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Global Observations Identify Two Reinforcing Reasons Why the
Aleutian-Alaska Subduction Zone is Prone to Rupture in High
Magnitude Earthquakes
David W. Scholl1,2,*, STEPHEN H. KIRBY1,3 ROLAND VON HUENE1
1
U.S. Geological Survey, Emeritus, Menlo Park, California 94025, USA
2
Department of Geology and Geophysics, Emeritus, University of Alaska Fairbanks,
Fairbanks, Alaska 99775, USA
3
Research Center for Earthquake and Volcanic Eruption Prediction, Visiting
Seismologist, Tohoku University, Sendai, Japan.
*[email protected]
A Common Physical Setting of High-Magnitude Subduction Zone Earthquakes
Important questions have been raised about the possible influence that geologic
setting has on the subduction zone occurrence of great (>Mw 8.0), giant (>Mw 8.5) and
supergiant (>Mw 9.0) megathrust earthquakes. A large database of global observations,
one that grew rapidly in the past 12 years, identifies two significant factors concerning
the smoothness of the subducting surface of the underthrusting plate that, respectively,
can inhibit or promote large-magnitude, long-runout ruptures of tsunamigenic
earthquakes. These are:
*An Inhibiting Factor--underthrusting of high, areally extensive bathymetric
relief terminates, hinders, or significantly modulates rupture continuation and the
generation of high-magnitude megathrust earthquakes. For a comprehensive discussion
of the inhibiting factor, see Wang and Bilek (2014)
*A Promoting (favoring) Factor--underthrusting thick sediment or smooth lower
plate relief promotes rupture continuation and the generation of large-magnitude
megathrust earthquakes. Using a small and somewhat inaccurate database, Ruff (1989)
was among the first to explore the smoothness factor. A relation was conjectured to
exist, but it was not a statistically compellingly one.
Rechecking the Smoothness Factor
To check the correctness of the Ruff conjecture, we compiled a much larger
global database of vetted instrumental era (1898 through January, 2013) megathrust
earthquakes and precise sediment thickness measurements (Scholl et al. 2015). This
compilation statistically supported the correctness of the inference that high-magnitude
megathrust earthquakes tend to occur where the interplate surface beneath the
submerged forearc is made physically smooth by:
(1) Subduction of a long length (~>300-400 km) of thick (>1.0 km), reliefsmothing trench sediment, and
(2) Subduction of a long length (~>300-400 km) of bathymetrically smooth (low
relief) seafloor.
Where subducting sediment is thin and much less than ~1.0 km, interplate
smoothing can be effected by a subduction channel thickened by basal forearc
subduction erosion. For a full discussion of these observations, see Scholl et al. (2015).
Graphic displays of the elevated occurrence of high magnitude megathrust
earthquakes at thick- vs thin-sediment trench sectors are presented on Figures 1 and 2.
Figure 1 is a diagram showing the number and percent of all instrumentally-recorded
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megathrust events that nucleated at or above a plotted magnitude (total event number =
176). Figure 2, is an occurrence diagram of the number and relative percent of all
events but compensated in the Mw range 7.5-8.4 for the ~11000-km longer (58%)
global length of thin- vs. thick-sediment trenches (~19000 vs. ~8000 km). Prior to
January 2013, most (93%) megathrust earthquakes occurred in this magnitude range
(n=164 of 176 events).
Large Mw8.0-9.0 megathrust earthquakes (n=23) also nucleate at trenches with
sediment fills much less than ~1.0 km. As noted, these earthquakes are associated with
the subduction of low-relief ocean floor and where the debris of subduction erosion
thickens (smooths) the plate-separating subduction channel to favor the characteristic
lengthy rupture continuation of high-magnitude megathrust earthquakes. See, for
example, the thin-sediment plots on Figures 1 and 2 registering the 1952 Kamchatka
and 2011 Tohoku-Oki Mw9.0 events where the incoming seafloor is relatively smooth.
A Concerning Look at the Aleutian-Alaska Subduction Zone
Both inhibiting and promoting factors work in tandem to foster the repeated
rupturing of large-magnitude megathrust earthquake along the Aleutian-Alaska
subduction zone (Fig 3). Inhibiting factor 1, subducting high bathymetric relief,
principally occurs at widely spaced fracture zones and seamount chains entering the
subduction zone. These linear zones of relief tend to segment the margin into rupture
zones. Promoting (favoring) factor 2, subduction of thick sediment, occurs along
virtually the ~3500-km length of the Aleutian-Alaska subduction zone. Sediment sources
are primarily the glaciated drainages of SE Alaska that supplied the west-sloping trench
axis with a ~2-km-thick section of subducting sediment (Fig. 4). Active subduction
erosion also further thickens and smooths the subduction channel separating the North
American plate from the underthrusting Pacific plate.
It is worth noting that characteristically intra-oceanic subduction zones, e.g., the
SW Pacific Izu-Bonin-Mariana (IBM) and Tonga-Kermadec arc-trench systems, tend not
to rupture in great megathrust earthquakes. This circumstance is not true of the interoceanic Aleutian subduction zone (Figs. 3 and 4). The SW Pacific subduction zones are
only thinly sedimented and they are entered by numerous large, rupture-inhibiting
bathymetric elements, in particular the IBM system.
As recently reemphasized by Ryan et al., (2012a,b), Butler (2012), von Huene
(2014, 2015, and in press), certain segments or sectors of the Aleutian-Alaska
subduction zone are also prone to launching very destructive near-field Alaska and farfield transoceanic tsunamis. In addition certain segments, for example the 700-km long
Fox Island segment, have not ruptured in a great Mw earthquake at least as far back as
the beginning of Russian exploration and documentation in the mid 1700s (Fig. 3).
Equally concerning, sectors of the Shumagin-Semidi segment last ruptured in a great
megathrust earthquake in 1938 and, before that, perhaps in 1788 (Kirby et al., 2013; Fig.
3).
Tsunamis launched from the Fox and Shumagin-Semidi segments can be
expected to cause extensive damage to Alaska coastal communities, the west coast of
North America and Hawaii and potentially, as demonstrated by the 1946 Scotch Cap
tsunami, to island communities far to the south (Okal et al., 2002). Considering societal
importance, these segments are prime targets for on-shore and offshore paleoseismic
and geodetic research and the determination of their tsunami-launching potential. This
potential also applies to the Andreanof segment of the 1957 Aleutian Mw8.6 megathrust
that generated a destructive Alaska and trans-oceanic tsunami (Figs. 3 and 4).
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References
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Report 2013-1170 [http://pubs.usgs.gov/of/2013/1170/b/].
Ruff, L., 1989, Do trench sediments affect great earthquake occurrence in subduction zones?: Pure and Applied Geophysics, v. 129,
p. 263–282.
Ryan, H., von Huene, R. Scholl, D. W., and Kirby, S, 2012a: Tsunami hazards to U.S. coasts from giant earthquakes in Alaska
Alaskan-Aleutian earthquakes: EoS, Transactions, American Geophysical Union, v. 93, no. 19, 8 May, p. 185-186.
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FIGURE 1: Occurrence diagram of the number and relative percent of all instrumentallyrecorded megathrust events (n=176) that before January 2013 nucleated at a magnitude of
Mw7.5 and higher at thin-and thick sediment trenches. The global length of thin-sediment trench
at which >Mw7.5 events occurred is ~21500 km, that for thick-sediment trenches is ~14000 km.
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FIGURE 2: Same data plot of Figure 1 except for thin-sediment trenches, the number of events
in the Mw range 7.5-8.4 was proportionally reduced to compensate for the ~11000-km longer
global length of thin- vs thick-sediment trenches (~19000 vs ~8000 km). Prior to January 2013,
most (93%) megathrust earthquakes occurred (n=164 of 176 events) in this magnitude range.
FIGURE 3: Great megathrust earthquakes have repeatedly nucleated along the Aleutian-Alaska
subduction zone. Causative factors linked to this phenomenology include that the trench axis is
thickly (~2 km) charged with subducting sediment and the fact that large bathymetric elements
of seamount chains and fracture zones that limit megathrust earthquake ruptures are widely
spaced.
FIGURE 4: E-W longitudinal cross-section along the axis of the Aleutian-Alaska Trench
displaying it’s ~2-km-thick body of turbiditic sediment largely supplied by glaciated eastern
Alaska drainages.