Ikari, Matt
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Item-typ:Veröffentlichung, The transition from stable to slow to fast earthquake slip: the influence of surface morphology, fault normal stiffness and lithology(2021-11-05); ; ; Over the last decades, new types of earthquakes have been discovered. The most well-known group of ordinary earthquakes might be the most dangerous as they emit the largest amount of seismic radiation and cause ground-shaking, but repeating slow earthquakes can also damage buildings and infrastructure. Ordinary earthquakes occur when movement on a fault is unstable and a run-away process accelerates the movement to seismogenic velocities. During slow earthquakes, there are also clearly defined phases of faster slip along the fault, but the maximum slip velocity reached during these phases is lower. Then, there are aseismic faults, where slip accumulates constantly by stable creep at a rate close to the far-field stressing rate. The mechanisms that control the nature of sliding behavior of faults are multiple and studied in more or less detail. In this thesis, I explore how three factors influence fault stability: fault surface roughness and roughness anisotropy, fault-normal stiffness and stiffness contrasts across a fault, and the lithological controls on the extraordinary shallow slow slip events in the Hikurangi subduction zone margin (New-Zealand). Here, I present results using direct shear experiments, while varying one of the studied variables. To study the influence of fault surface morphology, I use two materials; a velocity-weakening and therefore potentially unstable pure quartz powder, and Rochester shale powder, which is velocity-strengthening and therefore likely to show stable sliding. Fault surface morphology evolves with displacement and its influence on frictional behavior is therefore studied by varying the amount of displacement on the samples. To test the influence of host-rock stiffness, the testing device is fitted with springs of variable stiffness in both the shear-parallel and fault-normal directions. Testing occurs on the intrinsically unstable quartz powder and I analyze both the frictional properties as well as the slip instabilities that occur. For the study about the Hikurangi margin, I use samples of the sediments on the incoming plate and use realistically low deformation rates, to study the frictional behavior and the occurrence of spontaneous slow slip events during the experiments. The results show rough, isotropic faults can host slip instabilities, because these show the required velocity-weakening frictional behavior. Striated, smooth surfaces are velocity-strengthening and promote stable sliding. The formed fault surfaces obey the typical self-affine fractal scaling, that make these results directly applicable to natural faults. Reducing the fault-normal stiffness causes the fault to become less velocity-weakening and would therefore promote stable sliding. However, slip instabilities occur when the fault-normal stiffness is reduced, which I explain by a different mechanism that requires a stiffness asymmetry. The asymmetry is the result of reducing the fault-normal stiffness on one side of the fault. The plate-rate shear experiments on Hikurangi sediments show spontaneous slow slip events occur in the calcite-rich lithologies, whereas the weakest lithologies are velocity-strengthening. Altogether, the results presented in this thesis suggest unstable sliding will occur on rough, isotropic fault patches. The slow slip events in the Hikurangi margin can only occur when the slow slip event-hosting lithologies are introduced into the deformation zone. This could be explained by a geometrically complex deformation zone due to subducting seamounts. Stiffness contrasts, due to lithological contrast across a fault or due to asymmetric damage, may cause slip instabilities that are not explained by the traditional critical stiffness theory. I show the three studied variables are closely linked and fault surface roughness, fault stiffness and stiffness contrast, as well as fault zone lithology may affect each other.Dissertation422 584 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, The seismic potential of the shallow portions of the northern Cascadia and the North Sumatra subduction zones: insights from laboratory friction experiments(2021-12-10); ; ; 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.Dissertation241 156 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Nankai Trough fault slip behavior analyzed in-situ and in shear experiments(2019-12-20); ; ; The Nankai Trough subduction zone hosts various modes of fault slip from slow to megathrust earthquakes. Slow earthquakes release energy slowly over days to years and can only be recorded geodetically or by borehole observatories. It is not well understood how they connect to regular earthquakes. In contrast, megathrust earthquakes are rapid events that often generate destructive tsunamis, documented for several centuries in the Nankai Trough. Successful earthquake mitigation strategies can only be developed with a better understanding of fault slip behavior and deformation processes within the seismogenic zone and the overlying accretionary prism.Dissertation543 237