Scientists drilled two miles into the tectonic plate to understand Japan’s ‘great earthquake’

“It changes the way we’re thinking about stress in these systems,” added Saffer, who co-led the research and scientific mission that drilled the fault.

In geology, a fault is a fracture along which the blocks of crust on either side have moved relative to one another parallel to the fracture.

The results, documented in the journal Geology, are intriguing because it was thought that the fault was getting ready for a significant earthquake, which usually happens almost every century.

Despite the Nankai fault being stalled for years, the analysis reveals that there aren’t any significant indications of tectonic stress that haven’t yet been released.

But “that doesn’t alter the long-term outlook for the fault, which last ruptured in 1946—when it caused a tsunami that killed thousands—and is expected to do so again during the next 50 years,” warned Saffer.

Understanding the relationship between tectonic forces and the earthquake cycle

The discoveries will help researchers better understand the relationship between tectonic forces and the earthquake cycle, which could improve earthquake forecasts for both the Nankai and other megathrust faults like Cascadia in the Pacific Northwest.

“Right now, we have no way of knowing if the big one for Cascadia—a magnitude nine scale earthquake and tsunami—will happen this afternoon or 200 years from now,” said Harold Tobin, a researcher at the University of Washington and the first author of the paper.

“But I have some optimism that with more and more direct observations like this, we can start to recognize when something anomalous is occurring and that the risk of an earthquake is heightened in a way that could help people prepare.”

According to scientists, there is currently no accurate technique to predict when and where the next major earthquake or tsunami will occur, despite the fact that megathrust faults like Nankai and the tsunamis they produce are among the most destructive on the planet.

The researchers hope to measure the tectonic stress directly—the force felt when tectonic plates are pushed against one another—to determine when a major earthquake is about to occur.

The huge earthquake faults are located kilometers beneath the seafloor in deep oceans due to the tectonic makeup of the earth, making it extremely difficult to monitor them directly. The closest the scientists have gotten is with Saffer and Tobin’s drilling operation.

Chikus expedition in the deeper sea

The Chikyu, a Japanese scientific drilling ship, was used in their record-breaking effort in 2018. The Chikyu bored two miles into the tectonic plate before it became too unstable to proceed, stopping a mile short of the fault.

However, the information the researchers gathered regarding the tension in the subsoil near the fracture was vital.

They first calculated how much the borehole’s form changed as the earth compressed it from the sides, and then they pumped water through it to determine how much force was required to push its walls back out.

The strength and direction of the horizontal stress experienced by the plate pushing on the fault were revealed by this method.

Contrary to expectations, the horizontal stress that was supposed to have increased since the most recent large earthquake was almost zero, as if its stored energy had already been released.

The researchers offered a number of explanations: It may simply require less stored energy than previously assumed for the fault to slip after a strong earthquake, or the stresses may be lying closer to the fault than the drilling could access.

Alternatively, it’s possible that the tectonic push will occur suddenly over the next several years. In any case, the researchers claimed that the drilling demonstrated the necessity for additional research and ongoing fault monitoring.

Study abstract:

Stress state is a long-sought but poorly known parameter on subduction megathrusts

and in overlying accretionary wedges in general. We used direct observations made during

drilling of Integrated Ocean Drilling Program (IODP) borehole C0002 to a depth of 3058 m

below the seafloor (mbsf) in the Nankai subduction zone of southwestern Japan to constrain

in situ pore pressure and stress state in the deep interior of an accretionary wedge for the first

time. These data included downhole pressure, active pumping tests, and logging and sample

measurements. We found a nearly linear gradient in minimum horizontal principal stress

(Shmin) and show that it remained consistently smaller than the vertical stress (Sv), definitively

ruling out a thrust-faulting stress regime to at least 3 km depth, and to within ∼2 km above

the subduction megathrust. At 3000 mbsf, the estimated effective stresses were: Sv = 33 MPa,

SHmax = 25–36 MPa, and Shmin = 18.5–21 MPa. We therefore interpret that the stress state

throughout the drilled interval, which lies entirely in the hanging wall of the active mega-

thrust, lies in a normal or strike-slip faulting regime (Sv ≥ SHmax > Shmin). Total differential

stresses are below ∼18 MPa. We conclude that (1) basal traction along the megathrust must

be small in order to permit both locking (and frictional sliding at failure) of the décollement

and such low differential stresses deep within the upper plate; and (2) although differential

stresses may remain low all the way to the plate boundary at ∼5000 mbsf, SHmax must transi-

tion to become greater than the vertical stress—either spatially below the base of the borehole

or temporally leading up to megathrust fault rupture—in order to drive thrust motion along

the plate boundary as observed in great earthquakes and in recurring very low-frequency

earthquakes and slow-slip events