Studying ancient rocks can help us better understand the movements of tectonic plates during the years between major earthquakes

A research team from Penn State and Brown University has discovered that rocks that were once buried deep in ancient subduction zones could provide valuable information about how these zones behave between major earthquakes. This discovery was made by studying rock formations in Alaska and Japan. The scientists developed a new model to predict the pressure solution activity in subduction zones based on clues from these rocks. Sedimentary rocks consist of grains surrounded by water-containing pores. When rocks are subjected to great pressure, the grains dissolve at their boundaries into the water present in pores, forming pressure solution. This process allows the rocks to deform and change shape, which can influence how the tectonic plates slide past each other. The researchers published their findings in the journal Science Advances.

“It’s like when you go ice skating — the blade on the surface ends up melting the ice, which allows you to glide along,” said corresponding author Donald Fisher, professor of geosciences at Penn State. “In rocks, what happens is quartz grains dissolve at stressed contacts and the dissolved material moves to cracks where it precipitates.”

The most powerful earthquakes in the world take place in subduction zones, where one tectonic plate moves underneath another. As these plates become wedged together, stress accumulates in the Earth’s crust, much like a rubber band being stretched. When the stress reaches a critical point and overcomes the resistance holding the plates together, an earthquake occurs, similar to a rubber band snapping.

“We’ve shown that pressure solution is a fundamental process during the interseismic period in subduction zones,” Fisher said. “The occurrence of this pressure solution can really affect the amount of elastic strain that accumulates in different parts of the seismogenic zone.”

Exploring pressure solution in a laboratory is challenging because it happens very slowly over thousands to millions of years. To accelerate the process, higher temperatures are required, which cause other changes in rocks that affect the experiments. Hence, instead of using laboratory methods, scientists studied rocks that have experienced tectonic pressures and have been brought to the surface by geological processes. These rocks show microscopic breaks caused by strain, which contain textures that provide evidence for pressure solution, according to the scientists.

“This work allows us to test a flow law, or model, that describes the rate of pressure solution in ancient rocks that were once down at the plate boundary and have been exhumed to the surface,” Fisher said. “And we can apply this to active margins that are moving today.”

Scientists created a detailed model that considers factors such as the rocks’ grain size and solubility to link stress and strain rate.

“We were able to parameterize the solubility as a function of temperature and pressure, in a practical way that hadn’t been done before,” Fisher said. “So now we can plug in numbers — different grain sizes, different temperatures, different pressures and get the strain rate out of that.”

The strain occurring within the seismogenic layer, which is the depth range where most earthquakes happen, can be revealed through the obtained results. The Cascadia Subduction Zone, an active fault that spans from northern California to Canada and passes through major cities such as Portland, Oregon, Seattle and Vancouver, British Columbia, was used by the researchers as a case study. The temperature and strain buildup along the plate boundary in this area have been well researched, and the scientists confirmed that their model’s findings align with the crustal movements observed through satellite observations.

“Cascadia is a great example because it’s late in the interseismic period — it’s been 300 years since the last major earthquake,” Fisher said. “We may experience one in our lifetime, which would be the biggest natural disaster that North America can anticipate in terms of the potential for shaking and resulting tsunami.”

This news is a creative derivative product from articles published in famous peer-reviewed journals and Govt reports:

1. Fisher, Donald M., and Greg Hirth. “A pressure solution flow law for the seismogenic zone: Application to Cascadia.” Science Advances 10.4 (2024): eadi7279.
2. J. Gomberg, Slow-slip phenomena in Cascadia from 2007 and beyond: A review. Geol. Soc. Am. Bull. 122, 963–978 (2010).
3. G. C. Beroza, S. Ide, Slow earthquakes and nonvolcanic tremor. Annu. Rev. Earth Planet. Sci. 39, 271–296 (2011).
4. W. M. Behr, R. Bürgmann, What’s down there? The structures, materials and environment of deep-seated slow slip and tremor. Philos. Trans. A. Math. Phys. Eng. Sci. 379, 20200218 (2021).
5. C. B. Condit, M. E. French, J. A. Hayles, L. Y. Yeung, E. J. Chin, C. T. A. Lee, Rheology of metasedimentary rocks at the base of the subduction seismogenic zone. Geochem. Geophys. Geosyst. 23, e2021GC010194 (2022).

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