Laboratory Earthquake Ruptures Contained by Velocity Strengthening Fault Patches

Many natural faults are believed to consist of velocity weakening (VW) patches surrounded by velocity strengthening (VS) sections. Numerical studies routinely employ this framework to study earthquake sequences including repeating earthquakes. In this laboratory study, we made a VW asperity, of length L, from a bare Poly(methyl methacrylate) PMMA frictional interface and coated the surrounding interface with Teflon to make VS fault sections. Behavior of this isolated asperity was studied as a function of L (ranging from 100 to 400 mm) and the critical nucleation length, h & lowast;, which is inversely proportional to the applied normal stress (2-16 MPa). Consistent with recent numerical simulations, we observed aseismic slip for L/ h & lowast; < 2, periodic slip for 2 < L/h & lowast; < 6, and non-periodic slip for 10 < L/h & lowast;. Furthermore, we compared the experiments where L was contained by VS material to standard stick -slip events where L was bounded by free surfaces (i.e., L = the total sample length). The free surface case produced -10 times larger slip during stickslip events compared to the contained fault ruptures, even with identical L/h & lowast;. This disparity highlights how standard, complete-rupture stick -slip events differ from contained events expected in nature, due to both the free surface conditions and the heterogeneous normal stress along the fault near the free ends, as confirmed by Digital Image Correlation analysis. This study not only introduces the Teflon coating experimental technique for containing laboratory earthquake ruptures, but also highlights the utility of L/ h & lowast; as a predictive parameter for earthquake behavior.