Earthquake rupture propagation inferred from the spatial distribution of fault rock frictional properties

The frictional properties of fault rocks that rupture during earthquakes are expected to affect the nucleation and propagation of seismic slip, but these properties and their variation under in-situ earthquake conditions are typically unknown. Here, we present experimental results on the variation of frictional properties of fault rocks from the Alhama de Murcia Fault (AMF) in SE Spain, which ruptured in the 2011 Lorca earthquake. In the epicentral area, the fault zone is characterized by the presence of abundant phyllosilicate-rich gouges that surround more competent, fractured lenses of the phyllitic basement. Measurements of lineaments and orientations of R-shears in the gouges are consistent with CMT-solutions of the 2011 Lorca earthquake, suggesting that the bulk of displacement on the AMF was accommodated within similar gouges at depth. In order to evaluate the frictional properties of the fault rocks of the AMF, we performed rotary shear experiments under hydrothermal conditions of samples obtained from surface outcrops of the AMF zone, progressively simulating deeper levels in the crust by stepping temperature, effective normal stress and fluid pressure. A negative velocity-dependence of friction, expressed as a negative value of the Rate-and-State Friction (RSF) parameter (a - b) and which is a prerequisite for the nucleation of an instability, was observed only for samples derived from competent lenses under hydrothermal conditions. Gouge-derived samples exhibited only velocity-strengthening properties, i.e. positive values of (a - b), which increase with deeper conditions, in particular with increasing effective normal stress. Combined with our outcrop observation of the anastomosing nature of gouges surrounding more competent lenses of fractured protolith, our results suggest that the 2011 Lorca earthquake nucleated in the competent lenses, followed by propagation of slip into the frictionally weaker gouges. The inferred upward propagation direction of the 2011 Lorca earthquake is consistent with propagation of the rupture into the velocity-strengthening gouges which at shallower levels provide a smaller barrier to seismic slip due to lower values of effective normal stress and (a - b). Our results suggest that the spatial variation of the frictional properties along the AMF was an important factor controlling the nucleation and propagation of seismic slip which, together with the shallow hypocenter close to the city of Lorca, led to serious damage. We infer that understanding of such variations in frictional properties may significantly improve seismic hazard evaluations in tectonically active regions