Fault Roughness May Not Set the Ceiling for Great Earthquakes

For decades, seismologists have searched for a physical explanation for one of the most unsettling facts about subduction zones: some of them produce magnitude 9 monsters while others, apparently similar in many respects, seem to top out at far smaller events. One of the most influential ideas in this debate has been roughness. The boundary between the descending oceanic plate and the overriding plate is not a smooth surface; it is a corrugated landscape of ridges, seamounts, fracture-zone scars and abyssal-hill fabric, buried kilometers beneath the seafloor. The intuitive argument has been that a rough interface is like a landscape full of bumps and notches that resist sliding, fragmenting the fault into small patches and limiting how far a rupture can run, whereas a smooth interface allows ruptures to propagate unimpeded for hundreds of kilometers and grow into giant earthquakes. A new study published in Nature Geoscience challenges that intuition directly, concluding that the maximum earthquake magnitude a subduction zone can produce is unlikely to be controlled by interface roughness.

The appeal of the roughness hypothesis has always been its visual plausibility. On bathymetric maps, the regions that hosted the largest recorded events, such as the 2004 Sumatra-Andaman earthquake, the 2011 Tohoku-oki earthquake and the 1960 Chile earthquake, seemed to correspond with segments of the trench where the incoming plate appeared comparatively smooth, while rougher margins like those off parts of Central America or Japan’s older, sediment-starved trenches appeared to host only smaller ruptures. The hypothesis gained traction because it offered a forecasting shortcut: map the bumps on the incoming seafloor before it subducts, and you have a proxy for how big the next earthquake might be. Given how difficult it is to observe the actual fault surface at depth, a measurable feature of the incoming plate seemed like a gift to hazard assessors.

Yet the new analysis finds that the correlation does not survive careful, systematic testing. Rather than selecting a few celebrated case studies, the researchers assembled a globally consistent dataset of subduction interface roughness measurements and paired them with the maximum magnitudes that each margin has produced, constrained by both the instrumental record and, where available, longer historical and paleoseismic evidence. When roughness is quantified uniformly, using the same topographic measures of relief and spectral character of the incoming plate across all margins, the expected relationship between rough interfaces and smaller maximum earthquakes largely dissolves. Margins with prominently rough seafloor have still hosted very large ruptures, and some comparatively smooth margins have not delivered the giant events the hypothesis predicts.

The authors point to several reasons why the roughness argument fails as a control on maximum magnitude. First, roughness measured on the seafloor before subduction is a poor guide to roughness on the fault at seismogenic depths. As the plate descends, sediments smear into the topographic lows, hydrothermal alteration and mineralization smooth the interface, and the accretionary wedge redistributes material. A seamount that looks like a formidable asperity at the trench may be largely underplated or subducted within a weaker sedimentary layer by the time it reaches the depth range where most seismic moment is released. The fault that ruptures in a great earthquake is therefore not the same rough surface that geodesists and marine geologists can map from shipboard sonar.

Second, and more fundamentally, the maximum magnitude of an earthquake is a geometric property of how large a patch can rupture in a single event, and that is governed by the lateral and downdip extent of coherent locking, the segmentation of the plate boundary, and the accumulated slip deficit, not necessarily by centimeter- to kilometer-scale frictional heterogeneity. A rupture can jump or creep past small roughness elements if the surrounding fault is sufficiently stressed and strongly coupled. Conversely, a smooth interface segmented by major structural boundaries such as fracture zones or tear faults may still be unable to host a rupture longer than a few hundred kilometers. In other words, the features that actually arrest or release ruptures may be structural discontinuities with tens of kilometers of offset, not the relief on the incoming plate.

The study also re-examines the physical reasoning behind the roughness hypothesis itself. Laboratory and theoretical work shows that roughness influences frictional behavior most strongly at small scales, affecting the onset of slip and the distribution of aftershocks, but its effect on the total possible rupture area at the scale of a magnitude 9 earthquake is weak. Scale considerations matter: an earthquake of magnitude 9 ruptures a fault patch on the order of 1,000 kilometers long. Bumps a few kilometers across are simply too small to act as persistent barriers to a rupture front carrying enormous elastic strain energy. The comparison is often made to tearing a sheet of paper: small wrinkles in the paper do not determine where the tear stops if the sheet is being pulled hard enough.

The implications for seismic hazard assessment are significant and uncomfortable. If roughness cannot be used as a proxy for maximum magnitude, then several regional hazard models that factor seafloor roughness into estimates of maximum credible earthquakes may need revision. The study suggests that hazard assessors should weight other evidence more heavily, including geodetic measurements of plate coupling, the distribution of past ruptures inferred from historical accounts, tsunami deposits and coral microatolls, and the structural segmentation of the margin. It also argues against using roughness to declare any margin incapable of producing a giant earthquake, a conclusion with direct consequences for coastal communities and infrastructure planning along subduction margins worldwide.

The researchers emphasize that their findings do not make roughness irrelevant. Interface roughness still shapes where within a rupture the largest slip occurs, how strong ground shaking is distributed, and possibly the frequency of smaller-to-moderate events. What the study removes is the assumption that roughness imposes a hard ceiling on rupture size. Distinguishing between influences on the distribution of slip and controls on maximum magnitude is, the authors argue, an essential refinement that the field has too often glossed over. Roughness may sculpt the earthquake, but it does not appear to cap it.

The work also speaks to a broader lesson in earthquake science: the danger of building predictive frameworks on a handful of spectacular case studies. The great earthquakes of the past century are few, and any global comparison involving fewer than a dozen giant events is statistically fragile. By compiling a comprehensive, uniformly processed dataset, the new study reduces the risk of overfitting a narrative to memorable examples. The result is a more sober picture of subduction zones: nearly any of them, given enough time to accumulate strain, may be capable of producing ruptures larger than their instrumental records suggest, and the size of the next great earthquake may be far less predictable from the shape of the seafloor than scientists once hoped.

Subject of Research: The relationship between subduction interface roughness and maximum earthquake magnitude

Article Title: Maximum earthquake magnitude unlikely to be controlled by subduction interface roughness

Article References: Yang, X., Bell, R. E., Whittaker, A. C., Xu, H., Han, X., Knowlson, A. R., & Locher, V. A. (2026). Maximum earthquake magnitude unlikely to be controlled by subduction interface roughness. Nature Geoscience. https://doi.org/10.1038/s41561-026-02093-z

Image Credits: AI Generated

DOI: 10.1038/s41561-026-02093-z

Keywords: subduction zones, earthquake magnitude, fault roughness, seismic hazard, plate boundaries, rupture segmentation, great earthquakes, seismology, geodesy, marine geophysics, plate coupling, Nature Geoscience