Motivation
An earthquake occurs when elastic strain builds up on faults in the Earth’s crust and is released as sudden, rapid slip. It is possible to measure the difference between the stress just before and after the earthquake using seismic data (among other types), which is commonly known as the stress drop. In simple terms, stress drop represents how much of the accumulated shear stress is released during the earthquake. It does not indicate how much absolute stress has accumulated on the fault before it slips, but intuition suggests that large, accumulated stress may lead to large stress release, and vice versa.
Several studies have suggested that earthquake stress drop may be related to fault strength, defined as the maximum shear stress a fault can withstand before it breaks. If this relationship holds true, stress drop should increase with depth, since rocks are thought to become stronger deeper in the brittle part of the Earth’s lithosphere. This hypothesis is intuitive: the deeper we go, the higher the pressure, and the stronger the rocks should be.
However, observations have not always supported this idea. Some studies show clear evidence of stress drop increasing with depth, while others find different trends. The dependence of stress drop on depth therefore remains debated and is not consistently observed across different regions and datasets.
Direct measurements of shear stress in the brittle lithosphere are extremely challenging to make and are sparsely distributed and/or limited to shallow depths. Thus, scientists infer shear stress indirectly through models, laboratory experiments, or seismological proxies such as stress drop.
To complicate matters, stress drop estimates themselves can vary by more than three orders of magnitude, even among earthquakes of similar size. The large variability in earthquake stress drop estimates, combined with the lack of direct stress measurements, makes it difficult to determine conclusively whether stress drop is truly related to maximum shear stress and whether it increases with depth.
Study region: Why northern Japan?
To investigate the relationship between stress drop, depth and shear stress, we focus on one of the most seismically active and well-instrumented regions on Earth: Northeastern Japan, which lies along a tectonic plate boundary where the Pacific Plate is subducted beneath the North American Plate. The study region experienced intense aftershock activity following the Mw 9.0 2011 Tohoku-Oki earthquake, one of the largest earthquakes ever recorded.
We focus on northeastern Japan because it offers exceptionally high-quality seismological data, including a dense network of borehole seismometers, which provide high signal-to-noise recordings of ground motion, even for small events. Aftershock seismicity following the Tohoku-Oki earthquake was intense and spanned a depth range of about 60 kilometers, providing an ideal dataset to investigate how stress drop varies with depth.
Main findings
Our study analyzes stress drop values from thousands of aftershocks recorded in the northern Japan forearc (i.e., the area between inland Japan and the deep-sea trench ~220 km off the coast). We observe that median stress-drop values increase with depth across the 60-kilometer range. This pattern suggests that deeper earthquakes tend to release larger amounts of stress than shallower ones.
To explore the physical reason for this trend, we use finite-element numerical models of force balance to estimate the maximum absolute shear stress in the brittle forearc lithosphere. These models compute the total stress in the forearc resulting from the superposition of topographic (i.e., gravity) and tectonic stresses. Comparing the modeled stress values with the observed stress drop values shows a clear positive correlation between median stress drop and maximum shear stress.
Interpretation and implications
Our results provide quantitative evidence that earthquake stress drop is linked to the maximum shear stress a fault can sustain before failure. In other words, stronger faults or faults at greater depth, can sustain higher stresses before they rupture. As a result, the earthquakes that occur there tend to have larger stress drops.
Our results provide a physical basis for the relationship between stress drop and depth, and offer a consistent framework that can reconcile some of the conflicting results reported in previous studies.
Establishing a quantitative link between stress drop and fault strength has broad implications for understanding earthquake physics and crustal mechanics. It suggests that seismological data, which are routinely collected worldwide, can be used to infer relative crustal strength, a property that is otherwise nearly impossible to measure directly.
Citation:
Bocchini, G.M., Dielforder, A., Kemna, K.B. et al. Earthquake stress-drop values delineate spatial variations in maximum shear stress in the Japanese forearc lithosphere. Commun Earth Environ 6, 858 (2025). https://doi.org/10.1038/s43247-025-02877-y