USU Geologist Studying Seismic Behavior of California’s San Andreas Fault

Alex DiMonte working in a lab.

Arriving at Utah State University in Fall 2019, geosciences graduate student Alexandra DiMonte doesn’t see a global pandemic disrupting classes and research activities.

“I was fortunate to have the opportunity to participate in a field trip to the San Andreas Fault with USU Geosciences Professor Susannah Jänecke during the Spring Semester 2020, before the university had to close classes and human activities,” said DiMonte, who earned a master’s degree in geology from USU in 2022, and is currently working toward a doctoral degree. “The visit to the fault sparked my interest in continuing to study the area for my graduate research.”

With his master’s thesis, DiMonte explored how some faults, including the San Andreas Fault and the Wasatch Fault in Utah, slip slowly between earthquakes.

“From the excavated faults, brought to the surface of the Earth from the depths, we can observe a rich record of the fault processes and know the fault rock structures, textures and chemistry,” he said.

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In particular, DiMonte analyzed hematite, a common iron oxide mineral found on fault surfaces that preserves a chronological record of past fault activity, using a technique called uranium-thorium-helium dating.

“Hematite can be analyzed using a radioisotopic system called (U-Th)/He thermochronometry, which can constrain the slip history of a fault,” he said.

Between some faults, such as the Wasatch Fault here in Utah, the intense heat from the friction of the geologic movement on both sides gives a signature, like a fingerprint, to the hematite, DiMonte said. However, in the faults of the San Andreas fault system, hematite textures and thermochronometry data do not show these signatures, suggesting that they are slowly lost.

“This event prompted me to look for other signatures of earthquakes and slow slip on the San Andreas Fault,” he said.

For his doctoral dissertation, DiMonte studied the material properties of clay-rich rocks and sediments on the southern San Andreas fault, to determine how they influence the behavior of the fault.

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“The area I’m studying includes fault material that once lived less than a kilometer below the Earth’s surface,” he said. “We analyzed rocks from a geologically recent period of time – less than a million years old – to understand how the fault behaves today, and if and how an earthquake occurs in coming.”

The last major earthquake in the area occurred about 300 years ago, DiMonte said. Triggered and episodic shallow creep events occurring in the study area accommodate some of the slip, but understanding the behavior of the segment requires identification of material faults.

To that end, DiMonte used the scanning electron microscope, or SEM, at USU’s Microscopy Core Facility to closely examine research site samples and experimental samples from deformation apparatuses.

“We describe the friction of these materials, when stress is applied,” he said.

To advance his research, DiMonte, working with USU faculty member Alexis Ault, regularly travels to Brown University to conduct additional experiments and process data. Back at USU, he prepared clay samples to identify specific minerals with X-ray diffraction (XRD) and K-Ar dating, a technique for measuring the ratio of radioactive potassium to argon in samples.

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“Acquiring this data was essential to my dissertation research,” he said. “And I am grateful that I received the USU Department of Geosciences J. Stewart Williams Scholarship to fund this part of my work.”

He added: “We observed that, below the water table and above it during the rainy season, the influence of water on the mechanical behavior of clays and the effects of water between clay particles – or pore pressure – can dominate the shallow San Andreas fault network to enhance how earthquakes rupture at the surface.

DiMonte was one of the graduate students featured in the College of Science during its Oct. 5 “One” Day of Giving. The 2023 area of ​​the college to give focus is the graduate student scholarship.


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