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Seismic Wave Traveled 32 Km Under the Sea at the Nankai Fault, Revealing How Fluid and Pressure Affect Slow Earthquakes
Researchers from the University of Texas at Austin have recorded a slow slip earthquake in motion. The phenomenon was detected in a fault zone near the ocean floor off the coast of Japan. The discovery is considered important for understanding how stress accumulates and dissipates in the Earth’s tectonic plates.
The earthquake occurred at the Nankai Fault, an area known for its seismic activity and ability to generate tsunamis. The team used drilling sensors to monitor the phenomenon. The sensors were installed in a critical area where the fault is close to the ocean trench.
These sensors are sensitive enough to detect very small movements, of just a few millimeters. According to Demian Saffer, director of the university’s Institute of Geophysics (UTIG), this type of detection would not be possible with traditional GPS networks.
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Tectonic Waves Traveled Across the Ocean Floor
The phenomenon, described by scientists as a “wave” moving along the contact line between two tectonic plates, was observed in the fall of 2015.
A second similar event occurred in 2020. Both followed the same path and extended into shallow areas of the fault, where more superficial earthquakes can generate tsunamis.
The study leader was doctoral student Josh Edgington, who analyzed the data as part of his work at UTIG. The study was recently published in the journal Science.
Slow Slip Traveled 32 Km Over Weeks
In the article, the scientists state that the slip starts about 30 kilometers inland from the ocean trench.
The movement proceeds seaward at a speed between 1 and 2 kilometers per day. The events coincide with the onset of tremors and very low-frequency earthquakes.
The researchers used fluid pressure records in the pores obtained from three offshore drilling observatories.
This data allowed them to reconstruct the history of the two events and confirm that they occurred in regions of high fluid pressure and low stress.
Geological Fluids Are Key in the Process
This finding is relevant because it shows a direct link between the presence of fluid and slow slip earthquakes. Until now, this relationship was considered likely but still lacked clear observational evidence.
According to the statement released by the university, the waves originated about 48 kilometers off the coast of Japan. Over the course of weeks, they traveled about 32 kilometers before dissipating at the edge of the continental shelf.
Part of the Fault Acts as a Natural Buffer
The results indicate that this part of the fault acts as a tectonic buffer. This means that instead of accumulating energy to generate large earthquakes, this area releases pressure slowly and recurrently. Such behavior may reduce the risk of more destructive seismic events in that specific region.
Even so, the Nankai Fault has already experienced an earthquake of magnitude 8 in 1946. At that time, more than 36,000 houses were destroyed and over 1,300 people lost their lives.
A new large-scale earthquake is expected in the future, but researchers believe that slow events help relieve some of the accumulated pressure.
Study May Help Understand Other Faults
The location of the sensors is also relevant. It shows that the most superficial part of the fault can release energy independently of the rest of the structure.
This knowledge can be used to study other subduction zones, such as those that are part of the Pacific Ring of Fire — a region that concentrates the largest earthquakes and tsunamis on the planet.
The team now intends to use the collected information to investigate other areas of the fault. This will make it possible to better understand seismic behavior in similar regions and improve future risk assessment.
With information from Interesting Engineering.
