Richter Relation Research Articles

Theoretical Basis for the Observed Break inML/MwScaling between Small and Large Earthquakes

Abstract For a set of earthquakes in a perfectly elastic medium and provided that stress drop and rupture velocity do not vary systematically as a function of magnitude and that the instrument response has been corrected for, M L ∝ M w over the entire range for which M L can be determined. In practice, however, we observe that M L and M w deviate from this 1:1 scaling relation. Detailed analysis of a natural earthquake sequence and of a case of induced seismicity show that, for small events (in general, for M w below 2–3), M L ∝1.5 M w , in agreement with the results of other studies. This behavior can be reproduced with synthetic earthquake source time functions convolved with a causal Q operator using independently determined Q ‐values. A key result of both the data analysis and the modeling exercise is that, for small events below a certain magnitude, observed pulse widths and equivalently corner frequencies remain practically constant. Thus, in an attenuating medium, the signals of small earthquakes constitute essentially the impulse response of the medium, scaled by the seismic moment. A simple theoretical demonstration shows that under these circumstances the observed proportionality of 1.5 between M L and M w is a necessary consequence of the intrinsic scaling properties of amplitude and duration of the moment‐rate function versus seismic moment, as well as of the frequency response of an attenuating medium. As a consequence of this and of the bias introduced by the response of the Wood–Anderson seismometer for the larger events, the Gutenberg–Richter relation based on M L loses its physical justification and, with respect to M w , leads to different b ‐values for small and large events. Electronic Supplement: Figures of hypocenter locations of the induced seismicity below Basel and of seismograms showing the degree of similarity among signals.

Shape of the Magnitude–Frequency Distribution for the Canterbury Earthquake Sequence from Integration of Seismological and Geological Data

Abstract We combine geological and seismological data to describe the shape of the magnitude–frequency distribution for the Canterbury earthquake sequence. In doing so, we take the opportunity of the new datasets to address a long‐standing debate in the literature as to whether the seismicity of individual faults or fault zones is best described by the Gutenberg–Richter (GR) relationship or the characteristic earthquake (CE) model. We find that the magnitude–frequency distribution for the entire composite fault zone of the Canterbury earthquake sequence is adequately described by the GR relationship when uncertainties in the GR curve (fitted to the instrumental catalog data) and the range of paleoseismically derived recurrence intervals for large (mainshock) earthquakes on the Greendale fault are collectively considered. In contrast, the magnitude–frequency distribution for the smaller area of the Greendale fault is better described by the CE model. The difference is one of scale, in that the composite fault zone represented by the Canterbury earthquake sequence shows a GR distribution, whereas individual faults within the zone show CE distributions. Definition of magnitude–frequency distributions for seismic‐hazard modeling must therefore take scaling considerations into account, rather than simply assuming that a single magnitude–frequency distribution shape is applicable to all fault definitions.

Alternative representation of the Gutenberg–Richter relation in terms of the logarithmic mean annual seismicity rate and its standard deviation

Gutenberg and Richter developed an empirical relation, \(\log_{10} N(M) = a - bM\), to quantify the seismicity rate of various magnitudes in a given region and time period. They found the equation fit observed data well both globally and for particular regions. In conventional G–R relation, N(M) represents an arithmetic mean. As a result, the arithmetic standard deviation cannot be explicitly incorporated in the log-linear G–R relation. Moreover, this representation is susceptible to influence of spuriously large numbers of aftershocks of major earthquake sequences. To overcome these shortcomings, we propose an alternative representation of the G–R relation in terms of the logarithmic mean annual seismicity rate and its standard deviation. We select the crustal earthquake data from 1973 to 2011, as listed in the National Earthquake Information Center (NEIC) global catalog and the Central Weather Bureau (CWB) Taiwan regional catalog, to illustrate our methodology. We first show that by using the logarithmic annual seismicity rates we can significantly suppress the influences of spuriously large numbers of aftershocks following major earthquake sequences contained in the Taiwan regional catalog. More significantly, both the logarithmic mean annual seismicity rate and its standard deviation can be explicitly represented in the Gutenberg–Richter relation as follows: $${\text{For}}\,{\text{global}}\,{\text{crustal}}\,{\text{seismicity}}{:}\;\log_{10} N = 8.14 - 1.03M \pm (0.04M - 0.13);$$ $${\text{For}}\,{\text{Taiwan}}\;{\text{crustal}}\,{\text{seismicity}}{:}\;\log_{10} N = 5.62 - 0.90M \pm (0.02M + 0.17)$$ where log10N represents the logarithmic annual seismicity rate. Above analytical equations are very well constrained by observed global seismicity data with \(5.0 \le M \le 7.0\) and by Taiwan seismicity data with \(3.0 \le M \le 5.0\). Both equations can be extrapolated with confidence to simultaneously estimate not only the median annual seismicity rates but also their uncertainties for large earthquakes for the first time since inception of the G–R relation. These equations can be used to improve the conventional probabilistic seismic hazard assessment by including the dispersion of the annual seismicity rate. Finally, the corresponding numerical median annual seismicity rate with its upper and lower bounds obtained from above equations for \(5.0 \le M \le 9.0\) is listed in Table 1. Table 1 Observed and estimated median annual seismicity rate and return period with their dispersions for Taiwan and global crustal earthquakes Magnitude Catalog Taiwan catalog (CWB) Taiwan catalog (CWB) Global catalog (NEIC) Global catalog (NEIC) Annual rate (event/year) Return period (year) Annual rate (event/year) Return period (year) M ≥ 5.0 24.55 13.18 7.08 0.041 0.076 0.14 1148.16 977.24 831.76 0.0009 0.001 0.0012 M ≥ 5.5 8.91 4.68 2.45 0.11 0.21 0.41 367.28 298.54 242.66 0.0027 0.0033 0.0041 M ≥ 6.0 3.24 1.66 0.85 0.31 0.60 1.18 117.49 91.20 70.79 0.0085 0.011 0.014 M ≥ 6.5 1.17 0.59 0.30 0.85 1.69 3.33 37.58 27.86 20.65 0.027 0.036 0.048 M ≥ 7.0 0.43 0.21 0.10 2.33 4.76 10.0 12.02 8.51 6.03 0.083 0.12 0.17 M ≥ 7.5 0.15 0.074 0.036 6.67 13.51 27.78 3.85 2.60 1.76 0.26 0.38 0.57 M ≥ 8.0 0.056 0.026 0.012 17.86 38.46 83.33 1.23 0.79 0.51 0.81 1.27 1.96 M ≥ 8.5 0.020 0.009 0.004 50.00 111.11 250.0 0.39 0.24 0.15 2.56 4.17 6.67 M ≥ 9.0 0.0074 0.0033 0.0015 135.14 303.03 666.67 0.13 0.074 0.04 7.69 13.51 25.00 Observed value is shown in bold number, estimated value in regular number \(\log_{10} N = 5.62 - 0.90M \pm (0.02M + 0.17)\) for Taiwan crustal earthquakes \(\log_{10} N = 8.14 - 1.03M \pm (0.04M - 0.13)\) for global crustal earthquakes

Seismogenesis and earthquake triggering during the 2010–2011 Rigan (Iran) earthquake sequence

This study assesses the aftershock activity of two earthquakes that occurred on December 20, 2010 with magnitude of MN 6.5 (Global CMT Mw 6.5) and January 27, 2011 with magnitude of MN 6.0 (Global CMT Mw 6.2) in the Rigan region of southeastern Iran. This study has been done by assessing the statistical properties of the aftershock sequences associated with each of these earthquakes, namely b-value of Gutenberg–Richter relation, partitioning of radiated seismic energy, p-value of modified Omori law and the DC-value associated with the fractal dimension. The b-values of b = 0.89 ± 0.08 and b = 0.88 ± 0.08 were calculated for first main shock and second main shock sequence respectively. This suggests that this region is characterized by large differential stress; the genesis of large aftershock activity in a short time interval gives power this. Further, 2.2% of the whole energy is related with the aftershocks activity for first main shock sequence while 97.8% is associated with main shock; for second sequence, 20% of the total energy is associated with the aftershocks activity while 80% is associated with main shock. The p-values of 1.1 ± 0.12 and 1.1 ± 0.1 were calculated for first and second main shocks sequence respectively, which imply fast decay rate of aftershocks and high surface heat flux. A value of the spatial fractal dimension (Dc) equal to 2.34 ± 0.03 and 2.54 ± 0.02 for first and second main shocks sequence respectively, which reveals random spatial distribution and source in a two-dimensional plane that is being filled-up by fractures. Moreover, we then use the models to calculate the Coulomb stress change to appraise coming seismic hazard by inspecting the static Coulomb stress field due to these two main shocks for the recognition of the conceivable regions of aftershocks activity. The first main shock increased stress by more than 0.866 bars at the hypocenter of the second main shock, thus promoting the failure. In addition, the cumulative coseismic Coulomb stress changes due to the reveals that most of the aftershocks happened in the region of increased Coulomb stress.

Statistical analysis of aftershock sequences related with two major Nepal earthquakes: April 25, 2015, MW 7.8, and May 12, 2015, MW 7.2

Present study describes the statistical properties of aftershock sequences related with two major Nepal earthquakes (April 25, 2015, MW 7.8, and May 12, 2015, MW 7.2) and their correlations with the tectonics of Nepal Himalaya. The established empirical scaling laws such as the Gutenberg–Richter (GR) relation, the modified Omori law, and the fractal dimension for both the aftershock sequences of Nepal earthquakes have been investigated to assess the spatio-temporal characteristics of these sequences. For this purpose, the homogenized earthquake catalog in moment magnitude, MW is compiled from International Seismological Center (ISC) and Global Centroid Moment Tensor (GCMT) databases during the period from April 25 to October 31, 2015. The magnitude of completeness, MC, a and b-values of Gutenberg–Richter relationship for the first aftershock sequence are found to be 3.0, 4.74, 0.75 (±0.03) respectively whereas the MC, a and b-values of the same relationship for the second aftershock sequence are calculated to be 3.3, 5.46, 0.90 (±0.04) respectively. The observed low b-values for both the sequences, as compared to the global mean of 1.0 indicate the presence of high differential stress accumulations within the fractured rock mass of Nepal Himalaya. The calculated p-values of 1.01 ± 0.05 and 0.95 ± 0.04 respectively for both the aftershock sequences also imply that the aftershock sequence of first main-shock exhibits relatively faster temporal decay pattern than the aftershock sequence of second main-shock. The fractal dimensions, DC values of 1.84 ± 0.05 and 1.91 ± 0.05 respectively for both the aftershock sequences of Nepal earthquakes also reveal the clustering pattern of earthquakes and signifies that the aftershocks are scattered all around the two dimensional space of fractured fault systems of the Nepal region. The low b-value and low DC observed in the temporal variations of b-value and DC before the investigated earthquake (MW 7.2) suggest the presence of high-stress concentrations in the thrusting regimes of the Nepal region before the failure of faults. Moreover, the decrease of b-value with the corresponding decrease of DC observed in their temporal variations can primarily act as an indicator for possible prediction of major earthquakes in the study region.

The Dependency of Probabilistic Tsunami Hazard Assessment on Magnitude Limits of Seismic Sources in the South China Sea and Adjoining Basins

The South China Sea (SCS) and its adjacent small basins including Sulu Sea and Celebes Sea are commonly identified as tsunami-prone region by its historical records on seismicity and tsunamis. However, quantification of tsunami hazard in the SCS region remained an intractable issue due to highly complex tectonic setting and multiple seismic sources within and surrounding this area. Probabilistic Tsunami Hazard Assessment (PTHA) is performed in the present study to evaluate tsunami hazard in the SCS region based on a brief review on seismological and tsunami records. 5 regional and local potential tsunami sources are tentatively identified, and earthquake catalogs are generated using Monte Carlo simulation following the Tapered Gutenberg–Richter relationship for each zone. Considering a lack of consensus on magnitude upper bound on each seismic source, as well as its critical role in PTHA, the major concern of the present study is to define the upper and lower limits of tsunami hazard in the SCS region comprehensively by adopting different corner magnitudes that could be derived by multiple principles and approaches, including TGR regression of historical catalog, fault-length scaling, tectonic and seismic moment balance, and repetition of historical largest event. The results show that tsunami hazard in the SCS and adjoining basins is subject to large variations when adopting different corner magnitudes, with the upper bounds 2–6 times of the lower. The probabilistic tsunami hazard maps for specified return periods reveal much higher threat from Cotabato Trench and Sulawesi Trench in the Celebes Sea, whereas tsunami hazard received by the coasts of the SCS and Sulu Sea is relatively moderate, yet non-negligible. By combining empirical method with numerical study of historical tsunami events, the present PTHA results are tentatively validated. The correspondence lends confidence to our study. Considering the proximity of major sources to population-laden cities around the SCS region, the tsunami hazard and risk should be further highlighted in the future.

Energetic and spatial characterization of seismicity in the Algeria–Morocco region

We estimate the energetic and spatial characteristics of seismicity in the Algeria–Morocco region using a variety of seismic and statistical parameters, as a first step in a detailed investigation of regional seismic hazard. We divide the region into five seismotectonic regions, comprising the most important tectonic domains in the studied area: the Moroccan Meseta, the Rif, the Tell, the High Plateau, and the Atlas. Characteristic seismic hazard parameters, including the Gutenberg–Richter b-value, mean seismic activity rate, and maximum possible earthquake magnitude, were computed using an extension of the Aki–Utsu procedure for incomplete earthquake catalogs for each domain, based on recent earthquake catalogs compiled for northern Morocco and northern Algeria. Gutenberg–Richter b-values for each zone were initially estimated using the approach of Weichert (Bull Seismol Soc Am 70:1337–1346, 1980): the estimated b-values are 1.04 ± 0.04, 0.93 ± 0.10, 0.72 ± 0.03, 0.87 ± 0.02, and 0.77 ± 0.02 for the Atlas, Meseta, High Plateau, Rif, and Tell seismogenic zones, respectively. The fractal dimension D 2 was also estimated for each zone. From the ratio D 2/b, it appears that the Tell and Rif zones, with ratios of 2.09 and 2.12, respectively, have the highest potential earthquake hazard in the region. The Gutenberg–Richter relationship analysis allows us to derive that in the Tell and Rif, the number of earthquake with magnitude above Mw 4.0, since 1925 normalized to decade and to square cell with 100-km sides is equal to 2.6 and 1.91, respectively. This study provides the first detailed information about the potential seismicity of these large domains, including maximum regional magnitudes, characteristics of spatial clustering, and distribution of seismic energy release.

Estimation of Earthquake Hazard Parameters from Incomplete Data Files. Part III. Incorporation of Uncertainty of Earthquake‐Occurrence Model

Most probabilistic seismic‐hazard analysis procedures require that at least three seismic source parameters be known, namely the mean seismic activity rate λ , the Gutenberg–Richter b ‐value, and the area‐characteristic (seismogenic source) maximum possible earthquake magnitude m max. In almost all currently used seismic‐hazard assessment procedures that utilize these three parameters, it is explicitly assumed that all three remain constant over time and space. However, closer examination of most earthquake catalogs has indicated that significant spatial and temporal variations existed in the seismic activity rate λ , as well as in the Gutenberg–Richter b ‐value. In this study, the maximum likelihood estimation of these earthquake hazard parameters considers the incompleteness of the catalogs, the uncertainty in the earthquake magnitude determination, as well as the uncertainty associated with the applied earthquake‐occurrence models. The uncertainty in the earthquake‐occurrence models is introduced by assuming that both the mean seismic activity rate λ and the Gutenberg–Richter b ‐value are random variables, each described by the gamma distribution. This approach results in the extension of the classic frequency–magnitude Gutenberg–Richter relation and the Poisson distribution of the number of earthquakes with their compounded counterparts (Benjamin, 1968; Campbell, 1982, 1983). The proposed procedure was applied in the estimation of the seismicity parameters in an area that had experienced the strongest and most devastating earthquake in contemporary South African history, namely the 29 September 1969 M w 6.3 Ceres–Tulbagh event. In this example, it was shown that the introduction of uncertainty in the earthquake‐occurrence model reduced the mean return periods, leading to an increase of the estimated seismic hazard. Additionally, this study confirmed that accounting for magnitude uncertainties had the opposite effect, that is, it brought about increases in the return periods, or, equivalently, a reduction of the estimated seismic hazard.

Loss estimation of steel buildings to earthquake mainshock–aftershock sequences

Following a large earthquake numerous aftershocks can be triggered due to the complex stress interaction between and within tectonic plates. Although aftershocks are normally smaller in magnitude, their ground motion intensity can be large and have different energy content than the mainshock. Even seemingly undamaged buildings may be damaged as a result of aftershocks. The mainshock-damaged buildings with deteriorated structural properties are more susceptible to damage.This paper proposes a framework for loss estimation of steel structures subjected to mainshock–aftershock sequences. The analysis is based on a typical 4-story steel frame with a deterioration model. Mainshocks are modeled as a homogeneous Poisson process, while aftershocks are simulated from non-homogeneous Poisson process, magnitudes of which are characterized by the Gutenberg–Richter relationship. The proposed framework is applied to examine the effects of aftershocks on seismic loss. The expected seismic loss of the building subjected to two levels of earthquakes, the Design Earthquake (DE) and the Maximum Considered Earthquake (MCE), followed by aftershocks are examined considering both transition cost and downtime cost. Monte Carlo Simulation (MCS) with Latin Hypercube Sampling (LHS) is applied to examine the uncertainty in the loss estimation. Uncertainty in earthquake ground motions, structural model, damage and loss are all considered. It was found that even if aftershocks have little effect on structural response, they may still have a significant impact on seismic loss due to the uncertainty of the damage state and cost estimation. This methodology can be used to mitigate seismic loss and evaluate the current building design.

Nepal earthquake 25 April 2015: source parameters, precursory pattern and hazard assessment

ABSTRACTSpatio-temporal variations of seismicity within 300 km of the main Nepal earthquake of 25 April 2015 showed seismic quiescence since 2007. Decadal changes in b-value using the Gutenberg–Richter relation showed a well-marked decrease during the period January 2005–April 2015 preceding the main earthquake. Stress drop of this earthquake in the inter plate region was found to be 3.4 MPa which is much lower than the intra plate Bhuj earthquake, 2001. The un-ruptured portion of the seismic gap in western Nepal lies between longitude 82.5°E and 84.5°E, whose 200 km length (if assumed to rupture entirely in one earthquake) coupled with locked zone of about 100 km from GPS data, may generate an earthquake of magnitude about 8 although no historical data for a major earthquake is as yet available.

Site specific design response spectrum proposed for the capital city of Agartala, Tripura

ABSTRACTThe design response spectrum is smooth in shape compared to other response spectra. The objective of design spectra is to estimate the possible earthquake lateral loads which may experience on a particular structure during its design life. In this study, an attempt has been made to develop, design response spectra for the Agartala city, Tripura which is one of the North Eastern states in India considered at the highest level of seismic activity in the country having a zone factor 0.36g as per Indian seismic code (BIS 1893-2002). Based on the present data set collected from 1731-2011, the region is characterized and the seismicity parameters estimated separately for each source zone by Gutenberg and Richter (G-R) relationship. The two ground motion models were used for the hazard prediction. However, hazard curve, and uniform hazard response spectra (UHRS) (2% and 10% probability level for 50 years) has been developed for the Agartala at seismic bedrock level condition. Further, the NEHRP (National Earthquake Hazards Reduction Program) site classes D and E-types have been identified based on the average shear wave velocity at an upper 30 m depth of the subsurface soil profile. The direct shear wave velocity profiles were obtained through multi channel analysis of surface wave tests conducted at 27 locations at Agartala city. The design response spectra have been developed at the surface level for both the site classes. The results could be used as direct inputs for earthquake resistant design of civil engineering infrastructures of the study area.

Frequency distributions of magnetic storms and SI+SSC-derived records at Kakioka, Memambetsu, and Kanoya

The Japan Meteorological Agency keeps records of geomagnetic phenomena observed at Kakioka (magnetic latitude, 27.47°), Memambetsu (magnetic latitude, 35.44°), and Kanoya (magnetic latitude, 22.00°). We used these records to examine the cumulative frequency distribution of magnetic storms, sudden impulses, and storm sudden commencements. The distributions of magnetic storms resemble the Gutenberg–Richter relation between earthquake frequency and magnitude used in seismology. The coefficients determined with the maximum likelihood method show that when the H-range of a magnetic storm at Kakioka is doubled, the frequency of the magnetic storm is about one seventh, for example. Intense magnetic storms occur less frequently than calculated by the functions. This statistical analysis proves that there are no significant differences between slopes of the frequency distribution functions of the magnetic phenomena at Kakioka, Memambetsu, and Kanoya.

Stress drop at the Kephalonia Transform Zone estimated from the 2014 seismic sequence

The Kephalonia Transform Zone (KTZ) is a seismically active dextral transform fault decoupling the extensional deformed area of the Ionian Abyssal Plain and the compressional deformed area of the Mediterranean Ridge. We estimate a prominent steady state strain rate of 225±20nstrain/year across the KTZ from the mean velocities of permanent Global Navigation Satellite System (GNSS) stations in East Italy and West Greece, which confirms previous estimates. Based on the regional statistical seismicity and using the local Gutenberg–Richter relation we investigate the energetic balance between stress accumulated as a consequence of the continuous dextral shear deformation, and the average stress released by shallow seismicity. If the maximum expected magnitude is Mw=7.4, the estimated a and b parameters of the local Gutenberg–Richter relation and the geodetically determined shear-strain rate set an upper limit to the regional stress drop ∆σ≤0.4MPa.We verify these values by analyzing a seismic sequence on the KTZ between January and February 2014, which culminated in the Mw=6.09 event of Jan. 26, and the Mw=6.14 event of Feb. 3. The estimated epicenters lie within a few kilometers from each other, on the western side of the Kephalonia Island, at a hypocentral depth≤10km. The measured coseismic displacements of GNSS stations in this area fit the expected surface dislocation, which can be predicted for an elastic half-space using the measured fault-plane solutions as input. If the hypocenters of the aftershocks are taken as indicator of optimal dextral shear-stress orientation, then a low regional deviatoric stress of 0.4MPa, that is comparable with the maximum regional stress drop estimated above, is required for the Coulomb stress to match the pattern of the aftershocks of the 2014 sequence. As a consequence, we conclude that the regional deviatoric stress and the seismically released shear stress are in close balance in the KTZ seismic province.

Violations of Gutenberg–Richter Relation in Anthropogenic Seismicity

Anthropogenic seismicity (AS) is the undesired dynamic rockmass response to technological processes. AS environments are shallow hence their heterogeneities have important impact on AS. Moreover, AS is controlled by complex and changeable technological factors. This complicated origin of AS explains why models used in tectonic seismicity may be not suitable for AS. We study here four cases of AS, testing statistically whether the magnitudes follow the Gutenberg–Richter relation or not. The considered cases include the data from Mponeng gold mine in South Africa, the data observed during stimulation of geothermal well Basel 1 in Switzerland, the data from Acu water reservoir region in Brazil and the data from Song Tranh 2 hydropower plant region in Vietnam. The cases differ in inducing technologies, in the duration of periods in which they were recorded, and in the ranges of magnitudes. In all four cases the observed frequency–magnitude distributions statistically significantly differ from the Gutenberg–Richter relation. Although in all cases the Gutenberg–Richter b value changed in time, this factor turns out to be not responsible for the discovered deviations from the Gutenberg–Richter-born exponential distribution model. Though the deviations from Gutenberg–Richter law are not big, they substantially diminish the accuracy of assessment of seismic hazard parameters. It is demonstrated that the use of non-parametric kernel estimators of magnitude distribution functions improves significantly the accuracy of hazard estimates and, therefore, these estimators are recommended to be used in probabilistic analyses of seismic hazard caused by AS.

Long-term earthquake forecasting model for northeast India and surrounding region: seismicity-based model

An earthquake forecasting model has been proposed for northeast India and surrounding region in a specified time span by exploiting the temporal and the spatial variations in the Gutenberg and Richter parameters (a and b). The study region has been divided into grid with a grid interval of 0.1° in latitude and longitude and 14 zones for the seismic activity density and the seismicity parameters estimation, respectively, while the time span of the used earthquake catalog has been divided into time grids with a fixed time interval of 5 years. The seismic activity rate at a grid point has been estimated from seismic activity density at the grid point and seismic activity in zone comprising the grid. The normalized multivariate Gaussian mixture model is considered to estimate the seismic activity density, while seismicity parameters have been estimated using the Gutenberg and Richter relation and the method of maximum likelihood estimation. The seismic activity rates at each grid point in all time grids have been fitted to an autoregressive model of order three, and the inferred coefficients of the model has been used to forecast seismic activity rate in next time grid, e.g., in time grid from 2011 to 2015 including.

Variation of b and p values from aftershocks sequences along the Mexican subduction zone and their relation to plate characteristics

Aftershock sequences along the Mexican subduction margin (between coordinates 110ºW and 91ºW) were analyzed by means of the p value from the Omori–Utsu relation and the b value from the Gutenberg–Richter relation. We focused on recent medium to large (Mw > 5.6) events considered susceptible of generating aftershock sequences suitable for analysis. The main goal was to try to find a possible correlation between aftershock parameters and plate characteristics, such as displacement rate, age and segmentation. The subduction regime of Mexico is one of the most active regions of the world with a high frequency of occurrence of medium to large events and plate characteristics change along the subduction margin. Previous studies have observed differences in seismic source characteristics at the subduction regime, which may indicate a difference in rheology and possible segmentation. The results of the analysis of the aftershock sequences indicate a slight tendency for p values to decrease from west to east with increasing of plate age although a statistical significance is undermined by the small number of aftershocks in the sequences, a particular feature distinctive of the region as compared to other world subduction regimes. The b values show an opposite, increasing trend towards the east even though the statistical significance is not enough to warrant the validation of such a trend. A linear regression between both parameters provides additional support for the inverse relation. Moreover, we calculated the seismic coupling coefficient, showing a direct relation with the p and b values. While we cannot undoubtedly confirm the hypothesis that aftershock generation depends on certain tectonic characteristics (age, thickness, temperature), our results do not reject it thus encouraging further study into this question.

Microseismic Activity and Basement Controls on an Active Intraplate Strike‐Slip Fault, Ceres–Tulbagh, South Africa

Earthquakes far from plate boundaries are poorly understood, because of few well‐studied examples and uncertainty of what controls the location of these events. In 1969 a damaging local magnitude (ML) 6.3 strike‐slip earthquake, with no surface expression, occurred in the Ceres–Tulbagh region in the South African stable continental interior. Here, we present a microseismic study of the Ceres–Tulbagh area, conducted over three months in 2012, in which 172 events recorded on at least three stations follow a Gutenberg–Richter relationship for −1.5<ML<0.5. The events delineate a 5 km wide, subvertical zone that is microseismically active to a depth of 15 km. This fault zone is subparallel to the 1969 aftershock zone and at low angle to the regionally inferred greatest horizontal stress. We argue that the microseismically active zone is guided by inherited structures in the basement geology. This and similar structures may represent significant earthquake hazard in plate interiors.

Earthquake hazard analysis for East Anatolian Fault Zone, Turkey

The aim of this study was to investigate the earthquake hazard of the East Anatolian Fault Zone by determining the a and b parameters in a Gutenberg–Richter magnitude–frequency relationship. For this purpose, the East Anatolian Fault Zone is divided into five different source zones based on their tectonic and seismotectonic regimes. We calculated the b value, which is the slope of the frequency–magnitude Gutenberg–Richter relationship, from the maximum likelihood method (ML). Also, we estimated the mean return periods, the most probable maximum magnitude in the time period of t years and the probability for an earthquake occurrence for an earthquake magnitude ≥M during a time span of t years. We then produced a and b value maps using the ML. We obtained the lowest b value in Region 1 covered Karliova triple junction. This conclusion is strongly supported from the probability value, which shows the largest value (90 %) for an earthquake with magnitude greater than or equal to 6.0. The mean return period for such a magnitude is the lowest in this region (43 years). The most probable magnitude in the next 100 years was calculated, and we determined the highest value around Karliova triple junction. According to these parameters, Region 1 covered the Karliova triple junction and is the most dangerous area around the East Anatolian Fault Zone.

Study of the seismicity temporal variation for the current seismic hazard evaluation in Val d'Agri, Italy

Abstract. This study examines the temporal variation of the seismicity in the Val d'Agri (southern Italy) and adjacent areas, for the current seismic hazard evaluation. The temporal variation of the seismicity is expressed as time series of the number of earthquakes, b value of Gutenberg–Richter relationship or b value of the frequency–magnitude distribution and the seismic energy released in the form of logE2/3. The analysis was performed by means of a new research tool that includes visualizing techniques, which helps the interactive exploration and the interpretation of temporal variation changes. The obtained time series show a precursory seismicity pattern, characterized by low and high probability periods, which preceded earthquakes of magnitude M ≥ 4.0. The 75% of the examined cases were successfully correlated with a change in seismicity pattern. The average duration of the low and the high probability periods is 10.6 and 13.8 months respectively. These results indicate that the seismicity temporal variation monitoring in a given area and the recognition of the low and high probability periods can contribute to the evaluation, in regular monthly intervals, of current seismic hazard status.

Evaluation of the seismicity for the Marmara region with statistical approaches

The Marmara region has been divided into six sub-regions having regard to the certain seismotectonic characteristics of the region, to study the seismicity. For this, the data belonging to both the historical and instrumental period has been used. The probabilities of earthquake recurrences were obtained by using Poisson statistical distribution models. Gutenberg–Richter (GR) relationship for each sub-region has been computed and, a- and b-values known as seismic hazard scaling parameters have been determined by linear least square (LLS) method and maximum likelihood (ML) method. The highest and the lowest \(b\)-values determined by LLS are found for sub-region 4 (Saros Gulf, \(0.74\pm 0.08\)) and sub-region 1 (Duzce, \(0.56\pm 0.06\)), respectively. The highest \(a\)-value for sub-region 4 as \(5.05\pm 0.05\) and the lowest \(a\)-value for sub-region 3 (Tekirdag) as \(4.12\pm 0.02\) are found. Likewise, the calculated lowest and highest \(b\)-values with ML are \(0.55\pm 0.05\) for sub-region 1 and \(0.68\pm 0.07\) for sub-region 4, respectively. Also, \(b\)-value for all region (108 years and \(M_{S}\ge 4.0\)) are estimated as \(0.71\pm 0.03\) with LLS and \(0.65\pm 0.02\) with ML. The highest earthquake occurrence probabilities of \(M_{S} \ge 7.0\) in the next 100 years according to the Poisson distribution are found as 74.4 % for sub-region 1 (Duzce), 58.6 % for sub-region 2 (Izmit), 54.3 % for sub-region 3 (Tekirdag) and 49.7 % for sub-region 5 (Bandirma). The shortest recurrence period for the earthquake with the same magnitude is determined as 73 years for sub-region 1.