Episodic Slow Slip Research Articles

Seasonal slow slip in landslides as a window into the frictional rheology of creeping shear zones.

Whether Earth materials exhibit frictional creep or catastrophic failure is a crucial but unresolved problem in predicting landslide and earthquake hazards. Here, we show that field-scale observations of sliding velocity and pore water pressure at two creeping landslides are explained by velocity-strengthening friction, in close agreement with laboratory measurements on similar materials. This suggests that the rate-strengthening friction commonly measured in clay-rich materials may govern episodic slow slip in landslides, in addition to tectonic faults. Further, our results show more generally that transient slow slip can arise in velocity-strengthening materials from modulation of effective normal stress through pore pressure fluctuations. This challenges the idea that episodic slow slip requires a narrow range of transitional frictional properties near the stability threshold, or pore pressure feedbacks operating on initially unstable frictional slip.

Laboratory Study on Frictional Behavior of Rock Blocks of Meter Scale. Methods and Preliminary Results

—The paper introduces a new, unique for Russia, meter-scale laboratory setup created in the Institute of Geosphere Dynamics of the Russian Academy of Sciences (IDG RAS) to study the development of different sliding regimes on rock discontinuities. The experimental procedure is described and the results of the first series of tests aimed at studying the formation of different sliding regimes on rock faults are presented. The laboratory fault was a loaded contact of two 75-cm long blocks made of diabase. The fault was filled with granular material (a fault gouge). Normal stresses on the fault can reach 10 MPa. By varying fault gouge composition and loading rate, we reproduced a wide range of sliding regimes: sliding with constant velocity, regular stick-slip, and aperiodic slow slip episodes. It is shown that a variation in the loading rate can cause a significant change in the sliding regime. Intense frictional crushing of gouge grains is detected in the experiments at relatively low normal pressure of 2 MPa. In the case of high-amplitude stick-slip, besides crushing of the gouge material, also structural phase transformations of quartz grains corresponding to a local temperature increase up to 700°C are revealed. A possible set of the problems related to deformation processes in seismogenic fault zones that can be addressed by modeling on such setups—the pre-seismic stage of inelastic behavior of the main fault zone at critical stress—is outlined.

Episodic Slow Slip Hosted by Talc‐Bearing Metasomatic Rocks: High Strain Rates and Stress Amplification in a Chemically Reacting Shear Zone

AbstractEpisodic tremor and slow slip (ETS) downdip of the subduction seismogenic zone are poorly understood slip behaviors of the seismic cycle. Talc, a common metasomatic mineral at the subduction interface, is suggested to host slow slip but this hypothesis has not been tested in the rock record. We investigate actinolite microstructures from talc‐bearing and talc‐free rocks exhumed from the depths of modern ETS (Pimu'nga/Santa Catalina Island, California). Actinolite deformed by dissolution‐reprecipitation creep in the talc‐free rock and dislocation creep ± cataclasis in the talc‐bearing rock. This contrast results from stress amplification in the talc‐bearing rock produced by high strain rates in surrounding weak talc. We hypothesize that higher strain rates in the talc‐bearing sample represent episodic slow slip, while lower strain rates in the talc‐free sample represent intervening aseismic creep. This work highlights the need to consider fluid‐mediated chemical change in studies of subduction zone deformation and seismicity.

Seasonal Slow Landslide Displacement Is Accommodated by mm‐Scale Stick‐Slip Events

AbstractLike faults, landslides can slip slowly for decades or accelerate catastrophically. However, whereas experimentally derived friction laws provide mechanistically based explanations for similarly diverse behavior on faults, little monitoring exists over the temporal and spatial scales required to more clearly illuminate the mechanics of landslide friction. Here we show that displacement of an active slow landslide is accommodated primarily through mm‐scale stick‐slip events that recur on timescales of minutes to hours on asperities that are small (<100 m) relative to the landslide. The frequency of slip events tracks both landslide velocity and pore fluid pressure. The stick‐slip nature demonstrates by itself that slow slip is governed, at least in part, by velocity‐weakening frictional asperities. This observation, in combination with the sensitivity of slow slip to pore fluid pressure and the small relative scale of asperities, suggests similarities between slow slip in landslides and episodic slow slip along faults.

Megathrust Shear Modulated by Albite Metasomatism in Subduction Mélanges

AbstractAseismic megathrust slip downdip of the seismogenic zone is accommodated by either steady creep or episodic slow slip events (SSEs). However, the geological conditions defining the rheology of megathrust slip remain elusive. We examined exhumed subduction mélanges on Kyushu, Japan, which deformed at ∼370–500°C under greenschist to epidote‐amphibolite facies conditions, comparable to warm‐slab environments. The mélanges recorded fluid release and viscous shear localization associated with metasomatic reactions between juxtaposed metapelitic and metabasaltic rocks. Metasomatic reactions caused albitization of metapelite, resulting in depth‐dependent changes to megathrust rheology. In a mélange deformed at ∼370°C, very fine grained reaction products (metasomatic albite) facilitated grain boundary diffusion creep at stresses of ∼45 MPa, less than those in the surrounding metabasalt. Mineralogical and chemical changes during metasomatic reactions, and their field content, imply an onset of albite metasomatism at ∼350°C. Albite metasomatism therefore potentially contributed to decreased megathrust strength around the inferred thermally controlled base of the seismogenic zone. In a mélange deformed near the mantle wedge corner at ∼500°C, metasomatic reactions promoted local quartz vein formation and localized viscous shear at slow slip strain rates, during which the coarse‐grained metasomatic albite behaved as relatively rigid blocks in a viscous matrix. We suggest that albite metasomatism can facilitate changes in a megathrust slip mode with depth and may explain why slip mode changes from creep to SSEs with tremor with increasing depth.

Effects of episodic slow slip on seismicity and stress near a subduction-zone megathrust

Slow slip phenomena deep in subduction zones reveal cyclic processes downdip of locked megathrusts. Here we analyze seismicity within a subducting oceanic slab, spanning ~50 major deep slow slip with tremor episodes over 17 years. Changes in rate, b-values, and stress orientations of in-slab seismicity are temporally associated with the episodes. Furthermore, although stress orientations in the slab below these slow slips may rotate slightly, in-slab orientations 20–50 km updip from there rotate farther, suggesting that previously-unrecognized transient slow slip occurs on the plate interface updip. We infer that fluid pressure propagates from slab to interface, promoting episodes of slow slip, which break mineral seals, allowing the pressure to propagate tens of km further updip along the interface where it promotes transient slow slips. The proposed methodology, based primarily on in-slab seismicity, may help monitor plate boundary conditions and slow slip phenomena, which can signal the beginning stages of megathrust earthquakes.

Slab dehydration in warm subduction zones at depths of episodic slip and tremor

Non-volcanic tremor (NVT) and episodic slow slip events (SSEs) have been observed below the seismogenic zone of relatively warm subduction zones for the past 20 years. Geophysical and geologic observations show that this portion of the subduction interface is fluid-rich, and many models for these slip behaviors necessitate high pore fluid pressures. However, whether these fluids are sourced from local dehydration reactions in particular lithologies, or require up-dip transport from greater depths is not known. We present thermodynamic models of the petrologic evolution of four lithologies typical of the plate interface (average MORB, seafloor altered MORB, hydrated depleted MORB mantle, and metapelite) along predicted plate boundary pressure–temperature (P-T) paths at several warm subduction segments where NVT and SSEs are observed at depths between 25-65 km. The models suggest that 1-2 wt% H2O is released at the depths of NVT/SSEs in Jalisco-Colima, Guerrero, Cascadia, and Shikoku due to punctuated dehydration reactions within MORB, primarily through chlorite and/or lawsonite breakdown. These reactions produce sufficient in-situ fluid across a narrow P-T range to cause high pore fluid pressures at NVT/SSE depths. Dehydration of hydrated peridotite is minimal at these depths for most margins, and metapelite releases H2O (<1.5 wt%) gradually over a wide depth range compared to MORB. We posit that punctuated dehydration of oceanic crust provides the dominant source of fluids at the base of the seismogenic zone in these warm subduction zones, and up-dip migration of fluids from deeper in the subduction zone is not required.

Offshore Postseismic Deformation of the 2011 Tohoku Earthquake Revisited: Application of an Improved GPS‐Acoustic Positioning Method Considering Horizontal Gradient of Sound Speed Structure

AbstractOne of the important issues on the GPS‐acoustic (GPS‐A) observation for sea bottom positioning is how to address the horizontal heterogeneity of the sound speed in oceans. This study presents an analysis method of GPS‐A data in the presence of a sloping sound speed structure. By applying this method and revising the analysis scheme to make full use of existing data, we reevaluated the horizontal postseismic deformations occurring ~1.5–5 years after the 2011 Tohoku earthquake. The revised horizontal movements have more uniform directions and rates between neighboring sites, suggesting enhancement of the array positioning accuracy. The revised displacement rate of the site on the incoming Pacific plate, located ~100 km northeast of the main rupture zone, was decreased significantly; it was only slightly, by 1.4 cm/year larger than the global motion of the Pacific plate, suggesting a relatively small effect of viscoelastic relaxation. The horizontal movements of the near‐trench sites above the main rupture zone were generally landward and were significantly faster than the Pacific plate motion, indicating a viscoelastic relaxation of 5–10 cm/year. The distribution of the fast landward movements peaked near 38°N at an updip of the mainshock hypocenter and extended significantly farther to the north than to the south. This implies the existence of a secondary coseismic slip patch in the northern area in addition to a primary slip patch at ~38°N. The occurrence of episodic slow slip in early 2015 to the north of the main rupture zone was also verified from the GPS‐A analyses.

Tremor and Inferred Slow Slip Associated With Afterslip of the 2011 Tohoku Earthquake

AbstractCharacterizing shallow interplate slip is essential for modeling the potential generation of megathrust earthquakes and tsunamis. Large postseismic crustal deformation caused by the 2011 Tohoku‐Oki earthquake suggests the occurrence of shallow afterslip, but it is unclear what kind of slip is dominant at the shallow plate interface. Here we report episodic tremor activity south of the primary rupture area based on ocean bottom seismometer observations. Five tremor episodes with recurrence intervals of ~60 days show migration behavior and shear mechanisms consistent with plate subduction, indicating the occurrences of the episodic slow slip events in the vicinity of the afterslip area. The association of slow slip events and afterslip sheds new light on the frictional nature and stress state of shallow megathrusts and may help predict the possible rupture extent of future earthquakes.

Episodic Slow Slip Events and Overlying Plate Seismicity at the Southernmost Ryukyu Trench

AbstractShallow slow slip events accompanied by seismicity has been increasingly reported in subduction zones during the last decade; however, the relationship between shallow slip events and seismicity is unclear. We report episodic slow slip events and seismicity at the southernmost Ryukyu Trench. The inversion results from GPS‐derived cumulative displacements indicate that the slow slip events occur offshore in northeastern Taiwan on the shallow subduction interface, where high seismic VP/VS ratios applies. Seismicity is observed above the subduction interface for locations of significant variations in the VP/VS ratio and does not follow the migration of peak slip along the subduction interface. We calculated the temporal evolution between the seismicity and the propagation of slow slip events and determined that seismicity is not primarily driven by slip propagation. The spatiotemporal relationship, coupled with the VP/VS ratio, suggests that high‐pressure fluid activity may be crucial to episodes of slow slip events and seismicity.

Two-Dimensional Probabilistic Cell Automaton Model for Broadband Slow Earthquakes

Slow tectonic events, such as deep tectonic tremor, low-frequency earthquakes, very low frequency earthquakes, and slow slip events, are regarded as “broadband slow earthquakes”. Various observational properties of these events have previously been explained using a one-dimensional (1D) stochastic model that mimics Brownian motion in the time domain. Here we present a simple two-dimensional (2D) slow earthquake model using a probabilistic cell automaton (PCA), where each cell possesses one of two states (slip or hold) at a given time step. The status of each cell is updated as a Markov process dependent on its own status and that of neighboring cells. The seismic moment rate of the event in the 2D PCA model is assumed to be proportional to the number of slipping cells, which is approximated using a stochastic differential equation equivalent to the previous 1D Brownian model approach. This 2D PCA model is, therefore, able to explain the various observational properties of slow earthquakes. Moreover, it includes an additional degree of freedom to separate the effects of loading and dissipation, which enables us to explain the variations in episodic slow slip activity. The size of the slow earthquake is limited by either the system size or the characteristic size, which is controlled by the degree of dissipation. Such a simple representation of slow earthquakes is useful for characterizing the broad spectrum of slow earthquake behavior in different environments.

Episodic creep events on the San Andreas Fault caused by pore-pressure variations.

Recent seismic and geodetic observations indicate that interseismic creep rate varies in both time and space. The spatial extent of creep pinpoints locked asperities, while its temporary accelerations, known as slow-slip events, may trigger earthquakes. Although the conditions promoting fault creep are well-studied, the mechanisms for initiating episodic slow-slip events are enigmatic. Here we investigate surface deformation measured by radar interferometry along the central San Andreas Fault between 2003 and 2010 to constrain the temporal evolution of creep. We show that slow-slip events are ensembles of localized creep bursts that aseismically rupture isolated fault compartments. Using a rate and state friction model, we show that effective normal stress is temporally variable on the fault, and support this using seismic observations. We propose that, compaction-driven elevated pore fluid pressure in hydraulically isolated fault zone and subsequent frictional dilation cause the observed slow slip episodes. We further suggest that the 2004 Mw6 Parkfield earthquake might have been triggered by a slow-slip event, which increased the Coulomb failure stress by up to 0.45 bar per year. This implies that while creeping segments are suggested to act as seismic rupture barriers, slow-slip events on these zones might promote seismicity on adjacent locked segments.

Repeated drainage from megathrusts during episodic slow slip

Pore-fluid pressure levels are considered to regulate the frictional strength and slip behaviour at megathrusts, where the largest earthquakes on Earth occur. Some analyses have suggested that the breaking of permeability seals during megathrust earthquakes causes subsequent drainage from the megathrust. However, it is poorly understood whether drainage follows frequent occurrences of episodic slow slip events. Here we analyse seismic waveform data beneath Kanto, Japan, for the period from 2004 to 2015 and show that seismicity rates and seismic attenuation above the megathrust of the Philippine Sea slab change cyclically in response to accelerated slow slip. These observations are interpreted to represent intensive drainage during slow slip events that repeat at intervals of approximately one year and subsequent migration of fluids into the permeable overlying plate. Our observations suggest that if slow slip events occur under an impermeable overlying plate, fluids draining due to slow slip events could be forced to channel within the megathrust, potentially enhancing pore-fluid pressure at an up-dip, locked seismogenic megathrust. This process might increase the potential to trigger large earthquakes near slow slip areas. Although stress transfer is recognized as an important factor for triggering megathrust failure, fluid transfer accompanied by episodic slow slip events will thus play an additional and crucial part in megathrust weakening. Slow slip events may cause fluids to drain from the plate boundary into the overlying plate at subduction zones, according to seismic analyses of events recorded in Kanto, Japan.

Areas prone to slow slip events impede earthquake rupture propagation and promote afterslip

At subduction zones, transient aseismic slip occurs either as afterslip following a large earthquake or as episodic slow slip events during the interseismic period. Afterslip and slow slip events are usually considered as distinct processes occurring on separate fault areas governed by different frictional properties. Continuous GPS (Global Positioning System) measurements following the 2016 Mw (moment magnitude) 7.8 Ecuador earthquake reveal that large and rapid afterslip developed at discrete areas of the megathrust that had previously hosted slow slip events. Regardless of whether they were locked or not before the earthquake, these areas appear to persistently release stress by aseismic slip throughout the earthquake cycle and outline the seismic rupture, an observation potentially leading to a better anticipation of future large earthquakes.

Tensile overpressure compartments on low-angle thrust faults

Hydrothermal extension veins form by hydraulic fracturing under triaxial stress (principal compressive stresses, σ1 > σ2 > σ3) when the pore-fluid pressure, Pf, exceeds the least compressive stress by the rock’s tensile strength. Such veins form perpendicular to σ3, their incremental precipitation from hydrothermal fluid often reflected in ‘crack-seal’ textures, demonstrating that the tensile overpressure state, σ3′ = (σ3 − Pf) < 0, was repeatedly met. Systematic arrays of extension veins develop locally in both sub-metamorphic and metamorphic assemblages defining tensile overpressure compartments where at some time Pf > σ3. In compressional regimes (σv = σ3), subhorizontal extension veins may develop over vertical intervals <1 km or so below low-permeability sealing horizons with tensile strengths 10 < To < 20 MPa. This is borne out by natural vein arrays. For a low-angle thrust, the vertical interval where the tensile overpressure state obtains may continue down-dip over distances of several kilometres in some instances. The overpressure condition for hydraulic fracturing is comparable to that needed for frictional reshear of a thrust fault lying close to the maximum compression, σ1. Under these circumstances, especially where the shear zone material has varying competence (tensile strength), affecting the failure mode, dilatant fault–fracture mesh structures may develop throughout a tabular rock volume. Evidence for the existence of fault–fracture meshes around low-angle thrusts comes from exhumed ancient structures and from active structures. In the case of megathrust ruptures along subduction interfaces, force balance analyses, lack of evidence for shear heating, and evidence of total shear stress release during earthquakes suggest the interfaces are extremely weak (τ < 40 MPa), consistent with weakening by near-lithostatically overpressured fluids. Portions of the subduction interface, especially towards the down-dip termination of the seismogenic megathrust, are prone to episodes of slow-slip, non-volcanic tremor, low-frequency earthquakes, very-low-frequency earthquakes, etc., attributable to the activation of tabular fault–fracture meshes at low σ3′ around the thrust interface. Containment of near-lithostatic overpressures in such settings is precarious, fluid loss curtailing mesh activity.Graphical abstract.

Cascadia subduction tremor muted by crustal faults

Deep, episodic slow slip on the Cascadia subduction megathrust of western North America is accompanied by low-frequency tremor in a zone of high fluid pressure between 30 and 40 km depth. Tremor density (tremor epicenters per square kilometer) varies along strike, and lower tremor density statistically correlates with upper plate faults that accommodate northward motion and rotation of forearc blocks. Upper plate earthquakes occur to 35 km depth beneath the faults. We suggest that the faults extend to the overpressured megathrust, where they provide fracture pathways for fluid escape into the upper plate. This locally reduces megathrust fluid pressure and tremor occurrence beneath the faults. Damping of tremor and related slow slip caused by fluid escape could affect fault properties of the megathrust, possibly influencing the behavior of great earthquakes.

Seismicity rate increases associated with slow slip episodes prior to the 2012 Mw 7.4 Ometepec earthquake

The March 20, 2012 Mw 7.4 Ometepec earthquake in the Oaxaca region of Southern Mexico provides a unique opportunity to examine whether subtle changes in seismicity, tectonic tremor, or slow slip can be observed prior to a large earthquake that may illuminate changes in stress or background slip rate. Continuous Global Positioning System (cGPS) data reveal a 5-month-long slow slip event (SSE) between ∼20 and 35 km depth that migrated toward and reached the vicinity of the mainshock a few weeks prior to the earthquake. Seismicity in Oaxaca is examined using single station tectonic tremor detection and multi-station waveform template matching of earthquake families. An increase in seismic activity, detected with template matching using aftershock waveforms, is only observed in the weeks prior to the mainshock in the region between the SSE and mainshock. In contrast, a SSE ∼15 months earlier occurred at ∼25–40 km depth and was primarily associated with an increase in tectonic tremor. Together, these observations indicate that in the Oaxaca region of Mexico shallower slow slip promotes elevated seismicity rates, and deeper slow slip promotes tectonic tremor. Results from this study add to a growing number of published accounts that indicate slow slip may be a common pre-earthquake signature.

The Pelona–Orocopia–Rand and related schists of southern California: a review of the best-known archive of shallow subduction on the planet

ABSTRACTThe Pelona–Orocopia–Rand and related schists of southern California are an archetypal example of an exhumed shallow subduction complex. ‘The schist’ comprises mainly trench materials underthust beneath continental arc rocks during Late Cretaceous–early Cenozoic collision of one or more oceanic plateaux with southern California. The arc-on-trench relationship, without intervening mantle or lowermost crust, implies that significant subduction erosion accompanied shallow subduction. Upsection increases in metamorphic grade (~150 ± 100°C/km) and spatial variations in age and peak temperature provide an ~50 million year long record of tectonic underplating within a cooling system. Evidence for palaeoseismic events in earliest formed and hottest (locally transitional granulite grade) schists provides a possible field-based record of episodic tremor and slow slip events such as detected in several modern shallow subduction zones. Structural ascent of the schist was achieved in distinct Late Cretaceous–early Eocene and late Oligocene–early Miocene extensional pulses, the first during collapse of gravitationally unstable upper plate assemblages and accompanied by trench-directed (top-NE) lower plate extrusion and the second corresponding temporally, spatially, and in character with core complex formation in the SW United States. The line between schist and core complex belts is blurred by the recent discovery of schist within 40 km of the nearest core complex and containing synkinematic Miocene intrusions, a hallmark of SW U.S. core complexes. The history of schist assembly, metamorphism, and exhumation provides the most complete field-based record of thermo-mechanical processes, subduction erosion and tectonic underplating in particular, that operated during a shallow subduction event. Future cross-disciplinary investigations of, and comparisons between, the schist and other possible ancient (e.g. Swakane gneiss, Sanbagawa belt, Qiangtang terrane) and modern (e.g. Cascadia, SW Japan, central Mexico, Chile) shallow subduction zones will yield new insights into the tectonic and petrologic processes that operate within such systems.

Triggering of the 2014 Mw7.3 Papanoa earthquake by a slow slip event in Guerrero, Mexico

Since their discovery two decades ago, slow slip events have been shown to play an important role in accommodating strain in subduction zones. However, the physical mechanisms that generate slow slip and the relationships with earthquakes are unclear. Slow slip events have been recorded in the Guerrero segment of the Cocos–North America subduction zone. Here we use inversion of position time series recorded by a continuous GPS network to reconstruct the evolution of aseismic slip on the subduction interface of the Guerrero segment. We find that a slow slip event began in February 2014, two months before the magnitude (M_w) 7.3 Papanoa earthquake on 18 April. The slow slip event initiated in a region adjacent to the earthquake hypocentre and extended into the vicinity of the seismogenic zone. This spatio-temporal proximity strongly suggests that the Papanoa earthquake was triggered by the ongoing slow slip event. We demonstrate that the triggering mechanism could be either static stress increases in the hypocentral region, as revealed by Coulomb stress modelling, or enhanced weakening of the earthquake hypocentral area by the slow slip. We also show that the plate interface in the Guerrero area is highly coupled between slow slip events, and that most of the accumulated strain is released aseismically during the slow slip episodes.

Tectonic tremor and slow slip along the northwestern section of the Mexico subduction zone

The southwestern coast of Mexico is marked by active subduction of the Rivera and Cocos plates, producing megathrust earthquakes that tend to recur every 50–100 yr. Herein, we use seismic and GPS data from this region to investigate the potential relationship between earthquakes, tectonic (non-volcanic) tremor, and transient slip along the westernmost 200 km of the Mexico subduction zone. Visual examination of seismograms and spectrograms throughout the 18-month-long MARS seismic experiment reveals clear evidence for frequent small episodes of tremor along the Rivera and Cocos subduction zones beneath the states of Jalisco, Colima, and Michoacán. Using a semi-automated process that identifies prominent energy bursts in envelope waveforms of this new data, analyst-refined relative arrival times are inverted for source locations using a 1-D velocity model. The resulting northwest–southeast trending linear band of tremor is located downdip from the rupture zones of the 1995 Mw 8.0 Colima–Jalisco and 2003 Mw 7.2 Tecoman subduction-thrust earthquakes and just below the regions of afterslip triggered by these earthquakes. Despite the close proximity between tremor and megathrust events, there is no evidence that the time since the last great earthquake influences the spatial or temporal pattern of tremor. A well-defined gap in the tremor beneath the western Colima Graben appears to mark a separation along the subducted Rivera–Cocos plate boundary. From the position time series of 19 continuous GPS sites in western Mexico, we present the first evidence that slow slip events occur on the Rivera plate subduction interface. Unlike the widely-recorded, large-amplitude, slow slip events on the nearly horizontal Cocos plate subduction interface below southern Mexico, slow slip events below western Mexico have small amplitudes and are recorded at relatively few, mostly coastal stations. The smaller slow slip beneath western Mexico may be due to the steeper dip, causing a narrower zone where the conditions for slow slip are met. The prominent afterslip following the M=8 1995 Colima–Jalisco and M=7.2 2003 Tacoma megathrust earthquakes may also shrink the area available for episodic slow slip.