A team of scientists puts together the whole picture on what caused the extreme magnitude and slip during the 2011 magnitude-9 Tohoku earthquake, and overturns a long-held scientific law.
By Elisabeth Nadin Ph.D., University of Alaska Fairbanks
Citation: Nadin, E, 2020, A surprisingly weak fault led to a massive earthquake, Temblor, http://doi.org/10.32858/temblor.135
If you’ve ever tried to push a heavy couch across a floor, you know that you can shove with all your might without it moving an inch, until it suddenly slips forward. Faults are remarkably similar — stress builds until slip begins, then there’s a sudden drop in stress as the fault ruptures in a massive earthquake. A fundamental question that seismologists grapple with is, what will cause a fault to ultimately slip, such as the one that led to the 2011 Tohoku earthquake?
This was one question that led a large team of scientists to drill into this fault a year after the devastating earthquake. A recently published Annual Reviews study reveals a surprisingly low level of friction of the fault, and a coherent picture of stress before, during and after rupture. Scientists found that a combination of clay minerals in the fault and a surprisingly fast rupture speed triggered a spike in fluid temperature and pressure within the fault, causing it to weaken dramatically.
A law of the land
Faults have smooth surfaces for the most part, but little burrs stick into the fault plane. These “asperities” cause the rocks on either side to stick until stress overcomes friction in a catastrophic earthquake. Since the 1970s, earth scientists have understood that a particular law exists governing the typical level of friction that exists on a fault plane. This friction is what prevents slip along a fault — the process that causes earthquakes. Byerlee’s Law, as it is known, states that the same coefficient of friction (a number related to the resistance of motion) exists for almost all rock types.
An unusual quake
The Tohoku earthquake was unusual because of its large magnitude, and also because of how much motion occurred along the fault during the quake. The main fault that ruptured in the quake intersects the surface approximately 45 miles (72 kilometers) off the eastern shore of Japan. The quake registered as a magnitude-9.0, one of the largest ever recorded, and an astounding 203 feet (62 meters) of total slip occurred along the fault. This motion produced the devastating tsunami that hit the Japanese coast, causing widespread flooding and damage to the Fukushima Daiichi nuclear power plant.
“It was the largest instrumentally recorded slip ever in any earthquake,” notes Emily Brodsky, an earth science professor at U.C. Santa Cruz and lead author on the study. Her team of scientists wanted to know what led to such a large amount of slip. One way to do that is to look at the fault itself.
Drilling into a fault
In order to get up close to the fault, the team partnered with the International Ocean Discovery Program (IODP) to collect samples from within the fault, half a mile (800 meters) below the seafloor. Drilling into the fault was a monumental undertaking. As the Chikyu — the Japanese drilling vessel member of the IODP fleet — hovered over 7 kilometers (nearly 4.3 miles) of Pacific Ocean off the coast of Japan, a drill pipe descended to the deep-sea trench. “As a scientist, it’s unbelievable to have that kind of power available to answer the questions you want to answer,” Brodsky says of the program. “[the drilling project] was so on the edge of what could possibly be done, and [the ship’s crew] managed to do it.”
The team needed to get a handle on what friction on the fault was like before, during and after it slipped in the earthquake to see how much the friction really changed. To measure the pre-slip friction, samples of rock were retrieved from near the fault and brought to a lab, where an apparatus sheared them until they fractured under monitored conditions. Friction levels during the Tohoku earthquake were measured in several ways including by back-calculating rupture temperature from measured temperature dissipation in a borehole that was installed across the fault in 2013, as well as with magnetic and biomarker signals. The results indicated that friction was much lower than expected. “It was so surprising to so many people, that the friction was so low,” says Brodsky. “And the array of friction measurements were all shockingly overlapping in range.” A low level of friction means a fault can more easily slip once it is triggered.
According to Byerlee’s Law, the coefficient of friction is 0.6 – 0.8. “This is demonstrably not true here,” Brodsky says. Her team measured 0.08 again and again — an order of magnitude lower than expected. This means that scientists need to reevaluate how strong or weak faults are during rupture. “I grew up with Byerlee’s Law, but we should be out of the business of assuming it applies to all faults,” Brodsky says.
Another surprise from the measurements was that because of the low level of friction during slip, stress on the fault dropped to zero during the earthquake. This is like your couch hovering slightly above the ground when you push against it, allowing you to glide it with no resistance. This result means that scientists can formulate a relationship between stress on a fault and an earthquake’s magnitude, which brings them closer to forecasting the size of an event if they can estimate the stress buildup beforehand. This, Brodsky notes, “is why we spent so much time, energy and money on understanding the friction.”
Brodsky, E. E., Mori, J. J., Anderson, L., Chester, F. M., Conin, M., Dunham, E. M., … & Ikari, M. J. (2020). The state of stress on the fault before, during, and after a major earthquake. Annual Review of Earth and Planetary Sciences, 48.
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