Earthquake Initiation From Laboratory Observations and Implications for Foreshocks

This paper reviews laboratory observations of earthquake initiation and describes new experiments on a 3‐m rock sample where the nucleation process is imaged in detail. Many of the laboratory observations are consistent with previous work that showed a slow and smoothly accelerating earthquake nucleation process that expands to a critical nucleation length scale Lc, before it rapidly accelerates to dynamic fault rupture. The experiments also highlight complexities not currently considered by most theoretical and numerical models. This includes a loading rate dependency where a “kick” above steady state produces smaller and more abrupt initiation. Heterogeneity of fault strength also causes abrupt initiation when creep fronts coalesce on a stuck patch that is somewhat stronger than the surrounding fault. Taken together, these two mechanisms suggest a rate‐dependent “cascade up” model for earthquake initiation. This model simultaneously accounts for foreshocks that are a by‐product of a larger nucleation process and similarities between initial P wave signatures of small and large earthquakes. A diversity of nucleation conditions are expected in the Earth's crust, ranging from slip limited environments with Lc < 1 m, to ignition‐limited environments with Lc > 10 km. In the latter case, Lc fails to fully characterize the initiation process since earthquakes nucleate not because a slipping patch reaches a critical length but because fault slip rate exceeds a critical power density needed to ignite dynamic rupture. Plain Language Summary In uniquely large‐scale laboratory experiments, a 3‐m rock sample is squeezed until earthquake‐like slip events spontaneously develop on a planar fault cut through the sample. This paper describes the initiation of those slip events—where one part of the fault begins to slip a fraction of a second before the rest of it ruptures (i.e., preslip). The laboratory observations are compared to theoretical models, computer simulations, and field studies of foreshock sequences and other earthquake precursors. Many observations are consistent with previous work that showed slow and smoothly accelerating earthquake initiation—a process termed earthquake nucleation. When the preslip region grows larger than a critical length scale Lc (~1 m), it accelerates unstably and radiates seismic waves like an earthquake. However, some observations show an order of magnitude variation in apparent Lc. The initiation process is sensitive to details su