By Claire Discenza
When a major earthquake hits, Scripps Institution of Oceanography geophysicist Frank Vernon watches resulting terrestrial waves flow across the middle of the United States, bounce off of mountains and oceans, and return after rippling around the world and back.
Vernon presented his work on earthquake mapping at the March installment of the Jeffrey B. Graham “Perspectives on Ocean Science” lecture, hosted by the Birch Aquarium.
In his talk titled, “Seismic Networks and Arrays: Observation from Near and Far,” Vernon gave an overview of EarthScope, a National Science Foundation project studying the causes and propagation of earthquakes across North America. As part of EarthScope, Vernon and his team are working with the Array Network Facility (ANF), a group charged with deploying and managing some 400 portable broadband seismic stations to study movement of the earth’s crust underneath the United States.
Starting in 2004, the ANF inserted its first stations in the ground along the West Coast. Each station records the movement of the Earth at millimeter resolutions, and lasts approximately two years before it is removed, recycled, and relocated farther East. In this way, a rolling network of seismometers is slowly migrating across the county, collecting real-time data along the way.
The project will continue until 2013, when the sensors are scheduled to reach the Eastern Seaboard.
“With this seismic array, we’re getting great images, and a better understanding of the Earth’s crust,” said Vernon. “We can also take the raw data and use it to model how a rupture happens. This is the type of capability we’re developing — a better understanding of how earthquakes work.”
During his talk, Vernon presented data collected from the array during major events worldwide. “After an earthquake, the whole Earth rings,” described Vernon, showing an animation of the array data following the 9.0 magnitude Tohoku Earthquake in Japan last March. “Wave phases go around the world and wrap around 10, 20 times. It’s pretty astounding in a large earthquake.”
In the animation, an arrow at each of the stations displayed the magnitude and direction of the crust’s movement after the quake. “We don’t just measure the up-down movement of the Earth, but also back-forth and East-West,” Vernon explained. He and his team were able to study the two different types of perpendicular surface waves emanating from Japan, and watch as they formed swirling and rhythmic patterns of motion across the array.
Vernon’s team also looked at local earthquakes, and examined how the waves make their way back to the epicenter.
“In a perfectly homogeneous world, energy goes around and comes back and re-phases together in exactly the same spot,” Vernon said. Yet this did not occur — when the waves returned from their circumglobal journey, they collided in a slightly different location.
“I guess we don’t have a perfect world after all,” he quipped.
The team encountered many surprises and unexpected earthquake hotspots as the sensors moved across the country. “We saw a sequence of events in Oklahoma, Arkansas, and Texas. Who knows? These are not things I expected to see. I expected it to be kind of quiet there.”
While the ANF is in the process of recording seismic activity from the entire country, other EarthScope and ANF subprojects are exploring detailed movement of the West coast states where most U.S. epicenters are recorded.
In particular, the Cascadia project is looking at activity along the Cascadia Fault Line, which extends from Vancouver, Canada to northern California. “A major earthquake is something we don’t want to see in our backyard,” said Vernon. “Unfortunately, there’s an opportunity in the Cascades.”
During the question-and-answer period following the talk, one audience-member asked: “If you were sitting at the table with the president of the United States, what would you advise that he do as far as putting in nuclear power plants?”
“Don’t!” Vernon replied. “You can’t say there’s any place devoid of earthquake possibility in the United States. Make sure you understand and assess potential dangers. I’d definitely not want to be building plants right next to faults.”