Earthquake Size Distribution Research Articles

Statistically significant difference between earthquake size distributions of independent and triggered seismicity

The Alto Tiberina Fault system, located in Central Italy, is an active structure about 60 km long composed of a principal low-angle normal fault and several minor synthetic and antithetic splays. The system is monitored by a dense seismic network, giving us the opportunity to construct high-definition seismic catalogs with a low completeness magnitude. We analyze the clustering properties of the 2010-2015 seismicity by using a 3D stochastic declustering algorithm that also includes the earthquakes’ depth. We demonstrate that the earthquake size distribution is strongly correlated with the clustering of seismic events and their depth; in particular, the principal fault and secondary faults show an opposite behavior both in terms of earthquake size distribution and clustering properties.

Earthquake size distributions are slightly different in compression vs extension

The earthquake size distribution is described by an exponential function governed by the b-value parameter. It has already been proven that the b-value depends on the differential stress and tectonic settings. Here, we propose a new method to group earthquakes using the kinematics of the interseismic geodetic strain rates and horizontal stress directions. We select the Italian peninsula as a case study, and we find that the b-value is significantly larger in the extensional setting than in the compressional one, although these differences are much smaller than previously reported. We also show that spatial fragmentation of uniform tectonic regimes leads to inaccurate b-value estimation due to the undersampling of earthquake size distribution. Given these results, we conclude that stress directions and geodetic data complement other geological or geophysical information and reduce the arbitrariness in drawing zones for a seismotectonic model.

Variation of b-Value of Normal, Reverse and Strike-Slip Faulting Earthquakes with Focal Depth

A crucial factor in evaluating seismic activity, seismotectonics, and seismic hazard assessment is the distribution of earthquake sizes (b-value). The goal of this research is to explore the relation between depth and the b-constant values of normal, inverse, and strike-slip faulting earthquakes. The earthquake catalog used in this work was extracted from the Global Centroid Moment Tensor Catalog (GCMTC) and covers the period from 2008 to 2017. The focal depth of the selected tremors ranges from zero to 700 km. The magnitude of completeness for normal and reverse faulting events is 5.2 and 5.1 for strike-slip faulting events. The value of the b- constant was estimated using the maximum likelihood technique. The findings of regression analysis showed that there is an insignificant negative poor correlation relation between the b- value of the normal and strike-slip faulting events and depth, while that relation is an insignificant positive poor correlation for reverse faulting. The depth of the turning point for the b-constant value of normal and reverse faulting earthquakes is near the mantle seismic discontinuities 410 and 520 km. The depth of inflection points of b-constant value strike-slip faulting earthquakes is near to lithosphere-asthenosphere boundary (LAB) at depth of 250km and the mantle seismic discontinuity of 520km.

Reliability of earthquake-size distribution and stress regime relationships for fluid-injection-induced seismicity

Induced seismicity caused by fluid injection associated with unconventional gas and geothermal projects, poses a significant hazard that requires effective mitigation and management. This issue is of concern to local and indigenous communities, regulatory bodies, operators and their insurers, as it poses risks to health, safety, and project economics and success. To assess the likelihood and potential magnitude (i.e., severity) of fluid-injection-induced seismicity events, empirical relationships for natural earthquakes have been co-opted that relate event size distributions (b-value) to focal mechanisms and inferred in-situ stress regimes. However, it remains unclear whether these associations can be generalized and applied to fluid-injection- induced seismicity. Analyses presented here using a global dataset of induced seismicity events suggest they can't. Unlike natural earthquakes where tectonic forces generally activate the faults most critically aligned to the far-field in-situ stresses, fluid-injection-induced seismicity is driven mainly by localized pressure perturbations that may activate a fault with a non-critically aligned orientation. These findings are further investigated and supported using 3-D Mohr-circle slip tendency analyses and 3-D numerical models employing a fully-coupled hydro-mechanical bonded-lattice methodology via the commercial hydraulic fracturing numerical simulation program XSite. The combined empirical, analytical and numerical simulation results indicate that under a transform stress regime (SH > SV > Sh), for typical reservoir depths (i.e., 1–6 km), a larger area of the fault affected by the injected fluid pressures is critically stressed compared to a compressional stress regime (SH > Sh > SV), resulting in a larger slip area and an increased likelihood of larger magnitude fluid-injection-induced seismicity events. These findings have important implications for the development of effective hazard mitigation strategies and risk management protocols for fluid-injection-induced seismicity.

TheK−MSlope: A Potential Supplement forb-Value

AbstractThe b-value of the Gutenberg–Richter law describes the relationship of size and frequency distribution of earthquakes. Its variations may be related to stress state, hence has been used for short-term earthquake forecasting. However, estimation of b-value faces many uncertainties, making it difficult to interpret b-value changes as tectonic signals or statistical artifacts. Some recent studies have suggested that the b-values of some seismic catalogs are proportional to the K−M slope (KMS) obtained from the visibility graph analysis. If confirmed, the KMS may provide additional constraints to the b-value and its variations. In this study, we used synthetic seismic catalogs that obey various probability distributions to demonstrate that the proportional relationship between KMS and b-value is universal and stable, and that the KMS/b ratio is dependent on the catalog size. We found that the KMS estimation can perform better than the commonly used b-value estimation methods, especially when the catalogs are incomplete or have variations in the magnitude–frequency relations. We proposed an improved KMS method to ensure that the KMS value provides pure magnitude information and is not affected by the occurrence time or orders of the events. We used the KMS to estimate the spatiotemporal variations of b-value in the northeastern Tibetan plateau; the results are consistent with those derived from traditional b-value estimation methods. We suggest that the KMS can be used as a verification of or a supplement to the b-value.

A non-extensive approach to probabilistic seismic hazard analysis

Abstract. We modify the probabilistic seismic hazard analysis (PSHA) formulation by replacing the Gutenberg–Richter power law with the SCP (Sotolongo-Costa and Posadas) non-extensive model for earthquake size distribution and call it NEPSHA. The SCP claimed to model the regional seismicity better than the classical models. The proposed method (NEPSHA) is implemented in the Tehran region, and the results are compared with the classic PSHA method. The hazard curves show that NEPSHA gives a higher hazard, especially in the range of practical return periods. The uniform hazard spectra of NEPSHA provide more spectral accelerations, especially for the medium-height buildings, which are the most common urban structures.

Transient evolution of the relative size distribution of earthquakes as a risk indicator for induced seismicity

Induced earthquakes pose a substantial challenge to many geo-energy applications, and in particular to Enhanced Geothermal Systems. We demonstrate that the key factor controlling the seismic hazard is the relative size distribution of earthquakes, the b-value, because it is closely coupled to the stress conditions in the underground. By comparing high resolution observations from an Enhanced Geothermal System project in Basel with a loosely coupled hydro-mechanical-stochastic model, we establish a highly systematic behaviour of the b-value and resulting hazard through the injection cycle. This time evolution is controlled not only by the specific site conditions and the proximity of nearby faults but also by the injection strategy followed. Our results open up new approaches to assess and mitigate seismic hazard and risk through careful site selection and adequate injection strategy, coupled to real-time monitoring and modelling during reservoir stimulation.

Data-driven spatiotemporal assessment of the event-size distribution of the Groningen extraction-induced seismicity catalogue

For induced seismicity, the non-stationary, heterogeneous character of subsurface stress perturbations can be a source of spatiotemporal variations in the scaling of event sizes; one of the critical parameters controlling seismic hazard and risk. We demonstrate and test a systematic, statistical, penalized-likelihood approach to analysing both spatial and temporal variations in event size distributions. The methodology used is transferable to the risk analysis of any subsurface operation, especially for small earthquake catalogues. We explore the whole solution space and circumvent conventional, arbitrary choices that require a priori knowledge of these variations. We assess the effect of possible bias in the derivation, e.g., due to tapering of the earthquake-size distribution, correlation between the b-value and the magnitude of completeness and correlation between the b-value and the largest magnitude observed. We analyse the spatiotemporal variations in the earthquake-size distribution of the Groningen induced seismicity catalogue (December 1991–November 16, 2021). We find statistically significant spatial variations without any compelling, statistical evidence of a temporal variation. Furthermore, we find that the largest magnitudes observed are inconsistent with the sampling statistics of an unconstrained earthquake-size distribution. Current risk assessment models likely overestimate the probability of larger magnitude events (M ≥ 3.0) and thus the risk posed.

Spatio‐Temporal Clustering of Seismicity Enabled by Off‐Fault Plasticity

AbstractWhile significant progress has been made in understanding earthquake source processes in linear elastic domains, the effect of more realistic rheologies including plasticity is poorly understood. Here, we simulate the sequence of earthquake and aseismic slip of a 2D antiplane rate‐and‐state fault embedded in a full‐space elastic‐plastic bulk. We show that off‐fault plasticity may lead to partial ruptures as well as temporal clustering of seismic events. Furthermore, the interaction of fault slip and off‐fault plasticity results in pockets of slip deficit. While the energy dissipated through plastic deformation remains a small fraction of the total energy budget, its impact on the source characteristics is disproportionally large through the redistribution of stresses and viscous relaxation. Our results suggest a new mechanism of dynamic heterogeneity in earthquake physics that may have important implications on earthquake size distribution and energy budget.

A unified perspective of seismicity and fault coupling along the San Andreas Fault.

The San Andreas Fault (SAF) showcases the breadth of possible earthquake sizes and occurrence behavior; in particular, the central SAF is a microcosm of such diversity. This section also exhibits the spectrum of fault coupling from locked to creeping. Here, we show that the observations of aseismic slip, temporal clustering of seismicity, and spatial variations in earthquake size distributions are tightly connected. Specifically, the creep rate along the central SAF is shown to be directly proportional to the fraction of nonclustered earthquakes for the period 1984–2020. This relationship provides a unified perspective of earthquake phenomenology along the SAF, where lower coupling manifests in weaker temporal clustering, with repeating earthquakes as an end-member. This new paradigm provides additional justification for characterizing the northwest ∼75 kilometers of the creeping segment as a transition zone, with potential implications for seismic hazard.

Comment on “High-Definition Mapping of the Gutenberg–Richter b-Value and Its Relevance: A Case Study in Italy” by M. Taroni, J. Zhuang, and W. Marzocchi

Abstract Taroni et al. (2021) published a statistical framework to reliably estimate the b-value and its uncertainties, with the goal being the interpretation in a seismotectonic context and improving earthquake forecasting capabilities. In this comment, we show that the results presented for the Italian region and the conclusions drawn by the authors, are heavily biased due to quarry-blast events in the Italian earthquake catalog used in the analysis. Without removing this anthropogenic component in the data, a meaningful analysis of the earthquake-size distribution for natural seismicity is, in our opinion, not possible. This comment highlights the need for basic data quality analysis before sophisticated statistical tools are applied to a dataset.

Reliability of Earthquake-Size Distribution and Stress Regime Relationships for Fluid-Injection-Induced Seismicity

Reliability of Earthquake-Size Distribution and Stress Regime Relationships for Fluid-Injection-Induced Seismicity

Modelling the seismic potential of the Indo-Burman megathrust

The Indo-Burman arc is the boundary between the India and Burma plates, north of the Sumatra–Andaman subduction zone. The existence of active subduction in the Indo-Burman arc is a debatable issue because the Indian plate converges very obliquely beneath the Burma plate. Recent GPS measurements in Bangladesh, Myanmar, and northeast India indicate 13–17 mm/y of plate convergence along a shallow dipping megathrust while most of the strike-slip motion occurs on several steep faults, consistent with patterns of strain partitioning at subduction zones. A short period of instrumentally recorded seismicity and sparse historical records are insufficient to assess the possibility of great earthquakes at the Indo-Burman megathrust. Using the advantage of the Block-and-Fault Dynamics model allowing simultaneous simulation of slow tectonic motions and earthquakes, we test the hypothesis whether the India-Burma detachment is locked and able to produce great earthquakes, or it slips aseismically? We have shown that the model of locked detachment is preferred because it more adequately reproduces observed tectonic velocities. The integral characteristics of synthetic seismicity, the earthquake size distribution, and the rate of seismic activity are consistent with those derived from observations. Our results suggest that the megathrust is locked and can generate great M8+ earthquakes. The estimated average return period of great events exceeds one thousand years. Earthquakes of this size pose a great threat to NE India, Bangladesh and Myanmar, the most densely populated areas of the world.

The Effect of the Wenchuan and Lushan Earthquakes on the Size Distribution of Earthquakes along the Longmenshan Fault

Changes in the stress state of faults and their surroundings is a highly plausible mechanism explaining earthquake interaction. These stress changes can impact the seismicity rate and the size distribution of earthquakes. However, the effect of large earthquakes on the earthquake size distribution along the Longmenshan fault has not been quantified. We evaluated the levels of the b value for the stable state before and after the large earthquakes on 12 May 2008 (Wenchuan, MS 8.0) and 20 April 2013 (Lushan, MS 7.0) along the Longmenshan fault. We found that after the mainshocks, the size distribution of the subsequent earthquakes shifted toward relatively larger events in the Wenchuan aftershock zone (b value decreased from 1.21 to 0.84), and generally remained invariable in the Lushan aftershock zone (b value remained at 0.76). The time required for the b value to return to stable states after both mainshocks was entirely consistent with the time needed by the aftershock depth images to stop visibly changing. The result of the temporal variation of b values shows decreasing trends for the b value before both large earthquakes. Our results are available for assessing the potential seismic risk of the Longmenshan fault as a reference.

The Reliance of the Earthquake B-Value on Depth and Focal Mechanism

The earthquake size distribution (b-value) is a significant factor to recognize the seismic activity, seismotectonic, and seismic hazard assessment. In the current work, the connection of the b-constant value with the focal depth and mechanism was studied. The effect of the study scale (global, regional and local) on the dependence of b-value on the focal mechanisms was investigated. The database is quoted from the Global Centroid Moment Tensor catalog. The selected earthquakes are the shallow normal, reverse and strike-slip events. The completeness magnitude (Mc) is 5.3. The maximum likelihood method is utilized to compute the b-value. The obtained results show that the b-value is decreasing with depth to range 10-20 km, then increases to the depth of 40km. The turning point of b-value (increasing of b-value) locates at the depth of the transition brittle-ductile zone. Globally and regionally, low, moderate, and high b-values are associated with reverse, strike-slip, and normal focal mechanisms, respectively, while locally, the relation between b-values and focal mechanisms shows different association trends, such as low, moderate, and high b-values are associated with normal, strike-slip, and reverse focal mechanisms and so on.

The relationship between heat flow and seismicity in global tectonically active zones

AbstractThis study aims to analyze the complex relationship between heat flow and seismicity in tectonically active zones worldwide. The problem was quantitatively analyzed by using a geographic detector method, which is well suited for analyzing nonlinear relationships in geography. Moreover,β-value that describes the frequency-magnitude distribution is used to represent the seismicity. The results showed that heat flow (HF) = 84 mW/m2is a critical point for the relevant mechanisms of heat flow with seismicity in these zones. When HF < 84 mW/m2, the heat flow correlates negatively with theβ-value, with a correlation degree of 0.394. Within this interval, buoyant is a primary control on the stress state and earthquake size distribution. Large earthquakes occur more frequently in subduction zones with younger slabs that are more buoyant. Due to zones with a high ratio of large earthquake corresponds to lowβ-values, high heat flow values correspond to lowβ-values. When HF > 84 mW/m2, the heat flow correlates positively with theβ-value, with a correlation degree of 0.463. Within this interval, the increased heat flow decreases the viscosity of the rock plate and then reduces the stress. Lower stress would correspond to a smaller earthquake and then a higherβ-value. Therefore, high heat flow values correspond to highβ-values. This research would be conducive to understand the geologic activity and be helpful to determine the accuracy and timeliness of seismic hazard assessment.

The Impact of Large Erosional Events and Transient Normal Stress Changes on the Seismicity of Faults

AbstractThe long‐term erosion of steep landscapes is punctuated by dramatic erosional events that can remove significant amount of sediments within a timescale shorter than a seismic cycle. However, the role of such large erosional events on seismicity is poorly understood. We use QDYN, a quasi‐dynamic numerical model of earthquake cycles to investigate the effect of a large erosional event on seismicity. The progressive evacuation of landslide sediments is modeled by a transient normal stress decrease. We show that erosional events with a shorter duration compared with the duration of a seismic cycle can significantly increase the seismicity rate, even for small stress changes. Moreover, large erosional events with a shorter period compared with the earthquake nucleation timescale can change earthquake size distribution by triggering more small events. Those results suggest that large erosional events can significantly affect seismicity, illustrating in turn the short‐term impact of surface processes on tectonics.

Condition of Occurrence of Large Man-Made Earthquakes in the Zone of Oil Production, Oklahoma

Man-made seismicity is a response of the brittle crust to fluid injection at depth and to the subsequent increase in pore-pressure and stress field perturbations. In Oklahoma, where the sharp increase in earthquake rate correlates with injection operations, we show that the earthquake-size distribution can differ significantly on the volume of injected fluid. The size distribution of M 2) may be documented for larger magnitude ranges. This change shows statistically significant positive dependence on injection activity. In addition, largest events occur at the border of the injection area at some distance from massive injection, and in the periods of steady injection rate. These observations suggest that a deficit of large induced earthquakes under conditions of high injection rate can be accompanied by an overall increase of natural seismicity along pre-existing faults in the surrounding volume, where large events are more likely to be triggered over longer space-time scales.

GR_EST: An OCTAVE/MATLAB Toolbox to Estimate Gutenberg–Richter Law Parameters and Their Uncertainties

AbstractThe estimation of the earthquake size distribution parameters is one of the most important parts in any seismic hazard study. GR_EST toolbox is a source code written for OCTAVE/MATLAB (Eaton et al., 2019; MATLAB, 2019) that allows estimating these parameters in a proper way, including the estimation of the associated uncertainties. The toolbox contains functions to make the parameter estimation both for instrumental and historical seismic catalogs, also considering time-varying completeness for magnitudes. Different functional forms for the magnitude–frequency distribution and different strategies for the estimation of its parameters and relative uncertainty are included. To guide the seismologists into the use of this toolbox, a set of complete examples is provided, to be used as “how to” use cases.

Pseudoprospective Evaluation of the Foreshock Traffic-Light System in Ridgecrest and Implications for Aftershock Hazard Assessment

AbstractThe Mw 7.1 Ridgecrest earthquake sequence in California in July 2019 offered an opportunity to evaluate in near-real time the temporal and spatial variations in the average earthquake size distribution (the b-value) and the performance of the newly introduced foreshock traffic-light system. In normally decaying aftershock sequences, in the past studies, the b-value of the aftershocks was found, on average, to be 10%–30% higher than the background b-value. A drop of 10% or more in “aftershock” b-values was postulated to indicate that the region is still highly stressed and that a subsequent larger event is likely. In this Ridgecrest case study, after analyzing the magnitude of completeness of the sequences, we find that the quality of the monitoring network is excellent, which allows us to determine reliable b-values over a large range of magnitudes within hours of the two mainshocks. We then find that in the hours after the first Mw 6.4 Ridgecrest event, the b-value drops by 23% on average, compared to the background value, triggering a red foreshock traffic light. Spatially mapping the changes in b values, we identify an area to the north of the rupture plane as the most likely location of a subsequent event. After the second, magnitude 7.1 mainshock, which did occur in that location as anticipated, the b-value increased by 26% over the background value, triggering a green traffic light. Finally, comparing the 2019 sequence with the Mw 5.8 sequence in 1995, in which no mainshock followed, we find a b-value increase of 29% after the mainshock. Our results suggest that the real-time monitoring of b-values is feasible in California and may add important information for aftershock hazard assessment.