In‐Situ Vp/Vs Reveals Fault‐Zone Material Variation at the Westernmost Gofar Transform Fault, East Pacific Rise.

Ocean transform faults often generate characteristic earthquakes that repeatedly rupture the same fault patches. The westernmost Gofar transform fault quasi‐periodically hosts ∼M6 earthquakes every ∼5 years, and microseismicity suggests that the fault is segmented into five distinct zones, including a rupture barrier zone that may have modulated the rupture of adjacent M6 earthquakes. However, the relationship between the systematic slip behavior of the Gofar fault and the fault material properties is still poorly known. Specifically, the role of pore fluids in regulating the slip of the Gofar fault is unclear. Here, we use differential travel times between nearby earthquakes to estimate the in‐situ Vp/Vs of the fault‐zone materials. We apply this technique to the dataset collected by an ocean‐bottom‐seismometer network deployed around the Gofar fault in 2008, which recorded abundant microearthquakes, and find a moderate Vp/Vs of 1.75–1.80 in the rupture barrier zone and a low Vp/Vs of 1.61–1.69 in the down‐dip edge of the 2008 M6 rupture zone. This lateral variation in Vp/Vs may be caused by both pore fluids and chemical alteration. We also find a 5%–10% increase in Vp/Vs in the barrier zone during the 9 months before the mainshock. This increase may have been caused by fluid migrations or slip transients in the barrier zone. Plain Language Summary: Oceanic transform faults are ideal natural laboratories for studying earthquake physics due to their regular behaviors. The westernmost Gofar transform fault near the East Pacific Rise generates magnitude six earthquakes every 5–6 years on two rupture zones, respectively, which are separated by a barrier zone repeatedly stopping ruptures on the adjacent segments. The distinct behavior of the barrier zone might be due to differences in material properties from the rupture zones. To explore this hypothesis, we analyze thousands of small earthquakes in 2008 and find that the barrier zone has a higher ratio between P and S velocities than the rupture zone. This difference indicates that the barrier and rupture zones differ in their fluid content and chemical composition, which may have regulated their distinct slip behaviors. We also find an increase in the ratio between P and S velocities in the barrier zone during the 9 months before the magnitude six earthquake in 2008, which may reflect fluid flows or aseismic fault slips. Our findings suggest that pore fluids and the chemical composition of fault‐zone materials likely play a crucial role in regulating the slip behaviors of oceanic transform faults. Key Points: Rupture barrier zone has a moderate Vp/Vs of 1.75–1.80Down‐dip edge of the 2008 M6 mainshock has a low Vp/Vs of 1.61–1.69Vp/Vs in the rupture barrier zone increased in the 9 months before the mainshock [ABSTRACT FROM AUTHOR]