Most of the major natural hazards associated with geological activity—including major earthquakes, volcanic eruptions and tsunamis—occur when the rigid tectonic plates in the planet’s lithosphere, or outer layer, collide. Beneath the plates lies the asthenosphere, a 200-mile-thick layer of the Earth’s upper mantle made of very hot material, part solid rock and part viscous magma, which lubricates the movement of the plates. Since the discovery of plate tectonics 50 years ago, scientists have made significant progress understanding these processes, but much is still unknown about this extremely complex system, particularly how the characteristics of the asthenosphere contribute to seismic and volcanic events.
Professor James Gaherty and associate professor Ryan Porter of Northern Arizona University’s School of Earth and Sustainability were recently awarded $473,000 by the National Science Foundation for a study of the Cocos oceanic plate beneath Central America. NAU is collaborating on a large project led by Samer Naif of the Georgia Institute of Technology. Gaherty and Porter will lead the seismology portions of the expedition, while Naif will lead an effort to conduct electromagnetic imaging. The NAU team, including graduate students and early-career scientists, will participate in two research expeditions in the Pacific Ocean in the next four years to investigate the tectonic and geodynamic processes of this region that control faulting, magmatism, surface deformation and their associated human impacts.
The team will use unique seafloor sensors to collect data on seismic signals and bring it back to NAU’s geophysics lab, where graduate and undergraduate students will gain research experience by analyzing the data. The resulting seismic images of the Cocos plate and underlying asthenosphere will be integrated with complementary electromagnetic images constructed by Naif and his team.
“Much of the research being done in Earth sciences is focused on the processes taking place—looking at why an earthquake starts and stops where and when it does, for example, or exactly how magma rises from tiny melting pockets deep in the mantle and accumulates in a magma chamber just below the surface,” Gaherty said.
“Understanding these processes at any given location, however, still requires a better understanding of the larger system, including the forces driving the plates, the rock types involved, the conditions that cause those rocks to break—forming faults and earthquakes—and the conditions under which they flow like Silly Putty or melt, potentially forming volcanos.”
He noted that the asthenosphere plays a huge role in controlling how plates move and provides the source of the melt for most volcanism, and by better understanding how weak the asthenosphere is and how much melt it produces, scientists can better understand the processes that underlie volcanic activity and the movement of the plates that lead to earthquakes.
Using a fleet of ocean-bottom seismometers (OBS) and ocean bottom electro-magnetometers (OBEM), the team will focus on data from the seafloor in an area off the west coast of Central America, across a feature called the Nicaragua Fracture Zone (NFZ). The water in this region ranges from 3,000 to 4,500 meters (9,800 to 14,700 feet) deep. The OBS and OBEM will operate autonomously on the seafloor, recording natural seismic and electromagnetic signals for approximately one year.
“By using passive-source seismic and electromagnetic imaging to quantify the shear velocity and electrical resistivity of the lithosphere and asthenosphere on both sides of the NFZ, the team will address a number of key questions associated with the state of the asthenosphere and the nature of intraplate volcanism near where the Cocos, Nazca and Pacific plates meet, Gaherty said.
Ultimately, the team hopes to provide a new understanding of how much melt exists in the asthenosphere and what role that plays in volcanic processes and the mobility of the plates.
Gaherty, who joined NAU in 2020, has worked on several other projects that use similar approaches in other parts of the Pacific, but this is the first time he’s worked in this region. To learn more about Gaherty’s previous expeditions, visit this research blog with entries written by several scientists and students.
“I love having the opportunity to collect data that sample portions of the Earth that have never been imaged in any other way,” Gaherty said. “Seismic networks historically are limited to on land and in many cases in highly developed regions. Over my career, I’ve had the opportunity to probe a wide range of tectonic systems that have never previously been explored using these types of seismic techniques, including remote locations in the Northwest Territories of Canada, islands on the eastern tip of Papua New Guinea and in the depths of Lake Malawi in east Africa. The expanses of the ocean basins are particularly special, and this will be the fifth seismic investigation in the Pacific that I’ve led. The data have led me and my students and coworkers to a much better understanding of the processes driving evolution of plates. They also live on as unique data for a wide range of other researchers to exploit. NASA emphasizes the excitement of exploring new planets; I take great satisfaction in exploring the depths of Earth’s ocean basins in ways that have never been done before.”
Team will work with international partners to share data, improve understanding
The team will collaborate directly with international partners in the region who represent the national networks charged with monitoring earthquakes in El Salvador, Nicaragua and Costa Rica. Representatives from each organization will be invited to join the two expeditions, gaining unique training and experience in oceanographic data collection and ocean-bottom geophysical instrumentation and enabling a host of scientific analyses that will improve understanding of seismic and tsunami hazards in the region.
Kerry Bennett | University Marketing