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Nuclear Explosion Seismology
Nuclear explosion monitoring research is focused on lowmagnitude
events (m b
≤ 4.0) over broad areas, particularly
in Eurasia. Monitoring normally requires observations of
events at regional distances (< 1500 km) where signals are
best observed at relatively high frequencies (0.05–10 Hz).
Propagation through the heterogeneous crust and upper
mantle has a strong impact on these signals, and requires
calibration to account for path-specific seismic observables
(e.g., travel times, amplitudes, surface wave dispersion,
and regional phase propagation characteristics). The
PASSCAL facility provides instrumentation for research
experiments related to nuclear monitoring, as well as an
archived global data set that indirectly supports nuclear
explosion monitoring research by constraining crust-mantle
structural models, particularly in the critical Eurasian
region, and by improving empirical calibration methods.
Specific experiments that have contributed to our knowledge
of seismic structure and seismic monitoring calibration
include: 1991–1992 Tibet (Owens et al., 1993), Tanzania
(Nyblade et al., 1996), INDEPTH-II (Nelson et al., 1996),
1995–1997 Saudi Arabia (Vernon and Berger, 1998), Eastern
Turkey Seismic Experiment (Sandvol et al., 2003), and Iraq
(Ghalib et al., 2006). The value of archived PASSCAL data is
illustrated here; although these experiments were generally
supported to address fundamental scientific objectives, they
nonetheless provide data that benefit applied seismology
for nuclear monitoring.
Underground nuclear explosion monitoring is the main
theme of verification research, but source phenomenology is
a second important area of interest. In this vein, PASSCAL
instrumentation has been used in experiments with the
specific goal of improving understanding of large chemical
explosions, such as the nuclear analog Non-Proliferation
Experiment (Zucca, 1993; Tinker and Wallace, 1997), as well
as the Source Phenomenology Experiment, which examined
mining explosions (Leidig et al., 2005; Hooper et al., 2006).
Polar Efforts
Seismology in polar regions is a rapidly developing component
of PASSCAL-supported science. Antarctic, Greenland,
and past continental ice sheets and sea ice have dramatically
affected climate and sea level throughout Earth’s history. Yet,
great extents of these key regions are largely inaccessible
to geologic study, and Antarctica remains a tectonic terra
incognita. IRIS PASSCAL-supported seismology, principally
funded by the NSF Office of Polar Programs (OPP), is
enhancing fundamental understanding of basic crustal and
upper mantle structure as a part of larger interdisciplinary
studies, and is being used in novel studies of ice cap, glacial,
and iceberg-related seismic phenomena. Facility support for
these efforts requires significant new development efforts in
sensor, telemetry, and station design. Currently, this effort
is being accomplished through a joint IRIS PASSCAL/
UNAVCO OPP Major Research Instrumentation (MRI)
initiative, supplemented by a second IRIS MRI largely for
equipment procurement.
The far polar regions have the poorest seismographic
coverage of any region on Earth, and temporary PASSCAL
deployments at high latitudes not only provide regional
structure but also unique raypaths for constraining important
elements of deep structure in global tomographic models.
Broadband seismic recording in polar regions is uniquely
useful for constraining inner core anisotropy, because the
axis of inner core anisotropy is oriented approximately
parallel to Earth’s spin axis. The source of the anisotropy is
believed to be the preferred orientation of anisotropic inner
core iron crystals, but alignment mechanisms and crystallography
are unclear. Improved understanding the inner core is
key to understanding the evolution of the core system, core
heat flow, and magnetic field throughout Earth history.
A series of completed and ongoing PASSCAL experiments
(e.g., Figure 13) is interrogating the seismic structure of
the Antarctic lithosphere using specialized cold-weather
instrumentation. Little has been known about the origin
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