Deep earthquakes (35 - 45 km) in the East African Rift Zone: Frictional properties from shear experiments on rock samples from the Lake Malawi area
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Earthquakes in the southern part of the East African Rift occur at depths in excess of 30 km, up to 35 – 45 km, indicating a thick seismogenic crust. This is uncommon, especially in an extensional regime. Typically, earthquakes don’t nucleate at depths corresponding to a temperature above 350°C, the temperature at which quartz becomes ductile. In an extensional regime, the geotherm is usually elevated, and earthquake depth more shallow. Several hypotheses have been proposed to explain why this is not the case in the (southern part of the) East African Rift Zone. Mafic material has a high viscous strength and it is therefore expected to deform brittle to deeper depths than quartzo-feldspathic material. The presence of weak zones would locally increase the strain rate within a strong lower crust, lowering the brittle-ductile transition. Here we provide clues to the potential of seismic behavior at depth for three different rocks sampled from the Malawi rift zone. Rotary shear experiments are conducted at 100 MPa, 200 MPa and 250 MPa effective normal stress, at temperatures from room temperature up to 600°C. These experiments can say something about the strength of the rocks and the velocity dependence of frictional strength, expressed using the rate and state friction parameter (a-b). This gives clues to seismic behavior from shallow to deeper depths, and possible deformation processes playing a role in this. A calc-silicate sample containing mostly diopside shows a relatively decreasing trend in friction with temperature, with its highest friction at 200°C of 0.82, and 0.57 at 600°C. It shows no trends in (a-b). Its microstructures show areas of smaller grain size compared to the surrounding grains. Grain size reduction thus played a major role in this sample. The felsic sample (58% plagioclase) shows a possible dependence of friction on normal stress, indicating brittle behavior up to high temperatures. The friction coefficient of the mafic sample is relatively unaffected by changes in temperature and normal stress. It has negative (and decreasing) (a-b) values at high temperatures (> 400°C). The strength of the mafic material and its ability to localize shear to planes of weakness at depth, in combination with its velocity weakening properties at high temperatures may provide part of the answer as to why these deep earthquakes take place in an extensional setting and in which rock type the earthquakes nucleate.