Variations in mechanical properties control segmentation of oceanic transform faults

Fengzhou Tan, Wenyuan Fan, Peter M. Shearer, Mark Behn, & Jeff J. McGuire

Submitted September 7, 2025, SCEC Contribution #14813, 2025 SCEC Annual Meeting Poster #TBD

Oceanic transform faults exhibit concurrent slip behaviors, combining creep with characteristic earthquakes restricted to localized patches. At the Gofar transform fault on the East Pacific Rise, magnitude (M) 6 earthquakes are bounded by creeping segments that act as persistent rupture barriers. However, physical controls of this segmentation remain unclear. Here, we analyze one year (2008) of ocean bottom seismometer data and develop a focal mechanism catalog of 3,122 earthquakes on the westernmost Gofar transform fault to investigate faulting style and stress regimes. Pure strike-slip earthquakes comprise only 20.5% of events; most are oblique, reverse, or normal earthquakes. These non-strike-slip earthquakes are broadly distributed, and their occurrence correlates with seismicity-inferred fault segmentation. Reverse and oblique-reverse events cluster within the long-term creeping barriers. Using Coulomb failure modeling, we interpret this pattern as evidence of elevated pore‑fluid pressure and mechanical weakness within the rupture barriers. The 2008 M6 mainshock increased the focal mechanism diversity across the fault, indicating widespread secondary-structure activation. Our findings suggest that local mechanical properties govern slip mode and segmentation at Gofar, and likely other oceanic transform faults. Moreover, the prevalence of extension and compression reveals that these systems may behave as non-conservative plate boundaries within the oceanic crust.

Citation
Tan, F., Fan, W., Shearer, P. M., Behn, M., & McGuire, J. J. (2025, 09). Variations in mechanical properties control segmentation of oceanic transform faults. Poster Presentation at 2025 SCEC Annual Meeting.

Related Projects & Working Groups
Seismology