Researchers discover how the biggest earthquakes get so big

A new study suggests that the surprising behavior of the magnitude 9.1 Tohoku earthquake, which lurched, paused and lurched again multiple times, can be explained by examining how the properties of the fault itself changed during the event.

U.S. National Science Foundation-supported researchers led the study, published in Nature Communications, which helps refine forecasts and improve preparedness for future hazards.

The Tohoku earthquake struck eastern Japan over 15 years ago in March 2011, triggering a tsunami that reached the coast within 30 minutes. The tsunami resulted in over 18,000 deaths and disabled three nuclear reactors.

The Tohoku earthquake is one of the most studied and is only slightly smaller than the largest earthquake to strike the U.S., the 1964 magnitude 9.2 Alaskan earthquake.

How it happened

Researchers used advanced models and generated 3D simulations to better understand the megathrust fault holding the Pacific plate next to the North American plate. Modeling the earthquake rupturing revealed complex physics that seismic and geodetic instruments could not capture.

As the Pacific Plate moves under the North American Plate, stress builds along the megathrust interface. However, that stress is countered by friction created by crushed rock, minerals and fluids at high pressures and temperatures at the interface. When the accumulated stress exceeds the frictional strength, an earthquake happens.

The new study shows that the stresses on the interface between the Pacific and North American Plates were not uniform. The researchers also found that the friction at the interface changed during the earthquake in different ways depending on the depth of fault movement and the pre-existing stresses.

As the earthquake moved downward from the hypocenter, it started and stopped several times because the motion itself quickly decreased and then increased the friction, temporarily halting the motion.

In contrast, as the earthquake proceeded toward the surface, the rock was too weak to resist the motion, causing as much as 50 meters, or the length of an Olympic-size swimming pool, of displacement, contributing to the catastrophic tsunami.

These different behaviors occur using consistent rules for how faults weaken during slip. The differences arise because the fault responds differently with depth. The further down into the Earth, the heavier the overlying rock.

This study provides a better understanding of what makes an earthquake at a subduction zone grow so large, advancing efforts to assess hazards, particularly relevant to those in Alaska and the Pacific Northwest, and ultimately leading to better forecasting of hazards.