School of rock: Properties of rocks in fault zones contribute to earthquake generation

(Press-News.org) ANN ARBOR—Earthquakes occur along fault lines between continental plates, where one plate is diving beneath another. Pressure builds between each plate, called fault stress. When this stress builds enough to release, the plates slip and grind against each other, causing an earthquake. 

Researchers have long thought that this force is the central driver of earthquakes. But another force is also in the mix: the properties of the rocks in the fault zones along the plate interface. This includes both the structure of the rock as well as how the rocks are arranged along the zones. 

Now, a University of Michigan study looking at a small region in Japan has shown that the properties of fault zone rocks really matter for the generation of earthquakes. 

Yihe Huang, lead author and U-M associate professor of earth and environmental sciences, and her team looked at data from the eastern Kanto region of Japan, including Tokyo. The region is situated where the Philippine Sea Plate is sandwiched between the North American Plate and the Pacific Plate. 

Earthquakes that occur there tend to be small and occurring at depths of about 60 to 70 kilometers—fairly deep, according to Huang. The region generates about half of the earthquakes that can be felt in Tokyo.

The researchers found that the rock material along this fault line is foliated, which means the minerals within the rocks are arranged in distinct layers. Their findings, published in Science Advances, suggest that the properties of rocks may contribute equally to the generation of earthquakes, alongside fault stress.

Huang says finding active areas of subduction zones where earthquakes tend to occur, and examining why earthquakes occur in these areas, can help researchers devise better earthquake warning systems.

"When we think about where earthquakes occur, we are always thinking about a broader scale, such as subduction zones more broadly. But it's not like that. Even in subduction zones, there are certain locations that are more active and we need to find those spots," Huang said. "Our study provides a way for us to find those spots. If they are linked together spatially or temporally, then they may contribute to the generation of bigger earthquakes.

"The main takeaway is that the subduction zone plate boundary contains much more foliated, damaged rocks than what we thought about before at this kilometer scale, which is relevant to earthquake generation in all subduction zones that host earthquakes in localized spots." 

This foliation gives the rock what's called an anisotropic property, which means the rock exhibits different properties in different directions, similar to the difference in properties of wood along its grain compared to across it. This causes the rock to have different levels of strength in these directions, similar to how wood is stronger along its grain than across it. The rock layers themselves along this fault line are a mixture of different types of rocks.

The region the researchers examined is covered by a network of borehole seismometer stations placed about every 25 kilometers. Because the seismometers are placed about 100 meters deep in boreholes, the seismometers can pick up very small signals.

The researchers used data from the borehole seismometers to calculate the underlying rock's Poisson's ratio. This ratio measures the 3D elastic properties of rocks: think about a sponge, Huang says. If you squeeze a sponge end-to-end, the middle of the sponge stretches apart. If you stretch a sponge, the middle of the sponge will bunch up. In this way, the material inside the sponge becomes deformed.

The researchers found that in this region, when the rocks in this particular fault zone are squeezed, they don’t deform much in the other direction. This means that the rock is stronger in one direction than the other, again, similar to how wood is stronger along the grain.

"That's very strange, and we have not seen that in materials from Earth at this kilometer scale," Huang said. "We are hypothesizing that this must be related to a very local structural heterogeneity in the subduction zone."

She says that both understanding the underlying structures of rocks and how tectonic plates shift together is critical to understanding how and when earthquakes occur.

"Looking at these particular anomalous regions on subduction zones is probably key to understanding where earthquakes are occurring and why they are occurring there," Huang said.

Next, the researchers are planning to examine data from seismometers placed on the ocean floor directly in offshore Japan to probe into more rocks along the plate interface of the Japan subduction zone.

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