📄 earth_science

Width-dominant rupture of an Mw 7.4 earthquake off Aomori Prefecture, Japan, on 8 December 2025 and its relationship with fault scaling

This study analyzes the December 2025 Mw 7.4 earthquake off Aomori, Japan, revealing a unique width-dominant rupture pattern that deviates from standard fault scaling laws and suggests a complex plate interface geometry involving multiple asperities.

Original authors: Tatsuya Kubota, Hisahiko Kubo, Hiroaki Tsushima, Tatsuhiko Saito, Yusaku Ohta

Published 2026-07-23

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Tatsuya Kubota, Hisahiko Kubo, Hiroaki Tsushima, Tatsuhiko Saito, Yusaku Ohta

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

Imagine the Earth's crust not as a solid, unbreakable shell, but as a giant, slow-moving jigsaw puzzle. The pieces are tectonic plates, and where they rub against each other, they get stuck. Think of it like two pieces of sandpaper pressed together; they hold tight until the pressure builds up so much that they suddenly snap free. When that snap happens, it's an earthquake. But here's the tricky part: sometimes, that snap doesn't just happen in a straight line. Sometimes, the break spreads out in weird directions, or it might involve multiple "sticky spots" breaking all at once. Scientists study these breaks to understand how big they get and how much energy they release. This is crucial because the size and shape of the break determine how much the ground shakes and, more importantly for coastal areas, how big the resulting tsunami waves will be. If we can figure out the exact shape of the break, we can predict the danger much better.

Now, let's zoom in on a specific event that happened off the coast of Aomori, Japan, on December 8, 2025. A massive earthquake, rated at a magnitude of 7.4, shook the ocean floor. While other scientists were looking at the shaking waves recorded on land to guess what happened, a team of researchers decided to look at the ocean itself. They used a special network of pressure gauges sitting on the deep ocean floor, like underwater microphones listening to the ocean's heartbeat. By analyzing how the water pressure changed as the tsunami waves rolled over these gauges, they could reconstruct a map of the seafloor uplift.

Here is the surprising twist they found: usually, when an earthquake of this size happens, the broken area is longer than it is wide, kind of like a long, thin strip of tape. But this earthquake was different. The team discovered that the area that broke was actually much wider than it was long. They calculated the broken patch to be about 116 kilometers wide and only 40 kilometers long. That's nearly three times wider than it is long! It's as if you expected a long, skinny crack in a sidewalk, but instead, you found a huge, wide puddle of broken concrete.

The researchers suggest this unusual "width-dominant" shape happened because the earthquake didn't just break one single spot. Instead, it likely triggered a chain reaction, breaking several different "sticky spots" (called asperities) that were lined up along the dip of the fault, one above the other. Imagine a row of dominoes standing up; instead of just the first one falling, a whole section of them toppled over in a way that made the broken area look wide and squat rather than long and narrow. This specific area is also where the geometry of the tectonic plates gets complicated, which might be why the break behaved so strangely.

The team also compared this 2025 event to a famous earthquake in the same spot back in 1968. They found that the deeper part of the 2025 break overlapped with a "sticky spot" that had broken in 1968, suggesting that this specific patch of the fault is prone to breaking again. However, the 2025 event didn't reach all the way to the very shallow part of the fault where other slow, silent movements happen, which is a good thing for understanding the limits of the rupture.

What makes this study really stand out is that the team's model fits the ocean data much better than models based on land-based sensors. Land sensors sometimes miss the details of breaks that happen far out at sea, especially near the surface. By using the ocean-bottom pressure gauges, the researchers were able to see the full extent of the break, revealing that it stretched far toward the shore (the "up-dip" side). This finding challenges the standard rules scientists use to predict how big an earthquake's break will be based on its magnitude. This earthquake is an outlier, a weird exception that proves the rules aren't always perfect. It suggests that in this specific corner of the ocean, the tectonic plates might be interacting in a more complex, multi-layered way than we previously thought. While this doesn't mean every earthquake will be this wide, it does tell us that we need to keep our eyes open for these unusual shapes, especially in areas where the plate boundaries twist and turn.

Technical Summary: Width-Dominant Rupture of the 2025 Off-Aomori Earthquake

Problem Statement
On December 8, 2025, an interplate earthquake with a magnitude of $M_J$ 7.4 occurred off the coast of Aomori Prefecture, northeastern Japan. While seismic data provides essential information on earthquake sources, estimating the spatial dimensions of rupture areas for offshore events remains challenging due to the limited azimuthal coverage of onshore stations and the complex effects of coastal sites on tsunami waveforms. Previous studies of similar events in the Tohoku subduction zone, such as the 2011 Tohoku earthquake, have shown that rupture zones can extend significantly toward the trench (up-dip), a feature often under-resolved by land-based observations. Furthermore, the relationship between this 2025 event and the major $M_J$ 7.9 earthquake of 1968, which ruptured a nearby asperity, requires re-evaluation using modern data. The primary objective of this study is to reliably estimate the fault dimensions and slip distribution of the 2025 Off-Aomori earthquake using dense offshore tsunami data and to analyze how these dimensions relate to established fault scaling laws.

Methodology
The study utilizes data from the S-net (Seafloor Observation Network for Earthquakes and Tsunamis along the Japan Trench), a dense network of ocean-bottom pressure gauges (OBPs). The analysis proceeds in two main stages:

Tsunami Source Inversion:
- Raw OBP data were processed to remove tidal components using a theoretical tide model and high-frequency seismic noise via a 100-second lowpass filter. To address long-term drifts and large step-like pressure offsets (likely artificial) observed near the epicenter, the authors applied a highpass filter (3600 s cutoff) and utilized time-differentiated pressure waveforms ( $\partial p/\partial t$ ) for the inversion, following the method of Kubota et al. (2018).
- The inversion distributed unit source elements (20 km diameter) over a $200 \times 200$ km area with 10 km spacing. Tsunami propagation was simulated using a linear long-wave equation and ETOPO2022 bathymetry data.
- Spatial smoothing and damping constraints were applied to stabilize the inversion, with weights determined through trial and error.
Rectangular Fault Modeling:
- The estimated tsunami source distribution was treated as offshore vertical displacement data. A single rectangular fault model with uniform slip was estimated using a Bayesian framework and a Markov Chain Monte Carlo (MCMC) approach.
- Model parameters included fault geometry, length, width, and slip amount. The strike angle was fixed at 187°, consistent with the F-net moment tensor solution, as tsunami source data alone poorly constrains strike.
- The MCMC sampling involved $5 \times 10^4$ steps, discarding the first $1 \times 10^4$ as burn-in, to derive the posterior probability density function and identify the optimal solution.

Key Results

Tsunami Source Distribution: The inversion revealed a major uplift area extending approximately 100 km in the east–west direction and 50 km in the north–south direction. The epicenter was located at the western edge of this uplift, with the main area extending predominantly toward the up-dip (east) side.
Fault Dimensions: The optimal rectangular fault model has a length ( $L_0$ ) of 40 km and a width ( $W_0$ ) of 116 km, with a uniform slip of 0.6 m, corresponding to a seismic moment of $1.6 \times 10^{20}$ Nm ( $M_W$ 7.40).
Width-Dominant Rupture: The estimated fault width is nearly three times its length ( $W/L \approx 2.9$ ). This deviates significantly from common scaling relationships for interplate earthquakes, which typically exhibit length-dominant ruptures ( $L > W$ ).
Comparison with Other Models:
- Models derived from teleseismic data (e.g., USGS, JMA) and onshore GNSS data (GSI) tend to concentrate slip around the epicenter with dimensions closer to $60 \times 40$ km, failing to resolve the extensive up-dip slip.
- A model derived from onshore GNSS data by Tohoku University, which accounts for horizontal displacements along the Hokkaido coast, shows a width of ~90 km, aligning more closely with the tsunami-derived width.
- The deeper half of the 2025 fault model overlaps with the northern asperity of the 1968 $M_J$ 7.9 earthquake, suggesting re-rupture of this patch. However, the 2025 rupture extended further up-dip than the 1968 event.
Scaling Relationship: The 2025 event appears as an outlier in the $L$ vs. $W$ scaling plot (Allen and Hayes, 2017). The authors note that if the fault width were halved ( $W_1 = 0.5W_0$ ), the parameters would align with standard scaling relations, suggesting the observed width-dominance may represent the combined rupture of multiple subevents or asperities along the dip direction rather than a single asperity.

Significance and Claims
The paper claims that the dense offshore tsunami data from S-net provides superior constraints on the up-dip limit of offshore earthquake ruptures compared to onshore datasets, which often suffer from poor azimuthal coverage and sensitivity to shallow slip. The study highlights that the 2025 Off-Aomori earthquake exhibited a width-dominant rupture, likely reflecting the simultaneous or sequential rupture of multiple asperities along the dip direction. This behavior may be linked to the complex geometry of the plate interface in the source region, where the strike angle of the subducting plate rotates significantly.

The authors suggest that the deviation from standard fault scaling laws is not necessarily an error in estimation but may indicate a rupture process involving multiple subevents, similar to observations in the 2003 Colima and 2022 Michoacán-Colima earthquakes. The study underscores the importance of integrating offshore geodetic data to reassess the rupture extents of past major earthquakes (such as 1968 and 1994) and to better understand the accumulation and release of slip deficit in the Tohoku subduction zone.

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