Earthquake relocations and three-dimensional VP, VS and VP/VS along the fast-slipping Gofar oceanic transform fault, East Pacific Rise.

Fast-slipping mid-ocean ridge transform faults are characterized by quasi-periodic seismic cycles with typical inter-event times of 5 to 8 years. In particular, the Gofar transform fault (GTF) of the East Pacific Rise, generates a M_W ~ 6 earthquake every 5 to 6 years on short (~20 km) along-strike segments separated by a barrier zone. Therefore, the GTF presents the opportunity to investigate the relation between fault structure and material properties of this fault to earthquake processes. Here, we perform a joint inversion of P- and S-wave arrival times from local earthquakes to develop three-dimensional seismic velocity models (V_P, V_S and V_P/V_S) of the easternmost and westernmost segments (G1 and G3, respectively). The velocity models reveal that G3 is characterized by a more heterogeneous fault zone velocity structure compared to G1. Sharp velocity contrasts are observed along G3 interpreted to reflect along-strike variations in material properties. G1 is characterized by large low-velocity anomaly extending through the entire oceanic crust with subtle along-strike variations. The 2020 Mw 6.1 earthquake occurred within a low V_P, low V_S and high V_P/V_S patch along G1 whereas the 2008 Mw 6 earthquake occurred on sharp V_P, V_S and V_P/V_S contrast. We also note similarities between the two fault segments. In particular, rupture barrier zones are characterized by a high rate of seismicity and a rapid decrease following the mainshock. We also note the occurrence of deep seismicity in low V_P/V_S patches beneath the rupture barrier zones, which may indicate sea-water infiltration at 10 to 14 km depth below sea level.