Scientists have identified the physical mechanism that causes magnitude 6 earthquakes to occur with regularity every five to six years on the Gofar transform fault, located approximately 1,000 miles off the coast of Ecuador in the eastern Pacific Ocean. The findings were published in the journal Science.
Key Details
- The Gofar fault lies along the East Pacific Rise, where the Pacific and Nazca tectonic plates slide past each other at about 140 mm per year.
- For at least three decades, large earthquakes have occurred repeatedly in the same locations and stopped at the same locations, with intervening fault segments acting as barriers.
- The study analyzed data from two ocean-bottom seismometer experiments, one in 2008 and another from 2019 to 2022.
The Mechanism
- The barrier zones are structurally complex, with multiple fault strands and small offsets of 100–400 meters, creating local extension and allowing seawater infiltration.
- A process called “dilatancy strengthening” occurs: when a large earthquake rupture reaches the barrier, rapid movement causes the porous, fluid-saturated rock to lock up due to a sharp drop in pore pressure, stopping the rupture from growing larger.
- Before each major earthquake, the barriers showed intense small-earthquake activity, which ceased immediately after the main event.
“The barrier zones are structurally complex... rapid movement causes the porous, fluid-saturated rock to lock up due to a sharp drop in pore pressure.”
Implications
- The study suggests that similar barrier zones may be widespread on ocean-floor transform faults globally, naturally limiting the maximum size of earthquakes along these boundaries.
- This knowledge may improve seismic risk models for underwater faults near populated coastlines.
The research was funded by the National Science Foundation and the Natural Sciences and Engineering Research Council of Canada.