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Australia: The Land Where Time Began |
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Alaska
Subduction Zone – Tsunamic Structures in a Creeping Section According to Bécel et
al. there appear to be
structural characteristic configurations associated with subduction
zones that are capable of tsunamigenic earthquakes. Included among these
structures are heterogeneous plate interfaces, a small wedge of deformed
sediment situated at the toe of the plate that is overriding (the
frontal prism), and in the crust of the overriding plate splay faults
that root within the plate boundary megathrust. In this study Bécel et
al. used seismic reflection
imaging in to display the presence of these features within the creeping
segment of the Alaska subduction zone, the Shumagin Gap.
An active crustal-scale normal
fault system was identified that dips landwards which resembles that of
the 2011 Tohoku-oki earthquake in Japan. The Shumagin Gap was also found
to have a small frontal prism, a splay fault in deep water, and that
here the plate interface is rough and sedimented thinly. Lateral
propagation of rupture from a neighbouring segment into the Shumagin Gap
was proposed by Bécel et al.
to possibly explain a tsunamigenic earthquake that occurred there in
1788, and that in hazard assessments for the region tsunamigenic
potential should be considered. Structural configurations that are
similar to those in Tohoku are demonstrated by the results of this study
to possibly exist in other subduction zones, including within creeping
segments or segments that have no record of megathrust earthquakes. The
ability to identify regions that have similar configurations capable of
generating large tsunamis globally may improve tsunami forecasting. Very large tsunamis can be triggered by slip on
the shallow portion of the subduction zone plate boundary faults.
According to Bécel et al.
this can occur when large interplate earthquakes propagate to the trench
or during a special class of earthquakes, ‘tsunami earthquakes’, produce
larger tsunamis than expected for their moment magnitude (Kanamori, 1972
and are characterised by rupture velocities that are low and of long
durations. Subduction zones with a small frontal prism, a thin
subducting sediment layer and oceanic crust that is highly faulted are
where both types of event primarily occur (Polet & Kanamori, 2,000;
Tsuji et al., 2013; Okamura et al., 2008). The 2011 Tohoku-oki
earthquake (Tsuji et al., 2013) is the most dramatic recent example of
shallow, tsunamic slip occurring; more than 50 m of slip occurred near
the trench, which produced a local tsunami run-up of up to 30 m. Another
important feature that can be associated with tsunamigenic slip, a
localised normal fault, which is landwards dipping, branching from the
megathrust, was highlighted by this earthquake. When earthquake rupture
propagates to shallow depths extension is expected in the overriding
plate (Li et al., 2014; Xu et al., 2016). It is suggested by studies of
the Tohoku-oki earthquake that this normal fault may, alternatively,
promote shallow slip by decoupling the portion of the wedge that is
located seawards of the normal fault (McKenzie & Jackson, 2012; Ide,
Baltay & Beroza, 2011). In great earthquakes and/or tsunamigenic
earthquakes shallow slip is rare, though it is very destructive, which
makes it imperative to extract any information on structures that are
associated with these events, e.g. the Tohoku normal fault, which can
then be applied to the global subduction zone system. Only a few normal
faults that branch from a megathrust have been identified so far (Tsuji
et al., 2013; Tsuji et al., 2011; Klaeschen, 1994). Bécel et
al. suggest this may be a
result of these faults being steep, extending to great depths, and
straddle the shoreline in many subduction zones which makes them a
challenge to image and also, the significance of these was not
recognised until the Tohoku event. Evidence has been accumulating that tsunamigenic
earthquakes can be hosted even by creeping or weakly coupled subduction
zone segments, e.g. Java (Bilek & Engdahl, 2007); Nicaragua (Satake,
1994; Correa-Mora et al., 2009). The
M8.6 tsunami earthquake of
1957 is an example of such events along the Alaska-Aleutian margin that
propagated laterally through a shallow portion of the megathrust, that
was weakly coupled, offshore Unalaska Island, which caused a tsunami
that was unusually large, 23 m, (Nicolsky et al., 2016), and another
example is the Mw8.6
tsunami earthquake in 1946 (Lopez & Okal, 2006) that ruptured the weakly
coupled segment offshore Unimak Island. The importance of considering
such potentially tsunamigenic structural configurations that are thought
to be creeping or with no record of historical megathrust earthquakes is
highlighted by these and other events in segments that are weakly
coupled.
The Shumagin Gap – current knowledge In this study Bécel et
al. focussed on the Shumagin
Gap which is about 200 km wide and that has not ruptured in a great
earthquake in 150 years (Davies et al., 1981). It is indicated by data
from the Global Positioning System (GPS) that this system is creeping to
weakly locked, though on the shallow part of the plate boundary coupling
is not well resolved by the shore data (Fournier & Freymueller, 2007). A
few moderate earthquakes, M6.5
to M7.5, have occurred at
depths greater than 35 km in the area. It has been suggested by some
authors that rather than rupturing as a single large earthquake steady
creep has released most of the seismic moment and therefore a moderate
earthquake of M ⁓ 7 every ⁓40
years, as has been observed over the last century, is sufficient to
release the residual slip deficit (Fournier & Freymueller, 2007).
Earthquakes in 1788 and possibly 1847 are, however, inferred to have
ruptured laterally through part or all of the Shumagin Gap (Davies et
al., 1981). During the 1788 earthquake rupture occurred from the
neighbouring Semidi segment (Davies et al., 1981), which is strongly
coupled at the present. It is suggested by historical records that the
1788 earthquake, in particular, generated a large tsunami (Davies et
al., 1981). A recent tsunami scenario for the Alaska Subduction Zone
(Kirby et al., 2013) which showed that a large tsunami in this segment
could cause devastating consequences to local communities in Alaska as
well as around the Pacific has emphasised how important it is to
recognise the hazard that is posed by the Shumagin Gap that is coupled
weakly.
Shumagin Gap – structural configuration In this paper Bécel et
al. present new observations
of the Shumagin Gap in the Alaska Peninsula that were obtained from
multichannel seismic (MCS) reflection, wide angle reflection/refraction,
and bathymetric data that was acquired using the RV
Marcus G. Langseth in the
summer of 2011 during the ALEUT (Alaska Langseth Experiment to
Understand the megathrust) programme. Acquisition of the MCS data was
achieved by using 2 8-km-long seismic streamers and a 6,600 cu. in.
tuned airgun array. It is clearly revealed that the structural
configuration in the Shumagin Gap could make it prone to generating
transoceanic as well as local tsunamis, and that can explain events in
the past that were generated by this segment that has been assumed to
have been creeping, Bécel et al.
observed a large normal fault that was active that may slip either
coseismically which would allow displacement over a large area seawards
of the fault (Makenzie & Jackson, 2012) or be triggered by propagation
of a shallow earthquake rupture (Xu et al., 2016), as was determined for
the Tohoku earthquake (Tsuji et al., 2013; Tsuji et al., 2011), and
therefore could promote or result from shallow slip. They also observed
a heterogeneous character along the shallow plate boundary, a small
frontal prism and a thrust splay fault in deep water, all features that
are favourable to tsunamigenesis (Wang & Bilek, 2014). As was mentioned above, a large normal fault that
is landwards-dipping in the overriding plate that bounds the eastern
Sanak Basin is the most prominent feature that was imaged. This fault
system crosses the upper slope 75 km from the trench, dips ⁓40o-45o,
cutting the entire crust and connecting to the plate boundary fault at a
depth of ⁓35 km, near the intersection of the megathrust with the
forearc mantle wedge. At a depth of around ⁓6 km the fault splays into 2
branches, which breaches the seafloor at water depths of ⁓500-700 m. The
association of this fault with the Sanak Basin, which dates to the
Miocene, is ⁓6 km thick, which suggests that the fault had sustained
activity and accommodated significant total normal displacement. An
indication of the accumulative slip on the fault is the offset of the
acoustic basement which is very significant, ⁓2 km. The strongly coupled
Semidi segment, which is further to the northeast, ruptured in a
M8.2 earthquake in 1938, on
the upper slope the sediment is much thinner, and imaging shows no major
deep basin or normal fault system in the overriding plate (21). The 1938
Semidi earthquake epicentre was deep and the earthquake appeared to not
reach the trench and therefore only a small tsunami was produced
(Estabrook, Jacobs & Sykes, 1994). As presented here, the new data also demonstrate
that the fault that bounds the Sanak Basin and connects to the boundary
of the plate has been active recently. The youngest sediments are offset
by the fault and at the seafloor scarps about 5 high are observed in the
multibeam bathymetry data. Near the surface expression of the fault
pockmark features that are observed and the perturbed sediments could
suggest the presence of fluid flow that is localised. At the
intersection of this fault with the main plate boundary fault and the
occurrence of some seismicity also occurring within the upper plate near
the fault and further landwards (Abers, 1992). In the Shumagin Gap the properties of the shallow
plate boundary and structures in the overriding plate near the trench
are constrained by the seismic reflection data. These features are
consistent with settings that are known to generate tsunami earthquakes.
The oceanic plate that is incoming has a sediment cover that is thin,
50-800 m, and irregular that is disrupted strongly by the bending fault
(Shillington, 2015). It appears that only about 250-500 m of sediment
are being subducted, which can only be traced about 10 km from the
trench. The megathrust itself is marked further landwards by a
reflection that is discontinuous, and clear older thrust faults are
imaged in the outer wedge that merge with the top of the oceanic crust.
The thrust fault that is the most landward is located 20 km from the
trench and merges with the megathrust at a depth of about 12 km. This
prominent fault separates a highly deformed outer wedge with thrust
faults, which are clearly imaged, from ‘transparent’ basement rocks
further landwards that are overridden by tilted slope
Implications of imaged structure for tsunamigenesis A process for tsunamigenesis that accompanies
earthquakes in the Shumagin Gap, that is weakly coupled, could be
provided by the structures in the overriding plate
that are newly discovered and
properties of the plate boundary, when taken together (19,25). The
generation of a transoceanic tsunami would be favoured by the deep water
splay fault near the trench and or the rough, shallow plate interface
(Moore et al., 2007). The propagation of erratic slow rupture could be
favoured by the rough surface of the plate interface itself near the
trench with sediment that has been trapped in topography that was fault
generated at the top of the oceanic crust, as was proposed in 1986 for
the Sanriku tsunami earthquake (Tanioka, Ruff & Satake, 1997) or the
occurrence of slow slip events (Saffer & Wallace, 2015). Lithified
materials, including basement rocks, are present near the trench, and
which are implied by the narrow frontal prism and the storage of elastic
strain, could therefore permit the storage of elastic strain that could
lead to coseismic slip on the shallow part of the subduction zone and
that could favour large far-field tsunamis, as has been proposed for
other subduction zones (Polet & Kanamori, 2000; Contreras-Reyes, Flueh &
Grevemeyer, 2010). Several studies that were carried out since the
Tohoku earthquake have shown that a deep-rooted normal fault situated
within the overriding plate may be an indicator of propagation of
rupture to the trench (Tsuji et al., 2013; Li et al., 2014; Xu et al.,
2016) because large slip on the shallow plate interface would drive
extension in the overriding plate (Li et al., 2014; Xu et al., 2016). If
a large earthquake activated the normal fault system the decoupling of
the wedge seawards of the normal fault could be allowed, which could
therefore allow shallow slip and uplift of the seafloor (Makenzie &
Jackson, 2012) which would lead to the release of gravitational
potential energy over a large area or enabling overshoot (Ide, Baltay &
Beroza, 2011; Ito et al., 2011). According to Bécel et
al. it appears the normal
fault involved in the Tohoku earthquake separated parts of the seafloor
that experienced very different amounts of horizontal motion, 5-30 m of
horizontal motion landwards of the fault and 58-74 m of horizontal
motion seawards of the normal fault, though it appears the normal fault
itself slipped by only 1 m during the earthquake (Ito et al., 2011).
At steeply tapered outer wedges normal faults in the overriding
plate could be activated during large earthquakes by the reduction of
basal traction that is associated with drops in stress (Cubas et al.,
2013). According to Bécel et
al. a clear boundary in the
distribution of seismicity appears to be marked by the normal fault
system in the Shumagin Gap, were abundant seismicity associated with
plate interface is observed landward of the intersection of this fault
with the megathrust, though seawards of the intersection there is less
seismic activity. The seismicity change that is observed is robust and
is not related to station distribution. Where this fault intersects the
megathrust the sharp seismicity change implies that it could be
associated with a change in the physical properties of the upper plate
or conditions and frictional behaviour of the plate interface. It is
suggested by recent studies that changes in frictional parameters are
required in order to localise normal faulting along a major fault (Cubas
et al., 2013). These studies indicate, more specifically, a lower
effective friction on the plate interface updip of the normal fault than
downdip in this fault. It is known that a heterogeneous plate interface
facilitates activation of branching faults above the megathrust (Wendt,
Oglesby & Geist, 2009). It is also expected that slip on the major fault,
which is an actively landward-dipping normal fault system in the
Shumagin Gap, is located 75 km from the trench, is also expected to
produce vertical displacement of the seafloor near the fault, as well as
decoupling the seaward portion of the wedge, and could therefore enhance
the local tsunami, though this effect is known to be smaller than the
propagation of rupture to the trench (Li et
al., 2014) and depends
strongly on bathymetric effects. It is suggested by comparison of the thermal
models for the subduction zone off the Alaska Peninsula (Syracuse, van
Keken & Abers, 2010) with the structural configuration that was imaged
here and the estimated ruptures of earthquakes in the past that
temperature is not a primary control on slip behaviour here. The 150oC
isotherm, which was hypothesised by Bécel et
al. to represent the updip
limit of ruptures in some subduction zones (Hyndman & Wang, 1993), is at
a depth of 30 km about 120 km from the trench, which is deeper than
estimated great earthquake ruptures in this region (e.g. the 1938
earthquake) (Li et al., 2015; Estabrook, Jacob & Sykes, 1994). This is
similar to other subduction zones that are relatively cold, such as
Tohoku, where temperature also does not appear to be an important
control. The segment of the Shumagin Gap is made
particularly prone to producing tsunamigenic slip because of the
structural configuration that has been observed, though the causes of
coseismic slip capable of producing tsunamis to occur in a creeping or
weakly coupled section is not clear. It has been suggested by
earthquakes in the past that ruptures can propagate laterally into the
Shumagin Gap from neighbouring segments that are locked. A tsunamigenic
earthquake occurred on 7 August 1788 in the Shumagin segment (Davies,
Sykes, House & Jacob, 1981). Wave heights of 10s of metres were reported
on Sanak and Unga Islands, as based on historical descriptions from
Russian settlers, which is similar to the wave height that was recorded
for the 1946 Unimak near-field tsunami. It is suggested by Bécel et
al. that the shallow plate
boundary and/or splay faults in the overriding plate in this section
that is creeping and coupled weakly may have moved coseismically when
the 1788 earthquake propagated into the shallow Shumagin Gap. It is
demonstrated by other past events that asperities can ultimately break
through a neighbouring creeping to weakly coupled segment.
Significant seismological
evidence of the propagation of rupture laterally through creeping
segments (35) or updip,
through part of the subduction interface that is believed to be coupled
weakly (Noda & Lapusta, 2013), has been accumulating over the last 15
years. An example is the 1964 earthquake in Alaska in which 2 large
asperities were ruptured (Freymueller et al., 2008), and the Alaska
earthquake in 1957 when the weakly coupled Unalaska segment (Nicolsky et
al., 2016) ruptured. Bécel et
al. showed that there was a possibility of earthquakes nucleated in
the neighbouring locked Semidi segment propagating laterally into the
Shumagin Gap where they had shown that there was a structural
architecture that was favourable to large local and transoceanic
tsunamis, should be taken into account when considering hazards for this
region. The propagation of rupture through the creeping to weakly
coupled Shumagin Gap and activation of the normal fault system then
leading to either gravitational energy release or amplified motion
seawards of this fault by overshooting dynamically would result in the
tsunamigenic potential for this segment that is predicted by the elastic
coupling estimates (McKenzie & Jackson, 2012; Cubas, Avouac, Leroy &
Pons, 2013). If this is not taken into account this could lead to an
under-estimation of the seismic hazard. It is demonstrated by this study that normal
faults comparable to the one that was involved in the Tohoku earthquake
are present in other subduction zones. Bécel et
al. suggest such faults may
be of under-recognised significance around the world as a result of
their significance for tsunamigenesis previously not being appreciated
given the rarity of shallow slip in the short historical record, and as
imaging seismically requires the use of modern long-offset, MCS data,
that is available on only a small number of subduction zones. Also, such
structures would not be detected by imaging efforts if they straddled
the shoreline. Also suggested is that creeping regions might
have greater potential for tsunamis than has been recognised previously.
The identificatio0n and characterisation of active normal faults on a
crustal scale in the overriding plate as promotors or indicators of
rupture to the trench, and the shallow megathrust configuration is
therefore essential to a complete and comprehensive understanding of
hazards in the global subduction system.
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Author: M.H.Monroe Email: admin@austhrutime.com Sources & Further reading |