Earthquake Nucleation Research Articles

Earthquake forecast based on its nucleation stages and the ensuing electromagnetic radiations

A schematic spatial model of an earthquake nucleation is analyzed, and the three stages of the process are identified. Accordingly, the electromagnetic radiations emitted by the fracturing phenomena accompanying each stage are analyzed and it is demonstrated, that the changes of parameters in the persistent specific shape of these radiations during the stages are significant. Specifically, we analyze radiation changes in frequency, directionality and amplitude together with their relations to the fractures' properties across the nucleation process. These results allow us to present a possible method of forecasting, several hours before earthquakes of large magnitudes.

Pervasive Foreshock Activity Across Southern California

AbstractForeshocks have been documented as preceding less than half of all mainshock earthquakes. These observations are difficult to reconcile with laboratory earthquake experiments and theoretical models of earthquake nucleation, which both suggest that foreshock activity should be nearly ubiquitous. Here we use a state‐of‐the‐art, high‐resolution earthquake catalog to study foreshock sequences of magnitude M4 and greater mainshocks in southern California from 2008–2017. This highly complete catalog provides a new opportunity to examine smaller magnitude precursory seismicity. Seventy‐two percent of mainshocks within this catalog are preceded by foreshock activity that is significantly elevated compared to the local background seismicity rate. Foreshock sequences vary in duration from several days to weeks, with a median of 16.6 days. The results suggest that foreshock occurrence in nature is more prevalent than previously thought and that our understanding of earthquake nucleation may improve in tandem with advances in our ability to detect small earthquakes.

Useful predictions ahead of large earthquakes and lessons learned for future progress

A significant basis for this article is the outcome of 4 multinational research projects which were carried out in South-West Iceland from 1988–2006, focusing on crustal processes that preceded two magnitude-6.6 earthquakes in June 2000. The seismic activity that preceded a magnitude 6.3 double-earthquake in the same seismic zone in 2008 is also significant, as well as other research work which helps to understand how observable crustal processes lead to earthquakes. A significant outcome is that it cannot be assumed that any two earthquakes have the same precursory processes. Therefore, statistical analysis of precursors of past earthquakes is of limited value for predicting future earthquakes. On the other hand, with highly sensitive seismic monitoring it is possible to observe the nucleation process for each specific large earthquake for long enough time for earth-realistic modeling of it and extrapolating towards the earthquake in time and space. In the Iceland crust, with its fluid-rock interactions, pre-earthquake activity on a scale of years is expected. This allows a long-term approach to prediction. We apply historical information and sensor-based data to find probable sources of earthquake nucleation. We monitor the nucleation process for a possibly impending earthquake at these sources, and then model the process to find its governing factors and extrapolate those in time and space, aiming towards finding hypocenter, fault-size, impact, and time of the impending earthquake. We refine our models by predicting frequent medium-sized earthquakes and compare the predictions with measurements. We predict how the possibly impending large earthquake would trigger earthquakes at other locations. Given the complexity of the crust, we must take all observed changes into account when developing models of pre-earthquake processes. The development of a continuously operating geo-watching system is discussed to link scientific evaluations to warnings that can be used by emergency authorities.

Lower crustal earthquakes in the East African Rift System: Insights from frictional properties of rock samples from the Malawi rift

Earthquakes in the southern part of the East African Rift System (EARS) occur at depths up to 45 km in the lower crust, unusually deep for an extensional regime. Typically, earthquakes in continental crust nucleate at temperatures <350 °C, the temperature at which crystal plastic creep in quartz becomes efficient, corresponding to a depth of ~15 km with an average continental geothermal gradient. Several hypotheses have been proposed to explain the deep seismicity in the EARS, including the presence of high-viscosity mafic material, an abnormally low geotherm, locally elevated strain rates, or high fluid pressures. Here, we provide frictional properties of simulated fault gouges to address the seismic behavior of felsic and mafic crystalline basement rocks from the amagmatic Malawi Rift at the southern, propagating end of the EARS. Rotary shear experiments are conducted at 100, 200, and 250 MPa effective normal stress, a fluid pressure of 100 MPa and from room temperature to 700 °C. The friction of the felsic sample (58% plagioclase) is fairly constant up until 400 °C, after which it decreases. At temperatures of 600 °C and above, the relation between shear stress and effective normal stress deviates from linearity. The friction coefficient of the mafic sample is nearly constant in the considered range of temperature and normal stress and the sample shows a linear relation between normal stress and shear stress. The mafic sample also has negative, and decreasing, (a-b) values at temperatures >400 °C. These results are consistent with earthquake nucleation in mafic material, if present, at lower crustal P-T conditions. On the other hand, the data indicate that earthquake nucleation is unlikely in felsic material at temperatures above 500 °C, unless effective normal stress is low and strain rate is elevated by several orders of magnitude.

Analysis of Microseismicity Framing ML &gt; 2.5 Earthquakes at The Geysers Geothermal Field, California

AbstractPreparatory mechanisms accompanying or leading to nucleation of larger earthquakes have been observed at both laboratory and field scales, but conditions favoring the occurrence of observable preparatory processes are still largely unknown. In particular, it remains a matter of debate why some earthquakes occur spontaneously without noticeable precursors as opposed to events that are preceded by an extended failure process. In this study, we have generated new high‐resolution seismicity catalogs framing the occurrence of 20 ML &gt; 2.5 earthquakes at The Geysers geothermal field in California. To this end, a seismicity catalog of the 11 days framing each large event was created. We selected 20 sequences sampling different hypocentral depths and hydraulic conditions within the field. Seismic activity and magnitude frequency distributions displayed by the different earthquake sequences are correlated with their location within the reservoir. Sequences located in the northwestern part of the reservoir show overall increased seismic activity and low b values, while the southeastern part is dominated by decreased seismic activity and higher b values. Periods of high injection coincide with high b values and vice versa. These observations potentially reflect varying differential and mean stresses and damage of the reservoir rocks across the field. About 50% of analyzed sequences exhibit no change in seismicity rate in response to the large main event. However, we find complex waveforms at the onset of the main earthquake, suggesting that small ruptures spontaneously grow into or trigger larger events.

Effects of frictional properties of quartz and feldspar in the crust on the depth extent of the seismogenic zone

The depth extent of the crustal seismogenic zone is closely related to the size of earthquakes. The mechanisms that control the depth of the lower transition of the seismogenic zone are important issues in seismology and disaster mitigation. Laboratory studies have shown that the mechanism of earthquake nucleation is controlled by the frictional properties of fault materials. We measured the velocity dependences of the steady-state friction of quartz and feldspar, two major components of crustal rocks, under dry and wet conditions at temperatures up to 600 °C. In the presence of water, the temperature range over which the velocity dependence of steady-state friction was negative was wider for feldspar than for quartz, thus indicating that the temperature range of earthquake nucleation is wider for feldspar than for quartz. Considering that temperature increases with depth, our findings indicate that the material properties of feldspar likely play a dominant role in limiting the depth extent of the seismogenic zone.

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.

On behaviour and scaling of small repeating earthquakes in rate and state fault models

With abundant seismic data for small repeating earthquakes, it is important to construct a dynamic model that can explain various aspects of related observations. In this work, we study small repeating earthquakes on a fault governed by rate- and state-dependent friction laws. The earthquakes occur on a velocity-weakening patch surrounded by a much larger velocity-strengthening region. The whole fault is subject to long-term tectonic loading. The model with a circular patch and the aging form of rate- and state-dependent friction laws has been shown to reproduce the scaling of recurrence time versus seismic moment for small repeating earthquakes in a previous study. Here we investigate the behaviour of small repeating earthquakes in related models under different scenarios, including several forms of the state evolution equations in rate- and state-dependent friction laws, rectangular velocity-weakening patch geometries, quasi-dynamic versus fully dynamic representation of inertial effects and 2-D versus 3-D simulations. We find that the simulated scalings between the recurrence time and seismic moment for these different scenarios is similar while differences do exist. We propose a theoretical consideration for the scaling between the recurrence time and seismic moment of small repeating earthquakes. For patch radii smaller than or comparable to the full nucleation size, the scaling is explained by the increase of seismic to aseismic slip ratio with magnitude. For patch radii larger than the full nucleation size, the scaling is explained by the model in which the recurrence time is determined by the earthquake nucleation time, which is in turn determined by the time for aseismic slip to penetrate the distance of the full nucleation size into the patch. The obtained theoretical insight is used to find the combinations of fault properties that allow the model to fit the observed scaling and range of the seismic moment and recurrence time.

Beyond Byerlee friction, weak faults and implications for slip behavior

Some faults are considered strong because their strength is consistent with the Coulomb criterion under Byerlee's friction, 0.6<μ<0.85. In marked contrast, numerous studies have documented significant fault weakening induced by fluid-assisted reaction softening that generally takes place during the long-term evolution of the fault. Reaction softening promotes the replacement of strong minerals with phyllosilicates. Phyllosilicate development within foliated and interconnected fault networks has been documented at different crustal depths, in different tectonic regimes and from a great variety of rock types, nominating fluid-assisted reaction softening as a general weakening mechanism within the seismogenic crust. This weakening originates at the grain-scale and is transmitted to the entire fault zone via the interconnectivity of the phyllosilicate-rich zones resulting in a friction as low as 0.1<μ<0.3.Collectively, geological data and results from laboratory experiments provide strong supporting evidence for structural and frictional heterogeneities within crustal faults. In these structures, creep along weak and rate-strengthening fault patches can promote earthquake nucleation within adjacent strong and locked, rate-weakening portions. Some new frontiers on this research topic regard: 1) when and how a seismic rupture nucleating within a strong patch might propagate within a weak velocity strengthening fault portion, and 2) if creep and slow slip can be accurately detected within the earthquake preparatory phase and therefore represent a reliable earthquake precursor.

Induced Seismicity Driven by Fluid Diffusion Revealed by a Near‐Field Hydraulic Stimulation Monitoring Array in the Montney Basin, British Columbia

AbstractThis study presents observations using new data from a deployment of eight broadband seismometers surrounding a horizontal well pad at distances of ~1–3 km for the period before, during, and after a hydraulic fracturing treatment in the Montney Basin, British Columbia, Canada. We use a multistation‐matched filter detection and double‐difference earthquake relocation to develop a catalog of 350 events associated with hydraulic fracturing stimulation, with magnitudes ranging from −2.8 to 1.8 and estimated catalog completeness of approximately −0.2. The seismicity distribution suggests a statistically significant association with injection, and event migration can be described by a hydraulic diffusivity of ~0.2 m2/s. A comparison between daily seismicity rate and analytical stress evolution inferred from daily injection volumes implies that pore pressure diffusion largely controls earthquake nucleation at distances less than 1 km, whereas poroelastic stress transfer likely dominates at intermediate distances of ~1–4 km at time scales shorter than diffusion. Both mechanisms likely have a limited effect on stress perturbation at distances over 5 km.

Tidal Coulomb Failure Stresses in the northern Andean intermediate depth seismic clusters: Implications for a possible correlation between tides and seismicity

A recent statistical analysis of the relationship between tides and seismic activity in Colombia has suggested the existence of correlation anomalies for the case of intermediate depth events in the Bucaramanga nest and the Cauca cluster (Moncayo et al., 2019). In this work, we explore in detail the hypothesis that tides may be triggering seismic activity in these regions and extend the analysis to two other seismic clusters in northern-central South America, specifically in the areas of El Puyo (Ecuador) and Pucallpa (Peru). For this purpose, we use the available focal mechanism information for seismic events at these locations, and calculate for each event the Tidal Coulomb Failure Stress (TCFS) as obtained from estimations of the tidal strain tensor. Tidal strains are computed considering the Earth body tides and the effect of the ocean tidal loading. Since our purpose is to elucidate the role of tides in earthquake nucleation, calculations of the TCFS are conducted not only for the time of earthquake, but also for the time of the closest maximum strain within a window of a few hours before the events. Our results tend to support the hypothesis that tidal stresses are contributing to earthquake generation in all the studied areas; this trend is especially stronger when TCFS are calculated at pre-earthquake times. The physical explanation for the positive contribution of tides to earthquake triggering in intermediate depth clusters may lie in the fact that a wide diversity of fault plane orientations is possible within a relatively small volume of subducted lithosphere, so making the tides more likely to help loosening up the blocks at the fault plane to promote slip.

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).

Mechanical behaviour of fluid-lubricated faults

Fluids are pervasive in fault zones cutting the Earth's crust; however, the effect of fluid viscosity on fault mechanics is mainly conjectured by theoretical models. We present friction experiments performed on both dry and fluid-permeated silicate and carbonate bearing-rocks, at normal effective stresses up to 20 MPa, with a slip-rate ranging between 10 μm/s and 1 m/s. Four different fluid viscosities were tested. We show that both static and dynamic friction coefficients decrease with viscosity and that dynamic friction depends on the dimensionless Sommerfeld number (S) as predicted by the elastohydrodynamic-lubrication theory (EHD).Under favourable conditions (depending on the fluid viscosity (η), co-seismic slip-rate (V), fault geometry (L/H02) and earthquake nucleation depth (∝σeff)), EHD might be an effective weakening mechanism during natural and induced earthquakes. However, at seismic slip-rate, the slip weakening distance (Dc) increases markedly for a range of fluid viscosities expected in the Earth, potentially favouring slow-slip rather than rupture propagation for small to moderate earthquakes.

Microseismicity on Patches of Higher Compression During Larger‐Scale Earthquake Nucleation in a Rate‐and‐State Fault Model

AbstractWhile many large earthquakes are preceded by observable foreshocks, the mechanism responsible for the occurrence of these smaller‐scale seismic events remains uncertain. One physical explanation of foreshocks with growing support is that they are produced by the interaction of slow slip with fault heterogeneity. Inspired by the suggestion from laboratory experiments that foreshocks occur on fault asperities (bumps), we explore rate‐and‐state fault models with patches of higher normal stress embedded in a larger seismogenic region by conducting 3‐D numerical simulations of their behavior over long‐term sequences of aseismic and seismic slips. The models do produce smaller‐scale seismicity during the aseismic nucleation of larger‐scale seismic events. These smaller‐scale events have reasonable stress drops, despite the highly elevated compression assigned to the source patches. We find that the two main factors contributing to the reasonable stress drops are the significant extent of the rupture into the region surrounding the patches and the aseismic stress release just prior to the seismic events. The smaller‐scale seismicity can only occur if a sufficient separation in nucleation scales between the foreshock‐like events and mainshocks is achieved. Our modeling provides insight into the conditions conducive for generating foreshocks on both natural and laboratory faults.

Nucleation of Characteristic Earthquakes in Simulated Cycles Involving Deep Huge Slow Slip Events

AbstractThe 1944 Tonankai and 1946 Nankai megathrust earthquakes are likely to have been immediately preceded by a huge slow slip event (SSE) in the deeper brittle‐to‐ductile transition zone, which accumulates slip of a meter or more within several days. The present study investigates the interaction between such huge SSEs and the characteristic earthquakes through earthquake cycle simulations obeying the rate‐ and state‐dependent friction. To simulate huge SSEs, we employ a cutoff velocity (Vcx) above which the friction changes from velocity weakening to velocity strengthening. We assume that Vcx decreases from 10 to 10−9 m/s with depth over the transition zone located between 20‐ and 35‐km depths. Depending on the characteristic slip distance L of rate and state friction, simulation results showed three patterns of earthquake nucleation. If L is small, an SSE in the transition zone directly grows into an earthquake without experiencing a separate nucleation at the brittle zone (brittle nucleation). If L is moderate, an SSE triggers brittle nucleation, which develops into an earthquake. If L is large, an SSE occurs but ceases without triggering an earthquake nor brittle nucleation, then slip deficit accumulates further to finally start brittle nucleation without the direct help of SSE, and an earthquake occurs without a large precursor. In addition, isolated huge SSEs, resembling precursory ones, occur in all cases, at most a few times per cycle.

Evidence for Rupture Through a Double Benioff Zone During the 2017 Mw 8.2 Chiapas, Mexico Earthquake

AbstractWe combine multi‐array relative back‐projection of high‐frequency P waves and finite‐fault modeling of low‐frequency P waves from the 8 September 2017 Mw 8.2 Chiapas, Mexico earthquake to image unilateral rupture on a southeast‐northwest striking subvertical fault plane over depths of 10–70 km. Improved multi‐array relative back‐projection estimates of rupture speed and fault geometry provide key refinements to the slip model from finite‐fault modeling. Compilation of prior seismicity and aftershocks reveals the earthquake initiated in the lower plane of a double Benioff zone (DBZ) and ruptured with large slip across the entire DBZ. The previously aseismic interior does not appear to be due to lack of stress as both planes show downdip extension stresses. Instead, tomographic imaging of DBZs find high Poisson's ratio with fast velocities in the DBZ interior, so we propose that highly hydrated harzburgite generates velocity‐strengthening behavior that inhibits earthquake nucleation, yet it can slip seismically during dynamic weakening from a large throughgoing rupture.

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.

ULF Pre-Seismic Geomagnetic Anomalous Signal Related to Mw8.1 Offshore Chiapas Earthquake, Mexico on 8 September 2017.

In the last decade, the real time ground–based geomagnetic observations realized in correlation with the Vrancea seismicity in Romania, together with supplementary studies related to some earthquakes (Mw9.0 Tohoku, Japan on 11 March 2011 and Mw8.3 Coquimbo, Chile on 16 September 2015), enlarged our knowledge about the relationship between the pre-seismic anomalous phenomena and the final stage of the earthquake nucleation. To identify possible ultra-low-frequency (ULF) geomagnetic signals prior to the onset of an Mw8.1 earthquake, we retroactively analyzed the data collected on the interval 1 August–16 September 2017 at the Geomagnetic Observatories in Teoloyucan (TEO), Mexico and Tucson (TUC) USA, with the last taken as a reference. Daily mean distributions of the polarization parameter BPOL (geomagnetic polarization parameter) and standard deviation are obtained for both observatories using a fast Fourier transform (FFT) band-pass filtering in the ULF range (0.001–0.083 Hz). Further on, we investigated the singularity of the pre-seismic signal associated with an Mw8.1 earthquake and applied a statistical analysis based on a standardized random variable equation; results are presented as BPOL* time series on the interval 1–26 September. Finally, the hourly mean distribution, obtained as difference BPOL (TUC-TEO) on the interval 7–9 September emphasizes an anomalous signal with five hours before the onset of the Mw8.1 earthquake.

Earthquake Nucleation Size: Evidence of Loading Rate Dependence in Laboratory Faults.

Recent Global Positioning System observations of major earthquakes such as the 2014 Chile megathrust show a slow preslip phase releasing a significant portion of the total moment (Ruiz et al., 2014, https://doi.org/10.1126/science.1256074). Despite advances from theoretical stability analysis (Rubin & Ampuero, 2005, https://doi.org/10.1029/2005JB003686; Ruina, 1983, https://doi.org/10.1029/jb088ib12p10359) and modeling (Kaneko et al., 2017, https://doi.org/10.1002/2016GL071569), it is not fully understood what controls the prevalence and the amount of slip in the nucleation process. Here we present laboratory observations of slow slip preceding dynamic rupture, where we observe a dependence of nucleation size and position on the loading rate (laboratory equivalent of tectonic loading rate). The setup is composed of two polycarbonate plates under direct shear with a 30‐cm long slip interface. The results of our laboratory experiments are in agreement with the preslip model outlined by Ellsworth and Beroza (1995, https://doi.org/10.1126/science.268.5212.851) and observed in laboratory experiments (Latour et al., 2013, https://doi.org/10.1002/grl.50974; Nielsen et al., 2010, https://doi.org/10.1111/j.1365-246x.2009.04444.x; Ohnaka & Kuwahara, 1990, https://doi.org/10.1016/0040-1951(90)90138-X), which show a slow slip followed by an acceleration up to dynamic rupture velocity. However, further complexity arises from the effect of (1) rate of shear loading and (2) inhomogeneities on the fault surface. In particular, we show that when the loading rate is increased from 10−2 to 6 MPa/s, the nucleation length can shrink by a factor of 3, and the rupture nucleates consistently on higher shear stress areas. The nucleation lengths measured fall within the range of the theoretical limits L b and L∞ derived by Rubin and Ampuero (2005, https://doi.org/10.1029/2005JB003686) for rate‐and‐state friction laws.

Unstable Slip Pulses and Earthquake Nucleation as a Nonequilibrium First-Order Phase Transition.

The onset of rapid slip along initially quiescent frictional interfaces, the process of "earthquake nucleation," and dissipative spatiotemporal slippage dynamics play important roles in a broad range of physical systems. Here we first show that interfaces described by generic friction laws feature stress-dependent steady-state slip pulse solutions, which are unstable in the quasi-1D approximation of thin elastic bodies. We propose that such unstable slip pulses of linear size L^{*} and characteristic amplitude are "critical nuclei" for rapid slip in a nonequilibrium analogy to equilibrium first-order phase transitions and quantitatively support this idea by dynamical calculations. We then perform 2D numerical calculations that indicate that the nucleation length L^{*} exists also in 2D and that the existence of a fracture mechanics Griffith-like length L_{G}<L^{*} gives rise to a richer phase diagram that features also sustained slip pulses.