I’ve worked on a number of different subfields in seismology, including using tomography to image the structure of North America, characterizing the noise performance of seismic stations, and investigating the seismicity of the East Coast.

Tomographic imaging of stable North American lithosphere

Working with Suzan van der Lee, I’m using seismic tomography to image the structure of the upper mantle below the central and eastern US.  We are especially interested in the Midcontinent Rift (MCR), a 1.1 billion-year-old failed rift that now lies in the stable interior of North America.

Bouguer gravity anomaly map with seismic station locations.

Most of the Midcontinent Rift is buried beneath glacial sediments, but the rift’s dense, iron-bearing rocks are readily detected by gravity and magnetic studies. Here, a strong positive (red) gravity anomaly delineates the western arm of the rift, extending southwest from Lake Superior.

During about 20 Myr of extension and rifting, a volume of igneous rock was produced that is sufficient to form a Red Sea-sized ocean floor.  Yet for reasons unknown, rifting ceased before full-fledged seafloor spreading could begin.

  The area has been tectonically stable for at least the past billion years and displays no present-day
seismicity.  Continental- and regional-scale tomographic models show no velocity anomaly that significantly correlates with the rift, but the used data lack resolution near the MCR.  Little evidence seems to have remained of the thin lithosphere that must have allowed the asthenosphere to ascend and melt during rifting.  Furthermore, it is unknown how rifting and post-rift stabilization have affected the lithosphere.

    To illuminate the structure and evolution of the rift and the surrounding region, we will produce a new, high-resolution S-velocity model of central and eastern North America.  To do this, we are implementing a tomographic technique known as partitioned waveform inversion.  This new technique takes advantage of the densely-spaced data provided by arrays such as EarthScope’s Transportable Array and our own Superior Province Rifting Earthscope Experiment (SPREE).

 Long-period noise at SPREE stations

One unexpected result from SPREE came from an assessment (method described here) of our seismic stations’ noise characteristics.  During fieldwork, several SPREE team members had noticed that certain stations’ horizontal components recorded much more long-period noise than their neighbors.  Further investigation revealed that this noise varied both diurnally and seasonally. In this paper, we find that soil characteristics influence shallow seismic stations’ response to variations in atmospheric pressure.   SPREE stations in sandy soil showed the most consistent noise characteristics, while stations in finer-grained soils tended to be very noisy during summer (and especially during the day) and quieter in winter after the ground froze.

Seismicity of the North American passive margin

Map of eastern North America with earthquakes M > 4.

Moderate-to-large earthquakes (M>5) have occurred along the entire east coast of North America. This map shows seismicity from 1980-2009 (ANSS and Earthquakes Canada) and major historical events.

Working with Seth Stein, I conducted a study of the seismic activity observed on the east coast of North America.  Most earthquakes occur at the boundary between two tectonic plates.  On the East Coast, the continent and seafloor are part of the same plate, so we would expect no motion between them and thus no large earthquakes.  Yet in 1929, a M=7.2 earthquake caused a landslide and tsunami off the Grand Banks of Newfoundland, and in 1933 a M=7.3 tremor rocked Baffin Bay.  In this paper we discuss the 2011 M=5.8 Mineral, Virginia earthquake in the context of this “passive-aggressive” margin.

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