Anatomy of strike-slip fault tsunami genesis

Tsunami generation from earthquake-induced seafloor deformations has long been recognized as a major hazard to coastal areas. Strike-slip faulting has generally been considered insufficient for triggering large tsunamis, except through the generation of submarine landslides. Herein, we demonstrate that ground motions due to strike-slip earthquakes can contribute to the generation of large tsunamis (>1 m), under rather generic conditions. To this end, we developed a computational framework that integrates models for earthquake rupture dynamics with models of tsunami generation and propagation. The three-dimensional time-dependent vertical and horizontal ground motions from spontaneous dynamic rupture models are used to drive boundary motions in the tsunami model. Our results suggest that supershear ruptures propagating along strike-slip faults, traversing narrow and shallow bays, are prime candidates for tsunami generation. We show that dynamic focusing and the large horizontal displacements, characteristic of strike-slip earthquakes on long faults, are critical drivers for the tsunami hazard. These findings point to intrinsic mechanisms for sizable tsunami generation by strike-slip faulting, which do not require complex seismic sources, landslides, or complicated bathymetry. Furthermore, our model identifies three distinct phases in the tsunamic motion, an instantaneous dynamic phase, a lagging coseismic phase, and a postseismic phase, each of which may affect coastal areas differently. We conclude that near-source tsunami hazards and risk from strike-slip faulting need to be re-evaluated.

Tsunamis are classically defined as long, free-surface water waves generated by impulsive geological events (1). Tsunamis may be triggered by earthquakes, volcanoes, landslides or slumps, submarine gas releases, and meteorite impacts. Over the past century, tsunamis alone have been responsible for the loss of hundreds of thousands of lives and trillions of dollars in damage to the environment and built infrastructure (2, 3). This makes tsunamis among the most destructive natural hazards. Quantitative and predictive modeling of tsunamis is crucial for reducing the impact of these events and for enabling better preparedness plans.Generally, the tsunami impact is associated with the size of the vertical seafloor motion. Massive tsunamis are generally attributed to great earthquakes along subduction-zone plate boundaries, such as the 2004 M∼9.2 Sumatran and the 2011 M∼9.0 Tohoku-Oki events. Strike-slip faults, which generally generate small seafloor vertical displacements, are generally considered unfavorable for tsunami generation (4). Field observations, however, suggest that in many cases (5–9), strike-slip motion can indeed generate tsunami waves, supposedly by triggering landslides (10). Even though a small fraction of all tsunamis studied this far are believed to be triggered by strike-slip motion (11), their possible devastating humanitarian impact warrants further investigation into this particular mechanism for tsunami genesis.The September 2018 Mw 7.5 Sulawesi earthquake occurred on the Palu-Koro (P-K) strike-slip fault system and caused an unexpected localized tsunami, atypical in its impact for this type of fault motion (12). Bao et al. (13) and Socquet et al. (14) were among the first to recognize the supershear nature of this earthquake (13, 14). Several authors (13, 15–17) have postulated that submarine landslides, triggered by the earthquake’s strong ground motion, were the primary source for the devastating tsunami. Ulrich et al. (18) argued that the earthquake displacements were critical to the tsunami generation. Using a three-dimensional (3D) dynamic rupture model that emulated the earthquake propagation on the geometrically complex P-K fault system and coupling it with the two-dimensional (2D) shallow-water wave equations, they demonstrated that the fault slip, which included a nonnegligible rake and dip slip components, may only trigger tsunami waves of the order of a meter. Their conclusions, however, are confounded by the specifics of the complex Palu Bay bathymetry and the complex geometry of the P-K fault system.Amlani et al. (19) utilized near-fault GPS data to first conclusively demonstrate that the P-K rupture was indeed supershear. They then recovered a crucial term in the shallow-water wave equation by including the time derivative of the seafloor vertical displacements and, thus, ensured correct mass conservation (20). This forcing was implemented in the context of a one-dimensional (1D) nonlinear shallow-water wave model and a simple bathymetry, driven dynamically by the vertical components of the motion computed from a 3D dynamic rupture simulation of a supershear earthquake along a strike-slip fault. They showed that explicitly accounting for the dynamic source effects uncovers high-frequency details in the early phases of the tsunami motion. They claimed these details may get missed if only the static seafloor displacements are used. However, they did not consider the effect of horizontal displacements in the ground motion on deforming the bathymetry. Furthermore, their 1D model could not account for the dramatic focusing effect introduced by water waves converging at, and subsequently reflected and refracted from, the apex of any narrow bay. These limitations possibly led to a noticeable underprediction of the calculated amplitude of the waves.The question, therefore, remains whether generic strike-slip faults, in the absence of secondary sources, such as coseismic underwater landslides, can generate large tsunamis. This question has important ramifications, as several metropolitan areas worldwide are located near bays (6, 7, 21–23) that are traversed by strike-slip faults similar to the P-K system. Furthermore, while early warning for far-field tsunamis (1, 24, 25) based on hydrodynamic inversions is now fairly routine, at least in the North Pacific, little or no early warning is possible for near-field tsunamis, in which the tsunami originates just a few kilometers away from the coastline. Most field scientists agree that, thus far, for coastal residents, earthquake shaking is the warning for an impending tsunami from a nearshore source (26, 27).To shed light on the basic mechanisms through which strike-slip faults may cause damaging tsunamis, we have developed a computational framework that integrates mechanistic models for earthquake rupture dynamics with hydrodynamic models for tsunami generation and propagation. Possibly with a few exceptions, the initial condition in tsunami models is computed by using the static algorithms of Mansinha and Smylie (28), subsequently parameterized by Okada (29), which translate finite fault slip models into static seafloor displacements. In recent years, there has been an increased interest in developing models that account for dynamic generation (18, 30–33). Here, we use our dynamic generation model and focus on a planar strike-slip fault traversing a shallow bay with a simple geometry. Tsunami evolution over a more complex bathymetry may hide the effects of the dynamic rupture. Our approach is thus designed to unravel the underlying physics governing tsunami generation due to the intrinsic nature of strike-slip faulting. In other words, we seek to understand the basic phenomenology first, before applying our model to complex geophysical geometries.The 2018 Palu earthquake and tsunami highlighted the complex dynamics of tsunamis generated by intersonic earthquakes. In supershear, or intersonic, earthquakes (34–40), the rupture tip propagates faster than the shear wave speed (41–43). This leads to the emergence of large localized deformation bands along the shear shock wave fronts, also known as Mach cones (34, 35, 38, 39, 44). Dunham and Bhat (38) proved the presence of a second Mach front associated with Rayleigh waves that carry significant vertical motion, moderately attenuated, to large distances. When such earthquakes occur within a narrow bay, the associated large horizontal displacements, as well as the moderately attenuated vertical displacements (34, 39, 45), along the shear and Rayleigh shock wave fronts may cause significant motion in the bay boundaries, which, just as with a paddle wavemaker, could lead to the displacement of large volumes of water. Furthermore, in these scenarios, the triggered tsunami may exhibit multiple characteristic time scales, ranging from a few seconds to several minutes.In the following sections, we investigate the synergistic interactions between rupture speed, seafloor ground motions, and bay geometry. We also examine several distinct features of the tsunami, including the emergence of an instantaneous dynamic phase and a slower coseismic phase, both of which lead to a gravity-driven postseismic phase.     

Pulse-like and crack-like ruptures in experiments mimicking crustal earthquakes

Theoretical studies have shown that the issue of rupture modes has important implications for fault constitutive laws, stress conditions on faults, energy partition and heat generation during earthquakes, scaling laws, and spatiotemporal complexity of fault slip. Early theoretical models treated earthquakes as crack-like ruptures, but seismic inversions indicate that earthquake ruptures may propagate in a self-healing pulse-like mode. A number of explanations for the existence of slip pulses have been proposed and continue to be vigorously debated. This study presents experimental observations of spontaneous pulse-like ruptures in a homogeneous linear-elastic setting that mimics crustal earthquakes; reveals how different rupture modes are selected based on the level of fault prestress; demonstrates that both rupture modes can transition to supershear speeds; and advocates, based on comparison with theoretical studies, the importance of velocity-weakening friction for earthquake dynamics.     

The Influence of Valve Leaflet Stiffness Variability on Aortic Wall Shear Stress

Aortic stenosis is a common cardiac condition that impacts the aorta’s hemodynamics downstream of the affected valve. We sought to better understand how non-uniform stiffening of a stenotic aortic valve would affect the wall shear stress (WSS) experienced by the walls of the aorta and the residence time near the valve. Several experimental configurations were created by individually stiffening leaflets of a polymer aortic valve. These configurations were mounted inside an in vitro experimental setup. Digital particle image velocimetry (DPIV) was used to measure velocity profiles inside a model aorta. The DPIV results were used to estimate the WSS and residence time. Our analysis suggests that leaflet asymmetry greatly affects the amount of WSS by vectoring the systolic jet and stiffened leaflets have an increased residence time. This study indicates that valve leaflets with different stiffness conditions can have a more significant impact on wall shear stress than stenosis caused by the uniform increase in all three leaflets (and the subsequent increased systolic velocity) alone. This finding is promising for creating customizable (patient-specific) prosthetic heart valves tailored to individual patients.

     

Dynamic rupture initiation and propagation in a fluid-injection laboratory setup with diagnostics across multiple temporal scales

Fluids are known to trigger a broad range of slip events, from slow, creeping transients to dynamic earthquake ruptures. Yet, the detailed mechanics underlying these processes and the conditions leading to different rupture behaviors are not well understood. Here, we use a laboratory earthquake setup, capable of injecting pressurized fluids, to compare the rupture behavior for different rates of fluid injection, slow (megapascals per hour) versus fast (megapascals per second). We find that for the fast injection rates, dynamic ruptures are triggered at lower pressure levels and over spatial scales much smaller than the quasistatic theoretical estimates of nucleation sizes, suggesting that such fast injection rates constitute dynamic loading. In contrast, the relatively slow injection rates result in gradual nucleation processes, with the fluid spreading along the interface and causing stress changes consistent with gradually accelerating slow slip. The resulting dynamic ruptures propagating over wetted interfaces exhibit dynamic stress drops almost twice as large as those over the dry interfaces. These results suggest the need to take into account the rate of the pore-pressure increase when considering nucleation processes and motivate further investigation on how friction properties depend on the presence of fluids.

The close connection between fluids and faulting has been revealed by a large number of observations, both in tectonic settings and during human activities, such as wastewater disposal associated with oil and gas extraction, geothermal energy production, and CO₂ sequestration (1–11). On and around tectonic faults, fluids also naturally exist and are added at depths due to rock-dehydration reactions (12–15) Fluid-induced slip behavior can range from earthquakes to slow, creeping motion. It has long been thought that creeping and seismogenic fault zones have little to no spatial overlap. Nonetheless, growing evidence suggests that the same fault areas can exhibit both slow and dynamic slip (16–19). The existence of large-scale slow slip in potentially seismogenic areas has been revealed by the presence of transient slow-slip events in subduction zones (16, 18) and proposed by studies investigating the physics of foreshocks (20–22).Numerical and laboratory modeling has shown that such complex fault behavior can result from the interaction of fluid-related effects with the rate-and-state frictional properties (9, 14, 19, 23, 24); other proposed rheological explanations for complexities in fault stability include combinations of brittle and viscous rheology (25) and friction-to-flow transitions (26). The interaction of frictional sliding and fluids results in a number of coupled and competing mechanisms. The fault shear resistance τres is typically described by a friction model that linearly relates it to the effective normal stress σ^n via a friction coefficient f:τres=f σ^n=f (σn−p),[1]where σn is the normal stress acting across the fault and p is the pore pressure. Clearly, increasing pore pressure p would reduce the fault frictional resistance, promoting the insurgence of slip. However, such slip need not be fast enough to radiate seismic waves, as would be characteristic of an earthquake, but can be slow and aseismic. In fact, the critical spatial scale h* for the slipping zone to reach in order to initiate an unstable, dynamic event is inversely proportional to the effective normal stress (27, 28) and hence increases with increasing pore pressure, promoting stable slip. This stabilizing effect of increasing fluid pressure holds for both linear slip-weakening and rate-and-state friction; it occurs because lower effective normal stress results in lower fault weakening during slip for the same friction properties. For example, the general form for two-dimensional (2D) theoretical estimates of this so-called nucleation size, h*, on rate-and-state faults with steady-state, velocity-weakening friction is given by:h*=(μ*DRS)/[F(a,b)(σn−p)],[2]where μ*=μ/(1−ν) for modes I and II, and μ*=μ for mode III (29); DRS is the characteristic slip distance; and F(a, b) is a function of the rate-and-state friction parameters a and b. The function F(a, b) depends on the specific assumptions made to obtain the estimate: FRR(a,b)=4(b−a)/π (ref. 27, equation 40) for a linearized stability analysis of steady sliding, or FRA(a,b)=[π(b−a)2]/2b, with a/b>1/2 for quasistatic crack-like expansion of the nucleation zone (ref. 30, equation 42).Hence, an increase in pore pressure induces a reduction in the effective normal stress, which both promotes slip due to lower frictional resistance and increases the critical length scale h*, potentially resulting in slow, stable fault slip instead of fast, dynamic rupture. Indeed, recent field and laboratory observations suggest that fluid injection triggers slow slip first (4, 9, 11, 31). Numerical modeling based on these effects, either by themselves or with an additional stabilizing effect of shear-layer dilatancy and the associated drop in fluid pressure, have been successful in capturing a number of properties of slow-slip events observed on natural faults and in field fluid-injection experiments (14, 24, 32–34). However, understanding the dependence of the fault response on the specifics of pore-pressure increase remains elusive. Several studies suggest that the nucleation size can depend on the loading rate (35–38), which would imply that the nucleation size should also depend on the rate of friction strength change and hence on the rate of change of the pore fluid pressure. The dependence of the nucleation size on evolving pore fluid pressure has also been theoretically investigated (39). However, the commonly used estimates of the nucleation size (Eq. 2) have been developed for faults under spatially and temporally uniform effective stress, which is clearly not the case for fluid-injection scenarios. In addition, the friction properties themselves may change in the presence of fluids (40–42). The interaction between shear and fluid effects can be further affected by fault-gauge dilation/compaction (40, 43–45) and thermal pressurization of pore fluids (42, 46–48).Recent laboratory investigations have been quite instrumental in uncovering the fundamentals of the fluid-faulting interactions (31, 45, 49–57). Several studies have indicated that fluid-pressurization rate, rather than injection volume, controls slip, slip rate, and stress drop (31, 49, 57). Rapid fluid injection may produce pressure heterogeneities, influencing the onset of slip. The degree of heterogeneity depends on the balance between the hydraulic diffusion rate and the fluid-injection rate, with higher injection rates promoting the transition from drained to locally undrained conditions (31). Fluid pressurization can also interact with friction properties and produce dynamic slip along rate-strengthening faults (50, 51).In this study, we investigate the relation between the rate of pressure increase on the fault and spontaneous rupture nucleation due to fluid injection by laboratory experiments in a setup that builds on and significantly develops the previous generations of laboratory earthquake setup of Rosakis and coworkers (58, 59). The previous versions of the setup have been used to study key features of dynamic ruptures, including sub-Rayleigh to supershear transition (60); rupture directionality and limiting speeds due to bimaterial effects (61); pulse-like versus crack-like behavior (62); opening of thrust faults (63); and friction evolution (64). A recent innovation in the diagnostics, featuring ultrahigh-speed photography in conjunction with digital image correlation (DIC) (65), has enabled the quantification of the full-field behavior of dynamic ruptures (66–68), as well as the characterization of the local evolution of dynamic friction (64, 69). In these prior studies, earthquake ruptures were triggered by the local pressure release due to an electrical discharge. This nucleation procedure produced only dynamic ruptures, due to the nearly instantaneous normal stress reduction.To study fault slip triggered by fluid injection, we have developed a laboratory setup featuring a hydraulic circuit capable of injecting pressurized fluid onto the fault plane of a specimen and a set of experimental diagnostics that enables us to detect both slow and fast fault slip and stress changes. The range of fluid-pressure time histories produced by this setup results in both quasistatic and dynamic rupture nucleation; the diagnostics allows us to capture the nucleation processes, as well as the resulting dynamic rupture propagation. In particular, here, we explore two injection techniques: procedure 1, a gradual, and procedure 2, a sharp fluid-pressure ramp-up. An array of strain gauges, placed on the specimen’s surface along the fault, can capture the strain (translated into stress) time histories over a wide range of temporal scales, spanning from microseconds to tens of minutes. Once dynamic ruptures nucleate, an ultrahigh-speed camera records images of the propagating ruptures, which are turned into maps of full-field displacements, velocities, and stresses by a tailored DIC) analysis. One advantage of using a specimen made of an analog material, such as poly(methyl meth-acrylate) (PMMA) used in this study, is its transparency, which allows us to look at the interface through the bulk and observe fluid diffusion over the interface. Another important advantage of using PMMA is that its much lower shear modulus results in much smaller nucleation sizes h* than those for rocks, allowing the experiments to produce both slow and fast slip in samples of manageable sizes.We start by describing the laboratory setup and the diagnostics monitoring the pressure evolution and the slip behavior. We then present and discuss the different slip responses measured as a result of slow versus fast fluid injection and interpret our measurements by using the rate-and-state friction framework and a pressure-diffusion model.     

Transition metal transport in the green microalga Chlamydomonas reinhardtii--genomic sequence analysis

Uptake and export systems play a major role in transition metal homeostasis. The objective of this study was to identify potential metal transport mechanisms in the green microalga Chlamydomonas reinhardtii. We concentrated on the four major transition metal transporter families found in plants and other organisms: the ZIP, CDF and Nramp families, and the CPx-ATPases. Using the information available for these protein families we performed comparative sequence analysis in the recently released genome of C. reinhardtii. Using this approach we were able to identify members of all four transporter families (four ZIPs, one CDF, two CPx-ATPases, and five Nramps). These findings advance our current knowledge of the metal transport processes present in C. reinhardtii. In addition, by subsequent in silico splicing of the genomic sequence we obtained cDNA sequences which led to the identification of ESTs (expressed sequence tags) in the C. reinhardtii EST database. These identified ESTs will be valuable for the cloning and characterization of several metal transporters utilized by the alga.