Gutenberg-Richter Research Articles

Breakdown of the Gutenberg‐Richter relation for microearthquakes induced by hydraulic fracturing: influence of stratabound fractures

ABSTRACTHydraulic fracturing, a powerful completion technique used to enhance oil or gas production from impermeable strata, may trigger unintended earthquake activity. The primary basis for assessment of triggered and natural seismic hazard is the classic Gutenberg‐Richter (G‐R) relation, which expresses scale‐independent behaviour of earthquake magnitudes. Here we use a stochastic approach to simulate and test magnitude‐distance trends expressed by microseismic catalogues derived from three hydraulic fracture monitoring programmes in North America. We show that a widely observed rapid fall‐off in large‐magnitude events, almost universally quantified using the G‐R b value, may in our case be an artefact of the strongly laminated character of the stimulated oil and gas reservoirs. We also show that, for the three reservoirs considered, mechanical bed thickness can be approximated by a lognormal distribution. For a stratabound fracture network, this leads asymptotically to a Gaussian decay for induced magnitudes. We show that the stratabound model provides a more significant correspondence with our observations. If applicable in general, this result has important implications for determining the energy balance of hydraulic fracture systems (i.e. radiated seismic energy versus injected energy) as well as hazard assessments based on the probability of occurrence of anomalous seismic events.

Power law statistics of force and acoustic emission from a slowly penetrated granular bed

Abstract. Penetration-resistant force and acoustic emission (AE) from a plunged granular bed are experimentally investigated through their power law distribution forms. An AE sensor is buried in a glass bead bed. Then, the bed is slowly penetrated by a solid sphere. During the penetration, the resistant force exerted on the sphere and the AE signal are measured. The resistant force shows power law relation to the penetration depth. The power law exponent is independent of the penetration speed, while it seems to depend on the container's size. For the AE signal, we find that the size distribution of AE events obeys power laws. The power law exponent depends on grain size. Using the energy scaling, the experimentally observed power law exponents are discussed and compared to the Gutenberg–Richter (GR) law.

Impact Analysis of Seismic Source Area Extent on Hazard Estimate for Chennai City

The present work is intended to study the effect of variation of seismic area source zone on hazard value for Chennai, India. In the conventional Cornell-McGuire approach to probabilistic seismic hazard analysis, the seismic activity rate for an area source is most often determined using the Gutenberg-Richter (G-R) recurrence law. The seismic area is delineated using geology, geography and/or seismotectonic characteristics of the region. However if the area lies in a distributed seismicity region, the delineation into different area zones introduces subjectivity. The impact of the areal extent of seismic area zone on the final hazard value is studied using two approaches - the conventional Cornell-McGuire and zone-free approaches. It is observed that in the conventional Cornell-McGuire approach, the peak ground acceleration decreases when the area source zone is increased while maintaining all other seismicity parameters same. In the kernel method, the PGA remained unchanged though the seismic source area is increased. This is due to the fact that in the kernel method, the seismic activity rate is a spatially varying parameter unlike the seismic activity rate determined from the G-R recurrence law, which assumes a homogeneous distribution of the activity rate for each of the area source zones. The uniform hazard spectra are obtained for various return periods by both the approaches. It is observed that in the Cornell-McGuire approach, the variation in the extent of seismic source area has little effect on longer return periods as compared to the smaller return periods.

혼합분포함수를 이용한 동해 지진자료의 확률론적 빈도해석

최근 국내에서는 지진의 발생빈도가 과거와 비교하여 점차적으로 증가하고 있으며, 주변국가인 일본과 중국에서도 대규모의 지진으로 인한 피해가 빈번하게 발생하면서 지진에 대한 위험도가 증가하고 있다. 본 연구에서는 지진으로 발생되는 피해에 대비하기 위하여 동해 연안의 지진자료에 확률론적 빈도해석을 적용하여 지진의 발생확률 및 발생빈도를 정량적으로 산정하였다. 연도별 최대값을 활용하는 수문빈도해석과는 다르게 연도별 지진의 발생유무를 반영하기 위하여 혼합분포함수를 활용하였으며, 분포모형으로는 Gumbel, Lognormal, Gamma, Weibull 분포를 적용하였다. 적용결과, 국내의 지진-규모식보다 혼합분포모형이 재현기간에 대하여 지진규모를 작게 산정되는 것을 확인할 수 있었으며, 분포모형 중 500년 이하의 재현기간에 대해서는 혼합 Gumbel, Weibull 분포모형이, 500년 이상의 재현기간에서는 혼합 Lognormal, Gamma 분포모형이 지진-규모식과 가장 근접한 결과를 도출하였다. 본 연구의 비교 분석을 위하여 사용된 규모-빈도식에서는 <TEX>$M_w$</TEX> 8.0 이상의 관측자료가 존재하지 않아 그 이상 규모의 지진에 대한 발생빈도의 정확도가 낮으며, 지진빈도해석 역시 31개의 적은 수의 지진 관측자료를 이용하여 1,000년 빈도의 지진규모를 추정하였으므로, 신뢰성이 낮은 한계가 존재한다. 하지만 본 연구는 국내의 관측자료를 활용하여 확률론적 지진빈도해석을 수행함으로써, 새로운 접근방법으로 지진의 발생빈도를 추정하였다는데 의의가 있다. In Korea, the occurrence frequency of earthquakes has recently increased compared to the past. In neighboring countries such as Japan and China, damage from major earthquakes has also increased leading to high earthquake risk. This study quantitatively estimated the occurrence probability and frequency of huge earthquake using statistical frequency analysis on instrumental earthquake data at East sea in order to cope with earthquake damage. After adapting Various distribution functions such as Gumbel, Lognormal, Gamma, and Weibull for earthquake data, we applied mixed distributions to reflect the characteristics of the occurrence of earthquakes. The results show that the mixed models estimated earthquake magnitude larger than the Gutenberg-Richter (G-R) equation, and the mixed Gumbel distribution estimated most closed values with the G-R equation. It was found that the G-R equation is very limited to estimate a huge earthquake since there is no observed data over magnitude 8.0 and the frequency analysis is also limited to be applied in practice because the lack of data induces high uncertainty on the estimates of 1,000 year-earthquake. However, this study has an experiential significance in that the statistical frequency analysis was applied to instrumental earthquake data at East sea as an alternative.

Analysis and Visualization of Seismic Data Using Mutual Information

Seismic data is difficult to analyze and classical mathematical tools reveal strong limitations in exposing hidden relationships between earthquakes. In this paper, we study earthquake phenomena in the perspective of complex systems. Global seismic data, covering the period from 1962 up to 2011 is analyzed. The events, characterized by their magnitude, geographic location and time of occurrence, are divided into groups, either according to the Flinn-Engdahl (F-E) seismic regions of Earth or using a rectangular grid based in latitude and longitude coordinates. Two methods of analysis are considered and compared in this study. In a first method, the distributions of magnitudes are approximated by Gutenberg-Richter (G-R) distributions and the parameters used to reveal the relationships among regions. In the second method, the mutual information is calculated and adopted as a measure of similarity between regions. In both cases, using clustering analysis, visualization maps are generated, providing an intuitive and useful representation of the complex relationships that are present among seismic data. Such relationships might not be perceived on classical geographic maps. Therefore, the generated charts are a valid alternative to other visualization tools, for understanding the global behavior of earthquakes.

Convergence of the frequency‐size distribution of global earthquakes

The Gutenberg‐Richter (GR) frequency‐magnitude relation is a fundamental empirical law of seismology, but its form remains uncertain for rare extreme events. Here, we show that the temporal evolution of model likelihoods and parameters for the frequency‐magnitude distribution of the global Harvard Centroid Moment Tensor catalog is inconsistent with an unbounded GR relation, despite if being the preferred model at the current time. During the recent spate of 12 great earthquakes in the last 8 years, record‐breaking events result in profound steps in favor of the unbounded GR relation. However, between such events the preferred model gradually converges to the tapered GR relation, and the form of the convergence cannot be explained by random sampling of an unbounded GR distribution. The convergence properties are consistent with a global catalog composed of superposed randomly‐sampled regional catalogs, each with different upper bounds, many of which have not yet sampled their largest event.

Probabilistic seismic hazard analysis of Tripura and Mizoram states

A probabilistic seismic hazard analysis for the states of Tripura and Mizoram in North East India is presented in this paper to evaluate the ground motion at bedrock level. Analyses were performed considering the available earthquake catalogs collected from different sources since 1731-2010 within a distance of 500 km from the political boundaries of the states. Earthquake data were declustered to remove the foreshocks and aftershocks in time and space window and then statistical analysis was carried out for data completeness. Based on seismicity, tectonic features and fault rupture mechanism, this region was divided into six major seismogenic zones and subsequently seismicity parameters (a and b) were calculated using Gutenberg-Richter (G-R) relationship. Faults data were extracted from SEISAT (Seismotectonic atlas of India, Geological Survey of India, New Delhi, 2000) published by Geological Survey of India and also from satellite images. The study area was divided into small grids of size 0.05A degrees A xA 0.05A degrees (approximately 5 km x 5 km), and the hazard parameters (rock level peak horizontal acceleration and spectral accelerations) were calculated at the center of each of these grid cells considering all the seismic sources within a radius of 500 km. Probabilistic seismic hazard analyses were carried out for Tripura and Mizoram states using the predictive ground motion equations given by Atkinson and Boore (Bull Seismol Soc Am 93:1703-1729, 2003) and Gupta (Soil Dyn Earthq Eng 30:368-377, 2010) for subduction belt. Attenuation relations were validated with the observed PGA values. Results are presented in the form of hazard curve, peak ground acceleration (PGA) and uniform hazard spectra for Agartala and Aizawl city (respective capital cities of Tripura and Mizoram states). Spatial variation of PGA at bedrock level with 2 and 10 % probability of exceedance in 50 years has been presented in the paper.

Time-dependent earthquake probability calculations for southern Kanto after the 2011 M9.0 Tohoku earthquake

S U M M A R Y Seismicity in southern Kanto activated with the 2011 March 11 Tohoku earthquake of magnitude M9.0, but does this cause a significant difference in the probability of more earthquakes at the present or in the future? To answer this question, we examine the effect of a change in the seismicity rate on the probability of earthquakes. Our data set is from the JapanMeteorological Agency earthquake catalogue, downloaded on 2012 May 30. Our approach is based on timedependent earthquake probabilistic calculations, often used for aftershock hazard assessment, and are based on two statistical laws: the Gutenberg–Richter (GR) frequency–magnitude law and the Omori–Utsu (OU) aftershock-decay law. We first confirm that the seismicity following a quake of M4 or larger is well modelled by the GR law with b ∼ 1. Then, there is good agreement with the OU law with p ∼ 0.5, which indicates that the slow decay was notably significant. Based on these results, we then calculate the most probable estimates of future M6–7-class events for various periods, all with a starting date of 2012 May 30. The estimates are higher than pre-quake levels if we consider a period of 3-yr duration or shorter. However, for statistics-based forecasting such as this, errors that arise from parameter estimation must be considered. Taking into account the contribution of these errors to the probability calculations, we conclude that any increase in the probability of earthquakes is insignificant. Although we try to avoid overstating the change in probability, our observations combined with results from previous studies support the likelihood that afterslip (fault creep) in southern Kanto will slowly relax a stress step caused by the Tohoku earthquake. This afterslip in turn reminds us of the potential for stress redistribution to the surrounding regions. We note the importance of varying hazards not only in time but also in space to improve the probabilistic seismic hazard assessment for southern Kanto.

Converting Magnitudes Based on the Temporal Stability of a- and b-Values in the Gutenberg-Richter Law

The problem of converting from one magnitude scale to another is revisited in light of a method that preserves the b ‐value of the Gutenberg–Richter (GR) law of one of the two magnitude distributions. This methodology allows for the use of all reliable magnitude data rather than data of only those events for which paired magnitudes are available. Our method avoids resolving incompatible magnitude ranges. The temporal stability of a ‐ and b ‐values and of the resulting linear relation coefficients is considered as a basis for selecting an optimum set of parameters and time intervals for the regression. This analysis also provides additional insight into the basic reporting characteristics through time for a particular network. The conversion of body‐wave magnitude ( m b) to surface‐wave magnitude ( M s) is considered, in particular, since that is the case often faced in many geographical areas that deal with limited data for M s. M s to M w conversion is then straightforward with the exception of very large events. Three regional data sets, drawn from the same source (International Seismological Centre [ISC] catalog) in order to avoid bias due to magnitude estimate procedures, are analyzed separately as test cases in order to show the advantages and possible limitations of the method. The results provide time intervals best suited for further analysis on the basis of seismic record integrity, in addition to offering further insight about the variability of the b ‐value as a function of data recording.

Temporal and spatial variations in the magnitude of completeness for homogenized moment magnitude catalogue for northeast India

Northeast India region is one of the most seismically active areas in the world. Events data for the period 1897–2010, used in this study has been largely compiled from global ISC, NEIC and GCMT databases. Historical seismicity catalogue of Gupta et al (1986) and some events data from the bulletins of India Meteorological Department are also used. Orthogonal regression relations for conversion of body and surface wave magnitudes to M w,HRVD based on events data for the period 1978–2006 have been derived. An Orthogonal Standard Regression (OSR) relationship has also been obtained for scaling of intensity estimates to M w,NEIC using 126 global intensity events with intensity VI or greater during the period 1975–2010. Magnitude of completeness and Gutenberg–Richter (GR) recurrence parameter values have been determined for the declustered homogenized catalogue pertaining to four different time periods namely, 1897–1963, 1964–1990, 1964–2000 and 1964–2010. The M c and ‘b’ values are observed to decrease and increase, respectively, with addition of newer data with time. The study region has been subdivided into nine seismogenic zones keeping in view the spatial variations in earthquake occurrence and prevalent tectonics. M c, ‘b’ and ‘a’ values have been estimated with respect to each zone, and the variations in the values of these parameters have been analysed.

A fault and seismicity based composite simulation in northern California

Abstract. We generate synthetic catalogs of seismicity in northern California using a composite simulation. The basis of the simulation is the fault based "Virtual California" (VC) earthquake simulator. Back-slip velocities and mean recurrence intervals are specified on model strike-slip faults. A catalog of characteristic earthquakes is generated for a period of 100 000 yr. These earthquakes are predominantly in the range M = 6 to M = 8, but do not follow Gutenberg-Richter (GR) scaling at lower magnitudes. In order to model seismicity on unmapped faults we introduce background seismicity which occurs randomly in time with GR scaling and is spatially associated with the VC model faults. These earthquakes fill in the GR scaling down to M = 4 (the smallest earthquakes modeled). The rate of background seismicity is constrained by the observed rate of occurrence of M &gt; 4 earthquakes in northern California. These earthquakes are then used to drive the BASS (branching aftershock sequence) model of aftershock occurrence. The BASS model is the self-similar limit of the ETAS (epidemic type aftershock sequence) model. Families of aftershocks are generated following each Virtual California and background main shock. In the simulations the rate of occurrence of aftershocks is essentially equal to the rate of occurrence of main shocks in the magnitude range 4 &lt; M &lt; 7. We generate frequency-magnitude and recurrence interval statistics both regionally and fault specific. We compare our modeled rates of seismicity and spatial variability with observations.

Statistical Analysis of the 2002 Mw 7.9 Denali Earthquake Aftershock Sequence

We study the scaling properties of the aftershock sequence of the 2002 M w 7.9 Denali earthquake. The sequence exhibits nontrivial scaling behavior in magnitude, aftershock decay rates, and aftershock inter-occurrence times. In particular we observe a marked variability in the Gutenberg–Richter (GR) exponent, the b -value, in space, between early and late times after the mainshock and over different magnitude ranges. We find strong indications that certain aspects of this variability might be ascribed to the occurrence of some of the aftershocks on and around a supershear rupture associated with this event. Particularly, we observe a statistically higher b -value for the aftershocks that occurred in the zone surrounding the supershear rupture. We further outline a method to obtain an average b -value, consistent with the theory and statistical properties of the data. We find a significant difference between the magnitude of the largest aftershock and the mainshock. This is analyzed and discussed using the modified Bath’s law. It is observed that the aftershock decay rate can be approximated by the modified Omori law. The distribution of inter-occurrence times is studied and we show that it can be explained in terms of a nonhomogeneous Poisson process through time. We further observe that the rescaled inter-occurrence time distributions for various magnitude thresholds collapse into a universal functional form which indicates that the aftershock sequence exhibits self-similarity in both magnitude and time. In particular, the statistical features of the aftershock sequence show that the rupture process or the preconditions that lead to it may influence aftershock statistics.

AE Monitoring and Numerical Simulation of a Two‐span Model Masonry Arch Bridge Subjected to Pier Scour

Abstract: A scaled model of a two‐span masonry arch bridge has been built to investigate the effect of the central pile settlement because of riverbank erosion. The bridge geometry and the structural details, included the masonry bricks and mortar joints, are realized in the scale 1 : 2. The model bridge has been equipped with different Non‐Destructive Testing (NDT) instruments, including accelerometers, displacement transducers LVTD, strain gages, optic fibres strain sensors and Acoustic Emissions (AE) transducers. The model bridge has been subjected to incremental settlement of the pile, which was sustained on a mobile support. The exact mechanism and temporal evolution of the pier scour has been investigated numerically and experimentally by mean of a hydraulic model. In this article, a detailed description of AE data and damage localization is provided. During the first stage of the settlement, the AE counting has been recorded. Based on the interpretation of the AE rate, it is possible to monitor the criticality of the ongoing process. In addition, thanks to the AE equipment, it has been possible to localize the main damaged zones. The statistical properties AE time series have been analysed using an estimation of the b‐value of the Gutenberg‐Richter (GR) law permitting to determine the damage level reached in the model. The damage evolution has been also interpreted numerically with the aid of a finite element programme able to predict the nucleation and propagation of fracture. In this way, some criteria for the monitoring and interpretation of full‐scale structure cracking are provided for the assessment in presence of riverbank erosion phenomena.

Acoustic Emissions at High and Low Frequencies During Compression Tests in Brittle Materials

Abstract: The damage process in a concrete specimen subjected to uniaxial compression test is investigated by detection of the propagating elastic waves because of micro‐ and macrocrack growth. Besides the high‐frequency acoustic emissions (AEs), the presence of low‐frequency elastic emissions (ELEs), from 1 to 10 kHz, is detected just before the specimen failure. A spectral analysis of the ELEs is performed by measuring with a calibrated transducer the local acceleration of the specimen surface. Quantitative information about the macrocrack effects in terms of released energy is thus obtained. Furthermore, the evolution of damage is followed through the analysis of the amplitude distribution of AE and ELE signals, distributed according to the Gutenberg–Richter (GR) statistics.

Statistical Variability and Tokunaga Branching of Aftershock Sequences Utilizing BASS Model Simulations

Aftershock statistics provide a wealth of data that can be used to better understand earthquake physics. Aftershocks satisfy scale-invariant Gutenberg–Richter (GR) frequency–magnitude statistics. They also satisfy Omori’s law for power-law seismicity rate decay and Bath’s law for maximum-magnitude scaling. The branching aftershock sequence (BASS) model, which is the scale-invariant limit of the epidemic-type aftershock sequence model (ETAS), uses these scaling laws to generate synthetic aftershock sequences. One objective of this paper is to show that the branching process in these models satisfies Tokunaga branching statistics. Tokunaga branching statistics were originally developed for drainage networks and have been subsequently shown to be valid in many other applications associated with complex phenomena. Specifically, these are characteristic of a universality class in statistical physics associated with diffusion-limited aggregation. We first present a deterministic version of the BASS model and show that it satisfies the Tokunaga side-branching statistics. We then show that a fully stochastic BASS simulation gives similar results. We also study foreshock statistics using our BASS simulations. We show that the frequency–magnitude statistics in BASS simulations scale as the exponential of the magnitude difference between the mainshock and the foreshock, inverse GR scaling. We also show that the rate of foreshock occurrence in BASS simulations decays inversely with the time difference between foreshock and mainshock, an inverse Omori scaling. Both inverse scaling laws have been previously introduced empirically to explain observed foreshock statistics. Observations have demonstrated both of these scaling relations to be valid, consistent with our simulations. ETAS simulations, in general, do not generate Bath’s law and do not generate inverse GR scaling.

An event-driven model simulating fundamental seismic characteristics with the use of cellular automata

This paper presents an extensive simulation tool based on a Cellular Automata (CA) system that models fundamental seismic characteristics of a region. The CA-based dynamic model consists of cells-charges and it is used for the simulation of the earthquake process. The simulation tool has remarkably accelerated the response of the model by incorporating principles of the High Performance Computing (HPC). Extensive programming features of parallel computing have been applied, thus improving its processing effectiveness. The tool implements an enhanced (or hyper-) 2-dimensional version of the proposed CA model. Regional characteristics that depend on the seismic background of the area under study are assigned to the model with the application of a user-friendly software environment. The model is evaluated with real data that correspond to a circular region around Skyros Island, Greece, for different time periods, as for example one of 45years (1901–1945). The enhanced 2-dimensional version of the model incorporates all principal characteristics of the 2-dimensional one, also including groups of CA cells that interact with others, located to a considerable distance in an attempt to simulate long-range interaction. The advanced simulation tool has been thoroughly evaluated. Several measurements have been made for different critical states, as well as for various cascade (earthquake) sizes, cell activities and different neighbourhood sizes. Simulation results qualitatively approach the Gutenberg–Richter (GR) scaling law and reveal fundamental characteristics of the system.

Parametric optimisation in a 2-D cellular automata model of fundamental seismic attributes with the use of genetic algorithms

A two-dimensional (2-D) cellular automata (CA) dynamic system constituted of cells-charges has been proposed for the simulation of the earthquake process. In this paper, the study is focused on the optimal parameterisation of the model introducing the use of genetic algorithm (GA). The optimisation of the CA model parameterisation, by applying a standard GA, extends its ability to study various hypotheses concerning the seismicity of the region under consideration. The GA evolves an initially random population of candidate solutions of model parameters, such that in time appropriate solutions to emerge. The quality criterion is realised by taking into account the extent that the simulation results match the Gutenberg-Richter (GR) law derived from recorded data of the area under test. The simulation results presented here regard regions of Greece with different seismic and geophysical characteristics. The results found are in good quantitative and qualitative agreement with the GR scaling relations.

Characteristics of long-term regional seismicity before the 2008 Wen-Chuan, China, earthquake using pattern informatics and genetic algorithms

Abstract. To understand the generation of the 2008 Wen-chuan, China earthquake, we developed a strategy to objectively identify possible seismic precursors. Based on the pattern informatics (PI) method, the pattern of seismic anomaly was identified by the aid of genetic algorithms (GA) to be highly similar to the spatial distribution of the Wen-chuan earthquake sequence. We found that smaller earthquakes (M &lt; 4.4) showed a linear relationship of Gutenberg-Richter (G-R) distribution. However, the frequency of the intermediate earthquakes (M ≥ 4.4) showed an uplift. This uplift supports the seismic activation hypothesis developed by Rundle et al. (2000b) and is similar to the case of the 1999 Chi-chi, Taiwan earthquake sequence reported by Chen (2003).

Earthquake forecast models for inland Japan based on the G-R law and the modified G-R law

The frequency-magnitude distribution expressed by the Gutenberg-Richter (G-R) law is the basis of a simple method to forecast earthquakes. The frequency-magnitude distribution is sometimes approximated by the modified G-R law, which imposes a maximum magnitude. In this study we tested three earthquake forecast models: Cbv (the Constant b-value model) based on only the G-R law with a spatially constant b-value, Vbv (the Variable b-value model) based on only the G-R law with regionally variable b-values, and MGR (the Modified G-R model) based on the modified G-R or G-R law (chosen according to Akaike Information Criterion) with regionally variable b-values. We also incorporated aftershock decay and minimum limits of expected seismicity in these models. Comparing the results of retrospective forecasts by the three models, we found that MGR was almost always better than Vbv; Cbv was better than Vbv for short-term (one year) forecasts; little difference between MGR and Cbv for short-term forecasts; and MGR and Vbv tended to be better than Cbv for long-term (three years or longer) forecasts. We propose the use of MGR in the earthquake forecast testing experiment by the Collaboratory for the Study of Earthquake Predictability for Japan.

Frequency-Magnitude Characteristics Down to Magnitude -4.4 for Induced Seismicity Recorded at Mponeng Gold Mine, South Africa

This article aims to investigate the frequency-magnitude characteristics and lower magnitude limits of the microseismic catalog recorded with a seismic network sensitive to high frequencies at Mponeng mine, South Africa. The network, composed of one three-component accelerometer and eight acoustic emission sensors, is located at a depth of 3.5 km below the surface and covers the limited volume of approximately 300×300×300 m. The three-component accelerometer was used to estimate the moment magnitude for the limited number of 135 events ( M w ranged from -4.1 to -0.3) well recorded by the network. We use the relation between the moment magnitude estimated from accelerometer data and radiated energy/moment magnitude estimated from acoustic emission sensors to extend the catalog to lower magnitudes. The magnitude of completeness of selected spatiotemporal subsets of the catalog was estimated for: (1) an aftershock sequence of an M w 1.9 event that occurred approximately 30 m from the network, and (2) postblasting activity during working days, located more than 80 m from the network. The data follow the Gutenberg–Richter (GR) frequency-magnitude relationship with no visible deviation from self-similar behavior of seismicity between M w -4.4 and -1.9 for the aftershock sequence and between -3.5 and -1.5 for the postblasting dataset. We estimated the magnitude of completeness of selected subset as low as -4.3 ( b =1.26) for the aftershock sequence and -3.4 ( b =1.17) for the postblasting activity. Differences in magnitude of completeness are attributed to location of recorded activity and site effects.