New study reveals the cause of a 9.0-magnitude earthquake in Japan

Engineers at Caltech used earthquake simulations in the lab to provide significant support for the type of seismic propagation now presumed to be responsible for the 9.0-magnitude earthquake that devastated the coast of Japan in 2011.

When tectonic plates grind against each other, fine pebbles form along fault lines, which are the boundaries of tectonic plates. The impact of these pebbles on earthquakes has long been a source of debate among scientists. Caltech researchers explain in a new report published in the journal Nature that small pebbles, known as rock pits, first stop the spread of earthquakes but later trigger seismic rebirth, leading to severe ruptures.

“Our new approach has enabled us to look at the earthquake process up close, revealing key features of rupture propagation and the evolution of friction in rock pits,” explains Vito Rubino, lead author. “One of the main findings of our study is that fault sections previously thought to act as barriers against dynamic rupture may in fact host earthquakes, as a result of the activation of joint seismic friction weakening mechanisms.”

It was previously thought to be “immune” from major ruptures due to its “stable” or “creeping” nature, Lawrence A., Professor of Mechanical Engineering and Geophysics. Hanson and Theodor von Karmann Professor of Aeronautics and Mechanical Engineering, Rubino and co-authors show that this is not the case. When tectonic plates move gently over each other without causing major earthquakes, these faults form (for example, the currently creeping section of the San Andreas Fault in central California).

On the other hand, the rock gouge has a complex behaviour. It acts as the first line of defense against rupture, collecting energy and preventing it from spreading. The rock-drill interface degrades and dramatically reduces the friction between the two plates when the plates slide over each other at a high enough speed, causing the earthquake to return to the surface. Regeneration is the term for this process.

“We know that rock pits can either strengthen through faulty slip and act as a barrier, or weaken and reinforce earthquake ruptures,” LaPosta says, citing a large body of previous rock friction tests. “However, these behaviors are usually viewed as separate in space, with weakening and strengthening at different fault sites. Our experiments show how these behaviors can combine at the same fault sites during the same slip event, over timescales of dynamic rupture, resulting in It leads to intermittent slip and possibly transforms the fault barrier into an earthquake-prone zone.”

The research published in Nature looks at the role of drilling rock, a granular material with dimensions of micrometers and larger, and how it responds to seismic activity. The researchers used the Caltech’s so-called seismic wind tunnel, which was created by Rosakis and former Caltech Seismological Laboratory chief Hiro Kanamori, and Professor Emeritus of Geophysics John E. and Hazel S. Smits, to mimic the effect of rock drilling on earthquake propagation. Engineers and scientists can use the facility, which has been around since 1999, to investigate large earthquakes of a more manageable size for them.

To create an earthquake simulation, the team first cut a meter-sized translucent block of Homalite in two. Dynamic rupture nucleation can occur in samples as small as tens of centimeters in diameter, while examination of the same phenomena in rocks requires samples of tens of meters in size.

The researchers then applied massive pressure and shear to either side of the Homalite, replicating the tectonic pressure that builds up along the fault line. A fine-grained quartz powder was inserted between the cuts as a precaution for fault tampering. The scientists then placed a short fuse between the two pieces, the “epicenter” they were simulating. High-speed imaging technology was used to document the evolution of the simulated earthquake in a millionth of a second at a time as it developed.

“Back in the late 1990s, when we were designing the Seismic Wind Tunnel, we could never have imagined that it had succeeded in discovering such a rich spectrum of physical phenomena related to the source processes of frictional earthquakes and that these phenomena could be precisely scaled up to explain,” Rosakis adds. Natural earthquakes of vastly different length scales around the world.” “This is evidence of the enormous strength of the system’s mechanics.”

Researchers are now looking into how fluids, which are naturally present in the Earth’s crust, affect friction in rocks.

Image Credit: Getty

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