The magnitude 8.3 earthquake on Sept.16, 2015 off the coast of Chile generated a tsunami that was felt throughout the Pacific. Serendipitously, a Scripps Institution of Oceanography, UC San Diego-led project has a broadband seismic array deployed on the Ross Ice Shelf (RIS) in Antarctica.
These seismic stations made the first large-scale broadband seismic array observations of the response of an ice shelf to tsunami arrivals. A team of Scripps researchers now in Antarctica is recovering seismic data from 34 seismic stations spanning the ice shelf. Strong signals generated by the tsunami impacting the shelf were detected at all stations from which data has been recovered, with the expectation that the entire ice shelf was rocked.
Because the shortest direct path for the tsunami to the RIS goes through West Antarctica, refraction and scattering by seafloor ridges and seamounts must have diverted the tsunami energy that impacted the RIS.
Ice shelves are slabs of ice that extend from land over the ocean like a half-cover on a jacuzzi. Ice shelves provide a buttressing effect, restraining the flow of grounded ice sheets to the sea. When this restraint is removed, the flow of land ice into the ocean accelerates, raising sea level. The Ross Ice Shelf is the largest ice shelf in Antarctica that covers an area of the Ross Sea roughly the size of Texas, and restrains West Antarctic grounded ice sheet that could contribute as much as three meters of sea-level rise.
The seismic survey studying the vibrations of the Ross Ice Shelf (RIS) in response to ocean wave impacts will provide information on the structure and strength of the RIS, giving baseline “state-of-health” ice shelf measurements that will be used to identify the magnitude of changes in its integrity over time.
The servicing of the stations installed in November 2015 involves flying by Twin Otter aircraft to the stations and uncovering the instrument recording boxes buried by about 3-4 feet of snow. The Scripps team, led by Peter Bromirski with Anja Diez, Zhao Chen, and Jerry Wanetick, swap out the disc drives that contain the full year of data. Temperatures at the stations during data recovery have ranged from about -15 to -26° C (5 to -15° F), with winds as high as 40 knots.
The National Science Foundation Division of Polar Programs-funded project will continue collecting seismic and GPS data for another full year, including through the austral winter.
The triggers that initiated the collapse of the Larsen B Ice Shelf in 2002 and the Wilkens Ice Shelf in 2008 have not been identified. While tsunamis were not factors in those events, West Antarctic ice shelves are exposed to circum-Pacific-generated tsunamis that could provide the trigger for the collapse of weakened ice shelves, removing their restraining influence.
Institutions participating in the study include Woods Hole Oceanographic Institution, Washington University in St. Louis, Colorado State University, and Penn State University.
– Peter Bromirski