New method to infer fine 3D Earth structure from seismic waveforms Imaging the lowermost mantle beneath Central America

February 17, 2014

© Adapated from Figure 19 and Figure 21, Methods for inversion of body-wave waveforms for localized three-dimensional seismic structure and an application to D” structure beneath Central America , Kawai et al., Geophysical Journal International, Vol. 199.
(a) Earthquakes (under South America), seismic stations (in North America), and great circle ray paths connecting them. (b) 3D S-velocity structure (percentage difference with respect to the PREM reference model) inferred in the lowermost 50 km of the mantle beneath Central America. (c-d) Cross sections of results of the inversion (percentage difference with respect to the PREM reference model) in the lowermost 400 km of the mantle. High velocity material with a horizontal size of about 250 × 250 km is surrounded by low velocity material. This can be interpreted as the subducted Farallon plate causing upsplashing of hot material in the thermal boundary layer immediately above.

The Earth from its surface to its core can be divided into layers ? the crust, mantle, outer core and inner core. The lowermost few hundred kilometers of the mantle just above the outer core is called the D” (D double prime) region. The D” region is composed of different rocks from the rest of the lower mantle and is a region where chemistry and temperature change rapidly as the distance to the liquid iron-alloy core decreases. The flux of energy and material through this layer is vital to understanding the Earth’s evolution, but its detailed structure remains unknown.

University of Tokyo Graduate School of Science Visiting Researcher Kenji Kawai (concurrently assistant professor at Tokyo Institute of Technology), Professor Robert Geller, and their co-workers have developed a new method of seismic analysis (a “big data” seismic waveform inversion method) for inferring the three dimensional (3D) fine structure of the D” region. By applying this method to the vast quantity of seismic data recorded by the North American USArray seismic observatory, they were able to infer the 3D fine structure of the D” region beneath central America. They detected remnants of the subducted Farallon plate, indicating that mantle convection has carried the Farallon plate from the Earth’s crust to the bottom of the mantle.

It is expected that applying this approach to data from other regions of the world will lead to a deeper understanding of the D” region and of the Earth’s evolution.p>

Press release [PDF]


Kawai, K., K. Konishi, R.J. Geller and N. Fuji,
“Methods for inversion of body-wave waveforms for localized three-dimensional seismic structure and an application to D” beneath Central America”,
Geophysical Journal International Online Edition: 2014/2/4, doi: 10.1093/gji/ggt520.
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