Station Corrections Research Articles

Sediment Corrections for Distributed Acoustic Sensing

AbstractOn continental margins, sediments cause significant and spatially variable delays in seismic phase arrival times. The strong impedance contrast of the sediment‐bedrock interface causes P‐wave splitting that is clearly seen on distributed acoustic sensing recordings of earthquakes, resulting in additional phase arrivals that must be picked separately. We introduce sediment corrections to correctly account for those additional phases in the hypocenter localization procedure. Conceptually, the sediment correction method differs from the commonly‐used station corrections; instead of introducing a mathematically optimal constant time delay for each station and each phase, the corrections are derived from a physical, first‐order modeling of the wave propagation in the sediments. To calibrate the sediment corrections, a two‐step procedure is adopted: (a) the delay between the P‐phase and the converted Ps‐phase is taken as a proxy of the sediment thickness; (b) the P‐ and S‐wave speeds are determined through inversion. We show that sediment corrections are able to account for most of the observed bias while considerably reducing the number of free parameters compared to classical station correction. Moreover, the observed local delays are almost fully explained by the presence of the sedimentary layer, rather than by the 3D velocity variations of the bedrock. We retrieve and values that are compatible with values commonly found for sediments. Given the simplicity and physical foundation of the proposed method, we recommend the use of sediment corrections over station corrections whenever significant P‐wave splitting can be observed.

A Coda-Duration Magnitude Scale for the Mesopotamian Seismological Network in Iraq

One of the routine tasks of local, regional, and global seismic networks is to calculate the earthquake magnitude using one or more scales. This study aims to derive a duration magnitude equation using the earthquake data recorded by the local Mesopotamian Seismological Network (MPSN) in Iraq. The seismic waveform data were collected from 13 broadband seismic stations of the local MPSN for the period June 2014 to October 2023 years. The source parameters of these earthquakes were extracted from the International Seismological Centre (ISC). To establish a coda duration magnitude scale, 476 events were used and 100 events were employed to test the derived equation. Coda-duration magnitude equations were derived using linear regression analysis. To ensure the applicability of the magnitude equation for the network, the station correction coefficient (Sc) was estimated. The established coda duration magnitude scales are as follows: (4 ≤ ML ≤ 6) (4 ≤ ML ≤ 5) There is a good agreement between the MD values calculated using the above equation with those estimated for the same events using coda-duration magnitude equation derived in previous study from data of the Iraqi Meteorological Organization and Seismology (IMOS). The significant strong scaling relationships derived between MD–ML, MD–mb, MD–Ms, and MD–MW can be used quickly and reliably in MD calculation for earthquakes recorded by the MPNS stations and vice versa.

Reference 1D Seismic Velocity Models for Volcano Monitoring and Imaging: Methods, Models, and Applications

Abstract Seismic velocity models of the crust are an integral part of earthquake monitoring systems at volcanoes. 1D models that vary only in depth are typically used for real-time hypocenter determination and serve as critical reference models for detailed 3D imaging studies and geomechanical modeling. Such models are usually computed using seismic tomographic methods that rely on P- and S-wave arrival-time picks from numerous earthquakes recorded at receivers around the volcano. Traditional linearized tomographic methods that jointly invert for source locations, velocity structure, and station corrections depend critically on having reasonable starting values for the unknown parameters, are susceptible to local misfit minima and divergence, and often do not provide adequate uncertainty information. These issues are often exacerbated by sparse seismic networks, inadequate distributions of seismicity, and/or poor data quality common at volcanoes. In contrast, modern probabilistic global search methods avoid these issues only at the cost of increased computation time. In this article, we review both approaches and present example applications and comparisons at several volcanoes in the United States, including Mount Hood (Oregon), Mount St. Helens (Washington), the Island of Hawai’i, and Mount Cleveland (Alaska). We provide guidance on the proper usage of these methods as relevant to challenges specific to volcano monitoring and imaging. Finally, we survey-published 1D P-wave velocity models from around the world and use them to derive a generic stratovolcano velocity model, which serves as a useful reference model for comparison and when local velocity information is sparse.

Machine-learning based location of the 2021 MW 7.4 Maduo, Qinghai, China earthquake sequence: Insight into intraplate seismogenesis

On 22 May 2021, an MW 7.4 earthquake occurred in Maduo County, Qinghai Province, China, which is located on the Kunlun Mountain Pass-Jiangcuo fault inside the Bayan Har block, providing a good opportunity to investigate seismogenesis of large intraplate earthquakes. We analyze two years of continuous seismic data from June 2021 to June 2023, which were recorded at 34 portable seismic stations of the MaduoArray deployed in the source zone by our group. The LOC-FLOW workflow of automatic detection and location is applied to construct a complete and high-precision seismic catalog for the region, which includes machine-learning phase picking (PhaseNet), earthquake phase association (REAL), velocity model updating and station correction (VELEST), absolute earthquake location (HypoInverse), and relative location (HypoDD). As a result, 78,832 Maduo aftershocks and other local earthquakes are detected and relocated precisely. Our results show that the length of the Maduo aftershock zone is ∼170 km, which is mainly distributed along the NWW-SEE oriented Kunlun Mountain Pass-Jiangcuo fault, and there is a horsetail bifurcation feature at the eastern end of the aftershock sequence. The seismogenic fault is nearly vertical, and local seismicity occurs on both sides of the fault. Our results also show that there is no seismic gap or aftershock sparse area in the region. Previous studies have revealed a low-velocity and high-conductivity anomaly below the source zone, reflecting fluids ascending from the lower crustal flow. These results provide new insights into the cause of the 2021 Maduo earthquake.

Energy-Magnitude Station Corrections across the Conterminous United States

Abstract The determination of source parameters requires corrections for propagation and site effects. The source parameter of interest for this study is the radiated seismic energy, which we use in turn to compile a global energy magnitude Me catalog. The energy estimated for P waves recorded at distances between 20° and 98° is corrected for propagation effects using the numerical Green’s function for the AK135Q model, in which the correction is performed in the spectral domain over the frequency range 0.012–1 Hz. To quantify the impact of site effects on Me, we perform a mixed-effects regression to isolate repeated station effects from event and propagation contributions to the overall magnitude variability. We find that the standard deviation of the interstation residuals is 0.19, whereas the standard deviation of the path-to-path residuals is 0.24. To discuss the spatial variability of the station-specific magnitude residuals, we analyze the results obtained for permanent and temporary networks installed in the conterminous United States. The high density of stations considered (4502 stations from 97 networks) allows us to compare a map of the station corrections with the physiographic classification of the United States. We find a good agreement, with positive magnitude corrections for stations installed in the Atlantic Plain and in the Interior Plains divisions; negative corrections characterize installations in the Rocky Mountains system, in the Interior Highlands, in the ranges of the Pacific Mountains division and in the Appalachian Highlands. Moreover, the alternation of positive and negative magnitude corrections corresponds well with the province and section subdivisions.

A local magnitude scale (ML) for Northern Algeria

This study presents a local magnitude scale (ML) based on the original Richter definition and designed for use within the Algerian Digital Seismic Network (ADSN). The magnitude scale is derived from the analysis of 17,377 zero-peak maximum amplitude traces extracted from the vertical component, simulated as Wood-Anderson seismograms. These traces are taken from a dataset of 1901 earthquakes recorded between January 1, 2010, and June 1, 2022, at a minimum of five stations in the ADSN network. To better account for the attenuation of direct and refracted waves in northern Algeria, amplitude decay analysis reveals the presence of two transition distances at 90 and 190 km, resulting in three segments. A distance correction term, −log10(A0), is introduced and described by the following trilinear function:−logA0=0.6747∗log10R+0.0002∗R+1.6306R≤901.7736∗log10R+0.0002∗R−0.516990<R≤1902.4580∗log10R+0.0002∗R−2.0765R>190R represents the hypocentral distance in kilometers. The derived distance correction formula provides a well-constrained ML relationship for northern Algeria that is valid over a distance range of 5 to 600 km. Compared to other local magnitude relationships, the methodology proposed in this study consistently gives ML values slightly higher than those calculated by the Southern California relationship over all distances, with an average difference of 0.2 units. We computed corrections for 72 stations by minimizing the ML residuals. These corrections range from −0.50 to 0.54, highlighting the influence of local site effects on the amplitude of the seismic signal. The magnitude residuals using our magnitude relationship and incorporating the station corrections, show that the standard deviation has improved significantly, from 0.34 to 0.24. An ML relationship specific to the northern Algerian region provides a valuable tool for seismic monitoring, hazard assessment, and earthquake research in the region.

A Pn magnitude scale mb(Pn) for earthquakes along the equatorial Mid-Atlantic Ridge

SUMMARY We developed a short-period Pn magnitude scale mb(Pn) for earthquakes along the equatorial Mid-Atlantic Ridge. Due to low signal-to-noise ratios, teleseismic body wave magnitude and long-period surface wave magnitude cannot be confidently determined for small earthquakes of mb &amp;lt; 4. Local magnitude scales are also not useful for these events because the oceanic environment does not allow the propagation of crustal phases. However, regional high-frequency Pn waves from these small- to moderate-size (mb 3–6) earthquakes are well recorded in the equatorial Atlantic region and can be used to assign magnitudes. We measured over 2041 Pn peak amplitudes on vertical records from about 21 stations in northeastern Brazil and 11 stations in western Africa in the distance range of 700–3700 km. We analysed data from 189 events from the global centroid moment tensor catalogue to tie our mb(Pn) scale to Mw so that seismic moments can be readily estimated. Pn arrivals show apparent group velocity between 7.9 km s−1 at short ranges (∼1000 km) and up to 9.1 km s−1 at 3500 km. The measured peak amplitudes have a frequency between 0.8 and 3 Hz at 1000–1800 km, but at greater distances, 1800–3700 km, they show a remarkably consistent frequency of about 0.8 Hz. The peak amplitude attenuates at a higher rate at short distances (∼0.65 magnitude units between 700 and 2000 km) but attenuates at a lower rate at long distances (∼0.35 magnitude units between 2000 and 3700 km). The low rate of amplitude decay with distance and nearly constant frequency content of the peak amplitudes suggest that Pn waves propagate efficiently in the lower part of the upper mantle in the equatorial Atlantic Ocean basins. These are important attributes of oceanic Pn waves that can be used to assign magnitude for small- to moderate-size earthquakes in the equatorial mid-Atlantic region. The estimated station corrections correlate well with upper mantle low-velocity anomalies, especially in Brazil.

AI based 1-D P- and S-wave velocity models for the greater alpine region from local earthquake data

SUMMARY The recent rapid improvement of machine learning techniques had a large impact on the way seismological data can be processed. During the last years several machine learning algorithms determining seismic onset times have been published facilitating the automatic picking of large data sets. Here we apply the deep neural network PhaseNet to a network of over 900 permanent and temporal broad-band stations that were deployed as part of the AlpArray research initiative in the Greater Alpine Region (GAR) during 2016–2020. We selected 384 well distributed earthquakes with ML ≥ 2.5 for our study and developed a purely data-driven pre-inversion pick selection method to consistently remove outliers from the automatic pick catalogue. This allows us to include observations throughout the crustal triplication zone resulting in 39 599 P and 13 188 S observations. Using the established VELEST and the recently developed McMC codes we invert for the 1-D P- and S-wave velocity structure including station correction terms while simultaneously relocating the events. As a result we present two separate models differing in the maximum included observation distance and therefore their suggested usage. The model AlpsLocPS is based on arrivals from ≤130 km and therefore should be used to consistently (re)locate seismicity based on P and S observations. The model GAR1D_PS includes the entire observable distance range of up to 1000 km and for the first time provides consistent P- and S-phase synthetic traveltimes for the entire Alpine orogen. Comparing our relocated seismicity with hypocentral parameters from other studies in the area we quantify the absolute horizontal and vertical accuracy of event locations as ≈2.0 and ≈6.0 km, respectively.

The Berkovići (BIH) ML = 6.0 earthquake sequence of 22 April 2022 – Seismological and seismotectonic analyses

In the southeast of Bosnia and Herzegovina, the Berkovići earthquake sequence started with the mainshock on 22 April 2022 21:07 UTC at focal depth 22 km with magnitude ML = 6.0 (Mw = 5.7). Our preliminary estimation of the mainshock's maximum intensity is VII EMS for Berkovići where 29% of buildings were damaged. We analysed the first nine months of this sequence, 22 April 2022–22 January 2023. The earthquakes were located using a guided grid-search algorithm with source-specific station corrections as a mean of solutions for 54 combinations of velocity models and program control parameters. The analysis of aleatory variation and epistemic uncertainty showed that they are very dependent on the station coverage, especially for focal depth. The event catalogue consists of 7217 earthquakes and can be considered complete for ML ≥ 1.3. Focal depths (15–30 km) are considerably larger than average for the Dinarides, but consistent within the zone of mid-crustal events where the earthquakes occurred. Focal mechanisms were determined with the first-motion polarity method for eight earthquakes: five of them, including the mainshock, were due to reverse faulting on faults striking in the Dinaric direction, with the preferred main fault gently dipping to the northeast. However, three events were due to normal faulting, unexpected for this area. We constructed a regional seismotectonic cross-section to delineate a potential seismogenic source of the mainshock, and it suggests that the mainshock occurred on the NE-dipping blind ramp of the basal thrust of the Dalmatian tectonic unit. Moreover, another NE-dipping and blind ramp of this basal thrust could be responsible for the Ston–Slano 1996 earthquake, located to the SW of the Berkovići mainshock hypocentre at the horizontal distance of c. 35 km.

Standardizing Earthquake Magnitudes for the 2022 Revision of the Aotearoa New Zealand National Seismic Hazard Model

ABSTRACT The 2022 revision of the New Zealand National Seismic Hazard Model—Te Tauira Matapae Pūmate Rū i Aotearoa—requires an earthquake catalog that ideally measures earthquake size in moment magnitude. However, regional moment tensor solutions, which allow the calculation of moment magnitude MwNZ, were introduced in New Zealand only in 2007. The most reported magnitude in the national New Zealand earthquake catalog is a variation of local magnitude ML. In New Zealand, ML is systematically larger than MwNZ over a wide magnitude range. Furthermore, the introduction of the earthquake analysis system SeisComP in 2012 caused step changes in the catalog. We address the problems by converting magnitudes using regressions to define a standardized magnitude as a proxy for MwNZ. A new magnitude, MLNZ20, has an attenuation relation and station corrections consistent on average with MwNZ. We have calculated MLNZ20 for nearly 250,000 earthquakes between 2000 and 2020. MLNZ20 is a reasonable proxy for MwNZ for earthquakes with ML&amp;lt;5.5. For earthquakes with ML&amp;gt;4.6, MwNZ is reliably available. We have applied ordinary least squares (OLS) regression for MwNZ and MLNZ20 on ML before and after 2012. We argue that OLS is the most appropriate method to calculate a proxy for MwNZ from individual ML measurements. The slope of the OLS regression compares well to the slope from the method of moments, which accommodates equation error that is present when there is scatter beyond measurement error, as is the case for our magnitude data. We have defined as a proxy for MwNZ a standardized magnitude Mstd, which is Mw when available, MLNZ20 with some restrictions as a second choice, and otherwise the magnitude derived from regression. Standardization of the magnitudes reduces the total number of earthquakes with a magnitude of ≥4.95 by more than half and corrects step changes in the spatial distribution of earthquakes between 2011 and 2012.

Research Catalog of Inland Seismicity in the Southern Korean Peninsula from 2012 to 2021 Using Deep Learning Techniques

Abstract A seismicity catalog spanning 2012–2021 is proposed for the inland and near-coastal areas of the southern Korean Peninsula (SKP). Using deep learning (DL) techniques combined with conventional methods, we developed an integrated framework for compiling a comprehensive seismicity catalog. The proposed DL-based framework allowed us to process, within a week, a large volume of data (spanning 10 yr) collected from more than 300 seismic stations. To improve the framework’s performance, a DL picker (i.e., EQTransformer) was retrained using the local datasets from the SKP combined with globally obtained data. A total of 66,858 events were detected by phase association using a machine learning algorithm, and a DL-based event discrimination model classified 29,371 events as natural earthquakes. We estimate source information more precisely using newly updated parameters for locations (a 1D velocity model and station corrections related to the location process) and magnitudes (a local magnitude equation) based on data derived from the application of the DL picker. Compared with a previous catalog, the proposed catalog exhibited improved statistical completeness, detecting 21,475 additional earthquakes. With the newly detected and located earthquakes, we observed the relative low seismicity in the northern SKP, and the linear trends of earthquakes striking northeast–southwest (NE–SW) and northwest–southeast (NW–SE) with a near-right angle between them. In particular, the NE–SW trend corresponds to boundaries of major tectonic regions in the SKP that potentially indicates the development of fault structures along the boundaries. The two predominant trends slightly differ to the suggested optimal fault orientations, implying more complex processes of preexisting geological structures. This study demonstrates the effectiveness of the DL-based framework in analyzing large datasets and detecting many microearthquakes in seismically inactive regions, which will advance our understanding of seismotectonics and seismic hazards in stable continental regions.

Velocity structure (1D) and earthquakes relocation in the flat-slab to normal plate transition zone in the Argentinean transarc

The Argentinean Andean foreland region between latitudes 31°S to 33.5°S presents an intense crustal and upper mantle seismic activity that was recorded by a local seismological network called CHARSME (Chile Argentina SeisMic Experiment), and was used to obtain new velocity determinations. In a first moment, from 4 P-wave velocity (VP) models proposed by other authors, the detail in the layers was increased, resulting in better models used as initial model in the investment. Finally, by carefully selecting a set of earthquakes, a simultaneous velocity inversion, hypocentre determination and station correction were performed using the VELEST code. In order to obtain better velocity models (VP and VS) and VP/VS ratio, different tests were carried out in all processes.The final VP model presented in this paper incorporates considerations made in other seismological studies. This model accurately determined velocities between 10 and 65 km depth. The upper mantle velocity (VP) was defined in 8.05 km/s. From the Wadatti diagram, which represents a line whose slope is related to the wave velocities and the Poisson's coefficient, a VP/VS value of 1.732 was obtained. Alternative VP/VS values were also considered to invert VP, VS and VP/VS simultaneously. The minimum detailed model of VP, VS and VP/VS decreased the RMS by 25% relative to the initial models, implying an improvement in the uncertainty of locations. The new and precise locations of crustal earthquakes allowed defining clusters within Cuyania, a tectonic terrain that collided to Gondwana in the Early Paleozoic. Although it is known that foreland seismicity is mainly concentrated in this terrain, this work shows that there are well-defined zones within Cuyania that are more seismically active. Earthquakes of intermediate depth (upper mantle) are strongly agglomerated in the flat-slab zone of the subducting Nazca Plate.The discontinuities present in our model suggest upper, middle and lower crustal structuring, as well as a marked contrast between the sediments and the underlying basement. It was possible to interpret an upper crust consisting of sediments and basement to a depth of 25 km, a middle crust between 25 and 45 km and a lower crust between 45 and 55 km. We also defined the Mohorovičić (Moho) discontinuity at 55 km depth. The crustal velocities are in agreement with previous seismological studies.

Active Deformation Constraints on the Nubia‐Somalia Plate Boundary Through Heterogenous Lithosphere of the Turkana Depression

AbstractThe role of lithospheric heterogeneities, presence or absence of melt, local and regional stresses, and gravitational potential energy in strain localization in continental rifts remains debated. We use new seismic and geodetic data to identify the location and orientation of the modern Nubia‐Somalia plate boundary in the 300‐km‐wide zone between the southern Main Ethiopian Rift (MER) and Eastern Rift (ER) across the Mesozoic Anza rift in the Turkana Depression. This region exhibits lithospheric heterogeneity, 45 Ma‐Recent magmatism, and more than 1,500 m of base‐level elevation change, enabling the assessment of strain localization mechanisms. We relocate 1716 earthquakes using a new 1‐D velocity model. Using a new local magnitude scaling with station corrections, we find 1 ≤ ML ≤ 4.5, and a b‐value of 1.22 ± 0.06. We present 59 first motion and 3 full moment tensor inversions, and invert for opening directions. We use complementary geodetic displacement vectors and strain rates to describe the geodetic strain field. Our seismic and geodetic strain zones demonstrate that only a small part of the 300 km‐wide region is currently active; low elevation and high‐elevation regions are active, as are areas with and without Holocene magmatism. Variations in the active plate boundary's location, orientation and strain rate appear to correspond to lithospheric heterogeneities. In the MER‐ER linkage zone, a belt of seismically fast mantle lithosphere generally lacking Recent magmatism is coincident with diffuse crustal deformation, whereas seismically slow mantle lithosphere and Recent magmatism are characterized by localized crustal strain; lithospheric heterogeneity drives strain localization.

Obtaining an Empirical Earthquakes Duration Magnitude Formula from the Data of the Iraqi Meteorological Organization and seismology (IMOS)

This paper calculated the Duration Magnitude (MD) equation using data from the Iraqi Meteorological Organization and seismology (IMOS). It is an empirically determined equation and expressed as: The epicentral distance and local geological conditions affect the duration of the coda. The data is obtained from 7 seismic stations in the network. The new proposed duration magnitude equation results from applying linear regression analysis to the data of a seismic signal duration with correlation coefficient R2=0.76; and a standard deviation value of 0.049. Station corrections indicated by Sc are also specified for the seismic stations that range from -0.024 to + 0.02.

Improving Earthquake Early Warning Initial Peak Ground Motion Magnitude Estimation with Station Corrections: A Case Study Using the P-Alert Network in Taiwan

ABSTRACT Earthquake early warning systems assess the size of an earthquake based on initial ground motion and provide warnings before the arrival of large waves. We employ the iterative regression to simultaneously estimate the station correlations and the corresponding linear relations for magnitude. We report the standard error reductions of the magnitude estimation based on the initial peak P-wave acceleration (), velocity (), and displacement () as , , and , respectively. Factor analysis of the stationary correction reveals its correlation with several factors. This study shows that station correction can help to improve the precision of magnitude estimation in the future.

A Statistical Approach to Estimate Seismic Monitoring Stations’ Biases and Error Levels

ABSTRACT Magnitudes are common and important measures for the size of seismic events. The International Data Centre (IDC) of the Comprehensive Nuclear-Test Ban Treaty Organization estimates an event magnitude by averaging the magnitudes calculated by individual stations that detected the event, excluding outliers. This approach assumes that all station magnitudes have the same error level and are unbiased, namely, they have no systematic errors. We show that the body-wave and surface-wave magnitudes published in the Reviewed Event Bulletin (REB) of the IDC are inconsistent with these assumptions. We thus consider a model where each station has an unknown bias and error level. Given a large collection of reported event magnitudes by a network of monitoring stations, we propose a novel approach to estimate each individual station’s bias and error level. From a statistical perspective, this is a challenging problem involving a huge number of variables, because in addition to the stations’ biases and error levels, the event magnitudes are also unknown. Our approach is based on analyzing differences between reported magnitude values at pairs of stations, which cancels out the unknown event magnitudes and allows us to derive a simple and computationally efficient algorithm. We use the estimated station biases as station correction terms and the estimated error levels to compute weights for event magnitude estimation. Using a large data set from the REB with millions of reported station magnitudes, we show that our approach yields more consistent station and event magnitudes.

Nature of the Himalayan Seismicity Belt in the Garhwal–Kumaun Segment and Its Tectonic Implications

Abstract High-quality data recorded by a dense network of 53 seismic stations in the Garhwal–Kumaun Himalaya between February 2017 and December 2021 is analyzed. A total of 813 local earthquakes are relocated using a newly developed regional 1D velocity model incorporating station corrections. In addition, focal mechanism solutions of M ≥ 3.8 events are estimated using waveform inversion. The relocated seismicity patterns along with the focal mechanism solutions are utilized to present a seismotectonic scenario of the region. Almost 95% of the relocated seismicity is found to be clustered along the Himalayan seismic belt (HSB), down to ∼24 km depth. Seismicity in this belt is interpreted to be caused due to interseismic stress loading associated with the ongoing India–Eurasia collision tectonics. A few scattered hypocenters in the deeper crust between 30 and 50 km depth attest the strength of the downgoing Indian plate. Focal mechanisms in the seismogenic upper crust reveal thrusting of the Indian plate beneath the Lesser Himalaya, with compression normal to the strike of the Main Central Thrust (MCT). The north-dipping thrust mechanisms can be associated with a near-horizontal Main Himalayan Thrust (MHT). In addition, more steeply dipping faults above it define the Lesser Himalayan duplex systems, similar to those in western and Nepal Himalaya. A prominent ∼50 km wide seismicity gap region observed within the HSB is probably due to (1) a locally varying locking width of the MHT; (2) an unruptured, ductile segment at the eastern end of the rupture zone of the great 1803 earthquake (Mw 7.8 ± 0.2); and (3) a slab tear in the MHT, similar to those in subduction zones.

Scale of local magnitudes ML for earthquakes in the Terek-Caspian trough

A scale (calibration function) to determine the local magnitude ML of earthquakes with epicenters in the central part of the Terek-Caspian trough has been developed. The scale is based on a formula that takes into account attenuation and geometric divergence with distance for the maximum amplitudes of the simulated recordings of the short-period Wood-Anderson instrument. To calculate it, a sample of 64 earthquakes that occurred in 2020–2022, during the period of activation in the Black Mountains in the Chechen Republic, was compiled. Earthquakes were recorded by 58 stations in the distance range of 25–526 km, located around the epicenters in various geological and tectonic area. The Magscale program developed at the GS RAS based on methodological recommendations set out in the New Manual for Observatory Practice (NMSOP) was used for the calculation. The values of the coefficients of the equation, which characterize the attenuation in the crust and upper mantle, are obtained. The station corrections are calculated. Their use made it possible to reduce the scale deviations from ±0.26 to ±0.15. The areal distribution of residuals probably reflects the geological features of the area.

Properties of the Petrinja (Croatia) earthquake sequence of 2020–2021 – Results of seismological research for the first six months of activity

We present locations of almost 14,000 events from the first six months of the Petrinja (Croatia) earthquakes sequence (mainshock 29 December 2020, Mw 6.4). The catalogue is estimated complete for ML ≥ 1.20. Initially sparse local seismograph network was densified a week after the mainshock, which reduced uncertainties of focal locations by about 75%. Source-specific station corrections were used in an iterative location scheme. The hypocentres were located with 30 different sets of velocity models and program control parameters in order to gain insight into the epistemic uncertainty of each earthquake location. The bulk of epicentres are located close to, and to the SE of the causative, dextral strike-slip Petrinja fault, but considerable activity has been triggered on smaller faults, up to 25 km away. About a half of the 78 first-motion polarity focal mechanism solutions (FMS) from the full first year of activity were found to have mechanisms similar to the foreshock, the mainshock and the largest of aftershocks, while the rest indicated almost pure reverse faulting. Strike-slip mechanisms occurred more often close to the main fault. The P-axes of FMS for aftershocks were found to be on the average rotated by 16° clockwise with respect to the P-axis of the mainshock, regardless of the style of faulting. The spatial distribution of aftershocks coincides very well with the areas of positive Coulomb stress change caused by the mainshock rupture. The inferred cut-off seismogenic depth in the area (7–10 km), estimated on the basis of published values of the geothermal gradient and the assumption of predominantly granitic upper crust, is considerably shallower than the depth of the deepest reliably located aftershocks (16–18 km). This fact may be explained by considering ophiolite-dominated crust, and by increased strain rate during the sequence, which raises the ability of crustal rocks for brittle failure.

Calibration of the local magnitude scale (Ml) for Central Southern Africa

The Central Southern Africa is an area of moderate seismic activity generally caused by the presence of the East Africa Rift System. To improve the quantification of seismicity in this region, we propose a local magnitude scale (Ml), based on the original Richter definition to be used by the Zimbabwe and Botswana national networks. The magnitude scale is developed using 621 seismic events that occurred between 1997–2000 and 2013–2018. These events were recorded by 61 broadband seismic stations located in Botswana, Zimbabwe, and South Africa. We evaluated 5306 traces of zero to peak maximum amplitude, recorded on the horizontal channel from simulated Wood-Anderson seismograms. All 5306 maximum amplitude measurements were inverted simultaneously to determine the attenuation constants. The resultant Ml is defined as $ M_{l} = \log _{10}A+0.80 \times \log _{10}(R)+0.00086 \times R-1.37 \pm S $ in which A is ground displacement amplitude determined from instrument corrected synthetic Wood-Anderson seismograms in nanometres, R is the epicentral distance (km), and S is the station correction. Station corrections were determined for all stations during analysis resulting in values ranging between − 0.44 and + 0.31. The range in station corrections suggests a strong influence of local site effects on the amplitude of the seismic signal. Without station correction, the overall standard deviation of the magnitude residuals using our magnitude relation is 0.32, while using station corrections, the standard deviation 0.17. A comparison of our local magnitude relation with published mblg relationships for Southern Africa shows that these two magnitude scales are almost equivalent. The Ml equation for Central Southern Africa derived in this study has good correlation with mb values reported by the ISC and NEIC and attains systematically lower magnitude values than the South African relationship.