Scott Burdick

Model Update May 2016: Upper‐Mantle Heterogeneity beneath North America from Travel‐Time Tomography with Global and USArray Data

ABSTRACT P ‐wave travel‐time residuals from USArray helped improve the scale and consistency with which the mantle beneath North America is resolved. Beginning in 2008, we published a series of P ‐wave velocity models based on a global ray theoretical inversion of USArray and global catalog data. Here, we present the final model update, MITP_USA_2016MAY, which includes the complete set of travel‐time residuals from USArray Transportable Array (TA) in the contiguous United States. In this model, the area of high resolution extends to the eastern margin of the continent, allowing us to better estimate the location and extent of slow features in Central Virginia and New England. An increasing number of data from the TA in Alaska also allows us to recover the structure of subducting Pacific plate and Yakutat terrane. In addition to highlighting new features in the final model, we visualize and discuss the improvements to the model due to the addition of USArray data through time.

Investigation of Firn Aquifer Structure in Southeastern Greenland Using Active Source Seismology

In spring of 2011, a perennial storage of water was observed in the firn of the southeastern Greenland ice sheet, a region of both high snow accumulation and high melt. This aquifer is created through percolation of surface meltwater downward through the firn, saturating the pore space above the ice-firn transition. The aquifer may play a significant role in sea level rise through storage or draining freshwater into the ocean. We carried out a series of active source seismic experiments using continuously refracted P-waves and inverted the first P-arrivals using a transdimensional Bayesian approach where the depth, velocity, and number of layers are allowed to vary to identify the seismic velocities associated with the base of the aquifer. When our seismic approach is combined with a radar sounding of the water table situated at the top of the firn aquifer, we are able to quantify the volume of water present. In our study region, the base of the aquifer lies on average 27.7±2.9 m beneath the surface, with an average thickness of 11.5±5.5 m. Using a Wyllie average for porosity, we found the aquifer has an average water content of 16±8%, with considerable variation in water storage capacity along the studied east-west flow line, 40 km upstream of the Helheim glacier terminus. Between 2015 and 2016, we observed a 1-2 km uphill expansion of the aquifer system, with a site dry in summer 2015 exhibiting a water content of 530 kg m-2 in summer 2016. We estimate the volume of water stored in the aquifer across the entire region upstream of Helheim glacier to be 4.7±3.1 Gt, approximately 3% of the total water stored in firn aquifers across the Greenland ice sheet. Elucidating the volume of water stored within these recently discovered aquifers is vital for determining the hydrological structure and stability of the southeastern Greenland ice sheet.