Along-strike Variation in Fault Structural Maturity Dictates Location of Largest Earthquake Slip and Rupture Speed

Abstract:

We analyzed the slip distributions of ~30 large continental earthquakes for which the long-term propagation of the causative faults is known. The lengthening of faults over their long-term growth induces systematic changes of their structural maturity along their length. We find that, independent of where ruptures nucleated on the faults, largest earthquake slips systematically occurred on the most mature parts of the broken fault sections, whereas coseismic slip systematically tapered linearly over most of the rupture length in the direction of long-term fault propagation, i.e., decreasing maturity. Also, ruptures systematically propagated faster, up to supershear, along the most mature parts of the broken sections. We suggest that earthquake slip asymmetry and rupture acceleration are mainly governed by along-strike changes in fault zone strength, due to evolution of fault segmentation and off-fault damage with increasing maturity. The generic location of largest coseismic slip in the most mature part of a rupture zone is likely a static effect, resulting from the intimate elasto-static relation between stress drop, slip and elastic modulus of the faulted medium; even if stress drop is constant over the rupture, the strong reduction of elastic modulus in the zone of most intense damage off- the most mature fault section leads to significantly larger slip. Additionally, the pinning of fault slip by inter-segments decreases in direction of increasing maturity. The faster rupture speeds along the most mature rupture parts show that earthquake dynamics is also influenced by along-strike changes of fault maturity. The enhancement of rupture speed is likely due to interactions between rupture and trapped waves in the damage zone. Since the direction(s) of long-term fault propagation can generally be determined from geological evidence, it should be possible to anticipate in which direction earthquake slip will increase and accelerate, and possibly become a Big One.