"Dilatancy strengthening" identified as natural brake: how fluid-filled fractured rock limits earthquake size and creates a recurring pattern.
The Gofar Fault System
The Gofar fault lies along the East Pacific Rise, marking the boundary between the Pacific and Nazca tectonic plates. These plates slide past each other at a rate of approximately 140 millimeters per year.
For at least three decades, the fault has produced magnitude 6 earthquakes every five to six years, with ruptures repeatedly occurring on the same sections and stopping at consistent locations. The intervening segments act as barriers and do not produce earthquakes above magnitude 5.5.
Research Methods and Data
Scientists analyzed data from two ocean-bottom seismometer experiments. The first was deployed in 2008, and the second operated from 2019 to 2022. These instruments were placed directly on the seafloor to record ground movement.
Researchers detected tens of thousands of small earthquakes occurring before and after two magnitude 6 events. In the barrier zones, seismic activity increased in the days and weeks before a major earthquake and ceased immediately after the main event.
The Identified Mechanism
Barrier zones are not inactive. They are structurally complex, consisting of multiple fault strands with bends, splits, and small offsets of 100 to 400 meters. These features create local extension that allows seawater to infiltrate the fractured rock.
The proposed mechanism is called "dilatancy strengthening." During a large earthquake, when the rupture reaches a barrier zone, the rapid movement causes the porous, fluid-saturated rock to expand. This expansion causes a sharp drop in pore pressure. The resulting pressure change temporarily locks the rock, preventing further sliding and halting the rupture from growing larger.
Implications for Other Faults
The study suggests that similar barrier zones may be common on oceanic transform faults worldwide. These zones may act as natural brakes, limiting the maximum size of earthquakes along these tectonic boundaries. This could explain why transform faults often produce smaller earthquakes than their tectonic conditions would otherwise permit.
Future Research and Applications
The Gofar fault is located far from populated coastlines and does not pose a direct threat. However, the findings may improve seismic hazard models for underwater faults located near populated coastlines. Researchers have suggested that future studies could involve seafloor drilling to investigate barrier zones in more detail.
Study Authors and Funding
The research was led by Jianhua Gong of Indiana University Bloomington. Collaborators included researchers from the Woods Hole Oceanographic Institution, Scripps Institution of Oceanography, the U.S. Geological Survey, Boston College, the University of Delaware, Western Washington University, the University of New Hampshire, and McGill University.
The study was funded by the U.S. National Science Foundation and the Natural Sciences and Engineering Research Council of Canada.