Rupture Initiation in 3-meter Laboratory Rock Experiments: The Role of Heterogeneity, Rate Dependence, and a “Cascade Up” Model for Earthquake Initiation

Abstract:

I describe laboratory observations of earthquake initiation on a 3‐m rock sample where the nucleation process is imaged in detail. Many of the laboratory observations are consistent with previous laboratory 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. Consistent with theory, Lc depends on normal stress and ranges from about 0.5 m at 10 MPa to 2 m at 2 MPa. This type of nucleation is often observed at the intersection of creeping and locked fault segments, but also within the interior of a locked segment of the 3 m granite/granite fault. The experiments also highlight complexities not currently considered by most theoretical and numerical nucleation 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. The heterogeneity expected on natural faults motivates a “cascade up” model for earthquake initiation whereby a small dynamic event (smaller than a fault-averaged value of Lc) can jump-start a larger dynamic rupture. 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.