: 1 JAN 1989
Print ISBN: 9780875904566
Online ISBN: 9781118666388
A detailed study of the recent seismicity in northern 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. Based on an analysis of the asymptotic velocity of the direct waves, the P-wave velocity in the focal region of even the deepest earthquakes is about 6.2 km/s. 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 a consistent orientation of P- and T-axes, but without any systematic dependence on focal depth.
The pattern of focal-depth distributions is compared to the results of two-dimensional temperature field calculations, based on new heat-flow data and on plausible lower-crustal petrological models. the temperature modelling takes into account the temperature dependence of thermal conductivity as well as the depth dependence of radioactive heat production. Variations in mineral composition are accounted for with an experimentally established relation between seismic velocity, heat production and rock type. the calculated temperatures for the lower crustal seismogenic zone are above 450°C and are thus higher than what is generally considered compatible with brittle failure.