Slip Events Research Articles

Segmentation of Shallow Slow Slip Events at the Hikurangi Subduction Zone Explained by Along‐Strike Changes in Fault Geometry and Plate Convergence Rates

Over the last two decades, geodetic and seismic observations have revealed a spectrum of slow earthquakes along the Hikurangi subduction zone in New Zealand. Of those, shallow slow slip events (SSEs) that occur at depths of less than 15 km along the plate interface show a strong along‐strike segmentation in their recurrence intervals, which vary from ∼1 yr from offshore Tolaga Bay in the northeast to ∼5 yr offshore Cape Turnagain ∼300 km to the southwest. To understand the factors that control this segmentation, we conduct numerical simulations of SSEs incorporating laboratory‐derived rate‐and‐state friction laws with both planar and non‐planar fault geometries. We find that a relatively simple model assuming a realistic non‐planar fault geometry reproduces the characteristics of shallow SSEs as constrained by geodetic observations. Our preferred model captures the magnitudes and durations of SSEs, as well as the northward decrease of their recurrence intervals. Our results indicate that the segmentation of SSE recurrence intervals is favored by along‐strike changes in both the plate convergence rate and the downdip width of the SSE source region. Modeled SSEs with longer recurrence intervals concentrate in the southern part of the fault (offshore Cape Turnagain), where the plate convergence rate is lowest and the source region of SSEs is widest due to the shallower slab dip angle. Notably, the observed segmentation of shallow SSEs cannot be reproduced with a simple planar fault model, which indicates that a realistic plate interface is an important factor to account for in modeling SSEs.

Subduction earthquake sequences in a non-linear visco-elasto-plastic megathrust

SUMMARYWe present a 2-D thermomechanical computational framework for simulating earthquake sequences in a non-linear visco-elasto-plastic compressible medium. The method is developed for a plane-strain problem and incorporates an invariant formulation of the classical rate- and state-dependent friction law and an adaptive time-stepping, which allows the time step to vary by many orders of magnitude during a simulation. Long-term tectonic convergence is imposed by displacing a boundary at a constant rate, whereas temperature-dependent viscosity is solved using a rapidly converging Newton–Raphson scheme. The 2-D volume is discretized using finite differences on a fully staggered grid and marker-in-cell techniques. An adaptive free-surface approximation is used to modulate the air viscosity with the time step, which allows stresses to vanish on the free surface during the propagation of fast slipping events. We present a set of increasingly complex models in which we investigate how inertia, radiation damping, thermally activated non-linear rheology and off-megathrust splay-fault events affect sequences of seismic and aseismic slip on a simplified subduction megathrust. The new method provides a unique computational framework to analyse earthquake sequences and to connect forearc deformation with the dynamic properties of the megathrust, thus providing a physical link between observations spanning from slow interseismic strain accumulation to localized coseismic slip of individual earthquakes and post-seismic viscoelastic relaxation.

Automatic Detection of Slow Slip Events Using the PICCA: Application to Chilean GNSS Data

The detection of transient events related to slow earthquakes in GNSS positional time series is key to understanding seismogenic processes in subduction zones. Here, we present a novel Principal and Independent Components Correlation Analysis (PICCA) method that allows for the temporal and spatial detection of transient signals. The PICCA is based on an optimal combination of the principal (PCA) and independent component analysis (ICA) of positional time series of a GNSS network. We assume that the transient signal is mostly contained in one of the principal or independent components. To detect the transient, we applied a method where correlations between sliding windows of each PCA/ICA component and each time series are calculated, obtaining the stations affected by the slow slip event and the onset time from the resulting correlation peaks. We first tested and calibrated the method using synthetic signals from slow earthquakes of different magnitudes and durations and modelled their effect in the network of GNSS stations in Chile. Then, we analyzed three transient events related to slow earthquakes recorded in Chile, in the areas of Iquique, Copiapó, and Valparaíso. For synthetic data, a 150 days event was detected using the PCA-based method, while a 3 days event was detected using the ICA-based method. For the real data, a long-term transient was detected by PCA, while a 16 days transient was detected by ICA. It is concluded that simultaneous use of both signal separation methods (PICCA) is more effective when searching for transient events. The PCA method is more useful for long-term events, while the ICA method is better suited to recognize events of short duration. PICCA is a promising tool to detect transients of different characteristics in GNSS time series, which will be used in a next stage to generate a catalog of SSEs in Chile.

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 Spectrum of Slip Behaviors of a Granular Fault Gouge Analogue Governed by Rate and State Friction

AbstractCurrently, it is unknown how seismic and aseismic slip influences the recurrence and magnitude of earthquakes. Modern seismic hazard assessment is therefore based on statistics combined with numerical simulations of fault slip and stress transfer. To improve the underlying statistical models we conduct low velocity shear experiments with glass micro‐beads as fault gouge analogue at confining stresses of 5–20 kPa. As a result, we show that characteristic slip events emerge, ranging from fast and large slip to small scale oscillating creep and stable sliding. In particular, we observe small scale slip events that occur immediately before large scale slip events for a specific set of experiments. Similar to natural faults we find a separation of scales by several orders of magnitude for slow events and fast events. Enhanced creep and transient dilatational events pinpoint that the granular analogue is close to failure. From slide‐hold‐slide tests, we find that the rate‐and‐state properties are in the same range as estimates for natural faults and fault rocks. The fault shows velocity weakening characteristics with a reduction of frictional strength between 0.8% and 1.3% per e‐fold increase in sliding velocity. Furthermore, the slip modes that are observed in the normal shear experiments are in good agreement with analytical solutions. Our findings highlight the influence of micromechanical processes on macroscopic fault behavior. The comprehensive data set associated with this study can act as a benchmark for numerical simulations and improve the understanding of observations of natural faults.

Implications for megathrust slip behavior and pore pressure at the shallow northern Cascadia subduction zone from laboratory friction experiments

Geological records of past megathrust earthquakes on the Cascadia subduction zone give a valid reason to assume that another event will occur in the future. Both the magnitude and tsunamigenic potential of such an earthquake depend on the extent of seismic rupture and slip on the shallow megathrust, which are still under debate for Cascadia. The absence of shallow 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. A crucial factor controlling both the nucleation and propagation of an earthquake is the frictional slip behavior of the material in the fault system, of which little is currently known for this setting. Here, we report on laboratory friction experiments on all major lithologies of Juan de Fuca plate sediments being input to the northern Cascadia subduction zone. The material was obtained from cores recovered during Integrated Ocean Drilling Program Expedition 301 at Site U1301 in the depth range of 65 to 260 meters below the seafloor. From tests on both intact and powdered samples under in-situ effective normal stress and room temperature, we observe a significant decrease in friction coefficient from 0.76 to 0.19 with increasing depth. A frictionally weak hemipelagic illite-rich unit located directly above the basement is expected to host the décollement upon subduction. This unit reveals predominantly velocity-strengthening frictional behavior at slip velocities of ∼10−9 to 10−4 m/s, and exhibits no slow slip events in the laboratory, indicating a frictionally stable material. Coulomb wedge modeling based on our friction measurements and interpretation of décollement stratigraphic location suggests significant overpressure on the décollement near the trench. Our data offer a possible explanation for the lack of shallow slow slip activity on the northern Cascadia plate-boundary fault, and raise the possibility that the megathrust is not locked near the trench.

Slow slip in subduction zones: Reconciling deformation fabrics with instrumental observations and laboratory results

AbstractCataclasites are a characteristic rock type found in drill cores from active faults as well as in exposed fossil subduction faults. Here, cataclasites are commonly associated with evidence for pervasive pressure solution and abundant hydrofracturing. They host the principal slip of regular earthquakes and the family of so-called slow earthquakes (episodic slip and tremor, low to very low frequency earthquakes, etc.). Slip velocities associated with the formation of the different types of cataclasites and conditions controlling slip are poorly constrained both from direct observations in nature as well as from experimental research. In this study, we explore exposed sections of subduction faults and their dominant microstructures. We use recently proposed constitutive laws to estimate deformation rates, and we compare predicted rates with instrumental observations from subduction zones. By identifying the maximum strain rates using fault scaling relations to constrain the fault core thickness, we find that the instrumental shear strain rates identified for the family of “slow earthquakes” features range from 10−3s−1 to 10−5s−1. These values agree with estimated rates for stress corrosion creep or brittle creep possibly controlling cataclastic deformation rates near the failure threshold. Typically, pore-fluid pressures are suggested to be high in subduction zones triggering brittle deformation and fault slip. However, seismic slip events causing local dilatancy may reduce fluid pressures promoting pressure-solution creep (yielding rates of <10−8 to 10−12s−1) during the interseismic period in agreement with dominant fabrics in plate interface zones. Our observations suggest that cataclasis is controlled by stress corrosion creep and driven by fluid pressure fluctuations at near-lithostatic effective pressure and shear stresses close to failure. We posit that cataclastic flow is the dominant physical mechanism governing transient creep episodes such as slow slip events (SSEs), accelerating preparatory slip before seismic events, and early afterslip in the seismogenic zone.

Crustal deformation detection capability of the GNSS-A seafloor geodetic observation array (SGO-A), provided by Japan Coast Guard

The GNSS-A technique is an observation method that can detect seafloor crustal deformations with centimeter-level positioning accuracy. The GNSS-A seafloor geodetic observation array operated by the Japan Coast Guard (SGO-A) has been constructed near the Japanese Islands along the Nankai Trough and the Japan Trench. This observation array has detected several earthquakes’ displacements and episodic slow crustal deformation. To compare the detection results of SGO-A with other observation networks and expand the SGO-A coverage area, it is necessary to correctly understand its detection capability. In this paper, numerical simulations and statistical verifications were used to assess the capabilities of the present GNSS-A system using a manned vessel (observation frequency: 4–6 times/year, positioning accuracy: standard deviation = 1.5 cm) to detect (1) secular deformation only, (2) a transient slip event only and (3) secular deformation and a transient event together. We verified these results with appropriate thresholds and found the following features: When it is known that there is no transient event, the 95% confidence level (CL) for the estimation of secular crustal deformation rate with 4-year observation is about 0.5–0.8 cm/year; when the deformation rate is known, a signal of about 3.0 cm can be detected by observations of about 4 times before and after the transient event. When the deformation rate and the transient event are detected together, to keep the false positive low (about 0.05), the false negative becomes high (about 0.7–0.2 for detecting a signal of 4.5–6.0 cm). The determined rate and event variations are approximately 1.8 cm/year (95%CL) and 1.5 cm (standard deviation), respectively. We also examined the detection capability for higher observation frequency and positioning accuracy, to examine how the detection capability improves by technological advancements in the future. Additionally, we calculated the spatial range of event detectability using the determined values of detection sensitivity. Obtained results show that each seafloor site can detect a slip event of < 1.0 m scale within about 30 km radius, and approximately one-third of the subseafloor slip event over 100 km from land along the Nankai Trough can only be detected by SGO-A.

Accessing phase slip events in Nb meander wires

We report transport studies through Nb-based superconducting meander wires fabricated by focused ion beam milling technique. The effect of meandering on quantum transport has been probed experimentally by a direct comparison with the pristine thin-film device before meandering. The normal metal (NM) to superconductor (SC) phase transition becomes a wide and multi-step transition by meandering. Below the transition temperature (T c), the resistance-versus-temperature measurements reveal resistive tailing which is explained by the thermally activated phase slip (TAPS) mechanism. The TAPS fit indicates a selective region of the meander to be responsible for the resistive tailing. Besides, the phase slip (PS) mechanism in the meander is evident in its current–voltage characteristics that feature the stair-case type intermediate resistive steps (IRSs) during the SC–NM transition. The modulation of the IRSs is investigated with respect to temperature and external magnetic field. It is observed that the PS events are facilitated by magnetic fields up to about 250 mT. Further, the critical current varies strongly on the temperature and magnetic field for T < 0.5 T c and H ⩽ 100 mT where it fluctuates in an oscillatory manner. Finally, Nb based meander structures can be promising candidates for future PS based studies and applications.

Stress, microstructure, and deformation mechanisms during subduction underplating at the depth of tremor and slow slip, Franciscan Complex, northern California

Subduction zone shear stress is thought to influence the topography of accretionary wedges as well as earthquake magnitude and frequency, and it has traditionally been calculated indirectly using methods that include relating surface heat flow and wedge topography to shear stress on the megathrust. The Eastern Belt of the Franciscan Complex in northern California is part of an exhumed accretionary complex deformed and metamorphosed under high P/T ratio conditions. Its lack of a retrogressive overprint makes it an ideal location to examine the mechanics of accretion within a fossil subduction zone. Blueschist-facies metasediments were deformed by a combination of dislocation and pressure-solution creep. EBSD analysis of dynamically recrystallized quartz across both the upper and lower boundaries of an accreted packet of metasediments indicates shear stresses of 32.8–46.6 MPa along the bounding faults and ∼26 MPa within the main body of the accreted packet.A novel method of determining minimum strain due to pressure solution yields finite stretches of ∼1.6–1.9 and corresponds to strain rates generally on the order of 10−14 s−1 over the 2–6 m.y. period of deformation. A maximum strain rate on the order of 10−13 s−1 was calculated based on plate convergence rate and shear zone width. These independently determined strain rates are shown to be best predicted by a thin film pressure solution flow law. Deformation mechanism maps show that pressure solution and dislocation creep operated with similar strain rates, and that progressive differentiation then increased the favorability and domination of dislocation creep.Dilational microcracking associated with pressure solution produced opening increments in crenulation arcs of ∼ 1 mm. These events have the potential to produce microseismic events with an effective seismic moment comparable to that of low-frequency earthquakes. Concatenation of these events to form shear zones or folds on scales up to hundreds of meters could explain tremor bursts and slow slip events.

Dilatancy Toughening of Shear Cracks and Implications for Slow Rupture Propagation

AbstractDilatancy associated with fault slip produces a transient pore pressure drop which increases frictional strength. This effect is analyzed in a steadily propagating rupture model that includes frictional weakening, slip‐dependent fault dilation and fluid flow. Dilatancy is shown to increase the stress intensity factor required to propagate the rupture tip. With increasing rupture speed, an undrained (strengthened) region develops near the tip and extends beyond the frictionally weakened zone. Away from the undrained region, pore fluid diffusion gradually recharges the fault and strength returns to the drained, weakened value. For sufficiently large rupture dimensions, the dilation‐induced strength increase near the tip is equivalent to an increase in toughness that is proportional to the square root of the rupture speed. In general, dilation has the effect of increasing the stress required for rupture growth by decreasing the stress drop along the crack. Thermal pressurization has the potential to compensate for the dilatant strengthening effect, at the expense of an increased heating rate, which might lead to premature frictional melting. Using reasonable laboratory parameters, the dilatancy‐toughening effect leads to rupture dynamics that is quantitatively consistent with the dynamics of observed slow slip events in subduction zones.

Spatial Patterns in Frictional Behavior of Sediments Along the Kumano Transect in the Nankai Trough

AbstractFault slip activity at subduction zones is governed by sediment frictional properties, which in turn are affected by diagenetic processes. To study the spatial patterns in frictional properties across the Nankai Trough, SW Japan, and their relations to fault slip activity, we used sediment samples (10%–59% clay mineral content) along the Kumano transect covering a large spatial range from the inputs via the outer prism to the inner prism, including the deepest sample ever recovered to date. We performed laboratory friction experiments under in situ effective normal stresses and seawater‐saturated conditions. Our results generally demonstrate that the friction coefficient inversely correlates with the clay mineral content. However, the outer prism sediments show higher friction coefficients than sediments from the other locations for a comparable clay mineral content. All samples show velocity‐weakening behavior at low velocities, but the outer prism and the deep inner prism sediments show velocity strengthening at higher velocities. Based on the experimental results combined with a Coulomb wedge model, we propose that the lowest friction coefficient on the décollement occurs beneath the trenchward portion of the outer prism, whereas the minimum friction coefficient of the prism sediment occurs in the landward portion of the outer prism. In addition, the calculated critical nucleation length for slip instability suggests that the décollement beneath the outer prism area is more frictionally unstable than it is beneath the inner prism. This inference is consistent with the spatial distribution of very‐low‐frequency earthquakes and slow slip events along the shallow Nankai Trough.

Complexity of Deep Low‐Frequency Earthquake Activity in Shikoku (Japan) Imaged From the Analysis of Continuous Seismic Data

AbstractWe derive an extensive catalog of low‐frequency earthquakes (LFEs) in western and central Shikoku applying an automatic, coherency‐based detection and location method to 4‐year long continuous data from selected seismic stations. Our catalog is the first LFE catalog in this area that does not rely on a priory templates and results from a homogeneous data processing scheme. It allows investigating in detail main characteristics of LFE activity in space and time. We observe clear heterogeneity of the LFE space distribution in the along‐strike direction of the subducting Philippine Sea plate corresponding to segments with different sizes, some of which are highly productive in LFE generation. More detailed statistical and correlation‐based analysis of LFE occurrence patterns allows quantification of the along‐strike segmentation and examination of the migration and event interaction during the LFE sequences and inter‐sequence periods. The analysis indicates that a strong interaction exists among LFE sources during slow slip events, but it otherwise varies significantly among the along‐strike segments. We suggest that the observed segmentation of LFE activity is related to static heterogeneity, such as structural property variations along the subduction interface, or dynamic heterogeneity, corresponding to memory‐dependent stress variations or possible fluid transients. We also confirm that regions with the highest LFE productivity correspond to spots of tectonic tremor triggered by teleseismic earthquakes' surface waves supporting a localized fluid‐rich environment with possible fluid transients. Our results illustrate how high‐resolution LFE catalog can contribute to the characterization and quantification of slow earthquake processes through detailed statistical analysis of their activity.

Slow Slip Triggers the 2018 Mw 6.9 Zakynthos Earthquake Within the Weakly Locked Hellenic Subduction System, Greece

AbstractSlow slip events (SSEs) at subduction zones can precede large‐magnitude earthquakes and may serve as precursor indicators, but the triggering of earthquakes by slow slip remains insufficiently understood. Here, we combine geodetic, Coulomb wedge and Coulomb failure‐stress models with seismological data to explore the potential causal relationship between two SSEs and the 2018 Mw 6.9 Zakynthos Earthquake within the Hellenic Subduction System. We show that both SSEs released up to 10 mm of aseismic slip on the plate‐interface and were accompanied by an increase in upper‐plate seismicity rate. While the first SSE in late 2014 generated only mild Coulomb failure stress changes (≤3 kPa), that were nevertheless sufficient to destabilize faults of various kinematics in the overriding plate, the second SSE in 2018 caused stress changes up to 25 kPa prior to the mainshock. Collectively, these stress changes affected a highly overpressured and mechanically weak forearc, whose state of stress fluctuated between horizontal deviatoric compression and tension during the years preceding the Zakynthos Earthquake. We conclude that this configuration facilitated episodes of aseismic and seismic deformation that ultimately triggered the Zakynthos Earthquake.

Separation of Poroelastic and Elastic Processes of an Aquifer From Tectonic Phenomena Using Geodetic, Seismic, and Meteorological Data in the Pollino Region, Italy

AbstractVelocity variations obtained from ambient seismic noise are sensitive to many factors. We aimed to disentangle these processes in a 10‐year‐long recording of seismic noise from a single station in the Pollino region, in southern Italy. This region is characterized by aquifers and by a relatively short period of high seismic activity that included slow slip events and a earthquake that occurred on October 25, 2012. We apply two models that estimate the water level inside an aquifer, which show a good correlation with the measured , showing that the velocity variations are inversely proportional to the pore pressure inside the aquifer. Our interpretation is further confirmed by geodetic measurements that show that in a direction parallel to the strike angle of the fault rupture, the expansion‐contraction displacement of the zone follows the same patterns observed in the models and in the velocity variations, as a result of the pressure generated by the water on its interior. Going one step further, we analyze the nature of the small discrepancies between the measured and modeled velocity variations. These correlate well with the rainfall and with the vertical geodetic measures, which indicates an elastic response of the zone to the loading generated by the rainwater. Comparisons between these variables allow us to clearly identify the period of the seismic activity in the zone, which is represented by the characteristic drop in the seismic velocity in the period from the beginning of 2012 to mid‐2013.

Slow slip events precursory to the 2014 Iquique Earthquake, revisited with long-base tilt and GPS records

SUMMARY The M= 8.1, 1 April 2014 Iquique earthquake, which broke part of the northern Chile seismic gap, was preceded by a strong foreshock sequence starting early January 2014. The reported analysis of the continuous records of the nearby GPS stations from the Integrated Plate Observatory Chile, North Chile array lead to contradictory results concerning the existence and location of slow slip events (SSEs) on the interplate contact. Resolving this controversy is an important issue, as although many SSEs are reported in subduction zones, only a few were found to be precursory to large earthquakes. Here we show that the records of a long base tiltmeter installed near Iquique, when corrected for coseismic steps, long-term drift, tidal signals and oceanic and atmospheric loading, show significant residual signals. These can be modelled with a sequence of four SSEs located close to Iquique. Their signature was already reported on some GPS stations, but their source was then characterized with a very low resolution in time and space, leading to contradicting models. With the tilt records, we can rule out the previously proposed models with a single large SSE closer to the main shock. Combining tilt with GPS records greatly improves the resolution of GPS alone, and one could locate their sources 100–180 km south–southeast to the main shock epicentre, with moment magnitudes between 5.8 and 6.2, at the edge of the main aftershock asperities. These moderate SSEs thus did not directly trigger the main shock, but contributed to trigger the main foreshock and the main aftershock. Only the sensitivity and resolution of the tiltmeter, added to the GPS records, allowed us to describe with unprecedented accuracy this precursory process as a cascade of cross-triggered, short-term aseismic slip events and earthquakes on the interplate contact. This three months of precursory activation appears to be the final acceleration burst of a weaker, longer term SSE which started mid-2013, already reported, with a moment release history which we could quantify. From the methodological point of view, our study takes advantage of an interesting complementarity of tilt and GPS measurements, due to their different dependence in distance to the source of strain, which turns out to be very efficient for resolving location and moment of strain sources, even when both instruments are close to each other. It finally demonstrates the efficient removal of sequences of small or even undetected coseismic steps from high resolution tilt record signal in order to retrieve the purely aseismic signal, a presently impossible task for high time resolution GPS records due to low signal to noise.

Fourteen‐Year Acceleration Along the Japan Trench

AbstractAn acceleration of the background seismicity and a shortening of the slow slip events recurrence intervals in the Boso peninsula (Japan) suggest a slow unlocking of the Philippine Sea‐North America (PHS‐NAM) subduction interface from 1990 to 2011. Motivated by these previous observations, we used GPS (Global Positioning System) time series to study the 1997–2011 evolution of interface locking offshore Honshu with a specific focus on the Kanto region. We newly processed the GPS data in double difference and analyzed them with a trajectory model that accounts for seismic and aseismic variations, and that includes an inter‐seismic acceleration term. We inverted the surface acceleration obtained, on both the Pacific‐North America (PAC‐NAM) and the PHS‐NAM interfaces. The inverted slip rate changes over time compare well with previous studies: we infer slip deceleration between –N and acceleration between –N (PAC‐NAM), with a maximum amplitude of 1.75 mm/ corresponding to an equivalent geodetic locking change of 0.3 over the 14 years. More notably, our analysis reveals a novel and robust slip acceleration South of N that we interpret as an unlocking of the PAC‐NAM interface. It is located noticeably far from the 2011 Tohoku earthquake rupture and is therefore unlikely connected to it. The slip acceleration inverted from the surface displacements is comparable to the changes observed in the seismicity rate. Our results further demonstrate that inter‐seismic slip rate can significantly evolve over years to decades, and suggest a simple relationship between the seismicity and the slip on the subduction interface.

Phase slips dynamics in gated Ti and V all-metallic supercurrent nano-transistors

The effect of electrostatic gating on metallic elemental superconductors was recently demonstrated in terms of modulation of the switching current and control of the current phase relation in superconducting quantum interferometers. The latter suggests the existence of a direct connection between the macroscopic quantum phase (φ) in a superconductor and the applied gate voltage. The measurement of the switching current cumulative probability distributions is a convenient and powerful tool to analyze such relation. In particular, the comparison between the conventional Kurkijärvi–Fulton–Dunkleberger model and the gate-driven distributions give useful insights into the microscopic origin of the gating effect. In this paper, we summarize the main results obtained in the analysis of the phase slip events in elemental gated superconducting weak-links in a wide range of temperatures between 20 mK and 3.5 K. Such a large temperature range demonstrates both that the gating effect is robust as the temperature increases, and that fluctuations induced by the electric field are not negligible in a wide temperature range.

Estimating Effective Normal Stress During Slow Slip Events From Slip Velocities and Shear Stress Variations

AbstractTheoretical analyses using a rate‐ and state‐dependent friction law have shown quasistatic and stable slip behaviors following velocity weakening. We estimated spatiotemporal slip distributions for six slow slip events (SSEs) off the Boso Peninsula, Japan, from Global Navigation Satellite System data and plotted their shear stress variations vs. slip velocity, here called “trajectories.” Most trajectories are similar to theoretical stable trajectories, implying that the Boso SSEs represent a quasistatic and stable process, and that the gradient of the trajectories during slip acceleration is equal to the product of the friction coefficients of the slipping plates (a–b) and the effective normal stress (). Taking (a–b) = −0.003, we estimate that the effective normal stresses during the SSEs were 10–50 MPa at 12.5–20.5 km depth. These values are much smaller than the lithostatic pressures at those depths, implying high pore pressures on the slow slip interface.

Tectonic tremor as friction-induced inertial vibration

Slow slip events are often accompanied by tremor but how tremor is generated is yet elusive. In this study, we test the possibility that it is an inertial vibration. In the case of a single-degree-of-freedom spring and slider system with mass per unit area M, governed by rate-and-state friction with effective normal stress σ′, excitation of inertial vibrations emerges when normal stress (σ′) is low, and loading rate (Vl) is high. Accordingly, tremor can be excited in a low effective normal stress (σ′) zone, for example, in a zone of high pore pressure, when the loading rate (Vl) is temporally increased, as can happen during a slow slip event. A high loading rate helps to sustain the vibration, but a long-lasting attenuating tremor can still be excited even with a moderate velocity perturbation as long as the normal stress is sufficiently small. We use numerical simulations to verify that this hypothesis holds for a one-dimensional fault. The dominant frequency of the tremor is close to the fundamental frequency of resonance of the frictionless shear crack at a low sliding rate. Higher frequency modes are excited at higher sliding velocity. We show simulations of spontaneous slow slip events associated with tremor radiated from inertial vibration of a fault patch with locally low effective normal stress. This model provides a possible explanation for tectonic tremor associated with slow slip events.