CEE’s Xie researching improvements to seafloor motion monitoring for earthquake hazard assessment

An assistant professor at the Cullen College of Engineering has received a $690,000 grant to explore a new method to measure seafloor motion, an important component when it comes to assessing offshore earthquake and tsunami potentials.

Surui Xie, an assistant professor in the Civil and Environmental Engineering Department, is the PI for “Novel Designs of GNSS-Acoustic Surveying for Low-Cost Seafloor Geodesy.” This is a collaborative research grant from the National Science Foundation jointly with a research team at the University of California San Diego.

Xie pointed to the 2004 Indian Ocean earthquake and tsunami, and the 2011 Tōhoku earthquake and tsunami off the coast of Japan, as examples of devastating events that prove the need for more extensive monitoring. Similar events will likely hit the US Pacific Northwest and Alaska in the future. While there are methods to measure seafloor movement now, typically using a combination of GPS positioning and acoustic ranging, they are too expensive to implement at scale.

“This type of measurements is costly,” he said. “To determine the motion of a single site on the seafloor, at least three pieces of underwater equipment are needed, and each one costs approximately $50,000. The high cost makes it too expensive to install enough stations to cover large areas of the ocean.”

In contrast, Xie’s group will attempt to get reliable results by using just one piece of expensive monitoring equipment.

“To do so, we need to plan the underwater measurement paths carefully so that common sources of error cancel out. A major source of error is uncertainty in how fast sound travels through seawater, which changes with ocean temperature and salinity. To test methods that can minimize the influence of this type of error, our team will set up several experimental sites off the coasts of Oregon State and southern California, in water ranging different depths.”

He added, “The idea is to use multiple un-manned vehicles called Wave Gliders to form symmetric observational platforms, and let them measure their distances to the seafloor instrument simultaneously. This will largely cancel out the errors associated with inaccurate knowledge of the sound speed and its variation.”

Mark Zumberge, a research geophysicist at UCSD, will lead the research team at the University of California San Diego. Two Ph.D. students from UH — Guoli Li and Demirkan Oral — are participating in research cruises for instrument deployment, which Xie has also taken part in.

For Xie, it is the gap of information available about sea floor movement that makes it a compelling research subject. He is part of other UH teams focused on geodesy — the science of the size and shape of the Earth, and the exact position of points on its surface.

“Scientists can track the motion and deformation of land extremely accurately using space, airborne, and terrestrial techniques. In contrast, about 71% of the Earth's surface is covered by oceans, and this portion is not well monitored. There are significant data gaps. Seafloor geodesy is the key technique to fill the gaps and improve our understanding about how Earth works.”