New research reveals that the size of earthquake faults may have been previously underestimated, in findings that challenge long-held beliefs about regions most likely to be affected by seismic activity.
The new research, presented by geologists at the Seismological Society of America’s Annual Meeting, draws on global earthquake data to offer deeper insights into the range of fault systems.
According to researchers Christie Rowe of the University of Nevada, Reno, and Alex Hatem of the U.S. Geological Survey, the data reveal that fault zones are more complex than previously thought, branching into networks that span several hundred meters.
Incorporating data from regions like California, Turkey, and others that experience regular earthquake activity, the team has determined that earthquakes appear to activate a much larger array of fractures, rather than only single fault strands.
Rowe says this “suggests that significant parts of the broad array of fractures that [develop] over many earthquakes can be activated in a single earthquake,” an observation that appears to correspond with the width of California’s Alquist-Priolo zones, which are used to designate areas where construction must meet stricter seismic standards.
The team’s new findings could be profound regarding how scientists model earthquake activity dynamics.
“We want to know how this might change things like the shaking patterns that you would expect, or how much radiated energy you get from an earthquake,” Rowe said in a statement. “Because it’s not the same if you have slip distributed on many strands as when it is all on one strand of the fault.”
Drawing from rupture maps, geodetic surveys of creeping monuments, satellite imagery, aftershock locations, low-velocity damage zones, and the presence of specific rock types that form under seismic stress, the team assembled a large body of new data that offered unique insights into the size and dynamics of earthquake faults.
Among the team’s more intriguing discoveries is that although surface rupture zones may be wide, areas where a phenomenon known as “creep” occurs, involving slow, steady fault motion, appear far narrower by comparison.
Based on the new data, these creep zones range from 2 to 10 meters across, both at the surface and at depths as low as 25 kilometers, representing “the most localized behavior a fault does,” according to Rowe.
Rowe also emphasizes that future studies should focus on moving beyond two-dimensional models of faults, urging fellow scientists to consider the three-dimensional complexity of seismic zones.
“The sheer resistance, the strength or the friction, comes from a volume of rock that’s deforming during an earthquake or in between earthquakes,” she said. “So the size of that volume controls the strength of the fault in some really tangible ways.”
Rowe and Hatem’s findings also have implications for how scientists interpret the geologic record to help them estimate the recurrence and frequency of earthquakes. In the past, slip rates, a factor that governs how fast a fault moves, and recurrence intervals have often been gauged using surface measurements.
The findings suggest that such factors may not distinguish between sudden seismic slip and slower post-earthquake creep. As one example, Rowe cites a 2014 earthquake near Napa, California, where nearly half the observed fault movement appeared to occur gradually after the initial rupture.
“If the Napa earthquake occurred thousands of years ago and researchers came across its traces in the rock record, you would just see a bigger earthquake,” Rowe said. “You might lump all of that slip as a single event.”
Since creep isn’t always accounted for in paleoseismic analysis, Rowe says that “finding out that creep zones are quite narrow means that we should be aware that we could be convolving creep with seismic slip.”
Rowe and Hatem’s findings were presented at the 2025 Seismological Society of America Meeting.
Micah Hanks is the Editor-in-Chief and Co-Founder of The Debrief. He can be reached by email at micah@thedebrief.org. Follow his work at micahhanks.com and on X: @MicahHanks.