Seismic activity takes place along fault lines where tectonic plates meet, with one plate submerging beneath another. Pressure accumulates between the interacting plates, known as fault stress. When this accumulated stress reaches a critical point, the plates slip and grind against one another, resulting in an earthquake.
For many years, scientists believed that this force was the primary cause of earthquakes. However, another factor plays a significant role: the characteristics of the rocks in the fault zones along the tectonic boundary. This encompasses both the composition of the rock and the arrangement of these formations within the zones.
A recent investigation from the University of Michigan focusing on a small area in Japan has revealed that the characteristics of fault zone rocks are indeed crucial for the initiation of earthquakes.
Yihe Huang, the leading author and an associate professor of earth and environmental sciences at U-M, along with her team analyzed data from Japan’s eastern Kanto area, which includes Tokyo. This area lies between the Philippine Sea Plate, and the North American Plate and the Pacific Plate.
The earthquakes happening in this area are generally minor and occur at depths around 60 to 70 kilometers—significantly deep, according to Huang. This region accounts for approximately half of the seismic activity that can be noticed in Tokyo.
The scientists discovered that the rock composition along this fault line exhibits foliation, meaning the minerals within the rocks are aligned in distinct layers. Their results, published in Science Advances, indicate that rock characteristics could have an equal role in earthquake generation, alongside fault stress.
Huang emphasizes that identifying active areas in subduction zones prone to earthquakes and understanding the underlying reasons for seismic activity in these regions can assist researchers in developing more effective earthquake warning systems.
“When considering the locations of earthquakes, we typically think on a larger scale, such as encompassing entire subduction zones. However, it is more nuanced. Even among subduction zones, specific spots are more susceptible to activity, and identifying those locations is crucial,” Huang pointed out. “Our research offers a method to pinpoint those areas. If these spots are connected either spatially or temporally, they may lead to the occurrence of larger earthquakes.”
“The primary conclusion is that the subduction zone plate boundary harbors a far greater amount of foliated, compromised rocks than previously recognized at this kilometer scale, which is significant for earthquake generation within all subduction zones hosting seismic activity at particular localities.”
This foliation imparts what is termed an anisotropic characteristic to the rock, meaning that the rock presents varying properties in different directions, akin to how wood displays different features along its grain compared to across it. This leads to varying levels of strength in these orientations, similarly to wood being stronger along its grain versus across it. The rock layers along this fault line consist of a blend of various rock types.
The area under investigation by the researchers is fitted with a network of borehole seismometer stations located roughly every 25 kilometers apart. Since the seismometers are installed about 100 meters deep in boreholes, they are capable of detecting very subtle signals.
The researchers utilized data from these borehole seismometers to calculate the Poisson’s ratio of the underlying rock. This ratio evaluates the three-dimensional elastic properties of the rocks: consider a sponge, according to Huang. If you compress the sponge lengthwise, the sponge’s center bulges outward. Conversely, stretching a sponge causes the center to bunch up. In this manner, the material within the sponge undergoes deformation.
In this specific region, the researchers found that when rocks along this fault line are compressed, they exhibit minimal deformation in the alternate direction. This indicates that the rock has greater strength in one direction compared to another, resembling how wood is more robust along the grain.
“That is quite peculiar, and we have not encountered this behavior in earthly materials at this kilometer scale,” said Huang. “We hypothesize that this might relate to a pronounced local structural variability within the subduction zone.”
She emphasizes that comprehending the foundational structures of rocks and how tectonic plates interact is vital for recognizing how and when earthquakes take place.
“Investigating these specific anomalous regions on subduction zones is likely key to grasping where and why earthquakes occur,” asserted Huang.
Looking ahead, the researchers plan to analyze data from seismometers positioned on the ocean floor directly off the coast of Japan to further explore the rocks along the plate interface of the Japan subduction zone.