Thursday,
April 1, 1999, 12:30 p.m. in GWC Room 604
E. Garnero
Department of Geology
Using Seismology to Decipher Earth's Deep Interior
Abstract
Most of the Earth's interior is inaccessible, and must be studied
indirectly. Our detailed knowledge of the fundamental layering, and
distribution of heterogeneity in the Earth has primarily come from
seismology. The past couple of decades have seen many advances in our
understanding of the complexity of the interior, particularly near the
surface (the uppermost mantle) and at the boundary between the mantle and
core (the "core-mantle boundary", or CMB). Both of these boundaries have
large density contrasts, and marked differences in properties (e.g.,
chemistry, temperature, flow speeds, mineralogy) on either side of the
boundary. In the case of the CMB, the outer core is rapidly convecting and
heating the mantle from below. A resulting thermal boundary layer at the
base of the mantle is a host to a wide variety of detected structures.
In this talk, I'll show seismic data and analysis methods used to image
detailed features in the deep mantle. We are motivated to better understand
structure near the big discontinuities such as the CMB (and the surface)
because of their intimate relationship to mantle circulation, which at
present is still poorly understood. Our recent work has focused on two
interesting features: (1) a very thin layer (5-50 km thick) right at the
CMB, which is probably partially molten mantle; this layer underlies large
scale warm regions of the mantle, and surficial hot spot volcanism, and (2)
seismic wave speed anisotropy, which is the dependence of seismic wave speed
on wave propagation direction. Anisotropy may be related to alignment of
lowermost mantle minerals in strong boundary layer shear flow. Regions of
detected lowermost mantle anisotropy are typically beneath regions where the
oceanic plate is subducted down into the mantle. The primary method of
analysis is construction of synthetic seismograms to compare with
observations, in a forward modeling effort. The far reaching consequences
of the possibility of partial melt and anisotropy in the deep mantle will be
discussed.
Reference:
The core-mantle boundary and deep Earth dynamics", Nature, VOL 392, April
1998, 461-468.
