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from movement on geologic structures created by past
tectonic events. The regularity and consistency of the TA
make it possible to better understand these processes, which is
required to assess if time-dependent seismic hazard estimates
for sites of historical damaging earthquakes in central and
eastern North America is even feasible. Our understanding
will be widened with an enormous quantity of new data from
the arc-trench system on the southern margin of Alaska, the
active Aleutian volcanic arc, and the St. Elias Range where the
Yakutat terrane collides with coastal Alaska.
The availability of thousands of inexpensive and easily
deployed sensors and data acquisition systems in the FA pool
has facilitated rapid progress in studying episodic tremor and
slip, and motivated innovation in data processing such as new
multibeam back-projection methods that have been used to
map migration of tremors over days and weeks. Newly discovered
classes of earthquakes and patterns of tremor and slip
have profound and as yet only partially understood implications
for understanding the physics of fault friction and slip.
Resonance in basins and focusing of seismic waves by
surrounding structures are major contributors to geographic
Episodic Tremor and Slip
An exciting recent advance in seismology is recognition of
a wide range of rupture behaviors in and near major faults
zones, including “slow earthquakes” and regional seismic
tremor synchronous with geodetically measured episodic slip.
Seismic tremor was first recognized as a regional phenomenon,
rather than a probably anthropomorphic effect at a
single station, after deployment and joint analysis of dense
regional arrays, such as the Transportable Array and dense
permanent networks in Japan. The geodetic evidence shows
tectonic slip many orders of magnitude slower and often
deeper than “regular” earthquakes. Some tremor and slow slip
events occur regularly with periods of months to years, others
are aperiodic, and still others appear to be triggered by distant
earthquakes, suggesting that deeper portions of the fault are
only marginally stable.
High-resolution tremor
locations from multibeam
back-projection
during a major four-day
episodic tremor and slip
event under northern
Washington State. Each
black square represents
a subarray of seismic
stations. Circles show
tremor epicenters, colored
to indicate the time
of activity, from August
16 (dark blue) through
August 19 (dark red).
(Ghosh et al., 2010)
variability in earthquake shaking so, along with fault geometries,
high-resolution three-dimensional structural maps are
required to improve earthquake hazard models and strong
motion simulations. Compared to typical state-of-the-art
university-based seismology, the need for such maps across
broad areas compels a larger investment and innovative adaptation
of deployment strategies developed for commercial
applications. For this application, controlled-source data from
50˚
40˚
30˚
EarthScope and GeoPRISMS in
Eastern North America
The arrival of EarthScope’s Transportable Array on the East
Coast, Flexible Array deployments underway and planned in
the region, and availability of new geodetic data together
present an important opportunities to vastly improve the
resolution and accuracy of deformation models, more accurately
map seismic attenuation, raise awareness of earthquake
hazards, and motivate improved earthquake preparedness
in eastern North America. The Implementation Plan for
GeoPRSIMS identified eastern North America as a primary
site for the program because the record of multiple Wilson
cycles and significant along-strike geologic variations makes
it an excellent place to study mountain-building processes,
rift initiation, and the evolution, structure, and deformation
of a passive continental margin. The M W 5.8 earthquake near
Mineral, VA, during August 2011 was felt over a larger region of
the United States than any previous instrumentally recorded
earthquake; it serves as a reminder that passive margins are
tectonically active and that real seismic hazards on the East
Coast are poorly documented.
0
0
-90˚ -80˚ -70˚ -60˚ -50˚
The tectonic history of eastern North America is recorded in rift
basins, suture zones, and other geologic structures. Evidence of the
complex history is embedded in magnetic, gravity, and other geophysical
anomalies, as well as numerous seismically active fault zones.
(Shillington and Meltzer, pers. comm.)
0
0
0
Magnetic anomalies
Rift basins
select Paleozoic sutures
East Coast Magnetic
Anomaly
0 10 20 30
Earthquake depths (km)
I-10 VOLUME 1 | Section I | Scientific Justification