Lower-Crustal Earthquakes: Strain Rate Controls the Magnitude and Rate of Stress Amplification in Rigid Blocks

Abstract

Earthquakes in the dry lower continental crust are enigmatic, as they require very high deviatoric stresses. Field observations suggest that stress amplification in rigid blocks surrounded by ductile shear zones leads to seismic failure: the jostling block model. Here we quantify this model by systematically testing numerically how variations in geometry, material properties, and loading conditions impact magnitude and rate of stress amplification. We demonstrate that bulk strain rate is the dominant factor controlling stress amplification. High strain rates of 10⁻¹⁰–10⁻¹² s⁻¹ cause stresses on the 10²–10³–MPa level within 10⁰–10² years, while lower strain rates are insufficient to generate the stresses required for lower-crustal earthquakes. While geometries and material properties play a subordinate role in causing stress amplification, tests with varying loading conditions show that pure shear is more effective in generating high stress amplifications compared to simple shear in the case of the given geometry.