S13B-4455:
Teleseismic P-wave Delay Time Tomography of the southern Superior Province and Midcontinent Rift System (MRS) Region

Monday, 15 December 2014
Trevor A Bollmann1, Suzan van der Lee2, Andrew W Frederiksen3, Emily Wolin1, Ghassan I Aleqabi4, Justin Revenaugh5, Doulgas A Wiens6 and Fiona Ann Darbyshire7, (1)Northwestern University, Evanston, IL, United States, (2)Northwestern Univ, Evanston, IL, United States, (3)University of Manitoba, Winnipeg, MB, Canada, (4)Washington University in St Louis, Department of Earth and Planetary Sciences, St. Louis, MO, United States, (5)Univ Minnesota, Minneapolis, MN, United States, (6)Washington University in St Louis, St. Louis, MO, United States, (7)University of Quebec at Montreal UQAM, Centre de recherche GEOTOP, Montreal, QC, Canada
Abstract:
The Superior Province Rifting Earthscope Experiment (SPREE) and the northern midwest footprint of USArray’s Transportable Array recorded continuous ground motion for a period of 2.5 years. From around 400 M>5.5 teleseismic earthquakes recorded at 337 stations, we measured body wave delay times for 255 of these earthquakes. The P wave delays are accumulated over more than 45 thousand wave paths with turning points in the lower mantle. We combine these delay times with a similar number delay times used in previous tomographic studies of the study region. The latter delay times stem from fewer stations, including Polaris and CNSN stations, and nearly a thousand earthquakes. We combine these two sets of delay times to image the three-dimensional distribution of seismic velocity variations beneath the southern Superior Province and surrounding provinces.

 This combined data coverage is illustrated in the accompanying figure for a total number of 447 stations . The coverage and the combined delays form the best configuration yet to image the three-dimensional distribution of seismic P and S-wave velocity variations beneath the southern Superior and surrounding provinces. Closely spaced stations (~12 km) along and across the MRS provide higher resolving power for lithospheric structure beneath the rift system.

 Conforming to expectations that the entire region is underlain by thick, cool lithosphere, a mean delay of –.55 +/- .54 s. This is very similar to the mean delays -.6s +/- .37s measured for this region before 2012. Event corrections range from -.2 +/-.54 s and correlate with tectonics for 80% of the earthquakes.

 An inversion of these nearly one hundred thousand P and around thirty thousand S-wave delay times for high-resolution P and S-wave velocity structure, respectively, does not show structures that are obviously related to the crustal signature of the MRS. None of structures imaged, align with or have a similar shape to the high Mid-continent Gravity Anomaly (MGA). However, a low-velocity structure is imaged in the lithosphere just east of the MGA.