Download Resolving the fine scale structure of the core

Resolving the fine scale structure of the core-mantle boundary: A
global search for ultra-low velocity zones and D” structure
Background
The Earth's core-mantle boundary, where the molten iron of the core meets the silicates of the
mantle, is probably the most important internal boundary of the Earth. Strong elastic
heterogeneities have been mapped close to the core-mantle boundary (CMB) over the last 20
years. The large variety of heterogeneities found at or near the CMB is not unexpected for this
boundary layer that exhibits the largest temperature, viscosity and density contrasts within the
Earth. It has been proposed that the CMB influences and controls such diverse features as the
Earth’s magnetic field, hot-spot volcanism, super-continent break-up and plate tectonics.
Structures detected at the CMB using seismological methods include a sharp discontinuity on top
of the D” layer approximately 200 to 300 km above the CMB, strong reductions of seismic
velocities of up to 40% in thin layers (5 to 40 km) at
the CMB called ultra-low velocity zones (ULVZ) and
thin rigid layers beneath the CMB also known as corerigidity zones (CRZ) which are likely sediments
deposited beneath the CMB by the convection of the
liquid iron of the outer core. The origin, evolution and
dynamics of most of these structures is still poorly
understood.
The D” discontinuity is likely related to the recently
detected phase transition of perovskite, the main
mineral of the lower mantle, to post-perovskite at
lower mantle pressures and temperatures. The D”
Figure 1: Fine layers at the CMB can
discontinuity shows strong topography and can only
exist as partially molten material giving
be detected in specific regions of the Earth and many
rise to ULVZ. Sediments from core
questions regarding the thermal, chemical and
convection might collect beneath the
CMB forming core rigidity zones. ULVZ
dynamical history of this discontinuity remain open.
might influence the stability and location
ULVZ are probably one of the most enigmatic
of thermal mantle plumes. Image
features
found at the CMB. The large decrease in
courtesy of E. Garnero (ASU)
velocity in these very thin sheets atop the CMB
indicates the presence of partially molten material and
likely iron transported from the core to the mantle. The distribution and evolution of ULVZ and
their influence on mantle dynamics and mantle plume origin remain unclear. It has been
speculated that ULVZ form the base for mantle plumes that generate intra-plate volcanism such
as at Hawaii, but further research is necessary to clarify this point.
Project
To learn more about CMB structure, dynamics, and evolution, high-resolution seismological
studies are necessary. By using recordings of earthquakes at seismological arrays and detailed
waveform analysis of core reflected phases these high-resolution studies are possible. A seismic
array is a seismometer network that permits analysis of Earth structure through time series
stacking to enhance signal-to-noise ratios of coherent arrivals over incoherent energy. Using
waveform and traveltime information of seismic core phases (e.g. PcP, ScP) from stacked array
data allows us to resolve small-scale structures at the CMB. Indeed, array processing is necessary
to detect and study the subtle waveform variations due to the interaction of the core reflected
waves with CMB structure.
This PhD project will map global CMB structure
using data from globally distributed seismic arrays.
Most of the seismic arrays deployed today are part of
the International Monitoring System to monitor
compliance with the Comprehensive Test Ban Treaty
for underground nuclear explosions, but other data
sources for permanent and experimental arrays in
Europe, North-America, Australia and Asia will be
tapped. This will comprise a large dataset that will
allow an unprecedented dense sampling of the CMB
for high-resolution array studies with a special focus
on Circum-Pacific regions.
This project is very data intensive and the student
will learn about data collection, time-series analysis,
and archiving of seismic data. On the other hand, to
Figure 2: Seismic raypaths of the core
explain the seismic data, extensive waveform
reflected phases to be used in this
modeling using advanced methods to calculate
project. Layering at the CMB will be
detected by a suite of pre- and
synthetic seismograms in 1D, 2D and 3D is necessary.
postcursors to the main arrivals ScP and
The seismic modeling will show in unprecedented
PcP. Stacking is necessary to raise these
detail what kind of small-scale structures can be found
subtle arrivals out of the ambient noise.
at the CMB. The constraints from the seismological
study will be used in collaboration with mineral physicists, geochemists, and geodynamicist both
from the UK and the USA.
For more information about current research in this field please check
http://earth.leeds.ac.uk/~earsro or contact [email protected] .
References:
Garnero, E.J., Heterogeneities of the lowermost mantle, Annu. Rev. Earth Planet. Sci., 28, 509537, 2000.
Rost, S. and J. Revenaugh, Seismic detection of Rigid Zones at the Top of the Core, Science, 294,
1911-1914, 2001.
Rost, S. and J. Revenaugh, Small-scale ultra-low velocity zone structure resolved by ScP, Jour.
Geophys. Res. Solid Earth, 108, 10.1028/2001JB001627, 2003.
Rost, S., E.J. Garnero, Q. Williams and M. Manga, Seismic constraints on a possible plume root
at the core-mantle boundary, Nature, 435, 666-669, 2005
Garnero, E.J., M. Thorne, A. McNamara and S. Rost, Fine scale ultra-low velocity zone layering
at the core-mantle boundary and superplumes, in: Superplumes: Beyond Plate Tectonics;
edited by: D.A. Yuen et al. Springer, New York, in press, 2005.
Rost, S., E.J. Garnero, and Q. Williams, Fine scale ultra-low velocity zone structure from highfrequency seismic array data, 111, B09310, doi:10.1029/2005JB004088, Journal of
Geophysical Research, 2006.
Rost, S. and E.J. Garnero, Detection of an ultralow velocity zone at the CMB using diffracted
PKKPab, Journal of Geophysical Research, 111, B07309, doi:10.1029/2005JB003850, 2006.
Kito, T., S. Rost, C. Thomas and E.J. Garnero, New insights into the P- and S-wave velocity
structure of the D'' discontinuity beneath the Cocos plate, revision under review Geophysical
Journal International, 2006.