The mechanisms of deep earthquakes

• Main Author: Hughes, Andrew A.
• Published: University of Edinburgh 1999
• Subjects: 551.8
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.652694

High temperatures and pressures inhibit frictional sliding, normally restricting the occurrence of earthquakes to depths less than 30 km. However, in subduction zones, earthquakes occur down to depths approaching 700 km. In the literature, the process which enables unstable sliding at great depths is often linked to the occurrence of mineral reactions. In this thesis, both the pressure-temperature conditions of deep seismogenesis and the detailed kinematics of deep earthquake sources are investigated. Two dimensional thermal models are calculated for many subduction zones and are used to predict the spatial loci of various mineral reactions, and also to estimate the temperatures at which earthquakes nucleate. I show that the spatial distribution of seismicity is strongly controlled by temperature but is largely independent of pressure, suggesting that seismogenesis is unrelated to the occurrence of mineral reactions, which are dependent on both pressure and temperature. Moment tensor inversion using the polarities and relative amplitudes of P and its surface reflections, pP and sP, provides high-resolution constraint on the mechanisms of deep earthquakes. Using this technique, analysis of eight earthquakes revealed them all to be compatible with a double couple source (constant volume, simple shear). The most well constrained solution shows explicitly that any reduction in volume accounts for less than 2% of the total seismic moment. Variations in the duration of P and pP are modelled as directivity effects to invert for the geometry of rupture. The inferred rupture velocities vary widely between different events, with a minimum range 1.2 £ <i>V_r</i> £ 4.5 km s^-1, and are independent of pressure, temperature and fault plane orientation. The results described are in agreement with the hypothesis that instability develops as a result of dynamic grain size reduction and / or heating through viscous dissipation in a predominantly plastic deformation regime. This mechanism is consistent with published field observations, of pseudotachylytes in otherwise ductile shear zones, from the deep crust and shallow lithospheric mantle.