The seismic potential of the shallow portions of the northern Cascadia and the North Sumatra subduction zones: insights from laboratory friction experiments
Veröffentlichungsdatum
2021-12-10
Autoren
Betreuer
Gutachter
Zusammenfassung
Many regions that are prone to experience strong earthquakes and tsunamis are
densely populated, such as the coastlines of the Pacific Ocean and some of the Indian Ocean.
These regions are subduction zone settings, where one tectonic plate subducts beneath
another, which produces a gigantic fault – a megathrust fault. Subduction zone earthquakes
largely occur on such megathrust faults. They have cost an incredible number of lives, and
future events pose a constant threat to many more. Especially those megathrust earthquakes
that nucleate in or propagate to very shallow depths can cause large damage and tsunamis.
In general, the seismicity in the shallow portion of subduction zone megathrusts is low, but
recent events such as the 2004 Aceh-Andaman earthquake and tsunami offshore North
Sumatra have tragically shown the potential of shallow seismicity. Despite extensive
investigations of multiple geoscientific disciplines, the shallow extent of earthquake rupture and
slip of subduction zones around the world is still poorly constrained. Reasons for this lie in the
challenging nature of such investigations, because the shallow extent of subduction zone
earthquakes lies at sea and well below the ocean floor. Limited knowledge of this shallow
earthquake extent reduces the chance of meaningful earthquake and tsunami hazard
assessment and thus damage mitigation.
Because earthquakes are friction phenomena, a large body of work in earthquake
research is based on laboratory friction experiments. Early friction experiments have shown
that repetitive frictionally unstable stick-slip sliding on artificial faults in the laboratory
represents the small-scale equivalent of earthquakes on faults in nature. Friction on a fault
evolves with velocity, slip, and time (rate- and state-dependent friction) and thus can lead to
unstable sliding. Unstable sliding includes periods of fault locking and accumulation of elastic
energy, with intermittent periods of fault rupture and slip, which releases the stored energy.
The depth interval on the megathrust fault that is capable of unstable frictional sliding and thus
earthquake nucleation is called the seismogenic zone. Crucial to estimating the extent of the
seismogenic zone is knowledge of the variation of the velocity-dependent frictional behavior
with depth. Especially the velocity-dependent frictional behavior at plate tectonic rate has
shown to be crucial. This information can be derived from laboratory friction experiments and
application of so-called rate- and state-friction laws. Ideally, such experiments should be
conducted on fault-zone material. However, such material is difficult to obtain and its availability
is very limited. Subduction zone input materials, which are the marine sedimentary column on
the subducting plate, are less difficult to recover and hold important information on where a
megathrust forms or what intrinsic frictional behavior the fault-forming material has.
Measurements on input material are therefore a valuable alternative to measurements on fault zone material.
This thesis presents the results of laboratory friction experiments at room temperature,
under relatively low pressure, and driven at velocities starting from plate rate. These
experiments were designed to investigate the frictional behavior of subduction zone input
sediments and its implications on the fault slip behavior and seismic potential of the shallow
portions of two subduction zones. The first is the northern Cascadia subduction zone, located
along the West coast of North America, where a major earthquake is about to be due. The
second is the North Sumatra subduction zone, a region of the Sunda subduction zone and the
location of recent destructive earthquakes and tsunamis.
At northern Cascadia, the megathrust has so far not been sampled. Based on
measurements of frictional strength contrasts in the input sedimenatry column, we propose
that the megathrust fault will likely form in a weak illite-rich hemipelagic clay near the top of the
oceanic basement. Because this inference is in good agreement with interpretations of seismic
imaging, we focused on the frictional behavior of this specific material. The absence of shallow
non-destructive slow slip events at northern Cascadia has recently been interpreted to result
from a megathrust that is locked and potentially seismogenic all the way to the trench. In
contrast, the results presented in this work indicate that the shallow part of the megathrust is
not capable of producing slow slip events nor capable of locking and thus likely not
seismogenic. However, our friction data also indicate low resistance to a propagating
earthquake nucleating at greater depth. This low resistance is evident from substantially
elevated pore pressure, low frictional strength, and low cohesion. Therefore, the northern
Cascadia subduction zone holds the potential of shallow earthquake slip and tsunamigenesis.
At North Sumatra, seismic slip during the 2004 Aceh-Andaman subduction zone
earthquake was unexpectedly shallow and resulted in a devastating tsunami. Recent work
suggested that the cause is a very shallow seismogenic zone that may be created by
diagenetic strengthening of fault-forming input sediments prior to subduction. This thesis
presents the results of laboratory friction experiments designed to test this hypothesis. We
showed that input sediments to the North Sumatra subduction zone exhibit pronounced
frictional instability, offering evidence for a frictionally unstable and thus seismogenic shallow
megathrust and thus an explanation for shallow earthquake slip in the 2004 event. However,
our measurements indicate that the shallow megathrust is not seated in frictionally strong, but
in very weak sediments. The combination of weak and unstable sediments is striking because
a large number of previous friction studies have established that weak materials under low
temperature and pressure conditions are generally associated with stable frictional sliding. This
relationship offers an explanation for the observed general lack of seismicity in the shallow
portion of subduction zone megathrusts, where unconsolidated, clay-rich, weak materials are
typically encountered. We proposed that threshold amounts of dispersed hydrous amorphous silica in otherwise weak and clay-rich sediments are responsible for an unstable sliding
character, which can explain the shallow seismicity at North Sumatra.
To test the hypothesis that small amounts of hydrous amorphous silica induce unstable
sliding behavior, we designed friction experiments on artificial mixtures of weak shale and
biogenic opal, a type of hydrous amorphous silica. These experiments revealed pronounced
potentially unstable behavior in mixtures with ≥ 30 % opal that had low frictional strength.
Based on our results, we proposed that potential unstable sliding at low frictional strength can
be explained by the viscous behavior of frictional contacts of hydrous amorphous silica. This
highlights the necessity to reevaluate the strength-stability relationship. Our findings support
the hypothesis on the role of hydrous amorphous silica in unstable sliding behavior, which has
important implications for the potential of shallow seismogenesis at other subduction zones
where input sediments contain critical amounts of hydrous amorphous silica.
This thesis demonstrates that the northern Cascadia and the North Sumatra subduction
zone have very different intrinsic frictional fault slip behavior despite very similar extrinsic
properties and attributes, such as temperature or pressure. Thus, intrinsic factors are found to
be crucial to the estimation of the slip behavior of shallow megathrust faults, such as a mineral
composition of fault material with threshold amounts of hydrous amorphous silica. Hydrous
amorphous silica-bearing sediments could form megathrust faults due to intrinsically low
strength and potential of overpressure. The shallow portion of megathrust faults formed in such
sediments may thus be able to host large and slow earthquakes. This could for instance be
the case in the northern Barbados subduction zone, a setting that similar to the North Sumatra
subduction zone has been shown to have a porous, overpressured décollement and
predécollement consisting of material that contains elevated amounts of hydrous amorphous
silica. Thus, this thesis raises the possibility that subduction zones with a shallow seismogenic
zone may be more common than predicted by the seismogenic zone model. This inference
implies that earthquake and tsunami hazards could be highly underestimated at some
subduction zone settings.
densely populated, such as the coastlines of the Pacific Ocean and some of the Indian Ocean.
These regions are subduction zone settings, where one tectonic plate subducts beneath
another, which produces a gigantic fault – a megathrust fault. Subduction zone earthquakes
largely occur on such megathrust faults. They have cost an incredible number of lives, and
future events pose a constant threat to many more. Especially those megathrust earthquakes
that nucleate in or propagate to very shallow depths can cause large damage and tsunamis.
In general, the seismicity in the shallow portion of subduction zone megathrusts is low, but
recent events such as the 2004 Aceh-Andaman earthquake and tsunami offshore North
Sumatra have tragically shown the potential of shallow seismicity. Despite extensive
investigations of multiple geoscientific disciplines, the shallow extent of earthquake rupture and
slip of subduction zones around the world is still poorly constrained. Reasons for this lie in the
challenging nature of such investigations, because the shallow extent of subduction zone
earthquakes lies at sea and well below the ocean floor. Limited knowledge of this shallow
earthquake extent reduces the chance of meaningful earthquake and tsunami hazard
assessment and thus damage mitigation.
Because earthquakes are friction phenomena, a large body of work in earthquake
research is based on laboratory friction experiments. Early friction experiments have shown
that repetitive frictionally unstable stick-slip sliding on artificial faults in the laboratory
represents the small-scale equivalent of earthquakes on faults in nature. Friction on a fault
evolves with velocity, slip, and time (rate- and state-dependent friction) and thus can lead to
unstable sliding. Unstable sliding includes periods of fault locking and accumulation of elastic
energy, with intermittent periods of fault rupture and slip, which releases the stored energy.
The depth interval on the megathrust fault that is capable of unstable frictional sliding and thus
earthquake nucleation is called the seismogenic zone. Crucial to estimating the extent of the
seismogenic zone is knowledge of the variation of the velocity-dependent frictional behavior
with depth. Especially the velocity-dependent frictional behavior at plate tectonic rate has
shown to be crucial. This information can be derived from laboratory friction experiments and
application of so-called rate- and state-friction laws. Ideally, such experiments should be
conducted on fault-zone material. However, such material is difficult to obtain and its availability
is very limited. Subduction zone input materials, which are the marine sedimentary column on
the subducting plate, are less difficult to recover and hold important information on where a
megathrust forms or what intrinsic frictional behavior the fault-forming material has.
Measurements on input material are therefore a valuable alternative to measurements on fault zone material.
This thesis presents the results of laboratory friction experiments at room temperature,
under relatively low pressure, and driven at velocities starting from plate rate. These
experiments were designed to investigate the frictional behavior of subduction zone input
sediments and its implications on the fault slip behavior and seismic potential of the shallow
portions of two subduction zones. The first is the northern Cascadia subduction zone, located
along the West coast of North America, where a major earthquake is about to be due. The
second is the North Sumatra subduction zone, a region of the Sunda subduction zone and the
location of recent destructive earthquakes and tsunamis.
At northern Cascadia, the megathrust has so far not been sampled. Based on
measurements of frictional strength contrasts in the input sedimenatry column, we propose
that the megathrust fault will likely form in a weak illite-rich hemipelagic clay near the top of the
oceanic basement. Because this inference is in good agreement with interpretations of seismic
imaging, we focused on the frictional behavior of this specific material. The absence of shallow
non-destructive slow slip events at northern Cascadia has recently been interpreted to result
from a megathrust that is locked and potentially seismogenic all the way to the trench. In
contrast, the results presented in this work indicate that the shallow part of the megathrust is
not capable of producing slow slip events nor capable of locking and thus likely not
seismogenic. However, our friction data also indicate low resistance to a propagating
earthquake nucleating at greater depth. This low resistance is evident from substantially
elevated pore pressure, low frictional strength, and low cohesion. Therefore, the northern
Cascadia subduction zone holds the potential of shallow earthquake slip and tsunamigenesis.
At North Sumatra, seismic slip during the 2004 Aceh-Andaman subduction zone
earthquake was unexpectedly shallow and resulted in a devastating tsunami. Recent work
suggested that the cause is a very shallow seismogenic zone that may be created by
diagenetic strengthening of fault-forming input sediments prior to subduction. This thesis
presents the results of laboratory friction experiments designed to test this hypothesis. We
showed that input sediments to the North Sumatra subduction zone exhibit pronounced
frictional instability, offering evidence for a frictionally unstable and thus seismogenic shallow
megathrust and thus an explanation for shallow earthquake slip in the 2004 event. However,
our measurements indicate that the shallow megathrust is not seated in frictionally strong, but
in very weak sediments. The combination of weak and unstable sediments is striking because
a large number of previous friction studies have established that weak materials under low
temperature and pressure conditions are generally associated with stable frictional sliding. This
relationship offers an explanation for the observed general lack of seismicity in the shallow
portion of subduction zone megathrusts, where unconsolidated, clay-rich, weak materials are
typically encountered. We proposed that threshold amounts of dispersed hydrous amorphous silica in otherwise weak and clay-rich sediments are responsible for an unstable sliding
character, which can explain the shallow seismicity at North Sumatra.
To test the hypothesis that small amounts of hydrous amorphous silica induce unstable
sliding behavior, we designed friction experiments on artificial mixtures of weak shale and
biogenic opal, a type of hydrous amorphous silica. These experiments revealed pronounced
potentially unstable behavior in mixtures with ≥ 30 % opal that had low frictional strength.
Based on our results, we proposed that potential unstable sliding at low frictional strength can
be explained by the viscous behavior of frictional contacts of hydrous amorphous silica. This
highlights the necessity to reevaluate the strength-stability relationship. Our findings support
the hypothesis on the role of hydrous amorphous silica in unstable sliding behavior, which has
important implications for the potential of shallow seismogenesis at other subduction zones
where input sediments contain critical amounts of hydrous amorphous silica.
This thesis demonstrates that the northern Cascadia and the North Sumatra subduction
zone have very different intrinsic frictional fault slip behavior despite very similar extrinsic
properties and attributes, such as temperature or pressure. Thus, intrinsic factors are found to
be crucial to the estimation of the slip behavior of shallow megathrust faults, such as a mineral
composition of fault material with threshold amounts of hydrous amorphous silica. Hydrous
amorphous silica-bearing sediments could form megathrust faults due to intrinsically low
strength and potential of overpressure. The shallow portion of megathrust faults formed in such
sediments may thus be able to host large and slow earthquakes. This could for instance be
the case in the northern Barbados subduction zone, a setting that similar to the North Sumatra
subduction zone has been shown to have a porous, overpressured décollement and
predécollement consisting of material that contains elevated amounts of hydrous amorphous
silica. Thus, this thesis raises the possibility that subduction zones with a shallow seismogenic
zone may be more common than predicted by the seismogenic zone model. This inference
implies that earthquake and tsunami hazards could be highly underestimated at some
subduction zone settings.
Schlagwörter
Hydrous amorphous silica
;
Earthquake
;
Megathrust faults
Institution
Fachbereich
Dokumenttyp
Dissertation
Zweitveröffentlichung
Nein
Sprache
Englisch
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