Welcome to Temblor’s coverage of the American Geophysical Union (AGU) 2020 Fall Meeting! Today (Thursday, December 10, for those keeping track), we focus on student-led research that our readers might find interesting. Read on for some of our favorites from today.
By Alka Tripathy-Lang, Ph.D., science writer (@DrAlkaTrip)
Citation: Tripathy-Lang, A., 2020, American Geophysical Union meeting brings student research to light, Temblor, http://doi.org/10.32858/temblor.144
Subduction zone tremors
Institut de Physique du Globe de Paris doctoral student Gaspard Farge studies tremors, or low frequency earthquakes, in subduction zones. Tremors tell scientists about fluid movement, as opposed to high frequency earthquakes (the regular kind) that signal breaking rocks. In his poster, Farge and his colleagues show a simple model designed to replicate tremor activity at the interface between a subducting slab and the overriding tectonic plate.
In their model, the subduction interface is fed by a constant deep source of water. In real life, the water comes from dehydration reactions, where water-bearing minerals stable at surface conditions ride the subduction highway to the mantle, releasing water as they succumb to high pressures and morph into new molecules. This liberated water funnels into the channel between the two tectonic plates. Farge models the buildup of water pressure as a valve between zones of low and high permeability. These valves open when the pressure differential between the zones is high enough. Once the initial valve releases its water to the next section of the subduction channel, pressure builds on neighboring valves, triggering a cascade. Each opening valve functions as a seismic source.
Farge and his colleagues compare their model to the distribution of tremors along the subduction zone interface in Guerrero, Mexico, and find that they can replicate the tremors’ tendencies to migrate and cluster in both time and space. “Our title might sound a bit simplistic,” Farge told Temblor, referring to the query at the top of his poster: Tectonic tremor without slip? However, he and his colleagues are simply exploring the contribution of fluids to subduction processes, and differences in pore pressure seem at least as important as slip.
Searching for slip
Elizabeth Sherrill, a doctoral student at Indiana University, presented her work on the Nankai trough, which has produced two major megathrust earthquakes in the recent past — the 1944 magnitude-8.1 Tonankai earthquake and the 1946 magnitude-8.4 Nankai event. This section of the subduction zone off the coast of Japan is the most heavily instrumented in the world, and includes data reaching back into the 1940s.
Sherrill modeled slip both during and after these events, and although this research is ongoing, they’ve made some interesting observations. First, the slip budget, which Sherrill explained is the amount of movement that should occur on the interface between two moving plates, is met in certain places and below what it should be in others. In particular, her model showed that 30 years of afterslip, which is movement without seismicity that occurs after an earthquake, following the 1940s megathrust earthquakes helps to fill in some of the missing slip between 12 to 25 miles (20 to 40 kilometers) below the island of Shikoku. She also found that in western Shikoku, slip is missing at shallower depths. “It may be that there is additional aseismic slip that we have not been able to detect,” says Sherrill. “Or, that area … may rupture in a larger megathrust earthquake to make up for that slip deficit.”
Modeling an East African rift volcano and fault
Doctoral student Joshua Robert Jones of Virginia Tech, studies the East African rift zone. In his poster, he explored how changes at Ol Doinyo Lengai, an active, carbonatite volcano, can affect stress on the nearby Natron Fault. “The Natron Rift, where Lengai and the Natron Fault [are] located, is one of the youngest rift regions [in this part] of the East African Rift System,” Jones told Temblor. Because this segment is so young, he said, “understanding how stress is transferred … can help us investigate the initial processes of rifting.” Exploring stress transfer between an active volcano and fault will help scientists better understand how these “hazards are affected by stress change,” he said.
In his poster, Jones modeled the volcano and fault using PyLith, an open-source piece of software that lets users simulate crustal deformation and explore faulting. In his model, he varied the size of the magma chamber, topography, and material properties, which served as a proxy for lithology. Then, he compared the models and calculated stress difference between them. Based on these preliminary findings, the presence of topography should produce the most significant change in stress on the Natron fault, with the size of the magma chamber also potentially playing a role.
At the moment, Jones and his coauthors are comparing a flat model versus one that simulates Ol Doinyo Lengai. “We are not able to have a dynamic topography the changes with inflation and deflation,” he said, “but [seeing how] change in topography during an eruption cycle would affect stress transfer [would be interesting].” The size and plumbing of the magma chamber at Lengai is also not well-constrained, he said, which is why they modeled different magma chamber volumes. The next step of this work is to quantify uncertainties, and the project is ongoing.
Another virtual day at AGU
The above samples a small fraction of the interesting and important work presented by graduate students today at AGU. Other notable work includes this talk by Sydney Dybing, a student at the University of Oregon, who woke up at 4:00 a.m. to discuss her work on characterizing early earthquake rupture using instruments placed deep within boreholes that measure strain. She wants to know “how soon we can tell how big an earthquake is going to be.” A poster by post doc Simona Colombelli of the University of Naples Federico II discussed the same question as Dybing’s work, but focused on seismic data. Spoiler alert: she says it’s possible. Had she been in the actual poster hall, her next-door neighbor, Haoran Meng of the University of California, San Diego, would have been arguing the opposite a few feet away — that how an earthquake starts has little impact on how big it can become.
Thanks for joining Temblor’s coverage of the virtual AGU 2020 Fall Meeting.
