Geometry, structure, and tectonic regime of oceanic transform faults revealed by teleseismic earthquake focal mechanisms

Summary Oceanic transform faults (OTFs) have long been viewed exclusively as vertical, strike-slip structures offsetting mid-ocean ridges, yet their deep geometry and structural complexity remain poorly constrained. Thus, key questions persist, including whether OTFs are single-stranded and continuous, whether they maintain vertical dip angles, if they accommodate mixed-mode slip, and what factors control their geometry. This study addresses these questions through a global statistical analysis of teleseismic earthquake focal mechanisms from 150 OTFs across diverse tectonic settings. We introduce stack maps, a novel method that quantifies fault dip and rake, providing a graphical representation of average focal mechanisms. Our findings reveal that while OTFs tend to conform to the standard vertical, strike-slip model, nearly half exhibit deviations, either in dip or motion, challenging the classical view of these plate boundaries. We identify four distinct OTF categories: (1) those adhering to the standard model, (2) non-vertical faults with transtensive/transpressive components, (3) non-vertical faults accommodating strike-slip motion, and (4) vertical faults with a vertical component of motion. Tectonic regime shifts emerge as a primary driver of structural changes, with non-vertical geometries persisting even after the regime reverts to pure strike-slip motion. This structural memory suggests that fault geometry, once established, remains stable over geological timescales of several tens of Myr. By reconciling previously ’unusual’ focal mechanisms with fault structure and dynamics, this work demonstrates that global seismic catalogues, when analysed statistically, offer robust insights into OTF geometry and tectonic regimes.