The 1994 Mw 7.8 Sanriku-Haruka-Oki Earthquake and Complex Recurrence on a Weakly Segmented Megathrust
By integrating teleseismic, strong-motion, and GNSS data, this study reveals that the 1994 Mw 7.8 Sanriku-Haruka-Oki earthquake involved two distinct NW–SE aligned slip patches separated by a low-slip zone influenced by pre-existing structural heterogeneities, suggesting that such weak segmentation governs the complex recurrence patterns of large megathrust earthquakes in the region.
Original paper licensed under CC BY 4.0 (https://creativecommons.org/licenses/by/4.0/). This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer
Deep beneath the ocean floor where the Pacific Plate dives beneath the Japanese archipelago, a massive collision zone known as a megathrust stores immense energy. When this energy is released, it causes earthquakes that can reshape coastlines and trigger tsunamis. While these fault lines stretch for hundreds of miles, they do not always break all at once. Instead, the rupture often stops and starts, breaking into smaller sections or segments that behave differently depending on the rock types, temperature, and hidden faults within the Earth's crust. Understanding exactly how these segments break, and why some earthquakes jump across boundaries while others stop, is crucial for predicting the size and location of future disasters. In the waters off the Sanriku coast of northern Japan, the history of earthquakes is particularly complex, with large events occurring in overlapping patterns over decades rather than in a simple, repeating cycle.
For years, scientists have debated the details of a major earthquake that struck this region on December 28, 1994. Known as the Sanriku-Haruka-Oki earthquake, it registered a magnitude of 7.8. The prevailing view was that this event was a partial re-rupture of a much larger area that had broken during the 1968 Tokachi-Oki earthquake, and that the 1994 slip occurred in a single, continuous strip running roughly north to south. However, a new study by researchers from Tohoku University, Kyoto University, and the University of Tokyo challenges this long-held picture. By combining three different types of data—distant seismic waves that travel through the Earth's core, strong ground-motion records from nearby stations, and precise measurements of how the ground shifted on the surface—the team has reconstructed the 1994 event with much greater clarity. Their work reveals that the earthquake did not slide in one smooth sheet, but rather broke in two distinct, powerful bursts that moved in a specific direction.
The researchers found that the rupture began at the expected location but spent its first twenty-five seconds with very little movement. Then, the fault suddenly unleashed a massive amount of energy on a patch of the seafloor about 50 kilometers west of the starting point. This first major slip lasted roughly thirteen seconds. Shortly after, a second, equally powerful burst of energy occurred about 20 kilometers further to the northwest. This two-stage process created a distinct pattern in the seismic waves recorded around the world. Stations to the southeast heard two clear, separate pulses of shaking, while stations to the northwest heard a more blended signal, a difference that confirms the rupture traveled steadily toward the northwest. The total energy released was equivalent to a magnitude 7.83 earthquake, with the ground slipping by as much as 3.3 meters in the most intense areas.
This new model of two separate slip patches separated by a zone of weaker movement helps explain why the earthquake behaved the way it did. The two main areas of slipping are divided by a geological feature called the mantle-wedge corner, a point where the rock structure beneath the fault changes from the Earth's crust to the mantle. While this boundary is often thought to stop earthquakes, the 1994 rupture managed to cross it, jumping from a shallower patch to a deeper one. The researchers also noted that the edges of the 1994 rupture align perfectly with invisible boundaries in the Earth's crust, identified by variations in gravity. These boundaries seem to act as natural walls that guide where large earthquakes can grow and where they must stop.
Looking at the broader history of the region, the study suggests that the complex pattern of earthquakes off Sanriku is not random. The 1994 event occupied the central section of a larger zone that has produced three major earthquake regions aligned in a northwest-to-southeast direction. This alignment mirrors ancient, hidden faults in the overriding plate that run parallel to the coast. The evidence suggests that these pre-existing structures control how the megathrust breaks. While the 1994 earthquake managed to rupture both of the main patches identified in the new model simultaneously, historical records show that smaller earthquakes in the 1920s and 1930s likely broke only one of these patches at a time. This means that the same area can produce different types of disasters depending on whether the two patches break together or separately. The findings indicate that the physical architecture of the Earth's crust, including hidden faults and changes in rock composition, plays a decisive role in determining the size and shape of the next great earthquake in this region.
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