16.6 MB pdf - IRIS

Seismology plays key role in addressing many needs of
broader society, including energy resources, environment,
national security, and several natural hazards. In no area,
however, are the uniquely important impacts from seismological
research more evident than in addressing earthquake
hazard. National Earthquake Resilience: Research,
Implementation, and Outreach (NRC, 2011) details how seismologists
must contribute by making near-field measurements
to understand the physics of earthquake processes; evaluating
and testing earthquake early warning systems and methods
for operational earthquake forecasting; completing national
seismic hazard maps and creating urban seismic hazard maps;
enabling robust coupled simulations of fault rupture, seismic
wave propagation, and soil response to reliably estimate losses
and casualties; and contributing to scenarios so that communities
can visualize earthquake impacts.
Lowermost Mantle Global Structure
Seismologists are collaborating with geodesists, geodynamicists
and materials scientists to determine density, temperature
and other mantle properties that cannot be measured from
seismic wavespeeds alone (Simmons et al., 2010). This progress
comes, in part, from resolving the structure of large, low-shear
velocity provinces at the base of the mantle with the accumulation
of high-quality broadband data from the GSN and its international
counterparts, as well as dense PASSCAL and USArray
deployments. This work brings out the distinct average velocity
profiles within and outside irregularly shaped high- and lowvelocity
provinces in the lowermost mantle. Modeling of broadband
waves that propagate along and across the province
borders show that they are sharp, indicating that the distinctive
properties cannot be due only to temperature differences.
If large, chemically distinct reservoirs continue to exist in the
mantle, then occasional reorganization of mantle circulation
could profoundly alter cycling of volatiles between Earth’s interior
and the ocean and atmosphere.
Independent models of global shear wave speed in the lower mantle
are in increasingly good agreement. This map is composed of bins that
are color-coded according to how many of five well-regarded models
have a V S profile that cluster analysis identifies as distinctly slow. The
analysis demonstrates consensus on two large low-shear wave speed
provinces, the African and the Pacific, within a single, globally contiguous
faster-than-average lower mantle. (Lekic et al., submitted)
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In what follows, a set of key questions derived from
Seismological Grand Challenges in Understanding Earth’s
Dynamic System (Lay, 2009) is used to illustrate the ways in
which seismology, and the resources provided by the IRIS
facilities, contribute to a broad range of topics that enhance
our understanding of fundamental Earth processes and the
ways in which they impact our lives.
Thermo-Chemical Internal Dynamics
and Volatile Distribution
How do Earth’s temperature, composition, and internal boundaries
control mantle and core dynamics and the changing
morphology of our living environment? How do the lithosphere
and plate boundary systems evolve over Earth history?
The solid Earth is a complex and dynamic system with
processes that operate over spatial scales from nanometers
to tens of thousands of kilometers and time scales from
The Lithosphere-Asthenosphere
Boundary and Plate Tectonics
High-fidelity broadband records over long time spans at GSN
stations and on dense spatial arrays from PASSCAL and USArray
deployments have led to the surprising discovery of discontinuous
decreases in seismic wave speeds at depths of 50 to
130 km (Rychert and Shearer, 2009). In the ocean and in
tectonic areas, this discontinuity is likely related to the lithosphere-asthenosphere
boundary. But the discontinuity is also
observed midway through old continental lithosphere, where
it is associated with a several layers of distinct heterogeneity
and anisotropy measured from long-period surface wave and
overtone waveforms and SKS splitting data. Understanding this
structure, which is difficult to explain through normal thermal
mechanisms, will lead to fundamental new understanding of
how melting is distributed within Earth, how plate motions
affect and are influenced by variations in composition and rock
fabric, and how continents are formed.
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Layer 1
∆
FFC
Layer 2
Asthenosphere
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Latitude (°)
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Cratonic cross section showing the departure of the fast axis of azimuthal
anisotropy from the direction of absolute plate motion (APM)
of the North American Plate in the hotspot reference frame. The midlithospheric
discontinuity occurs in the depth range where a low velocity
layer is detected from receiver function studies (black dots). (Yuan
and Romanowicz, 2010).
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LAB
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Depth (km)
I-8 VOLUME 1 | Section I | Scientific Justification