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| Titel |
Constraining lowermost mantle structure using seismic observations of Earth's long period free oscillations |
| VerfasserIn |
Arwen Deuss, Paula Koelemeijer, Jeroen Ritsema |
| Konferenz |
EGU General Assembly 2013
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| Medientyp |
Artikel
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| Sprache |
Englisch
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| Digitales Dokument |
PDF |
| Erschienen |
In: GRA - Volume 15 (2013) |
| Datensatznummer |
250073579
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| Zusammenfassung |
| The Earth’s lowermost mantle (also called D”) is bounded by a thermal boundary layer
forming a discontinuous interface with the core. The range of seismic structures
found in this region, including ultra low velocity zones, anisotropy, discontinuities
and anti-correlations between shear and bulk sound velocity, rival the structures
found in the lithosphere, the mantle’s top thermal boundary layer. The majority of
these structures have been found using body wave observations, but normal mode
observations have been lacking. Even though seismic body wave studies of the core
mantle boundary region have revealed a range of structures, many fundamental
questions remain unanswered, including: Is there partial melting or compositional
heterogeneity? What causes ultra low velocity layers? Which is the importance of
post-perovskite?
Here, we will use long period whole Earth oscillations to study structures that have only
been studied using short period body waves before. The splitting of Earth’s free-oscillation
spectra is especially interesting, because they place important constraints not only on the
wave speed but also on the density structure of the Earth’s mantle. We present a new set of
splitting functions for mantle sensitive modes, of which almost half had not been measured
before. In particular, we have made measurements of Stoneley modes which are
uniquely sensitive to the core mantle boundary region, and have also added over
30 new modes which are predominantly sensitive to compressional velocity. Our
observations are derived from modal spectra up to 10 mHz for 91 events with Mw -¥ 7.4
from the last 34 years (1976–2010). Our data include the 23 June 2001 Peru event
(Mw=8.4), the Sumatra events of 2004 (Mw=9.0) and 2005 (Mw=8.6), the 2008
Wenchuan, China event (Mw=7.9) and the 2010 Chile event (Mw=8.8). The new
events provide significant improvement of data coverage particularly in continental
areas.
We are using our new measurements in a tomographic inversion for mantle shear and
compressional velocity, especially focussing on the lower most mantle region and D”. The
addition of compressional sensitive modes provides improved constraints on the large
scale compressional structure, and on the scaling ratio between δvs-vs and δvp-vp,
allowing us to estimate the amount of compositional heterogeneity in the lowermost
mantle. |
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