Dear All,
Our Smith Lecture speaker this week is Paula Koelemeijer, University of Cambridge. She is speaking on "The Earth has Music for Those Who Will Listen: Structure of the Deep Mantle from Earth’s Free Oscillations". Abstract below.
Our Smith Lecture speaker this week is Paula Koelemeijer, University of Cambridge. She is speaking on "The Earth has Music for Those Who Will Listen: Structure of the Deep Mantle from Earth’s Free Oscillations". Abstract below.
Smith Lectures are Friday afternoons from 4:00 to 5:00 pm, in Room 1528 C.C. Little Building. A reception is held following the lecture in 2540 C.C. Little. The events are free and open to the public. A full schedule for the term may be found on our website:
Best regards, -Anne
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Anne Hudon
Academic Student Services
Department of Earth and Environmental Sciences
University of Michigan
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Abstract:
At nearly 2900 km depth, the core-mantle boundary (CMB) separates the liquid iron core and the solid silicate mantle, marking a change in temperature and composition even larger than at the Earth’s surface. At this interface, dynamic processes occurring in the mantle and core interact, producing a range of structures at scales from tens to thousands of kilometers. The seismic properties of these structures provide an insight into the material properties and processes occurring in the deep Earth.
Two regions underneath the Pacific and Africa are characterized by low shear wave velocities (Vs), they extend laterally thousands of kilometers with heights up to 1000 km above the CMB. These large-low-shear velocity provinces (LLSVPs) are found consistently amongst different tomographic shear wave velocity models. The question remains whether these long wavelength features are dominantly thermal or thermochemical structures. Information on their density structure and shear to compressional wave velocity (Vp) ratio is vital for assessing their influence on mantle dynamics.
Normal modes or Earth’s free oscillations arise after large earthquakes when standing wave patterns occur along the surface and radius of the Earth and the entire Earth is ringing like a bell. They provide an invaluable tool for probing the Earth's deep interior since they are global in character and are affected by density structure in addition to velocity. In particular, Stoneley modes, which are modes confined to solid-liquid interfaces such as the CMB, are mainly sensitive to structures in the lowermost mantle. Observations of these modes increase the depth resolution and provide unique constraints on the long wavelength structures of the deep mantle.
Several large magnitude earthquakes have occurred in the last decade, significantly increasing the amount of available data for normal mode observations. We make use of a recent normal mode splitting function data set of 143 modes including 33 modes sensitive to compressional wave velocity and 9 CMB Stoneley modes. We combine our normal mode data with independent constraints from body waves and surface waves and invert independently for lateral Vs and Vp heterogeneity in Earth's mantle. We particularly focus our analysis on the relative distribution of Vs to Vp anomalies and compare the obtained seismic model to thermal and thermochemical models of mantle convection. In addition, we perform tests for possible density structures in the lowermost mantle in order to determine the long wavelength characteristics of the LLSVPs.
