Small Stress Perturbations Research Articles

Embrittlement Within Viscous Shear Zones Across the Base of the Subduction Thrust Seismogenic Zone

AbstractGeophysical observations indicate that patches of localized fracturing occur within otherwise viscous regions of subduction plate boundaries. These observations place uncertainty on the possible down‐dip extent of the seismogenic zone, and as a result the maximum magnitude of subduction thrust earthquakes. However, the processes controlling where and how localized fracturing occurs within otherwise viscous shear zones are unclear. We examined three exposures of exhumed plate boundary on Kyushu, Japan, which contain subducted sediments and hydrated oceanic crust deformed at ∼300 to ∼500°C. These exposures preserve subduction‐related viscous deformation, which in two of the studied exposures has a mutually overprinting relationship with quartz veins, indicating localized cyclical embrittlement. Where observed, fractures are commonly near lithological contacts that form viscosity contrasts. Mineral equilibrium calculations for a metabasalt composition indicate that exposures showing cyclical embrittlement deformed at pressure‐temperature conditions near dehydration reactions that consume prehnite and chlorite. In contrast, dominantly viscous deformation occurred at intervening pressure‐temperature conditions. We infer that at conditions close to metamorphic dehydration reactions, only small stress perturbations are required for transient embrittlement, driven by localized dehydration reactions reducing effective stress, and/or locally increased shear stresses along rheological contrasts. Our results show that the protolith composition of the subducting oceanic lithosphere controls the locations and magnitudes of dehydration reactions, and the viscosity of metamorphosed oceanic crust. Therefore, compositional variations might drive substantial variations in slip style.

The Influence of a Transitional Stress Regime on the Source Characteristics of Induced Seismicity and Fault Activation: Evidence from the 30 November 2018 Fort St. John ML 4.5 Induced Earthquake Sequence

ABSTRACTOn 30 November 2018, a sequence of seismicity including a felt (ML∼4.5) induced earthquake occurred ∼16 km southwest of Fort St. John, British Columbia. Using a local seismograph network around the epicentral region, we identified > 560 seismic events over a two-week period, incorporating two mainshock events within a 45 min time interval, both with ML>4.3. This seismicity occurred close in location and depth to ongoing hydraulic fracturing operations. Using previously unpublished data, our analysis suggests that events, including the largest mainshock, occurred at the interval of fluid injection, which is shallower than previously reported. The events showed a mix of reverse, oblique normal, and strike-slip mechanisms within a well-defined structural corridor that forms the southern margin of the Fort St. John graben. The two mainshock events reveal opposing mechanisms: one as a reverse (re)activation of a normal fault (ML 4.5) and the other an oblique normal mechanism (ML 4.3). Stress inversion and bootstrap analysis of 72 well-constrained focal mechanisms indicate that the maximum principal stress direction is horizontal, oriented in a north-northeast direction (3°–36°). However, the intermediate and minimum stress axes fluctuate between horizontal and vertical and are nearly equal in magnitude, indicating that both reverse and strike-slip regimes can occur in response to relatively small stress perturbations. Stress inversions using event subsets before and after the largest mainshock reveals an approximately 30° counter-clockwise coseismic rotation of the principal stress axes in the hypocentral region. Furthermore, the observed seismicity suggests that the largest mainshock event exceeded the calculated Mmax using models based on injected volumes, suggesting that it may be an example of runaway rupture. This has important implications for risk analysis, because small changes in the stress field may be induced through ongoing operations in this area, destabilizing different faults within a complex structural environment.

Earthquake Source Mechanisms and Stress Field Variations Associated With Wastewater‐Induced Seismicity in Southern Kansas, USA

AbstractThe strong increase of seismicity rates in the contiguous USA over the last 10 years is linked to the injection of huge amounts of wastewater from oil and gas production in unconventional hydrocarbon reservoirs. We calculated 549 moment tensors of induced earthquakes (MW ≤ 4.9) in southern Kansas to study their source mechanisms and their relation to injection activity. Seventeen percent of the events analyzed contained significant volumetric (ISO%) components, and these events mostly occurred near the two largest local earthquakes during the 4 months of largest active wastewater disposal. Mapping the local stress field, we determined that most of the region lies within a transtensional stress regime, with a maximum horizontal stress σHmax trending N75°E. In the epicentral area of the MW 4.9 Milan earthquake, the σHmax trend is rotated to about S80°E. Locally, two areas display a change in the stress field orientation with depth, from transtensional above 5.5 km depth to strike slip deeper in the basement. Relating the resolved fault geometries to the obtained local stress field orientation, we find that most of the activated fault planes were optimally oriented to the current stress field and thus small stress perturbations caused by the water injection could lead to failure.

Three-dimensional seismic velocity imaging of the Kachchh rift zone, Gujarat, India: Implications toward the crustal mafic pluton induced intraplate seismicity

Three-dimensional tomographic images of seismic velocities have been constructed for the 2001 Mw7.7 Bhuj earthquake region using data from a dense 58-station seismograph network, during 2001–16. Two distinct, prominent high-velocity anomalies (10–14% increase in Vp and Vs, and 10–11% decrease in Vp/Vs) have been mapped at 2–40 km depths below the Banni and Wagad uplift regions, which could be attributed to mafic plutons. We also image several high-velocity zones (intrusive mafic rocks) and some low-velocity patches (meteoric water/metamorphic fluids/volatile CO2) within the main rupture zone of the 2001 Bhuj earthquake, where maximum confinement of seismicity is noticed. We propose that stresses induced by mafic plutons in response to the N-S compression of the Indian plate might have resulted in a stress regime near to critical for thesub-parallel rift associated fault system within the main rupture zone, thereby, generating small to large earthquakes through small stress perturbations caused by crustal fluid flows and stress transfer.

Sensitivity of Acoustic Emission Triggering to Small Pore Pressure Cycling Perturbations During Brittle Creep

AbstractFluid‐induced stress perturbations in the crust at seismogenic depths caused by sources such as tidal or seasonal loading may trigger earthquakes. We investigate the role of small periodic pore pressure (Pp) perturbation in rupture nucleation by performing laboratory triaxial creep experiments on Fontainebleau sandstone, saturated in water, under sinusoidal Pp variations. Results show that recorded acoustic emissions (AEs) correlate with Pp as the rock approaches failure. More interestingly, AEs occur significantly more when Pp is decreasing, that is, when strain rate is maximum with a progressive increase of Pp‐AEs correlation in time as the rock approaches failure. This suggests that the correlation of small stress perturbations and AEs not only depends on Pp amplitude but also on the criticality of the rock. Observations at the laboratory scale support field observations where tidal loading may have modulated seismic rates during the nucleation phase of the 2004 Sumatra‐Andaman and 2011 Tohoku‐Oki earthquakes.

High resolution incremental stress testing of crushable granular materials

The incremental behaviour of crushable sands is investigated by means of a Discrete Element Method (DEM) based analogue. The DEM sample is subjected to a comprehensive set of small stress perturbations in the triaxial plane in order to identify the basic mechanisms contributing to the corresponding strain response. Three contributions to incremental strains are distinguished: elastic, plastic-structural and plastic-crushing. The behaviour observed appears to be consistent with the classic tenets of elasto-plasticity. It is also shown that high resolution probing is required to identify significant features such as elastic anisotropy and irreversible effects on the tangent bulk and shear moduli. As a consequence, computational efficiency is therefore a must for numerical studies of incremental response.

Repeating seismicity in the shallow crust modulated by transient stress perturbations

Recent studies have reported seismic phenomena that are modulated by small stress perturbations (∼10kPa), revealing their critically stressed nature. Such observations have been principally limited to plate interfaces with their occurrence linked to high fluid pore-pressure. In this study, we report observations of nine repeating seismic sources in the shallow crust in Guerrero, Mexico that emit events at rates comparable to other seismic phenomena in low stress environments. Testing their susceptibility to small stress perturbations, we find that all nine sources appear to be modulated by mining activity, tides, and a large slow slip event (Mw7.5). Our results suggest that the fault conditions necessary for low effective stress seismicity can occur away from plate interfaces.

Persistent shallow background microseismicity on Hekla volcano, Iceland: A potential monitoring tool

Hekla is one of Iceland's most active volcanoes. Since 1970 it has erupted four times with a period of quiescence of 14years since the last eruption. We detected persistent levels of background microseismicity with a temporary seismic network in autumn 2012. An amplitude based as well as an arrival-time based location method was applied to two populations of events and located them at shallow depths on the northern flank, close to the summit. This seismicity has not been identified previously by the permanent seismic network in Iceland as it is below its detection threshold. The detected events were either short, higher frequency events with distinct arrivals located beneath the summit on the northern flank of Hekla or longer, emergent, lower frequency events about 4km northeast of the summit at 200–400m depth below the surface. Estimated moment magnitudes were MW=-1.1 to -0.1 and MW=-0.9 to -0.0 and local magnitudes ML=-0.5 to +0.3 and ML=-0.3 to +0.3, respectively. This seismicity does not show any correlation with gas output but is located at the steepest slopes of the edifice. Hence we suggest that the current shallow microseismicity at Hekla is structurally controlled. This offers a possible opportunity of using near summit microseismicity as a tool for monitoring emerging unrest at Hekla. Microseismicity rates will be very sensitive to small stress perturbations due to magma migration at depth. Currently in the absence of microseismicity monitoring, Hekla switches from apparently quiescent to fully eruptive on the order of only 1h.

Inferring fault rheology from low‐frequency earthquakes on the San Andreas

AbstractFamilies of recurring low‐frequency earthquakes (LFEs) within nonvolcanic tremor (NVT) on the San Andreas fault in central California show strong sensitivity to shear stress induced by the daily tidal cycle. LFEs occur at all levels of the tidal shear stress and are in phase with the very small, ~400 Pa, stress amplitude. To quantitatively explain the correlation, we use a model from the existing literature that assumes the LFE sources are small, persistent regions that repeatedly fail during shear of a much larger scale, otherwise aseismically creeping fault zone. The LFE source patches see tectonic loading, creep of the surrounding fault which may be modulated by the tidal stress, and direct tidal loading. If the patches are small relative to the surrounding creeping fault then the stressing is dominated by fault creep, and if patch failure occurs at a threshold stress, then the resulting seismicity rate is proportional to the fault creep rate or fault zone strain rate. Using the seismicity rate as a proxy for strain rate and the tidal shear stress, we fit the data with possible fault rheologies that produce creep in laboratory experiments at temperatures of 400 to 600°C appropriate for the LFE source depth. The rheological properties of rock‐forming minerals for dislocation creep and dislocation glide are not consistent with the observed fault creep because strong correlation between small stress perturbations and strain rate requires perturbation on the order of the ambient stress. The observed tidal modulation restricts ambient stress to be at most a few kilopascal, much lower than rock strength. A purely rate dependent friction is consistent with the observations only if the product of the friction rate dependence and effective normal stress is ~ 0.5 kPa. Extrapolating the friction rate strengthening dependence of phyllosilicates (talc) to depth would require the effective normal stress to be ~50 kPa, implying pore pressure is lithostatic. If the LFE source is on the order of tens of meters, as required by the model, rate‐weakening friction rate dependence (e.g., olivine) at 400 to 600°C requires that the minimum effective pressure at the LFE source is ~ 2.5 MPa.

Tidal triggering of low frequency earthquakes near Parkfield, California: Implications for fault mechanics within the brittle‐ductile transition

Studies of nonvolcanic tremor (NVT) have established the significant impact of small stress perturbations on NVT generation. Here we analyze the influence of the solid earth and ocean tides on a catalog of ∼550,000 low frequency earthquakes (LFEs) distributed along a 150 km section of the San Andreas Fault centered at Parkfield. LFE families are identified in the NVT data on the basis of waveform similarity and are thought to represent small, effectively co‐located earthquakes occurring on brittle asperities on an otherwise aseismic fault at depths of 16 to 30 km. We calculate the sensitivity of each of these 88 LFE families to the tidally induced right‐lateral shear stress (RLSS), fault‐normal stress (FNS), and their time derivatives and use the hypocentral locations of each family to map the spatial variability of this sensitivity. LFE occurrence is most strongly modulated by fluctuations in shear stress, with the majority of families demonstrating a correlation with RLSS at the 99% confidence level or above. Producing the observed LFE rate modulation in response to shear stress perturbations requires low effective stress in the LFE source region. There are substantial lateral and vertical variations in tidal shear stress sensitivity, which we interpret to reflect spatial variation in source region properties, such as friction and pore fluid pressure. Additionally, we find that highly episodic, shallow LFE families are generally less correlated with tidal stresses than their deeper, continuously active counterparts. The majority of families have weaker or insignificant correlation with positive (tensile) FNS. Two groups of families demonstrate a stronger correlation with fault‐normal tension to the north and with compression to the south of Parkfield. The families that correlate with fault‐normal clamping coincide with a releasing right bend in the surface fault trace and the LFE locations, suggesting that the San Andreas remains localized and contiguous down to near the base of the crust. The deep families that have high sensitivity to both shear and tensile normal stress perturbations may be indicative of an increase in effective fault contact area with depth. Synthesizing our observations with those of other LFE‐hosting localities will help to develop a comprehensive understanding of transient fault slip below the “seismogenic zone” by providing constraints on parameters in physical models of slow slip and LFEs.

Reservoir-Triggered Seismicity at the Toulnustouc Hydroelectric Project, Quebec North Shore, Canada

Hydro-Quebec’s Toulnustouc hydroelectric complex is located approximately 120 km north of Baie–Comeau on the Quebec North Shore region. The project involved the construction of a 77-m-high dam and a 45-m-high dike that allowed the expansion of an existing 235 km2 reservoir by some 22 km2. When completely filled, the new part of the reservoir has a maximum depth of 75 m. Starting on 10 February 2005 and over a period of four days, the water of the original reservoir was used to start filling the new part. The water level of the entire reservoir increased by another 10 m after the spring snowmelt. The project also included a 9.8-km-long headrace tunnel constructed in Precambrian bedrock that creates a 170-m water head between the reservoir and the powerhouse. Electricity production started in May 2005. The complex is located in an area of low-level earthquake activity on the Canadian Shield. Four stations of the Canada National Seismograph Network (CNSN) located within 100 km of the reservoir provide a location threshold with a magnitude of about 1.2 ( m N ). To complement the CNSN, two portable seismographs were installed near the flooded area and the headrace tunnel about one week before flooding started. Between February and mid-May 2005, a total of four triggered earthquakes were recorded by the field instruments and the CNSN stations. The first event, a magnitude 1.4 ( m N ), occurred 16 days after flooding of the extension of the reservoir started. Three events were located beneath the reservoir and one was near the tunnel between the reservoir and the powerhouse where the water pressure is most intense. Numerous smaller but unlocatable events ( m N <1.0) were detected by one of the field stations. About two years later, two events separated by one minute and recognized as multiplets occurred near the reservoir. The small stress perturbations that led to the reservoir-triggered seismicity at Toulnustouc support the hypothesis that the mid to upper crust of intraplate areas is in a state of stress close to failure.

Surface loading and triggered earthquakes in the Koyna–Warna region, western India

Earthquakes of M≥5.0 continue to occur in the vicinity of Koyna reservoir in western India, the largest known case of Reservoir Triggered Earthquakes (RTS). The region remains seismically active even after four decades of the impounding of the Koyna reservoir. The earthquakes occur repeatedly along two major fault systems in NE–SW and NW–SE directions. We performed cross-correlation analysis between time series of the Koyna reservoir levels and the strain factor (energy 1/2) calculated for earthquakes of M≥3.0 occurring in the region during 1963–1999. Four time windows of 5 years or more are selected for cross-correlation to predict the periodicities, if any, in the seismic energy release caused by the annual reservoir water level fluctuations. A similar analysis was performed for the newly-impounded Warna reservoir, 25 km south of the Koyna dam. Our results suggest that the initial seismicity in the Koyna region during 1963 was triggered after the region attained steady state pore pressure by diffusion processes, particularly occurring along vertical strike-slip faults. Subsequently, major episodes of earthquake energy release until 1999 show a periodic behavior related to the annual filling of both the Koyna and Warna reservoirs. Two stages of earthquake energy release are evident till 1996 and coincide with annual filling and draining cycles of the reservoirs. Since 1996, the seismic energy release episodes correlate mostly with the draining cycle of the reservoir levels indicating a shift in the present day earthquake activity in the region, which may be due to a combined effect of the Koyna and Warna reservoirs. To explain the causal relationship between the reservoir water level fluctuations at the surface and earthquakes at hypocentral depths in terms of diffusion processes, we modeled the pore pressure front diffusion with time, in an inhomogeneous medium. It is seen that a water level change of the order of 1 m in 5 days in the surface loading can propagate 5–15% of pore pressure front, corresponding to 0.75–2.25 bar, to the hypocentral depth of 6–8 km in the presence of a vertical conducting fault. These small stress perturbations are sufficient to trigger seismicity on pre-existing, critically stressed faults in the Koyna–Warna region.

Internal stress field at Mount Vesuvius: A model for background seismicity at a central volcano

We propose a model to explain the background seismicity occurring at Somma‐Vesuvius in its present, mostly quiescent period. A finite element procedure has been used to simulate the stress field due to gravitational body forces in an axisymmetric volcano characterized by a central high‐rigidity anomaly. Results emphasize the important effect of axial high‐rigidity, which concentrates at its borders stresses resulting from the gravitational load of the volcanic edifice, as well as external (regional) stresses. The joint effect of the gravitational loading and of the presence of the anomaly produces stresses very close to or above the critical rupture threshold. The observed spatial concentrations of seismicity and moment release correlate well with peaks of computed maximum shear stress. Seismicity is then interpreted as due to small stress perturbations concentrated around the high‐rigidity core and added to a system already close, to the failure threshold. This model can explain the widely observed occurrence of background seismicity at central volcanoes worldwide.

Uniaxial stress measurements on the 1039.8 meV zero-phonon line in irradiated silicon.

A vibronic band in Si with its zero-phonon line at 1039.8 meV is a product of neutron irradiation and annealing at 500 to 800 K. We show that uniaxial stresses perturb the zero-phonon line as expected for a tetragonal centre. Since the TA(X) phonons are particularly important in forming the phonon sideband, we suggest that the tetragonal symmetry is a result of a naturally axial centre, and is not a Jahn-Teller distortion of a T d centre. No stress-induced reorientation of the centre occurs at small stress perturbations, but when the zero-phonon line splitting is comparable with the TA(X) energies the high energy stress-split component decreases rapidly in intensity. The zero-phonon line is negligibly affected by magnetic fields of up to 5 Tesla.