Earthquake Nucleation Process Research Articles

Nucleation Mechanism and Rupture Dynamics of Laboratory Earthquakes at Different Loading Rates

The loading rate of tectonic stress is not constant during long-term geotectonic activity and significantly affects the earthquake nucleation and fault rupture process. However, the mechanism underlying the loading rate effect is still unclear. In this study, we conducted a series of experiments to explore the effect of the loading rate on earthquake nucleation and stick–slip characteristics. Through lab experiments, faults were biaxially loaded at varying rates to produce a series of earthquakes (stick–slip events). Both shear strain and fault displacement were monitored during these events. The findings indicate a substantial effect of the loading rate on the recurrence interval and the shear stress drop of these stick–slip events, with the recurrence interval inversely proportional to the loading rate. The peak friction of the fault also decreases with the increasing loading rate. Notably, prior to the dynamic rupture of earthquakes, there exists a stable nucleation phase where slip occurs in a quasi-static manner. The critical nucleation length, or the distance required before the dynamic rupture, diminishes with both the loading rate and normal stress. A theoretical model is introduced to rationalize these observations. However, the rupture velocity of these lab-simulated earthquakes showed no significant correlation with the loading rate. Overall, this study enhanced our comprehension of earthquake nucleation and rupture dynamics in diverse tectonic settings.

Earthquake Nucleation Characteristics Revealed by Seismicity Response to Seasonal Stress Variations Induced by Gas Production at Groningen

AbstractDeterministic earthquake prediction remains elusive, but time‐dependent probabilistic seismicity forecasting seems within reach thanks to the development of physics‐based models relating seismicity to stress changes. Difficulties include constraining the earthquake nucleation model and fault initial stress state. Here, we analyze induced earthquakes from the Groningen gas field, where production is strongly seasonal, and seismicity began 3 decades after production started. We use the seismicity response to stress variations to constrain the earthquake nucleation process and calibrate models for time‐dependent forecasting of induced earthquakes. Remarkable agreements of modeled and observed seismicity are obtained when we consider (a) the initial strength excess, (b) the finite duration of earthquake nucleation, and (c) the seasonal variations of gas production. We propose a novel metric to quantify the nucleation model's ability to capture the damped amplitude and the phase of the seismicity response to short‐timescale (seasonal) stress variations which allows further tightening the model's parameters.

Foreshock properties illuminate nucleation processes of slow and fast laboratory earthquakes

Understanding the connection between seismic activity and the earthquake nucleation process is a fundamental goal in earthquake seismology with important implications for earthquake early warning systems and forecasting. We use high-resolution acoustic emission (AE) waveform measurements from laboratory stick-slip experiments that span a spectrum of slow to fast slip rates to probe spatiotemporal properties of laboratory foreshocks and nucleation processes. We measure waveform similarity and pairwise differential travel-times (DTT) between AEs throughout the seismic cycle. AEs broadcasted prior to slow labquakes have small DTT and high waveform similarity relative to fast labquakes. We show that during slow stick-slip, the fault never fully locks, and waveform similarity and pairwise differential travel times do not evolve throughout the seismic cycle. In contrast, fast laboratory earthquakes are preceded by a rapid increase in waveform similarity late in the seismic cycle and a reduction in differential travel times, indicating that AEs begin to coalesce as the fault slip velocity increases leading up to failure. These observations point to key differences in the nucleation process of slow and fast labquakes and suggest that the spatiotemporal evolution of laboratory foreshocks is linked to fault slip velocity.

The effect of temperature on the nonlinear elasticity of a fault rock in dynamic acoustoelastic testing (DAET) experiments

Summary Dynamic nonlinear elasticity in rocks may play an important role in earth processes, such as earthquake nucleation. In order to understand how nonlinear elasticity occurs within the shallow crust, experiments are required that simulate the in situ conditions of intact crustal rocks. Additionally, exploring the behaviour of nonlinear elasticity in response to changes in external parameters (e.g., temperature and wave frequency) acts as a means to further illuminate the complex mechanisms which give rise to nonlinear elasticity in rocks. In this study, we perform dynamic acoustoelastic testing (DAET) experiments on an intact cataclasite from the damage zone of the Alpine Fault, New Zealand. By performing pump-probe DAET experiments inside a temperature-controlled chamber, we are able to investigate a rich variety of nonlinear behaviour as a function of temperature. We find that the magnitude of averaging softening, cubic nonlinearity, and hysteresis tend to increase as temperature increases from 20 to 110○C. In contrast, quadratic nonlinearity decreases with increasing temperature. These observations support the hypothesis that at least two distinct mechanisms control nonlinear phenomena in rocks. Nonlinear parameters show little to no dependence on frequency over the 200-600 Hz pump range, although values of the nonlinear parameter α are found to be nearly two orders of magnitude smaller than those determined using ultrasonic perturbations. Additionally, an analysis using different time windows shows that the surface waves of the ultrasonic probe sense greater nonlinearity compared to the direct P-wave due to differences in the polarisation and propagation paths. As well as providing further insight into the mechanisms responsible for nonlinear elasticity in rocks, our experiments show that nonlinear softening will increase as temperature increases in the damage zones of faults. This has potential implications for understanding earthquake nucleation processes.

Triggering mechanism and brittle-ductile dynamics of active faults in the south-central Saurashtra horst, Gujarat, western India: A geospatial, geological, and geophysical approach

The seismically active Saurashtra horst is located within the intraplate volcanic continental margin of western India. The region is prone to moderate and low-magnitude earthquakes within the depth range of ∼ 3 to ∼ 24 km. We observed that the earthquakes in this region are associated with seismically active brittle and ductile crustal layers. To understand the dynamics of the earthquake generation process, we applied an integrated geological and geomorphological approach, supplemented by subsurface geophysical (magnetotelluric) studies. Additionally, the active surface deformation has been measured using the PSInSAR and GLA techniques. Based on the stream offset and geomorphic landform development pattern several NW-SE and NE-SW oriented strike-slip faults have been identified. The PSI-derived displacement analysis reveals that the area is deforming at the rate of ± 5 mm/yr. Furthermore, subsurface crustal heterogeneity with increasing depth has been identified using the magnetotelluric technique, which is reflected in the form of basaltic lava flows, plutonic emplacement within the granitic basement, and the presence of semi-crystallized magmatic bodies below the brittle-ductile level. Additionally, we proposed a model to depict the plutonic emplacement within the highly fractured/faulted granitic basement and their relationship to the earthquake generation process. Our model shows that crustal heterogeneity and the migration of hydrothermal fluid from the semi-crystallized magmatic body along the active fault cause earthquake nucleation processes within the brittle and ductile layers. We concluded that the upwelling magmatic fluid above the brittle-ductile transition (BDT) acted as a lubricant for the nucleation and triggering of the earthquake along the active faults. Similarly, the fractured ductile crust is weakened by fluid migration, which causes high fluid pressure in the ductile crust thereby decreasing the confining pressure and endorsing the velocity weakening in the aseismic layer, responsible for the shear instability that causes deep crustal earthquakes. More specifically, the lithological heterogeneity at brittle and ductile regimes is an important factor for the earthquake nucleation process in this part of the Indian plate.

Ubiquitous Earthquake Dynamic Triggering in Southern California

AbstractEarthquakes can be dynamically triggered by the passing waves of other distant events. The frequent occurrence of dynamic triggering offers tangible hope in revealing earthquake nucleation processes. However, the physical mechanisms behind earthquake dynamic triggering have remained unclear, and contributions of competing hypotheses are challenging to isolate with individual case studies. To gain a systematic understanding of the spatiotemporal patterns of dynamic triggering, we investigate the phenomenon in southern California from 2008 to 2017. We use the Quake Template Matching catalog and an approach that does not assume an earthquake occurrence distribution. We develop a new set of statistics to examine the significance of seismicity‐rate changes as well as moment‐release changes. Our results show that up to 70% of 1,388 global M ≥ 6 events may have triggered earthquakes in southern California. The triggered seismicity often occurred several hours after the passing seismic waves. The Salton Sea Geothermal Field, San Jacinto fault, and Coso Geothermal Field are particularly prone to triggering. Although adjacent fault segments can be triggered by the same earthquakes, the majority of triggered earthquakes seem to be uncorrelated, suggesting that the process is primarily governed by local conditions. Further, the occurrence of dynamic triggering does not seem to correlate with ground motion (e.g., peak ground velocity) at the triggered sites. These observations indicate that nonlinear processes may have primarily regulated the dynamic triggering cases.

Stress‐Based and Convolutional Forecasting of Injection‐Induced Seismicity: Application to the Otaniemi Geothermal Reservoir Stimulation

AbstractInduced seismicity observed during Enhanced Geothermal Stimulation at Otaniemi, Finland is modeled using both statistical and physical approaches. The physical model produces simulations closest to the observations when assuming rate‐and‐state friction for shear failure with diffusivity matching the pressure build‐up at the well‐head at onset of injections. Rate‐and‐state friction implies a time‐dependent earthquake nucleation process which is found to be essential in reproducing the spatial pattern of seismicity. This implies that permeability inferred from the expansion of the seismicity triggering front (Shapiro et al., 1997, https://doi.org/10.1111/j.1365-246x.1997.tb01215.x) can be biased. We suggest a heuristic method to account for this bias that is independent of the earthquake magnitude detection threshold. Our modeling suggests that the Omori law decay during injection shut‐ins results mainly from stress relaxation by pore pressure diffusion. During successive stimulations, seismicity should only be induced where the previous maximum of Coulomb stress changes is exceeded. This effect, commonly referred to as the Kaiser effect, is not clearly visible in the data from Otaniemi. The different injection locations at the various stimulation stages may have resulted in sufficiently different effective stress distributions that the effect was muted. We describe a statistical model whereby seismicity rate is estimated from convolution of the injection history with a kernel which approximates earthquake triggering by fluid diffusion. The statistical method has superior computational efficiency to the physical model and fits the observations as well as the physical model. This approach is applicable provided the Kaiser effect is not strong, as was the case in Otaniemi.

New Physical Implications From Revisiting Foreshock Activity in Southern California

AbstractForeshock analysis promises new insights into the earthquake nucleation process and could potentially improve earthquake forecasting. Well‐performing clustering models like the Epidemic‐Type Aftershock Sequence (ETAS) model assume that foreshocks and general seismicity are generated by the same physical process, implying that foreshocks can be identified only in retrospect. However, several studies have recently found higher foreshock activity than predicted by ETAS. Here, we revisit the foreshock activity in southern California using different statistical methods and find anomalous foreshock sequences, that is, those unexplained by ETAS, mostly for mainshock magnitudes below 5.5. The spatial distribution of these anomalies reveals a preferential occurrence in zones of high heat flow, which are known to host swarm‐like seismicity. Outside these zones, the foreshocks generally behave as expected by ETAS. These findings show that anomalous foreshock sequences in southern California do not indicate a pre‐slip nucleation process, but swarm‐like behavior driven by heat flow.

Testing Earthquake Nucleation Length Scale with Pawnee Aftershocks

Abstract The interpretation of precursory seismicity can depend on a critical nucleation length scale h*, yet h* is largely unconstrained in the seismogenic crust. To estimate h* and associated earthquake nucleation processes at 2–7 km depths in Oklahoma, we studied seismic activity occurring prior to nine M 2.5–3.0 earthquakes that are aftershocks of the 3 September 2016 M 5.8 Pawnee, Oklahoma, earthquake. Four of the nine M 2.5–3.0 aftershocks studied did not have detectable seismicity within a 2 km radius of their hypocenters in the preceding 16 hr time windows. For the other five events, which did exhibit foreshock sequences, we estimated the static stress changes associated with each event of each sequence based on precise earthquake relocations and magnitude estimates. By carefully examining the spatiotemporal characteristics, we found all five of these M 2.5–3.0 aftershocks, and 70% of our studied events were plausibly triggered via static stress transfer from nearby earthquakes occurring hours to seconds earlier, consistent with the cascade nucleation model and a small h* in this region. The smallest earthquakes we could quantitatively study were M −1.5 events, which likely have 1–2 m rupture dimensions. The existence of these small events also supports a small nucleation length scale h*≤1 m, consistent with laboratory estimates. However, our observations cannot rule out more complicated earthquake initiation processes involving interactions between foreshocks and slow slip. Questions also remain as to whether aftershocks initiate differently from more isolated earthquakes.

Creep-dilatancy development at a transform plate boundary

How tectonic plates slip slowly and episodically along their boundaries, is a major, open question in earthquake science. Here, we use offshore in-situ sediment pore-pressure acquired in the proximity of the active offshore Main Marmara Fault and onshore geodetic time-series data set from a single GPS station to demonstrate the pore-pressure/deformation coupling during a 10-month slow-slip event. We show that pore pressure fluctuations are the expression of hydro-mechanical process affecting the deep seismogenic zone and indicate that small disturbances in geodetic data may have important meaning in terms of transient deformations. These results have major implications in understanding the spatial impact of slow-slip processes and their role in earthquake cycles. We demonstrate that piezometers measuring along a transform fault can help define the time scale regulating the coupling between slow-slip events and earthquake nucleation process.

Aseismic Slip and Cascade Triggering Process of Foreshocks Leading to the 2021 Mw 6.1 Yangbi Earthquake

AbstractUnderstanding the nature of foreshock evolution is important for earthquake nucleation and hazard evaluation. Aseismic slip and cascade triggering processes are considered to be two end-member precursors in earthquake nucleation processes. However, to perceive the physical mechanisms of these precursors leading to the occurrence of large events is challenging. In this study, the relocated 2021 Yangbi earthquake sequences are observed to be aligned along the northwest–southeast direction and exhibit spatial migration fronts toward the hypocenters of large events including the mainshock. An apparent static Coulomb stress increase on the mainshock hypocenter was detected, owing to the precursors. This suggests that the foreshocks are manifestations of aseismic transients that promote the cascade triggering of both the foreshocks and the eventual mainshock. By jointly inverting both Interferometric Synthetic Aperture Radar and Global Navigation Satellite Systems data, we observe that the mainshock ruptured a blind vertical fault with a peak slip of 0.8 m. Our results demonstrate that the lateral crustal extrusion and lower crustal flow are probably the major driving mechanisms of mainshock. In addition, the potential seismic hazards on the Weixi–Weishan and Red River faults deserve further attention.

Foreshock Activity Promoted by Locally Elevated Loading Rate on a 4‐m‐Long Laboratory Fault

AbstractWe report laboratory experimental results to reveal the factors that control the occurrence and magnitude of foreshocks. We conducted rock friction experiments using an apparatus that can shear 4‐m‐long rock specimens. We observed many stick‐slip events, as well as very slow (several tens of μm/s) but long‐lasting slips between those main events. These long‐term slow slips individually initiated from both the leading and trailing edges of the fault, and kept propagating steadily toward each other. Such steady slips did not immediately trigger any seismic events, regardless of the accumulated slip amount. After the coalescence of the two long‐term slow slip fronts, a second phase of slow slip with higher slip rate (several hundreds of μm/s)—called precursory slow slip, began at the central area and was accompanied by the occurrence of small seismic events (foreshocks). Subsequently, the main fast rupture eventually developed. We propose that the asperities that hosted foreshocks had similar size to the local critical nucleation length; the asperities slipped stably when the local loading rate was low, but could also slip unstably and radiated seismic waves when the local loading rate became high. We further found a clear positive correlation between foreshock magnitude and local slip rate. These results suggest that local loading rate has a significant influence on the occurrence and magnitude of foreshocks. Therefore, its effect should be taken into account during the studies of earthquake nucleation process and other similar phenomena such as Episodic Tremor and Slip (ETS), icequakes, and repeating earthquakes.

Intense foreshock swarm preceding the 2019 MW 6.5 Ambon (Seram, Indonesia) earthquake and its implication for the earthquake nucleation process

We apply a template matching detection to report a swarm-like foreshock sequence concentrated near the northern edge of the strike-slip fault rupture of the 25 September 2019 MW 6.5 Ambon earthquake in Seram, Indonesia. The template matching helps to identify small earthquakes that the routine earthquake catalog has not listed. The magnitude of completeness decreases from 3.1 (Badan Meteorologi Klimatologi dan Geofisika – BMKG catalog) to 1.7 in the improved catalog. We detect 3201 foreshocks which are tens fold the number of foreshocks in the routine earthquake catalog. We did not find a significant temporal variation in the b-value of the frequency–magnitude distribution (FMD) of the foreshock seismicity prior to initiating the dynamic rupture of the mainshock. The foreshocks around the mainshock hypocenter start to be busy since the beginning of July 2019 and continuously accelerates, with a minor variation in the seismicity rates and b-values. Although possible, we did not suggest that the stochastic cascade effect plays a role in the mainshock nucleation process. We discuss and highlight the possibility of fluid triggering to explain the high b-values of foreshock seismicity.

Seasonal Seismicity in the Western Branch of the East African Rift System

AbstractSurface load variations from large water bodies alter the state of stress on faults and potentially affect earthquake nucleation processes. We report observational evidence for seasonal variations of seismicity on the border faults of the western branch in the East African Rift System. Relying on water level changes of Lake Victoria and the surrounding rift lakes and soil moisture from the Global Land Data Assimilation System model, we calculate the crustal stress changes using a finite element model. Our results suggest the seasonal hydrological variations produce a 0.2–0.8 kPa Coulomb stress change on the border faults. The primary annual peak in seismicity is coincident with the peak Coulomb stress with an insignificant time lag. The correlation suggests seasonal variations of seismicity rates in the western branch of East African Rift System are modulated by the hydrological loading from Lake Victoria and the large rift lakes in the surrounding region.

Earthquake Initiation From Laboratory Observations and Implications for Foreshocks

AbstractThis paper reviews laboratory observations of earthquake initiation and describes new experiments on a 3‐m rock sample where the nucleation process is imaged in detail. Many of the laboratory observations are consistent with previous work that showed a slow and smoothly accelerating earthquake nucleation process that expands to a critical nucleation length scale Lc, before it rapidly accelerates to dynamic fault rupture. The experiments also highlight complexities not currently considered by most theoretical and numerical models. This includes a loading rate dependency where a “kick” above steady state produces smaller and more abrupt initiation. Heterogeneity of fault strength also causes abrupt initiation when creep fronts coalesce on a stuck patch that is somewhat stronger than the surrounding fault. Taken together, these two mechanisms suggest a rate‐dependent “cascade up” model for earthquake initiation. This model simultaneously accounts for foreshocks that are a by‐product of a larger nucleation process and similarities between initial P wave signatures of small and large earthquakes. A diversity of nucleation conditions are expected in the Earth's crust, ranging from slip limited environments with Lc < 1 m, to ignition‐limited environments with Lc > 10 km. In the latter case, Lc fails to fully characterize the initiation process since earthquakes nucleate not because a slipping patch reaches a critical length but because fault slip rate exceeds a critical power density needed to ignite dynamic rupture.

Simulation of induced seismic ground motions using coupled geomechanical and seismic wave propagation models

Summary Numerical simulations of seismic wave propagation usually rely on a simple source model consisting of an idealized point location and a moment tensor. In general, this is a valid approximation when the source dimensions are small relative to the distance of points at which the seismic wave motions are to be evaluated. Otherwise, a more realistic spatio-temporal source representation is required to accurately calculate ground motions at the position of monitoring stations. Here, we present a generic approach to couple geomechanical simulations to seismic wave propagation models using the concept of the equivalent force field. This approach allows the simulation of seismic wave propagation resulting from the spatio-temporal dependent earthquake nucleation and rupture processes. Within the geomechanical package two separate geomechanics codes are used to simulate both the slow loading stage leading to earthquake nucleation as well as the successive dynamic rupture stage. We demonstrate the approach to a case of induced seismicity, where fault reactivation occurs due to production from a natural gas reservoir.

Depletion‐Induced Seismicity at the Groningen Gas Field: Coulomb Rate‐and‐State Models Including Differential Compaction Effect

AbstractWe implement a Coulomb rate‐and‐state approach to explore the nonlinear relation between stressing rate and seismicity rate in the Groningen gas field. Coulomb stress rates are calculated, taking into account the 3‐D structural complexity of the field and including the poroelastic effect of the differential compaction due to fault offsets. The spatiotemporal evolution of the Groningen seismicity must be attributed to a combination of both (i) spatial variability in the induced stressing rate history and (ii) spatial heterogeneities in the rate‐and‐state model parameters. Focusing on two subareas of the Groningen field where the observed event rates are very contrasted even though the modeled seismicity rates are of similar magnitudes, we show that the rate‐and‐state model parameters are spatially heterogeneous. For these two subareas, the very low background seismicity rate of the Groningen gas field can explain the long delay in the seismicity response relative to the onset of reservoir depletion. The characteristic periods of stress perturbations, due to gas production fluctuations, are much shorter than the inferred intrinsic time delay of the earthquake nucleation process. In this regime the modeled seismicity rate is in phase with the stress changes. However, since the start of production and for two subareas of our analysis, the Groningen fault system is unsteady and it is gradually becoming more sensitive to the stressing rate.

Source Localization of Microseismic Emissions During Pneumatic Fracturing

AbstractLocalization of signals is a widely applied technique used in different areas of science telecommunication, medicine, or seismology. In this work, we study microseismic emissions due to stick‐slip events during pneumatic fracture in a transparent setup at laboratory scale and apply a localization method “Estimated Source Energy Homogeneity.” The seismic location results are compared with the image correlation results for displacement maps corresponding to the event times. We have observed (using optics and acoustics) that the movement starts inside the porous medium and progresses toward the channel tips, eventually causing channels to grow further. This finding could be of interest in understanding fluid‐induced earthquake nucleation processes. Similar to in‐site applications of pneumatic or fluid‐related fracturing, it shows that the area influenced extends beyond the fracture tips. This also shows why even after the end of pumping, we may get earthquakes, such as in the Basel case (Haring et al., 2008, https://doi.org/10.1016/j.geothermics.2008.06.002).

Longer Migration and Spontaneous Decay of Aseismic Slip Pulse Caused by Fault Roughness

AbstractIt has been known that natural faults possess rough profiles, which may play a vital role in earthquake dynamics. Here we examine the effect of fault roughness in the earthquake nucleation process using the rate and state friction law with the slip evolution law. The nucleation process on rough faults behaves as accelerating and migrating aseismic slip pulse, which is similar to previous studies for flat faults. However, the migration distance on rough faults is much larger than flat faults. The effect of fault roughness on the aseismic slip pulse migration is described by a single parameter known as the roughness drag, which depends on the amplitude, the minimum wavelength, and the Hurst exponent of the fault roughness profile. We also show that aseismic slip pulse cannot accelerate to dynamic rupture and ceases spontaneously if the roughness drag exceeds the static stress drop.

Crustal deformation, spatial distribution of earthquakes and along strike segmentation of the Sagaing Fault, Myanmar

The ∼1200 km long north-south oriented Sagaing fault (plate boundary fault between the Burma and Sunda plates), entirely located in Myanmar, passes through some major cities, and has witnessed several major earthquakes since prehistoric time, including the recent Mw 6.8 Thabeikkyin earthquake in 2012. The Sagaing fault accommodates ∼50–55% of dextral motion of the India-Sunda plate motion. Combined, geodetic observations and rheological considerations using topography as a proxy, we propose along strike segmentation of the Sagaing fault from centrally locked segment to creeping segment further north along the Koma fault. Such segmentation facilitates stress transfer (or loading) from creeping segment to locked segment, and influences earthquake nucleation process and along strike spatial extent of great earthquake ruptures. Topographic margins and their relative strength, predicted from the Newtonian channel flow model, also support the along strike segmentation behaviour.