What Can Surface-Slip Distributions Tell Us about Fault Connectivity at Depth?

Abstract

ABSTRACT Fault systems with stepovers and gaps along strike are ubiquitous in nature, and many modern earthquakes (e.g., 1992 Landers, 1999 Hector Mine, 2016 Kaikōura, and 2019 Ridgecrest) have shown that ruptures can readily propagate across some disconnections, while being halted by others. It is quite possible, however, that many faults that appear discontinuous at the surface are in fact connected at depth, facilitating throughgoing rupture, and potentially increasing earthquake size. The present work explores whether the mapped surface slip in an earthquake is indicative of the connectivity of the fault system at depth. If there is a signal of subsurface connectivity in the surface-slip pattern, then the connectivity of the system could potentially be inferred. Through 3D dynamic rupture modeling of faults with along-strike gaps of various depths, I explore whether the amplitude or the spatial distribution of slip after an earthquake could be used to diagnose fault connectivity at depth. I find that, in general, fault segments that are connected up to shallow depths of 1–2 km and are relatively long along strike compared to the seismogenic depth tend to have higher slip gradients at their edges than faults that are connected at greater depth, or that are disconnected to the bottom of the seismogenic zone. Systematic slip gradient differences at fault segment edges have been recorded in past earthquakes, giving hope that the modeled effect can be detected in many cases, even though mapped surface slip is affected by a number of different sources of heterogeneity. The results provide an alternative explanation for observations that stepovers that allow throughgoing rupture tend to have higher slip gradients than those at which rupture terminates: perhaps many such stepovers are connected at depth, which could persistently favor throughgoing rupture. There may be implications for interpretation of apparent fault discontinuities worldwide.