Collaborative Research: Seismic and
Geodetic Imaging of Subducting Terranes Under North America (MOOS)
Principal
Investigators:
Geoffrey Abers, Boston University
Douglas Christensen and Jeff Freymueller, University of Alaska Fairbanks
Funded By
NSF-EAR Geophysics
Summary. The collision of thickened crust with subduction
zones significantly modifies subduction.
These accretion events lead to net growth of continents and drive much of
the subduction-related tectonism. Terrane
collision may also have a profound effect on the size, coupling, and rupture
characteristics of large intraplate earthquakes. The present accretion of
exotic terranes with the Alaska subduction system represents one of the few
examples of this process currently active. In this region, the collision of a region of thickened
crust, the Yakutat terrane, occurs at the largest rupture asperity known, part of
the 1964 Mw9.2 Alaska earthquake.
The collision produces mountains along the Alaska coast and perhaps far
inland, and may drive westward extrusion of distal parts of Alaska. Recently, an unusual layer,
perhaps thickened crust, has been imaged at the top of the subducting plate
beneath central Alaska from 70 to 150 km depth, using receiver functions from
the BEAAR (Broadband Experiment Across the Alaska Range) IRIS-PASSCAL
experiment. If continuous with the
shallow structure, this would represent the largest deeply-subducted fragment
of thickened crust yet observed. Subduction of such thick crust may help explain the size of
the 1964 asperity. However, the
lack of continuity between deep and shallow structures makes it difficult to
tell; have these signals imaged the largest piece of thick subducted crust on
the planet, or something else? In any case, what is the effect of subducting
terranes on mechanics of the thrust zone?
This project images the subducted plate, upper plate, and
intervening deformation in the region between the Alaska coastline and BEAAR. Here
subduction passes through and past the 1964 rupture zone. Broadband seismographs image the top of
the downgoing plate through and below the thrust zone. Integration with previous studies
providse the longest continuous transect of a subduction zone yet available,
over 700 km across strike, following a slab from the trench to coast to where
last seen at 150 km depth. In
parallel, a combination of geodesy and seismicity is used to image deformation
currently associated with the plate interface, where it ruptured in the
planetŐs second largest known earthquake. Modeling of deformation, when
integrated with the imaging, elucidates the nature of the locked zone, the
origin of the largest asperity, and the structural controls on interplate
thrust processes. These results are used to test ideas for the origins of
intermediate-depth earthquakes, by sampling at high resolution the transition
at the down-dip end of the thrust zone in seismicity, strain, and structure.
The experiment consists of a deployment of 30 broadband
seismographs at dense spacing, supplemented by short-period seismographs in
places where higher-resolution seismicity would provide most information, and
GPS measurements of surface deformation across this zone. Sparse permanent seismic and geodetic (PBO)
stations provide regional control. Many of the seismicity and GPS sites are
collocated, so there are cost savings to simultaneously conducting geodetic and
seismic field work. These data,
when integrated, will provide a thorough picture of terrane accretion and its
impact on the generation of great earthquakes.