Why deeper earthquakes pack more power than shallow ones

A team led by Prof. Dr. Armin Dielforder from the University of Greifswald and Dr. Gian Maria Bocchini from Ruhr University Bochum has found that the deeper an earthquake occurs, the more energy it releases. The discovery, published in Communications Earth & Environment, shows that rock strength is the key factor governing how much stress is released when faults rupture.

The researchers examined data from more than 10 000 earthquakes in northeastern Japan, recorded between 2011 and 2022, following the M9.0 Tohoku-Oki megathrust event. They discovered that earthquakes occurring at greater depths in the Earth’s crust experience a larger stress drop, meaning a greater release of built-up energy.

This depth dependence is not random. As rocks at depth are subjected to higher pressure, they become more compact and can bear greater stress before breaking. When failure occurs, the release is proportionally larger. These results confirm a long-suspected relationship between rock rigidity and seismic energy.

The analysis suggests that seismic data can serve as a window into the hidden strength of the Earth’s crust. By measuring the energy released in earthquakes, scientists can infer how resistant different regions of the crust are to deformation.

Measuring the strength of the deep crust

The team used advanced spectral analyses to estimate how much stress each earthquake released. This value, called the stress drop, reflects the difference between the stress before and after rupture. They combined two methods, single-spectrum fitting corrected for attenuation and spectral-ratio analysis, to improve precision.

To test whether these results matched the physics of the crust, the researchers compared them with computer models that simulate the distribution of stress in the forearc of Japan. These finite-element models calculated the maximum shear stress, known as τmax, throughout the upper 60 km (37 miles) of the lithosphere.

The observed stress drops increased steadily with depth, roughly 0.8 MPa for every 10 km (6 miles). This correlation means that the energy released in an earthquake is proportional to the maximum stress stored in the surrounding rock. Stronger rocks can accumulate more stress before they fail, producing more energetic earthquakes.

The pattern was consistent across two main study regions: one near Iwaki and another near Sendai. The Sendai transect, with slightly higher crustal stress, showed somewhat larger median stress drops. This agreement between field data and model predictions strengthens the case for a physical link between earthquake energy and rock strength.

A decade of seismic observation in Japan

The dataset used in this research was among the most detailed in the world. Japan’s dense network of Hi-net borehole seismometers provided continuous, high-quality recordings for more than a decade. The scientists analyzed over 10 000 well-located earthquakes with magnitudes of 2.5 and above, ensuring accurate depth and spectral data.

By examining 11 years of seismicity after the Tohoku-Oki event, they identified a consistent trend: stress drop values increased with depth and remained stable over time. Even as aftershocks diminished and tectonic stresses shifted, the relationship between depth and stress drop held steady.

This stability suggests that the physical properties of the faults did not change significantly. Despite the enormous strain released during the 2011 megathrust earthquake, the surrounding crustal rocks retained their strength. The finding implies that once faults reach a certain equilibrium state, they continue to behave consistently over long periods.

Prof. Dielforder noted that the stress drop values have hardly changed since 2011, suggesting that the solidity of faults remains constant. This consistency helps explain why aftershock sequences often follow predictable decay patterns over time.

How stress is stored and released

In simple terms, earthquakes occur when stress accumulated along a fault exceeds the rock’s ability to resist it. This breaking point, or failure stress, depends on the rock’s composition, the confining pressure, and the fluid content within its pores. The deeper the rock, the greater the confining pressure, and the stronger it becomes.

The Japanese data revealed that earthquakes at depths up to 60 km (37 miles) released between 10 and 30 percent of the total stress that had built up. This means that not all stored energy is released during a single rupture, allowing faults to reload and fail repeatedly in future events.

The models also showed that maximum shear stress values ranged from about 10 MPa near the surface to as high as 70 MPa in deeper sections of the crust. These stresses are much lower than theoretical estimates based on laboratory friction experiments, indicating that natural faults operate under lower effective friction due to fluids and temperature effects.

By combining seismological data and physical modeling, the study provides a more realistic picture of how stress accumulates and releases inside the Earth. It confirms that crustal strength is a key factor controlling earthquake energy, rather than simply the size of the rupture.

Why this discovery matters

The implications of this research extend far beyond Japan. Subduction zones around the world, from Chile to Alaska, share similar geological structures. If stress drop is proportional to rock strength in these regions as well, it may be possible to estimate the mechanical state of faults using only seismic data.

This approach could help scientists identify zones of unusually high or low crustal strength, which is key to assessing seismic risk. It might also help explain why some regions produce frequent moderate earthquakes while others accumulate stress for centuries before unleashing catastrophic megathrust events.

Understanding how stress varies with depth also refines models of the earthquake cycle. The finding that fault strength remains stable over time suggests that seismic hazard assessments can incorporate long-term mechanical properties rather than short-term variations.

In a broader sense, this study turns earthquakes into diagnostic tools. By analyzing how much energy they release, scientists can infer how strong the crust is and how close it may be to its next failure point. This insight brings us closer to understanding the physics behind one of Earth’s most powerful natural processes.

References:

¹ Earth’s Crust Under Stress: Researchers Decipher Energy Release During Earthquakes – University of Greifswald – November 20, 2025

² Earthquake stress-drop values delineate spatial variations in maximum shear stress in the Japanese forearc lithosphere – Gian Maria Bocchini et al. – Communications Earth & Environment – October 29, 2025 – https://doi.org/10.1038/s43247-025-02877-y – OPEN ACCESS

Reet Kaur

I’m a science journalist and researcher at The Watchers, contributing to the Epicenter edition, where I cover peer-reviewed scientific research and emerging discoveries across Earth and space sciences. With a background in astronomy and a passion for environmental science, I’ve worked in shark and coral conservation in Fiji, conducting reef and shark-behavior research, contributing to mangrove restoration, and earning PADI Open Water and Coral Reef Certifications. I bring a blend of scientific rigor and storytelling to illuminate the discoveries shaping our planet and beyond.

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