ABSTRACT
Seismic waves are continuous three-dimensional surfaces of associated ground
motions that propagate through the Earth. Seismometers record the passage of numerous
seismic waves through a given point near Earth’s surface, and classically these
seismograms are analyzed to deduce properties of the Earth’s structure and the seismic
source. Given a spatially dense set of seismic recordings, the signals can also be used to
visualize the actual continuous seismic waves, providing new insights into complex wave
propagation effects. Using signals recorded by an array of seismometers with
unprecedented station density and aperture deployed as part of the NSF-funded
EarthScope USArray project, we demonstrate innovative pedagogic and research
applications of visualizations of the seismic wavefield.
INTRODUCTION: DENSE RECORDING OF SEISMIC WAVES ON A
CONTINENTAL SCALE
Global seismologists have studied seismic wavefields for many years using sparse
networks of isolated stations and/or relatively small aperture, narrow-band seismometer
arrays. USArray, the primary seismological component of project EarthScope, will
transform many aspects of conventional seismological analysis. USArray includes 400
broadband seismographs being deployed in the Transportable Array (TA), which will
migrate across the United States over the next dozen years, occupying a total of 2000
sites for ~18 months each. The TA’s primary scientific objective is to collect seismic recordings over a continent-wide regularly spaced 70-km grid of sites to illuminate the
underlying lithosphere and deeper mantle structure with unprecedented resolution. This
project should revolutionize our understanding of the structure, evolution, and dynamics
of North America in particular, and continents in general.
Many other applications of the TA seismic recordings will be possible due to the
intrinsic multi-purpose nature of continuous, high dynamic range, broadband ground
motion recordings, all of which are openly available by internet
(http://earthscope). These applications include quantification of the rupture
process for large earthquakes around the world (Ammon et al., 2007), analyses of
structure in the lower mantle and core of the Earth (van der Hilst et al., 2007) and in
upper mantle regions remote from North America (Zheng et al., 2007), and detection and
analysis of signals from regional earthquakes (e.g. Herrmann, 2007, Dreger, 2007) and
exotic sources such as landslides, mine collapses (Ford et al., 2007), ocean storms (Rhie
and Romanowicz, 2006), submarine slumps, volcanic eruptions, surging glaciers
(Ekström et al., 2006), and large underground explosions (Ammon and Lay, 2006). The
TA also enables a transformative view of the seismic wavefield, densely sampled over
large spatial scale with high-quality seismometers. The dense sampling provides
opportunities to visualize the ground motions as a wave phenomenon rather than focusing
on point samples. This has previously only been viable for numerical models which
compute complete wavefields, but now we can see actual Earth signals. This can help
students of wave propagation on all levels to deepen their intuitive understanding of
fundamental seismic-wave interactions, along with revealing complexities that cannot be
recognized in individual seismograms


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