Not all earthquake faults behave the same. Some stick and snap, causing earthquakes. Others move slowly over time.
For years, the leading explanation for slow-moving faults has been that high-pressure fluids along the fault lubricate it, allowing the slabs to slide steadily rather than building up stress until that stress is eventually released in a large, destructive earthquake.
But in a new study of the Shumagin Gap, a quiet section of the Alaska-Aleutian subduction zone – the area where one tectonic plate dives below another – my colleagues and I found that the fault does not contain enough fluid to explain why it slides slowly. Scientists may need to rethink this assumption about subduction zones around the world.
Pinning down why faults creep matters for how scientists build models of the world’s most powerful earthquake zones to assess long-term earthquake and tsunami hazards, from Alaska to Japan to the Pacific Northwest. Knowing how earthquakes are likely to behave is essential for helping communities decide where and how to build homes and other infrastructure so they can withstand an earthquake and tsunami.
A topographic map of the Alaska-Aleutian subduction zone highlights the Shumagin Gap. The magnitude 7.8 earthquake in 2020 occurred at its inland edge, and a magnitude 8.2 earthquakes in 2021 struck nearby. Other large earthquakes are shown from 1938, 1946 and 1964.
Yinchu Li, et al., 2026
How earthquakes happen along faults
An earthquake fault is a break in Earth’s outer rock layer where two blocks of rock slide past each other. The way they slide determines what kind of shaking, if any, reaches the surface.
Some faults are “locked.” They do not budge until stress builds to a breaking point, then they release it all at once in a sudden rupture. This is what happens during most damaging earthquakes. Other faults “creep.” They glide past each other steadily, releasing stress gradually.
The biggest and most destructive earthquakes on Earth happen along subduction zones, where one tectonic plate dives beneath another. The Alaska-Aleutian margin, the Japan Trench, the subduction zone off Chile and the Pacific Northwest’s Cascadia zone are all examples. When a locked patch of a subduction fault suddenly slips, the seafloor can jolt upward and a tsunami can follow.
A quiet fault challenges a common assumption
Deep underground, fault behavior is hard to see directly, especially offshore where faults often sit beneath kilometers of seawater and sediment.
Scientists rely on measurements from GPS stations, seismometers and seafloor sensors, and then build computer models of what must be happening below. For decades, the leading explanation for creeping faults has been that high-pressure fluids along the fault reduce friction, the way a film of water causes tires to hydroplane.
