Exhumed faults are rough, often exhibiting topographic corrugations oriented in the direction of slip; such features are fundamental to mechanical processes that drive earthquakes and fault evolution. However, our understanding of corrugation genesis remains limited due to a lack of in situ observations at depth, especially at subducting plate boundaries. Here we present 3D seismic reflection data of the Costa Rica subduction zone that image a shallow megathrust fault characterized by (1) corrugated and (2) chaotic and weakly corrugated topographies. The corrugated surfaces extend from near the trench to several km down dip, exhibit high reflectionThis version of the article has been accepted for publication, after peer review (when applicable) and is subject to Springer Nature's AM terms of use, but is not the Version of Record and does not reflect post-acceptance improvements, or any corrections.

The physical properties of fault rocks govern the stability and strength of upper crustal faults in addition to the magnitude and distribution of radiated energy during earthquakes (e.g.,

Faults have grooves that are formed by abrasion and wear during slip. Recent observations indicate that this grooving is only a large‐scale feature, indicating brittle behavior has a length scale limit. The connection between this scale and earthquake behavior remains limited because no examples exist from a proven seismogenic fault. Here, we address this problem and analyze differences in this scale between lithologies to further our understanding of the underlying mechanics. This study uses samples from the Mt. Vettoretto fault collected after the Norcia earthquake of 2016. We imaged fault topography with a white light interferometer and 10 μm resolution structure from motion and then calculated a Monte Carlo version of root mean square roughness. We found a minimum scale of grooving of ~100 μm. In comparing this fault to the Corona Heights fault, we find that this minimum grooving scale is consistent with predictions based on material properties.

Faults can fail over a range of velocities and durations from plate-rate creep to stick-slip events. Aseismic slip is any fault movement that does not occur at seismic slip speeds, while slow slip occurs at speeds greater than plate-rate and slower than earthquake slip. We are studying an induced sequence in an area far from tectonic boundaries, such that the resolved velocity across this fault pre-induced seismicity should be effectively zero. Therefore, we interchangeably use slow slip and aseismic slip. The importance of aseismic and slow slip is demonstrated by the variety of ways it can interact with earthquakes. Slow slip events can occur as precursory slip that accelerates into large earthquakes (Kato et al., 2012(Kato et al., , 2016, independent events that trigger large earthquakes (Socquet et al., 2017;Uchida et al., 2016), or transient slip events that occur unrelated to a large earthquake (Shaddox et al., 2021). Aseismic slip can also occur as afterslip and triggered transient slip following moderate and large earthquakes (Hirao et al., 2021;Schwartz & Rokosky, 2007). The occurrence of aseismic slip following an earthquake redistributes stress concentrations and changes the seismic hazard of the area. Therefore, pinpointing the location of aseismic slip is necessary for determining where this redistribution takes place to better constrain the processes that control seismic slip behavior.Unraveling the role of aseismic slip in the accumulation and release of elastic strain can be difficult as there are limits to what slip amplitudes can be detected geodetically. Small to moderate sized slow slip events are typically spatially limited with small surface deformations, and thus must be inferred from other observations. Repeating

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