Srisharan Shreedharan , Luc Lavier, Chris Marone

Earthquake sequences in nature are complex, exhibiting a range of magnitudes and slip behaviors. In contrast, earthquake-like instabilities generated on frictional faults in the laboratory and in continuum numerical models are usually quasi-periodic with a smaller range of magnitudes and durations. The discrepancy, especially apparent for cm-sized samples used in lab friction experiments, has been attributed to complex multi-fault interactions in nature and heterogeneities in stress state or strength of seismogenic faults. Here, we provide another explanation by combining laboratory experiments and numerical models of fully deformable faults that show complex rupture sequences and fully confined slip events. We observe complex rupture sequences even on simple, initially homogeneous faults ranging from a few centimeters, in the lab, to tens of kilometers in numerical models. Our results show that self-generated heterogeneities on lab faults can produce slow and complex ruptures that may be fully confined on mm-scale faults, challenging the long-held idea that such lab faults fail only as rigid blocks. We also document complex behaviors including aperiodicity and significant variability in rupture properties over short timescales due to local, self-generated heterogeneities in stress and friction strength. Our simulations show that the ratio of fault zone thickness to the critical slip distance, Dc, controls the observed failure mode, with wider shear zones and larger Dc giving rise to slower slip events. We demonstrate (a) that complex rupture behaviors can arise even on initially homogeneous faults, and (b) that the same fault may accommodate a spectrum of earthquake slip modes at different scales.