First direct geodetic evidence of precursory shallow slow-slip associated with seismic swarms on oceanic transform faults

Oceanic transform faults (OTFs) accommodate a significant portion of global plate motions, yet the physical mechanisms governing their characteristically low seismic coupling and the initiation of earthquake swarms remain poorly understood. Here, we provide the first direct geodetic evidence of shallow slow slip events (SSEs) on an OTF. By leveraging the increase of seismic activity during earthquake swarms as temporal constraints on high-resolution, land-based continuous GNSS data from near the just-offshore Húsavík-Flatey Fault (HFF) in North Iceland, we utilized a signal-stacking strategy to isolate ultra-slow transients from stochastic noise. This approach detected SSEs that are several weeks in duration and with an average moment magnitude of Mw 5.34, systematically preceding the seismic swarms. The marked spatial complementarity between SSEs and swarms, combined with their temporal synchronicity, seem to indicate that the aseismic transients act as a mechanical trigger for the swarm-like activity along the western portion of HFF. The pronounced contrast between the weakly coupled western segment of the HFF—characterized by concurrent SSEs and earthquake swarms—and the strongly coupled eastern segment, which lacks moderate to large earthquake clusters, reveals a fundamental along-strike heterogeneity in fault behavior. The evidence of systematic aseismic slip release along the HFF accompanying swarm activity indicates that the seismic moment deficit of OTFs can be reconciled by aseismic slip transients. Our results corroborate that OTFs are complex faults where rheological and geometrical segmentation result in a complex interplay of slow and fast slip release.

Acknowledgements: We gratefully acknowledge Baptiste Rousset and Estelle Neyrinck for generously sharing their geodetic matched filter code for this analysis.