Download The Gulf of Alaska shear zone: a potential future Amphibious

The Gulf of Alaska shear zone: a potential future Amphibious Array scientific target
Jake Walter1, Gail Christeson1, Sean Gulick1, Robert Reece2
1
Institute for Geophysics, University of Texas at Austin
2
Dept. of Geology and Geophysics, Texas A&M University
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Scientific Justification
The Gulf of Alaska shear zone (GASZ) is a linear belt of intraplate seismicity in the Gulf
of Alaska that has been moderately active in the last 20 years. Significant seismicity occurred in
the late 1980s and early 1990s, and continues to this day. The north-striking belt of seismicity is
bounded by the eastward extent of the Aleutian arc and the Yakutat collision to the north and the
Kodiak-Bowie Seamount Chain to the south. It represents an intraplate zone of weakness and
possibly represents one mode for
−150˚
−138˚
−147˚
−141˚
−144˚
tectonic plate creation – the tearing
Magnitude
apart of an existing plate. Having
1
hosted a series of medium-sized
60˚
2
60˚
3
magnitude earthquake (Mw 7.2,
4
7.8, 7.7) during late 1987 and early
5
6
1988 (Pegler and Das, 1996), this
1987
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suture zone or tear in the plate is
1987
58˚
58˚
probably well-developed.
2012
1988
Seismicity studies offer
1992
important glimpses into active
deformational processes at plate
56˚
56˚
~54 mm/yr
margins and intraplate zones.
100 km
Figure 1 depicts the seismicity map
−150˚
−138˚
−147˚
of earthquakes in the AEC (Alaska
−141˚
−144˚
Earthquake Center) since 1965.
Within boxed area
The figure shows a clear linear
zone of seismicity, striking
100
northward, known as the Gulf of
80
60
Alaska Shear Zone (GASZ). The
40
GASZ exhibited significant activity
20
in the late 1980s and early 1990s
0
1965 1970 1975 1980 1985 1990 1995 2000 2005 2010
(Pegler and Das, 1996), and
7
features medium-sized earthquakes
6
5
(> magnitude 6) as recent as last
4
3
year. Figure 1b includes the
2
seismicity within the inset box on
1
1965 1970 1975 1980 1985 1990 1995 2000 2005 2010
Figure 1a. It should be noted that
the instrumental record was greatly
Year
improved in the 1990s with the
Figure 1: (Top panel) Map of historical AEC seismicity (1965-­‐
2013) and Global CMT larger earthquakes. The GASZ consists advent of digital data, and it is
of a north-­‐striking belt of seismicity and includes many likely that abundant seismicity
mapped features identified as faults (Reece et al., 2013). occurs today, though smaller events
(Middle panel) Seismicity rate during the time period 1965-­‐
are difficult to detect with the
2013. (Lower panel) E arthquake magnitudes during that time Fa
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ea
on
irw
tZ
ul
Fa
ASZ)
Zone (G
c
Alaska
Gulf of
Magnitude
eqs per 0.5 years
iak−
period. Bow
ie S
eam
sit
ion
Fa
ult
Shear
Ar
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tia
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Al
Kod
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an
au
rF
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current permanent land-based
network.
The Transition fault in the
Gulf of Alaska separates the thick
Yakutat block (crystalline crust
thickness 15-30 km; Christeson et
al., 2010; Worthington et al.,
2012) from oceanic crust of the
Pacific plate (Figure 2). The
deformation front, along which
the Yakutat block subducts
beneath the North American plate,
has moved to the east with time
from the Kayak Island fault to the
Figure 2: Proposed tectonic model of Gulick et al. (2007). Pamplona zone (Figure 2a) as the
Strike-­‐slip deformation is propagating westward along the Yakutat block resists subduction
Transition fault facilitated by a seaward extension of Queen (Gulick et al., 2013). The GASZ
Charlotte–Fairweather fault system. The thick Yakutat block, is approximately aligned with the
interpreted as an oceanic plateau, resists subduction unlike Pamplona zone, and separates two
the Pacific plate to the south. The GASZ divides w estern and localized blocks of the Pacific
eastern blocks of the Pacific plate.
plate (Figure 2; Gulick et al.,
2007). Reece et al. (2013) interpret the GASZ faults as increasingly transpressional to the north
and responsible for formation of the basement high, but note that there is no transpression
observed in the reported focal mechanisms of the GASZ. Reece et al. (2013) argue that the
GASZ is directly inline with the boundary between the subduction and collision regions and a
present-day GPS study (Elliott et al., 2013) confirms that the GASZ aligns with the onshore
separation of deformation styles. Reece et al. (2013) suggest that the boundary between the
subduction and collision regions became enhanced as the collision progressed and the thickest
part of the Yakutat (which thickens from west to east; Worthington et al., 2012) encountered the
North American margin during the Pleistocene, and that the increased stress may have initiated
the GASZ.
The GASZ robust activity is rather surprising as it occurs along an intraplate region rather
than at the plate boundary. This complex region is interpreted to have transpression increasing
from south to north along the GASZ (Reece et al., 2013), yet only strike-slip focal mechanisms
are observed. Transpression and strike slip events would be expected along the Transition fault
(Gulick et al., 2007), which instead exhibits little seismicity compared to the GASZ (Figure 1).
Plausibly there may exist a misalignment between regional stresses and some of the
transpressional features and thus the GASZ is an excellent opportunity to test theories about offfault damage in an area that has been mapped from multiple multi-channel seismic reflection
surveys (e.g. Reece et al., 2013). Close-by station spacing (e.g. ~25 km station spacing) should
address which percentage of seismicity occurs along the fault zone or “off the fault.” Just where
are the regions of active deformation and how does a plate break apart? The GASZ offers the
opportunity to address some of these fundamental questions in situ.
Oceanic intraplate zones are capable of producing some of the largest recorded strike-slip
earthquakes, for example, the Mw 8.7 April 11, 2012 earthquake in the Indian Ocean (Yue et al.,
2012). Detecting the smallest magnitude earthquakes and building a robust seismicity catalog
would allow investigation of some fundamental processes related to intraplate deformation. In
order to better study the GASZ and provide insight into the mechanics of intraplate shearing, we
urge the community to install an amphibious, dense (e.g. ~25 km station spacing) network of
OBSs in areas surrounding the GASZ and encompassing its juncture with the Transition Fault.
An OBS network would leverage USArray Transportable Array (TA) stations that will densify
the land-based seismic network throughout Alaska and the region (Southern Alaska).
Short Summary
In order to better study this deformational feature and provide insight into the mechanics
of intraplate shearing, we suggest that the GASZ be considered for a future OBS experiment.
The extension of USArray’s Transportable Array into Alaska offers a further complement to
closely study this feature. Such an experiment would allow delineation of seismicity along the
extent of the GASZ, to better understand micro-plate formation, as robust seismicity provides a
proxy for active brittle deformation. The location is unique in that it is one of the few places in
the world where micro-plate formation is likely ongoing and allows us to address fundamental
questions about how this tearing occurs. For example, do earthquakes occur along pre-existing
features or do they occur as off-fault damage – actively breaking the rock? Also, it could address
the role that far-field stresses play in determining the seismicity behavior on the heterogeneous
segments of the GASZ and the complicated junction with the Transition Fault. A number of
unforeseen discoveries may occur, such as delineating seismicity productivity differences, the
presence/absence of tidal triggering, and identifying zones that may experience tremor
accompanying slow slip. These and other first-order studies have implications for micro-plate
formation and for understanding earthquake rupture processes.
References
Christeson, G. L., S. P. S. Gulick, H. J. A. Van Avendonk, L. L. Worthington, R. S. Reece, and T. L. Pavlis (2010), The Yakutat
terrane: Dramatic change in crustal thickness across the Transition fault, Alaska, Geology, 38, 895-898, doi: 10.1130G31170.1.
Elliott, J., J. T. Freymueller, and C. F. Larsen (2013), Active tectonics of the St. Elias Orogen, Alaska, observed with GPS
measurements, J. Geophys. Res., 118, 5625-5642, doi: 10.1002/jgrb.50341.
Gulick, S. P. S., L. A. Lowe, T. L. Pavlis, J. V. Gardner, and L. A. Mayer (2007), Geophysical insights into the Transition fault
debate: Propagating strike slip in response to stalling Yakutat block subduction in the Gulf of Alaska, Geology, 35, 763-766, doi:
10.1130/G23585A.1.
Gulick, S. P. S., R. S. Reece, G. L. Christeson, H. J. A. Van Avendonk, L. L. Worthington, and T. L. Pavlis (2013), Seismic
images of the Transition fault and the unstable Yakutat-Pacific-North American triple junction, Geology, 41, 571-574, doi:
10.1130/G33900.1.
Pegler, G., and S. Das (1996), The 1987–1992 Gulf of Alaska earthquakes, Tectonophysics, 257(2–4), 111–136.
Reece, R. S., S. P. S. Gulick, G. L. Christeson, B. K. Horton, H. J. A. Van Avendonk, and G. Barth (2013), The role of farfield
tectonic stress in oceanic intraplate deformation, Gulf of Alaska, J. Geophys. Res., 118, 1862-1872, doi: 10.1002/jgrb.50177.
Worthington, L. L., H. J. A. Van Avendonk, S. P. S. Gulick, G. L. Christeson, and T. L. Pavlis (2012), Crustal structure of the
Yakutat terrane and the evolution of subduction and collision in southern Alaska, J. Geophys. Res., 117, B01102, doi:
10.1029/2011JB008493.
Yue, H, T. Lay and K. D. Koper (2012), En Echelon and Orthogonal Fault Ruptures of the 11 April 2012 Great Intraplate
Earthquakes. Nature, 490, 245-249, doi:10.1038/nature11492.