Upper Mantle Discontinuities

Cutaway of the Earth’s interior. The abundance of seismic data from numerous seismometers around the globe allows us to investigate detailed structure of the interior.

A key seismic tool for unraveling the origins of lateral heterogeneity inside the Earth comes from studying the depth and sharpness of abrupt discontinuous changes in elastic properties over depth.  These boundaries, known as seismic discontinuities, delineate the major vertical stratification within the Earth, including the Mohorovičić discontinuity at the base of the crust, the transition from solid mantle to molten outer core at the Core Mantle Boundary, and the Inner/Outer Core Boundary at the solid metallic core.  Aside from this major stratification in seismic properties, there are a variety of secondary discontinuities within the mantle, arising from a wide range of mechanisms, including changes in mineralogy, state, composition, melt content, anisotropy or a combination of the above.

The terrestrial component of research in our group centers on the seismological determination of the depth and sharpness of elastic contrasts that arise from high-pressure and temperature solid-to-solid phase changes in minerals, rheological transitions in material properties, the presence (or absence) of melt, and the chemical differentiation and stratification inside the Earth. We are developing multiple techniques for using large, high-quality, broadband, seismological array recordings of earthquakes to amplify low amplitude signals out of the background noise, both from existing datasets and new datasets collected in temporary field deployments of seismic arrays.  

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Our research brings together results from both new and established seismic techniques, providing an integrated and detailed image of discontinuity structure beneath well sampled regions, a result crucial for answering questions about the dynamics and evolution of Earth’s interior. Some of our research targets include the nature of the lithosphere-asthenosphere boundary, the lateral existence and character of “X” discontinuities, the global topography, sharpness, and impedance contrast of the 410 and 660 km discontinuities of the mantle, and intermittent boundaries, such as the 1000 km discontinuity, the 520, and the Lehmann.

The raypath geometry of the SS precursor seismic phase.

One particularly useful set of seismic phases for interrogating mantle structure are the SS precursors. These seismic body waves arrive several hundred seconds before their parent SS phase, which reflects off the crust at the center of its path. The precursors reflect from deeper interfaces, and are sensitive to discontinuity structure far away from sources and receivers, making them ideal for imaging the remotest places on the Earth. I have focused on using these phases to image many of the features listed above, and used the precursors to find new discontinuities in the Earth’s mantle. Our group is investigating other seismic phases that are sensitive to mantle structure and developing new algorithms for recovering Earth structure using seismology.

Variation in the depth of the upper mantle discontinuities from thermal and chemical heterogeneity.

Relevant Papers:

Schmerr, N.,  Imaging Mantle Heterogeneity with Upper Mantle Seismic Discontinuities, chapter in Mantle Heterogeneities, edited by. Khan, A. (accepted 2014).

Beghein, C., Yuan, K., Schmerr, N. & Xing, Z. Changes in Seismic Anisotropy Shed Light on the Nature of the Gutenberg Discontinuity. Science 343, 1237–1240 (2014). (pdf)

Porter, R. C., Fouch, M. J. & Schmerr, N. C. Dynamic lithosphere within the Great Basin. Geochem. Geophys. Geosyst. n/a–n/a (2014). doi:10.1002/2013GC005151. (pdf)

Rychert, C. A., Harmon, N. & Schmerr, N. Synthetic waveform modelling of SS precursors from anisotropic upper-mantle discontinuities. Geophysical Journal International 196, 1694–1705 (2014). (pdf)

Schmerr, N., Thorne, M., Kelly, B., (2013), Broadband Array Observations of the 300 km Seismic Discontinuity, Geophysical Research Letters, 40, 841–846, doi:10.1002/grl.50257. (pdf)

Rychert, C., Schmerr, N., Harmon, N., (2012), The Pacific Lithosphere-Asthenosphere Boundary: Seismic imaging and constraints on anisotropy from SS waveforms, Geochemistry, Geophysics, Geosystems, 13, doi:10.1029/2012GC004194. (pdf)

Long, M., Till, C., Druken, K., Carlson, R., Wagner, L., Fouch, M., James, D., Grove, T., Schmerr, N., Kincaid, C., (2012), Mantle dynamics beneath the Pacific Northwest and the generation of voluminous back-arc volcanism, Geochemistry, Geophysics, Geosystems, 13, doi:10.1029/2012GC004189. (pdf)

Schmerr, N., (2012), The Gutenberg Discontinuity: Melt at the Lithosphere-Asthenosphere Boundary, Science, 335, 1480-1483. (pdf) (supplemental)

Schmerr, N., Thomas, C., Subducted Lithosphere Beneath the Kuriles from Migration of PP Precursors (2011), Earth. Plan. Sci. Lett., 311, 101-111. (pdf)

Schmerr, N., Garnero, E., McNamara, A., (2010), Deep mantle plumes and convective upwelling beneath the Pacific Ocean, Earth. Plan. Sci. Lett., 294, 143-151. (pdf) (supplemental)

Schmerr, N., Garnero, E., (2007), Upper Mantle Discontinuity Topography from Thermal and Chemical Heterogeneity, Science, 318, 623-626. (pdf) (supplemental)

Schmerr, N., Garnero, E., (2006), Investigation of upper mantle discontinuity structure beneath the central Pacific using SS precursors, J. Geophys Res, doi:10.1029/2005JB004197. (pdf) (supplemental)

This research is supported by NSF Grants EAR-1361325 and EAR-1247608.

© Nicholas Schmerr 2014