Slip weakening as a mechanism for slow earthquakes

Slow slip forms part of the spectrum of fault behaviour between stable creep and destructive earthquakes1,2. Slow slip occurs near the boundaries of large earthquake rupture zones3,4 and may sometimes trigger fast earthquakes2. It is thought to occur in faults comprised of rocks that strengthen under fast slip rates, preventing rupture as a normal earthquake, or on faults that have elevated pore-fluid pressures5–7. However, the processes that control slow rupture and the relationship between slow and normal earthquakes are enigmatic. Here we use laboratory experiments to simulate faulting in natural rock samples taken from shallow parts of the Nankai subduction zone, Japan, where very low-frequency earthquakes—a form of slow slip—have been observed8–10. We find that the fault rocks exhibit decreasing strength over millimetre-scale slip distances rather than weakening due to increasing velocity. However, the sizes of the slip nucleation patches in our laboratory simulations are similar to those expected for the very lowfrequency earthquakes observed in Nankai. We therefore suggest that this type of fault-weakening behaviour may generate slow earthquakes. Owing to the similarity between the expected behaviour of slow earthquakes based on our data, and that of normal earthquakes during nucleation, we suggest that some types of slow slip may represent prematurely arrested earthquakes. Most slow fault slip has been observed near the downdip limit of the seismogenic zone at depths of ∼25–45 km (refs 2,11), but in well-instrumented areas it has also been detected at shallow depths (<10 km), updip of large earthquake ruptures8–10. Slow slip events at tectonic boundaries probably occur at the plate interface2,10, and therefore should be controlled by the frictional properties and conditions that characterize fault zones. For deep slow slip events, laboratory testing of natural material under in situ conditions is not feasible at present. Therefore, there is very little evidence to constrain the boundary conditions and friction laws that govern how these events nucleate, propagate and ultimately arrest12. Less attention has been paid to slow and transient slip at shallow depths; however, these regions are accessible to drilling and sampling. Here, we focus on the Nankai Trough, Japan, where mudstone samples have been recovered frommajor shallow fault zones13,14. Very low-frequency (VLF) earthquakes observed at Nankai have reverse-faulting focal mechanisms, and occur on splay faults within the accretionary prism or along the shallow décollement8–10. They seem to occur in regions of elevated pore-fluid pressure and therefore decreased effective stress11,15. The event locations correlate approximately with the estimated updip extent of rupture from the 1944 and 1946 Mw > 8 great earthquakes, and with the location of scientific drillsites (Fig. 1). Shallow portions of a major