Details of direct shear experiments on powdered gouge of ash and scoria samples from Anak Krakatau, Sunda Strait, Indonesia (Table 1)

We performed laboratory friction experiments on simulated fault gouges of ash and scoria samples from Anak Krakatau (Sunda Strait, Indonesia), representing the two main lithologies that can be found at Anak Krakatau and measured the velocity-dependence of friction in velocity-step tests. Moreover, we have directly measured cohesion on the samples of powdered gouge of ash and scoria. The experiments were carried out using a single-direct shear device at room temperature. For the shearing tests, all samples were crushed and sieved to a grain size < 125 µm. The ash sample was tested under fluid-saturated conditions (3.5 % NaCl brine) and the scoria sample was tested under fluid-saturated (3.5 % NaCl brine) and room dry conditions. The experiments were carried out at two different effective normal loads, 2.5 and 8 MPa, to simulate different depths of a proto shear plane within the volcanic edifice.