HR: 11:05h
AN: V32B-04    [Abstracts]
TI: Guided Seismic Waves: Possible Diagnostics for Hot Plumes in the Mantle
AU: Evans, J R
EM: jrevans@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AU: * Julian, B R
EM: julian@usgs.gov
AF: U. S. Geological Survey, 345 Middlefield Rd. MS 977, Menlo Park, CA 94025 United States
AU: Foulger, G R
EM: g.r.foulger@durham.ac.uk
AF: Dept. Earth Sciences, University of Durham, Durham, DH1 3LE United Kingdom
AB: Seismic waves potentially provide by far the highest resolution view of the three-dimensional structure of the mantle, and the hope of detecting wave-speed anomalies caused by hot or compositionally buoyant mantle plumes has been a major incentive to the development of tomographic seismic techniques. Seismic tomography is limited, however, by the uneven geographical distribution of earthquakes and seismometers, which can produce artificial tomographic wave-speed anomalies that are difficult to distinguish from real structures in the mantle. An alternate approach may be possible, because hot plumes and possibly some compositional upwellings would have low seismic-wave speeds and would act as efficient waveguides over great depth ranges in the mantle. Plume-guided waves would be little affected by bends or other geometric complexities in the waveguides (analogously to French horns and fiber-optic cables), and their dispersion would make them distinctive on seismograms and would provide information on the size and structure of the waveguide. The main unanswered question is whether guided waves in plumes could be excited sufficiently to be observable. Earthquakes do not occur in the deep mantle, but at least two other possible sources of excitation can be imagined: (1) shallow earthquakes at or near plume-fed hotspots; and (2) coupling of plume-guided waves to seismic body waves near the bottom of the mantle. In the first case, downward-traveling guided waves transformed to seismic body waves at the bottom of the waveguide would have to be detected at teleseismic distances. In the second case, upward-traveling guided waves generated by teleseismic body waves would be detected on seismometers at hotspots. Qualitative reasoning based on considerations of reciprocity suggests that the signals in these two situations should be similar in size and appearance. The focusing of seismic core phases at caustics would amplify plume waves excited by either mechanism (1) or (2) at particular epicentral distances. A failure to find such guided waves experimentally could mean either that the waveguides (plumes) do not exist or that the excitation mechanisms and/or seismometer networks are inadequate. Distinguishing these two possibilities would require careful analysis. Anticipated major improvements in seismic instrumentation, such as the EarthScope initiative, make this a propitious time to undertake a search for plume-guided waves in the mantle.
DE: 7208 Mantle (1212, 1213, 8124)
DE: 7260 Theory
DE: 8137 Hotspots, large igneous provinces, and flood basalt volcanism
DE: 8180 Tomography (6982, 7270)
SC: Volcanology, Geochemistry, Petrology [V]
MN: Fall Meeting 2005