Local Magnitude ML Research Articles

Seismicity associated with hydraulic fracturing in Changning shale gas field, China: Constraints from source mechanisms, stress field and fluid overpressure thresholds

Source properties and stress fields are critical to understand fundamental mechanisms for fluid-induced earthquakes. In this study, we identify the focal mechanism solutions (FMSs) of 360 earthquakes with local magnitude ML ≥ 1.5 in the Changning shale gas field from January 2016 to May 2017 by fitting three-component waveforms. We then constrain the directions of the maximum horizontal stress (σHmax) for four dense earthquake clusters using the stress tensor inversion method. The stress drops of 121 earthquakes with ML ≥ 1.5 are calculated using the spectral ratio method. We examine the spatiotemporal heterogeneity of stress field, and discuss the cause of non-double-couple (non-DC) components in seismicity clusters. Following the Mohr-Coulomb criterion, we estimate the fluid overpressure thresholds from FMS for different seismic clusters, providing insights into potential physical mechanisms for induced seismicity. The FMS results indicate that shallow reverse earthquakes, with steep dip angles, characterize most events. The source mechanisms of earthquakes with ML ≥1.5 are dominated by DC components (> 70%), but several earthquakes with ML > 3.0 and the microseismic events nearby during injection period display significant non-DC components (> 30%). Stress inversion results reveal that the σHmax direction ranges from 120° to 128°. Stress drops of earthquakes range between 0.10 and 64.49 MPa, with high values occurring on reverse faults situated at a greater distance from the shale layer, accompanied by a moderate rotation (≤ 25°) in the trend of σHmax. The seismic clusters close to the shale layer exhibit low fluid overpressure thresholds, prone to being triggered by high pore-pressure fluid. The integrated results suggest that the diffusion of high pore pressures is likely to be the primary factor for observed earthquakes. The present results are expected to offer valuable insights into the origin of anomalous seismicity near the shale gas sites.

Spatial variation of the source mechanisms in the Egyptian continental margin

The spatial variability of the focal mechanism solutions in the NW Egypt Cyrenaica and Levant-north Egypt domains of the Egyptian Continental margin is investigated based on the polarity of the P-wave first motions of eleven earthquakes with local magnitudes ML ranging from 4.2 to 6.2. The identified focal mechanism solutions in the Levant-north Egypt domain are evaluated in two sub-provinces with different geological settings. The first sub-province shows strike-slip faulting mechanisms with right-lateral motion along the NE-trending Rosetta-Qattara fault zone, while the NW-trending Temsah fault zone produces left-lateral strike-slip motion. The inland extension of the Rosetta-Qattara fault zone towards the Western Desert reflects the same sense of motion. Strike-slip faulting with thrust components dominates the focal mechanisms in the second sub-province but with a different stress orientation. A single thrust faulting mechanism event which occurred on the periphery of the continental shelf shows the same common orientation of the stress field at the Hellenic Arc. This type of mechanism in this locality can be explained by the mean of compressional stress transfer towards the coast generated by the accelerated southwest retreat of the Hellenic Trench. The complex regional stress along the Cyprian and Hellenic arcs, the current local structure features, and the dominant orientation of the pre-existing fault are the factors that contribute to the changeable stress pattern field in the Levant-north Egypt domain. A single event in the second domain, the NW Egypt Cyrenaica, is a strike-slip with a right-lateral sense of motion on a WNW-oriented plane, consistent with motion along the WNW tectonic fault which separates the continental shelf from the oceanic crust.

Сейсмические события в Воркутском углепромышленном районе в 2023 году

In 2023, regional seismic stations recorded 25 seismic events in the Vorkuta coal industrial district of the Komi Republic, with a local magnitude ML from 1.8 to 2.8. A significant contribution to the detection of weak, ML ≤ 2, events was made by data obtained from the temporary seismic station of the IG FRC Komi SC UB RAS, installed as part of expeditionary work in the summer of 2023 in the Polar Urals.
 Among them the seismic event on August 1, 2023, which had a macroseismic effect, is of particular interest. Residents of several districts of Vorkuta felt tremors similar to an earthquake. Seismic records from 5 stations with epicentral distances from 40 to 1070 km were processed. The following parameters of the epicenter were obtained: t0 = 18:39:07, 67.529N, 64.001E, h = 0 km, Rminor = 3.9 km, Rmajor = 6.7 km, AzMajor = 90°, Kp = 9.0, ML = 2.8, Ms = 2.35. The instrumental epicenter of the event is located in Vorkuta, near the mine field of the Vorkuta mine. We classified the seismic event as a rock burst. Macroseismic intensity calculated in accordance with the Seismic intensity scale-2017 using 57 questionnaires and 163 definitions of sensor categories was I0 = 4.73 ± 0.02. The calculated value of the macroseismic depth of the source was H = 0.5 ± 0.4 km, which corresponds to the instrumental definition and to the range of depths of mine workings of the Vorkuta coal deposit.

Application of seismic refraction and MASW methods for investigating the Spillway Fault trace along the western side of the Aswan High Dam, Egypt

AbstractAn earthquake of local magnitude ML = 4.6 occurred on November 7, 2010, 4.5 km northwest of the Aswan High Dam on the Spillway Fault. In the Aswan metropolitan region this earthquake was felt intensely. As no surface rupture was found, the focal mechanism and the distribution of seismic activity was one of the tools used for finding fault dimensions. The composite fault-plane solutions for the observed events on the Spillway Fault showed a left lateral strike-slip faulting with normal-fault component striking NNW-SSE. Also, remote sensing techniques were applied for the detection and identification of the geomorphology and geometry of the Spillway Fault. In this research, sub-surface layers and structures are delineated utilizing near-surface seismic techniques. Furthermore, the area’s supposed path and position of the Spillway Fault are also investigated. Two active seismic techniques, Seismic Refraction and Multi-Channel Analysis of Surface Waves (MASW), are utilized for recording near-surface seismic wave data at 9 sites. The seismic refraction profiles are conducted as a 2D cross-section on the trace of the detected Spillway Fault in the study area to evaluate the maximum depth of penetration of the P-wave for fault investigation. The constructed 2D seismic and structural sections from P-wave results show that the obtained average depth of about 30 m. In addition, the estimated P-wave velocities extend from 600 m/s to over 6500 m/s. Some lateral variation in the seismic wave velocities in all layers may represent fault zones. Moreover, the 1D MASW technique is conducted to estimate the velocities of the shear wave for the upper 30 m (Vs30) to provide the site classes and soil characteristics along both sides of the detected Spillway Fault trace in the study area. The calculated Vs30 values emphasized the idea of the existence of a normal dip-slip fault trace which divides the study area into two different lithological parts. The first part is located on the eastern side and characterized by almost class B (hard rock, according to NEHRP classification), while the other part is located to the west, and shows almost class type C (denoted as dense soil and soft rock soil).

Temporal Characteristics of the February 6, 2023 MW7.8, Turkey Earthquake Aftershock Sequence

The devastating earthquake with moment magnitude MW7.8, (local magnitude ML=7.5) which occurred in the East Anatolian Fault Zone, westwards of Gaziantep, Turkey on February 6, 2023 was followed by an intensive aftershock activity. Studying time distribution of aftershocks is important for understanding physics of the earthquake generation process.
 In the present study temporal pattern of aftershock sequence of the 2023 MW7.8 Turkey earthquake is analyzed. Because of the factors such as location and radiation pattern and the cumulative nature of building damage, aftershocks can cause more damage than the main shock. The temporal clustering of aftershocks is a dominant non-random element of the seismicity, so when the clusters are removed, the remaining activity can be modelled (as first approximation) as a Poisson process.
 On the assumption that aftershocks are distributed in time as a non-stationary Poisson process, we use the maximum likelihood method for estimating the parameters (K, c and p) of the modified Omori formula. For testing the goodness of fit between the aftershock occurrence and different statistical models, a transformation from the time scale t to a frequency-linearized time scale τ is applied. Akaike's information criteria is used to select the best model for the temporal distribution of aftershocks.
 It has been found that temporal distribution of the 2023 Turkey earthquake aftershocks is dominated by the classic power law decay in time and suggests the existence of secondary aftershock activity.

Reassessment of the Stability Conditions in the Lignite Open Pits of Oltenia (Romania) in Relation to the New Local Seismic Context as an Imperative for Sustainable Mining

Responsible mining considers the three pillars of sustainability, namely the environment, the economy and social welfare. As a result, exploitation of deposits of useful mineral substances, as an economic activity, must be carried out taking into account several requirements, among which is the generation of a reduced impact on the environment and local communities. Sliding of open pits and waste dumps slopes represents a major risk, which endangers workers and machinery, as well as the components of the natural and built environment in the influenced area. In order to avoid such phenomena and their consequences, it is imperative to analyze the stability conditions whenever their possible triggering factors appear (such as earthquakes). Between February and March of 2023, the region of Oltenia (south-west Romania) was affected by intense seismic activity, out of normal patterns. Considering this series of seismic events, in this paper we aimed at reevaluating the stability conditions of the slopes of the working fronts and of the internal dumps in the lignite open pits located in the region in this new context. Research focused on three lignite open pits, namely Peșteana North (Rovinari mining perimeter), Jilț North (Jilț mining perimeter), Berbești–Alunu (Berbești mining perimeter). After describing the general geology and tectonics of the areas under study, the seismic episode that affected the region at the beginning of 2023 (which in fact extended until November) is highlighted, with increased attention given to the earthquakes of 13 and 14 February 2023, with a local magnitude ML ≥ 5. The most important part of the study is represented by the stability analyses, carried out for normal conditions (considering the characteristics of the rocks at natural humidity and in the absence of the influence of external factors) and under seismic conditions, characterized by a peak ground acceleration equal to the maximum acceleration considered for the location area of the mining perimeters taken into study. The results of the study showed that, for most of the analyzed situations, a renewal of the technical exploitation documentation is required, which, taking into account the results of this study, must adopt new geometries of the excavation and deposition fronts, so that the objectives in terms of operational and workplace safety imposed by legislation are respected.

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<5.5. For earthquakes with ML>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.

SEISMICITY of AZERBAIJAN and ADJACENT TERRITORIES in 2018–2019

In 2018–2019, the Azerbaijan network of seismic stations continued to operate without changes and consisted of 35 digital stations. To estimate the magnitude of earthquakes in these years, only the local magnitude ML Azr was determined. The total number of earthquakes recorded by the Azerbaijan network of stations was 5387 in 2018 and 6880 in 2019, however, the catalog of earthquakes in Azerbaijan and adjacent territories, published in the appendix to this article, provides data on earthquakes only with a magnitude of ML Azr3.0, which ОБЗОР СЕЙСМИЧНОСТИ 66 number was 101 in 2018 and 115 in 2019. The number N of the representative level earthquakes (KR≥9) in the study area slightly increased in 2018 and 2019 compared to the long-term average annual value Nav for 1993– 2017, while the released seismic energy decreased relative to the background level by 6 and 7 times, respectively. The most significant seismic event on the territory of the Republic in 2018 was the earthquake of June 5 with ML Azr=5.5, and in 2019 – the earthquake of February 5, ML Azr=5.2. Both events were felt in the nearest settlements with an intensity of Ii=6.

Comparison of earthquake classification systems in local magnitudes ML in some regions of Northern Eurasia

The ML local magnitudes of the earthquakes of Armenia, Azerbaijan, the Arctic and the Baltic Shield (KOGSR and FCIAR station networks), the Urals and Altai and Sayan were analyzed. The regional dependences of ML on the mbISC “external” magnitude are established. It is shown that the magnitudes ML of weak and moderate seismic events registered by the seismic stations of these regions, with the exception of Altai and Sayan, are close at the same mbISC both among themselves and to ML according to the NAO, HEL, DDA, TEH networks of neighboring countries. For all regions, except for Altai and Sayan, a general dependence is established in the form: MLav=1.06⋅mbISC–0.34. A significant overestimation of the ML of Altai and Sayan earthquakes recorded by the regional network is found relative to other data. For the main group of data, the excess of local magnitudes ML=f(KR) over magnitude M in the known T.G. Rautian ratio, KR=lgE=1.8⋅M+4, by DM=0.2÷0.4 in the magnitude range of ML=1.0÷6.0 is shown. For Altai and Sayan earthquakes, the overestimation of local magnitudes is more significant and should be taken into account when calculating seismic energy using the regional ratio KR=f(ML). In general, magnitudes in the ratio KR=1.8⋅M+4 for weak and moderate earthquakes are closer to local magnitudes ML, and for strong events with KR>13 (Ms>5.0) – to surface wave magnitudes Ms.

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.

СИЛЬНОЕ ЗЕМЛЕТРЯСЕНИЕ 05.02.2022 (ML 5.5) ВБЛИЗИ НЕФТЕГАЗОВОГО МЕСТОРОЖДЕНИЯ НА СЕВЕРО-ВОСТОЧНОМ ШЕЛЬФЕ О. САХАЛИН

On 5 February, 2022 at 21:18 UTC (February 6 at 08:18 local time), an earthquake of local magnitude ML = 5.5 with a hypocenter in the upper crust occurred on the shelf of northeastern Sakhalin. The earthquake was caused by slip on the Piltun-Chaivinsky fault, which until recently was not considered a seismogenic source. The studied earthquake located in the area with Coulomb stress increment caused by the catastrophic 1995 Neftegorsk earthquake is most likely tectonic in origin. The accumulated tectonic stress associated with static stresses, triggered by the strongest earthquake in the northeast of the island, caused east-dipping reverse fault displacements. The focal mechanism of the earthquake is characterized by near-horizontal compression axes oriented sublatitudinally. The authors pioneered in collecting and analyzing instrumental data on strong ground motions near an earthquake, which are of value for engineering seismology. The macroseismic data incorporating the information received from local residents via the Internet were analyzed. Findings of the study are of practical relevance, since each year economic activities related to the subsoil development are expanding and intensifying. The present knowledge of active geological structures of the Sakhalin shelf contributes to the reassessment of seismic hazard in gas and oil producing areas.

A database of ground motion recordings, site profiles, and amplification factors from the Groningen gas field in the Netherlands

A comprehensive database that has been used to develop ground motion models for induced earthquakes in the Groningen gas field is provided in a freely accessible online repository. The database includes more than 8500 processed ground motion recordings from 87 earthquakes of local magnitude ML between 1.8 and 3.6, obtained from a large network of surface accelerographs and borehole geophones placed at 50 m depth intervals to a depth of 200 m. The 5%-damped pseudo-acceleration spectra and Fourier amplitude spectra of the records are also provided. Measured shear-wave velocity (VS) profiles, obtained primarily from seismic Cone Penetration Tests (CPTs), are provided for 80 of the ∼100 recording stations. A model representing the regional dynamic soil properties is presented for the entire gas field plus a 5 km onshore buffer zone, specifying lithology, VS, and damping for all layers above the reference baserock horizon located at about 800 m depth. Transfer functions and frequency-dependent amplification factors from the reference rock horizon to the surface for the locations of the recording stations are also included. The database provides a valuable resource for further refinement of induced seismic hazard and risk modeling in Groningen as well as for generic research in site response of thick, soft soil deposits and the characteristics of ground motions from small-magnitude, shallow-focus induced earthquakes.

Modeling Locations with Enhanced Earth’s Crust Deformation during Earthquake Preparation near the Kamchatka Peninsula

In seismically active regions of the Earth, to which the Kamchatka peninsula refers, pre-seismic anomalies are recorded in different geophysical fields. One of such fields is the acoustic emission of rocks, the anomalies of which are recorded 1–3 days before earthquakes at the distance of the first hundreds of kilometers from their epicenters. Results of joint acoustic-deformation measurements showed that growth of geoacoustic radiation intensity occurs during the increase in the level of deformations in rock masses by more than one order compared to the background values. Simulation studies of the areas with increased deformation are realized to understand the causes of anomalous acoustic-deformation disturbance occurrences before strong earthquakes. The model is based on the assumption that the Earth’s crust in the first approximation can be considered as a homogeneous isotropic elastic half-space, and an earthquake source can be considered as a displacements along a rectangular fault plane. Based on these assumptions, deformation regions of Earth’s crust were modeled during the preparations of two earthquakes with local magnitudes ML≈5 occurred on the Kamchatka Peninsula in 2007 and 2009. The simulation results were compared for the first time with the data of a laser strainmeter-interferometer installed at the Karymshina observation site (52.83∘ N, 158.13∘ E). It was shown that, during the preparation of the both earthquakes, the Karymshina observation site was within the region of shear deformations ≈10−7, which exceeded the tidal ones by an order. On the whole, simulation results corresponded to the results of the natural observations. Construction of an adequate model for the generation of acoustic-deformation disturbances before strong earthquakes is topical for the development of an early notification system on the threat of catastrophic natural events.

Earthquake on August 6, 2022 in the zone of the West Timan overthrust

On August 6, 2022, an earthquake was registered in Komi Republic, in the Udora district. Data from 17 seismic stations with epicentral distances from 227 to 1142 km were involved in processing the seismic event on August 6. The azimuth environment is uneven, the maximum azimuth gap is GAP=111.8°. As a result of instrumental processing, the following parameters were obtained: coordinates 64.023°N, 50.091°E, time at the source t0 =10:21:32 (UTC), depth h=14 km, energy class according to T.G. Rautian KR=10.2, local magnitude ML=3.6, error ellipse Azmajor=40°, Rminor=6.2 km, Rmajor=10.5 km. According to our calculations, the seismic event occurred in the middle reaches of the river Mezen, on the territory of the state natural reserve of regional significance “Udorsky”. The nearest settlements to the epicenter are located only 50 km away and the residents did not feel the earthquake. This area is not mining, so the seismic event is classified by us as a “tectonic earthquake”. The earthquake is confined to the West Timan thrust zone, which is a collision suture of the Russian and Pechora plates. The source at a depth of 14 km is located in the region of pericratonic subsidence covering the northeastern margin of the Russian Plate and the Kanino-Timan belt; on the boundary of the basement of the Mezen syneclise and the Timanid complex thrust over it. The earthquake may have been caused by tectonic pressure from the Pechora plate. This is the first earthquake in the West Timan thrust zone. Earthquakes occur mainly in the northeast of the Russian Plate, within the Volga-Ural anteclise, Mezen and Moscow syneclise. In addition, although the recorded earthquakes are mostly low-magnitude, they reflect the modern tectonic activation of the earth’s crust.

UGLYA-IV EARTHQUAKE on June 6, 2017 with KR=9.8, ML=2.7, І0=5 (Ukraine, Transcarpathia)

On June 6, 2017 at 17h27m in the mountains north of Uglya of the Tyachiv district of the Transcarpathian region, at a depth of h=5 km, a significant earthquake with energy class KR=9.8 and local magnitude ML=2.7 occurred. The earthquake was processed by records of 16 seismic stations of the Carpathian network. Immediately after the earthquake, information on its impact was collected through a telephone survey in 23 settlements located at distances of 5–30 km from the epicenter. As a result of processing the survey data from 23 settlements, an isoseismal map of the earthquake was constructed. The epicenter of the earthquake was located in an unpopulated mountainous area, where a calculated intensity was I0=5 by the MSK-64 scale. In the epicentral zone, it was felt with an intensity of 4–5 in 3 settlements: Prygid, Velikaya Ugolka and Fontynasy. According to a comparison with the map of the fault structures of the Solotvinskaya depression of the Transcarpathian trough and adjacent territories, it is clear that this earthquake is located in the zone between the Chernogolovsky and the Penin faults, where a lot of fault structures are noted, including an unclear morphology.

An alternative method to evaluate earthquake detection from synthetic Wood–Anderson seismograms: an application in Italy

SUMMARY Defining the regional variability of minimum magnitude for earthquake detection is crucial for planning seismic networks. Knowing the earthquake detection magnitude values is fundamental for the optimal location of new stations and to select the priority for reactivating the stations of a seismic network in case of a breakdown. In general, the assessment of earthquake detection is performed by analysing seismic noise with spectral or more sophisticated methods. Further, to simulate amplitude values at the recording sites, spectral methods require knowledge of several geophysical parameters including rock density, S-wave velocity, corner frequency, quality factor, site specific decay parameter and so on, as well as a velocity model for the Earth's interior. The simulation results are generally expressed in terms of Mw and therefore a further conversion must be done to obtain the values of local magnitude (ML), which is the parameter commonly used for moderate and small earthquakes in seismic catalogues. Here, the relationship utilized by a seismic network to determine ML is directly applied to obtain the expected amplitude [in mm, as if it were recorded by a Wood–Anderson (WA) seismometer] at the recording site, without any additional assumptions. The station detection estimates are obtained by simply considering the ratio of the expected amplitude with respect to the background noise, also measured in mm. The seismic noise level for the station is estimated starting from four waveforms (each signal lasting 1 min) sampled at various times of the day for a period of one week. The proposed method is tested on Italian seismic events occurring in 2019 by using the locations of 16.879 earthquakes recorded by 374 stations. The first results indicate that by evaluating the station noise level with 5-s windows, a representative sample of the variability in expected noise level is generated for every station, even if only 4 min of signal per day over a week of recordings is used. The method was applied to define the detection level of the Italian National Seismic Network (RSN). The RSN detection level represents a reference for the definition and application of guidelines in the field of monitoring of subsurface industrial activities in Italy. The proposed approach can be successfully applied to define the current performance of a local seismic network (managed by private companies) and to estimate the expected further improvements, requested to fulfil the guidelines with the installation of new seismic stations. This method has been tested in Italy and can be reproduced wherever the local magnitude ML, based on synthetic WA records, is used.

Brief communication: Seismological analysis of flood dynamics and hydrologically triggered earthquake swarms associated with Storm Alex

Abstract. On 2 October 2020, the Maritime Alps in southern France were struck by the devastating Storm Alex, which caused locally more than 600 mm of rain in less than 24 h. The extreme rainfall and flooding destroyed regional rain and stream gauges. That hinders our understanding of the spatial and temporal dynamics of rainfall–runoff processes during the storm. Here, we show that seismological observations from permanent seismic stations constrain these processes at a catchment scale. The analysis of seismic power, peak frequency, and the back azimuth provides us with the timing and velocity of the propagation of flash-flood waves associated with bedload-dominated phases of the flood on the Vésubie River. Moreover, the combined short-term average to long-term average ratio and template-matching earthquake detection reveal that 114 local earthquakes between local magnitude ML=-0.5 and ML=2 were triggered by the hydrological loading and/or the resulting in situ underground pore pressure increase. This study shows the impact of Storm Alex on the Earth's surface and deep-layer processes and paves the way for future works that can reveal further details of these processes.

Macroseismic survey of the 6 February 2016 KwaZulu-Natal, South Africa earthquake

AbstractOn the 6th of February 2016 at 11:00 hours local time (0900 UTC), KwaZulu-Natal was struck by an earthquake of local magnitude ML=3.8. The epicentre of the earthquake was located offshore in the Durban Basin. The earthquake shaking was widely felt within the province as well as in East London in the Eastern Cape province and was reported by various national media outlets. Minor structural damage was reported. A macroseismic survey using questionnaires was conducted by the Council for Geoscience (CGS) in collaboration with the University of KwaZulu-Natal (UKZN) which yielded 41 intensity data points. Additional intensity data points were obtained from the United States Geological Survey (USGS) Did You Feel It? programme. An attempt was made to define a local intensity attenuation model. Generally, the earthquake was more strongly felt in low-cost housing neighbourhoods than in more affluent suburbs.

Earthquakes Induced by Wastewater Disposal near Musreau Lake, Alberta, 2018–2020

Abstract Although hydraulic fracturing-induced earthquakes have been widely reported in Alberta, Canada, only one seismic cluster (the Cordel Field) has thus far been linked to wastewater disposal (WD). In this study, we report a statistically significant spatiotemporal correlation between recent earthquakes and nearby WD wells near Musreau Lake—the second disposal-induced earthquake swarm in Alberta. This newly occurred swarm contains five events with local magnitudes ML>3 from January 2018 to March 2020, forming into three tightly spaced clusters. The refined locations and focal mechanisms suggest a ∼10 km long northwest–southeast-trending rupture along the northern Rocky Mountains that developed over time, during which both poroelastic effects and static stress transfer played key roles. Through a statistical analysis of all reported induced earthquake clusters in the western Canada sedimentary basin (WCSB), we propose a linear predictive relationship (i.e., the “Interpolated Strike Orientation” model) between fault rupture direction and fault distance to the Rocky Mountains. This observation-based model, which is supported by both the focal mechanisms of the natural earthquakes and the nearby northwest-striking geological faults, is a new and useful reference for future assessments of seismic hazard in the WCSB.

The Impact of High-Energy Mining-Induced Tremor in a Fault Zone on Damage to Buildings

Seismic energy propagation from the hypocentre of mining-induced tremors usually causes an uneven distribution of the peak ground velocity PGVHmax in tectonically complicated structures, and consequently, an uneven distribution of damage to buildings located on the ground surface. This study aimed to estimate the impact of high-energy mining-induced tremors in fault zones on damage to buildings. In the study, we describe a case of one of the highest-energy mining-induced tremors E = 4.0 · 108 J (local magnitude ML = 3.6) that occurred in the Upper Silesian Coal Basin (USCB), Poland. The hypocentre of the tremor was most probably located in the Barbara fault zone, one of the larger faults in that western part of the USCB. Numerous damaged buildings on the terrain surface were registered, both in the epicentral zone and at a greater distance from the epicentre, mostly from the southern side of the Barbara fault zone. We calculated that the tremor was characterised by a normal slip mechanism associated with the same kind of fault as the Barbara fault. The azimuth of the nodal planes was similar to the west-east direction, which is consistent with the azimuth of the Barbara fault. From the focal mechanism, the greatest propagation of seismic energy occurred in south and west-east directions from the tremor hypocentre towards the surface. It was found that from the northern side of the hanging wall of the Barbara fault, there were 14 instances of damage (19%), and in the southern part of a hanging wall, there were 58 (81%). Therefore, the directionality of seismic energy propagation is aligned with the focal mechanism acting in the Barbara fault. It has also been concluded that a width of the zone of up to about 1200 m along the Barbara fault is the most threatening on the basis of registered building damage in the geological conditions of USCB. The study has shown that in assessing the impact of mining-induced tremors on buildings and the environment, the disturbance of seismic energy propagation by larger faults should be considered.