By Maia ten Brink
Miaki Ishii and Maureen Long are two young geophysicists whose early careers have been minted by the USArray data set. The USArray Transportable Array is an extensive network of seismometers deployed across North America that records ground vibrations from earthquakes. Ishii, Long, and other scientists and students can freely access those recordings in order to study a multitude of questions about the structures at and beneath Earth’s surface.
Ishii and Long utilize the same USArray data set; however, they study very different processes. Ishii’s research at Harvard University uses USArray’s earthquake recordings to create back-projections of what occurred at an earthquake’s epicenter. Long teaches at Yale University, where she uses USArray seismograms to study the mantle, the thick viscous layer directly beneath Earth’s crust.
Long knew she wanted to be a geophysicist back in 8th grade. “I took Earth Sciences,” she says. “We did a unit on plate tectonics and started mapping out that earthquakes and volcanoes happen on plate boundaries. I just thought it was the coolest thing I had ever heard in my life. I thought, “This is what I want to be when I grow up.” I was twelve years old.”
Her current research focuses on the dynamics of Earth’s mantle. “Right now we’re sitting here on continental crust, which is 35–40 kilometers thick,” she explains. “The mantle makes up about 80% of the volume of the Earth… If you look at a cross section of Earth, the crust is like the skin on an apple, and the mantle is everything else.”
“We want to know what’s going on down there because it turns out that processes in the mantle actually control a lot of what we see at the surface,” says Long. “We still don’t understand very well how continents are formed, why Earth has continents as part of its plate tectonics systems, and what processes led to the formation of the continental crust.”
The more scientists can learn about how the mantle interacts with tectonic plates, the more informed they can be about how, why, and when earthquakes occur. Those insights not only teach us about Earth’s geologic history, they may also help us prepare for earthquake hazards and prevent humanitarian disasters. “All of the plate tectonics we see at the surface, which affect natural hazards—we can’t understand that system unless we understand what’s going on in the deep Earth,” Long says.
It is too difficult to drill down and observe the mantle firsthand, so scientists like Long and Ishii rely on remote-sensing techniques from the field of seismology. They image the deep Earth using seismic waves from earthquakes in the same way that a doctor examines an X-ray to find out what’s going on in a patient’s body. “The fact that we can get such a crisp and detailed view of Earth’s interior from seismic waves, I just think is really cool,” says Long.
Earth’s mantle consists of rocks and minerals were compressed at such high temperatures and pressures that, at certain depths within Eath, they can flow like a viscous liquid. Using a technique called seismic anisotropy, Long can look at the different speeds of seismic waves recorded by USArray seismic stations in order to map out where the mantle flows faster or slower and in what directions.
Maps showing the topographies of mountain ranges and watershed systems of rivers already exist for the surface of North America. Long wants to map the shapes and flows of the underside of North America. “Now, with the continent-wide view of the mantle structure that we get from USArray, we can really ask how the deep structures and processes that we infer from seismology connect to the surface geology, and what that tells us about the evolution of the whole North American continent.”
Ishii studied physics as an undergraduate at the University of Toronto. Her first-year instructor, Jerry Mitrovica, was “probably one of the most entertaining people in the world… He basically told me, ‘Go be a geophysicist.’” She found that seismology combined the theoretical techniques she loved from physics with real-world applications.
Different kinds of earthquakes at different depths allow Ishii to look at different segments of the core, mantle, and crust. “I really need a deep, big earthquake to give me all the nice information about the deep inner core.” Non-scientists are often shocked when she tells them that earthquakes can be a useful tool. “I try to explain that we are not looking for disasters. My ideal earthquake is a magnitude 7.0 at 700 kilometers’ depth. People won’t feel too much at the surface unless they are sitting right on top of it.”
Ishii explains that the USArray is a particularly powerful seismic network for doing tomography—imaging Earth’s structure using seismic waves. The USArray Transportable Array seismic stations, densely packed every 70 kilometers across the surface of the United States, provide a very high-resolution image of a wavefront vibrating outward from an earthquake event. “The earth is not homogenous,” says Ishii. “We have continents and oceans at the surface. So waves passing through the continents are distorted differently than waves going through the ocean… Having dense stations [with the USArray] allows us to correct for that distortion.”
One of Ishii’s main projects using USArray data by aligning signals from earthquakes recorded at different stations in order to form a detailed visualization of how ruptures occurred. “That was something we didn’t think we could do, and it turned out to be very successful,” she says. “Now we are starting to see earthquakes that are actually not detected by conventional techniques.”
Imaging deep Earth structure using seismic tomography and doing back-projection analysis to “rewind” and visualize how faults rupture can help scientists learn what to expect next time a big earthquake hits. Scientists cannot predict earthquakes, but they can look at certain types of faults to figure out what kinds of motion and aftershocks to expect. They can also extrapolate from historical data about earthquakes to determine, within a large window of time, when another quake might hit. Their hypotheses inform building codes and evacuation procedures that can save lives. Ideally, Ishii says, they could image ruptures immediately in order to send out tsunami warnings or shaking announcements. “But without understanding how earthquakes happen, that’s very difficult.”
Ishii has been in several earthquakes herself, and she draws on those experiences to teach her students about magnitude and intensity scales. She still finds it amazing that, after an earthquake, seismic waves continue to travel through and around the planet, totally invisible to human senses. “When you have a big earthquake, it’s like hitting a bell. The earth rings,” she says.
The USArray first started making data freely available online just as Long and Ishii were finishing their graduate and postdoctoral studies. Long says, “It has been really formative for me and my contemporaries. Just as we were establishing our independent scientific careers, along comes this data set, this opportunity.”
“It completely changed the game,” she says. “It’s like if you had been an astronomer and all you had was a pair of binoculars looking up at the sky, and then somebody gave you this amazing telescope. All of a sudden you’re able to see so much more than before. That’s really what the USArray data set has done for seismology, specifically for looking at the structure beneath North America. It changed what we can do, what kind of imaging we can do, what kind of questions we can ask and answer.”
She and Ishii access USArray data sets, data products, and teaching tools on a daily and weekly basis through the website for the Incorporated Research Institutions for Seismology (IRIS). They can also borrow seismic stations from USArray to set up “flexible arrays,” small seismic networks for temporary experiments. Long has a Flexible Array study underway now in the Appalachians. “Before USArray,” says Long, “if you wanted to have data from a particular place, you had to do it yourself. As an individual scientist, you could never do something on the scale of USArray.”
Ishii hopes that, in the future, seismologists will be able to deploy a permanent observatory like the USArray Transportable Array for long-term earthquake monitoring. “We’ve seen how much the data set can do in advancing our studies and results. It definitely makes us think, how many things are we missing without having the TA [around permanently]?”
Long praises USArray for setting an example with its philosophy of open data access. “That’s really important for the geophysical community and particularly important for early career scientists.” She also thinks that “the USArray data set has pushed us seismologists to work in a more interdisciplinary and collaborative way.”
“I was not around when USArray was being conceived and planned,” she says. “At the time, people thought it was nuts, that it was too big. Whereas for me and people my age, we’ve grown up with this being a reality. You almost take it for granted that of course it’s possible to go out and collect a data set like USArray. So I think it has enabled people to think even bigger… Like, wow, if we can pull off USArray, what are the next crazy ideas that might turn out to actually be doable and pay off with amazing scientific rewards?”