2015 EarthScope/SSA Distinguished Lecturers

During the decade 2004-2014, 18 huge earthquakes with seismic magnitudes larger than 8.0 struck around the world, sometimes causing horrendous destruction and loss of life. The annual rate of such events was 2.5 times greater than had been experienced over the previous century of seismological observations.  Deployment of global networks of very high-quality seismic, geodetic, and tsunami recording systems had preceded most of these events, allowing unprecedented signals to be recorded for these great earthquakes.

Geophysicists have analyzed the recorded waves and ground motions to determine details of each earthquake, advancing our understanding of these dangerous events.  Most of the earthquakes have involved surprises, rapidly revising scientific ideas about the behavior of huge fault ruptures and indicating the need for improved mitigation efforts.

The continent of Antarctica supports the largest ice sheets in the world. The history of the ice sheets is intertwined with the geological history of continent, involving processes such as mountain building, the response of the land to the weight of glaciers, and heat flow melting and lubricating the bottom of the ice sheet. Very little is known about the history of Antarctica because the ice sheets preclude the usual geological sampling and mapping, so seismology offers a method to “see through the ice” and understand the continent beneath. Recent advances in technology now allow autonomous seismographs to be deployed across the continent for the first time.

In this presentation I will discuss projects that are installing seismographs across Antarctica and summarize recent discoveries. New results reveal that East Antarctica represents an ancient continent, with an average geological age of greater than one billion years and with highlands supported by thick continental crust.  In contrast, West Antarctica shows evidence of recent tectonic activity, and high heat flow that may lubricate the base of the ice sheet. The seismographs record earthquakes from magma movement associated with volcanoes beneath the ice. The high mantle temperatures suggest low mantle viscosity, such that the response of the land to ice mass changes will occur within a few hundred years. These insights are changing the way we model the recent history of the ice sheet.

Seismograms also provide important constraints on the movement and forces acting on the ice sheets. “Icequakes” ranging from very small cracking events near the surface of the ice sheet to massive crevassing and calving episodes produce unique seismic signals that help reveal the physics of ice movement. The largest signals come from a unique region on the Whillans Ice Stream, where a 100 mile-long section of the ice stream lurches forward twice a day, triggered by ocean tides, and sending seismic waves traveling across the entire continent. This unusual behavior may signal the slowing down of this ice stream in response to changes in the amount of water along the ice stream bed.