Lower crustal earthquakes in the East African Rift System: Insights from frictional properties of rock samples from the Malawi rift

Abstract

Earthquakes in the southern part of the East African Rift System (EARS) occur at depths up to 45 km in the lower crust, unusually deep for an extensional regime. Typically, earthquakes in continental crust nucleate at temperatures <350 °C, the temperature at which crystal plastic creep in quartz becomes efficient, corresponding to a depth of ~15 km with an average continental geothermal gradient. Several hypotheses have been proposed to explain the deep seismicity in the EARS, including the presence of high-viscosity mafic material, an abnormally low geotherm, locally elevated strain rates, or high fluid pressures. Here, we provide frictional properties of simulated fault gouges to address the seismic behavior of felsic and mafic crystalline basement rocks from the amagmatic Malawi Rift at the southern, propagating end of the EARS. Rotary shear experiments are conducted at 100, 200, and 250 MPa effective normal stress, a fluid pressure of 100 MPa and from room temperature to 700 °C. The friction of the felsic sample (58% plagioclase) is fairly constant up until 400 °C, after which it decreases. At temperatures of 600 °C and above, the relation between shear stress and effective normal stress deviates from linearity. The friction coefficient of the mafic sample is nearly constant in the considered range of temperature and normal stress and the sample shows a linear relation between normal stress and shear stress. The mafic sample also has negative, and decreasing, (a-b) values at temperatures >400 °C. These results are consistent with earthquake nucleation in mafic material, if present, at lower crustal P-T conditions. On the other hand, the data indicate that earthquake nucleation is unlikely in felsic material at temperatures above 500 °C, unless effective normal stress is low and strain rate is elevated by several orders of magnitude.