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Data from wells at plate boundaries could explain slow earthquakes


Drill preparation. The team is preparing the deep-sea drills for use on the sea floor. Credit: JAMSTEC-IODP

Slow earthquakes are long-term earthquakes that are not dangerous on their own, but are capable of causing more destructive earthquakes. Their origins lie in tectonic plate boundaries where one plate subsides under the other. Although the causal mechanism is already known, there has been a lack of data to accurately model the life cycle of slow earthquakes. For the first time, researchers have used deep-sea wells to measure the pressures below the sea floor. They hope that the data from this and future observations will help in understanding the evolution of earthquakes.

The Earth’s surface lies on giant tectonic plates. Its edges interact in different ways depending on the relative motion of the plates, their composition and density. When plates collide and one sink under the other it is known as a subduction zone, and is often the site of what are known as slow earthquakes. These are low-frequency earthquakes that release their energy over longer periods – hours to months – than the earthquakes we might feel shaking the ground beneath us, which can last from seconds to minutes.

It is important to understand that slow earthquakes, while not particularly dangerous per se, can cause larger short-term earthquakes, which can be very dangerous. Researchers believe that the pressure difference between the permeable regions of the subduction zone is the cause of the slow earthquakes. They speculated that excessive pressures beyond those types of rocks at those limits could withstand, may be responsible. Finally, solid data on these high pressure conditions were collected in a recent expedition of the Integrated Ocean Drilling Program (IODP), which included researchers from the University of Tokyo’s Earthquake Research Institute.

Drill inspection. A maneuverable robot arm inspects equipment underwater. Credit: Kinoshita et al.

“We think that the subduction fault zone is much weaker than the surrounding rock, and this can cause the fault zones to slip, which can lead to earthquakes,” said Professor Masa Kinoshita of the Earthquake Research Institute. “The high fluid pressure within the permeable rock faults, called aquifers, is one reason for this weakness. Our expedition to the Nankai Basin, a few hundred kilometers south of Osaka, involved doing tedious work measuring temperatures and pressures along the fault line. “

drilling. Metal poles extend the reach of the drill as it burrows under the sea floor. Credit: Kinoshita et al.

Typical, or “hydrostatic,” pressures below the sea floor in this region are about 60 MPa – roughly the pressure you’d feel if you lay on the ground and someone dropped the 200 Empire State Building on you. Well samples conducted by the researchers revealed pressures of about 5 MPa to 10 MPa greater than that in the vicinity of the same fault zone. The chosen area was ideal for making these types of observations. The team had foreknowledge of the existence of high temperature gradients that are likely to correlate with the changes in pressure they hope to detect. The team also included microbiologists who aim to uncover unseen microbial life in these previously unexplored regions.

“Although we obtained some very useful data, the first of its kind, the pressure readings had to be inferred, and in the future we would like to have monitoring stations on site that can transmit pressure and temperature data without the need for a vessel,” Kinoshita said. Now another expedition, this time west of Japan where there are frequent slow earthquakes. I’ve studied undersea heat flow since my graduation days. It’s exciting to actually see what was theoretical until very recently.”

Strange earthquake reveals hidden mechanism More information: T. Hirose et al, High-pressure Ben fluid patches below the bend: a possible source of slow earthquakes in the Nankai Basin off Cape Moroto, Journal of Geophysical Research: Solid Earth (2021). DOI: 10.1029/2021JB021831 Submitted by University of Tokyo

Citation: Data from wells in plate boundaries can explain slow earthquakes (2021, June 28) Retrieved June 28, 2021 from

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