Focal mechanism variations reveal mechanical controls on segmentation at oceanic transform faults

Abstract

Abstract Oceanic transform faults (OTFs) exhibit concurrent slip behaviors, with creep and characteristic earthquakes on localized patches. At the Gofar transform fault on the East Pacific Rise, magnitude 5.5+ earthquakes are bounded by creeping segments that act as persistent rupture barriers, yet the physical controls on this segmentation remain unclear. Here, from one year of ocean bottom seismometer data, we develop a focal mechanism catalog of 2323 earthquakes on the westernmost Gofar transform fault. This catalog enables robust statistical analyses of along-strike variations in faulting type and mechanical properties. Pure strike-slip earthquakes comprise ~31% of events; oblique earthquakes are pervasive; pure reverse and normal events also exist. Faulting types correlate with seismicity-inferred fault segmentation, with reverse and oblique-reverse events clustering in rupture barriers. Coulomb failure modeling shows that the pattern is diagnostic of elevated pore-fluid pressure and mechanical weakness within the barriers. The 2008 M6 mainshock increased focal mechanism diversity across the fault, indicating widespread activation of secondary structures through the seismic cycle. Comparisons with the Blanco and Chain transform faults suggest diverse faulting and along-strike mechanism clustering may be common OTF features, implying that local mechanical properties govern segmentation.