Constraining fault friction by re-examining earthquake nodal plane dips

We have assembled a catalogue of well-constrained focal mechanisms for earthquakes that occurred on continental dip-slip faults that have experienced only small displacements during their current phase of activity. Nodal planes for both reverse- and normal-faulting events are seen to vary between ∼30° and ∼60°, and are concentrated towards the centre of this range. The observed distributions suggest the reactivation of structures with a low coefficient of friction (less than ∼0.3, and possibly as low as ≤0.1). We propose that this low coefficient of friction corresponds to the presence of weak materials in pre-existing fault zones