Clusters Of Seismic Events Research Articles

Analysis of Spatial Clustering of Seismic Events in the Northwest Pacific Ocean

The spatial clustering of epicenters of seismic events in the Northwest Pacific Ocean is analyzed using the Discrete Perfect Sets (DPS) topological filtering algorithm. The results of the analysis are presented in this paper. Based on the data shallow earthquakes recorded from 1963 to 2022 by the seismic network of the Kamchatka Branch of the Geophysical Survey of the Russian Academy of Sciences, stable areas of epicenter clustering are identified. These areas are associated with the Northern and Southern segments of the Kuril-Kamchatka seismofocal zone and do not depend on the time of recording of the earthquakes. The characteristic clustering radius for the Kuril-Kamchatka seismofocal zone is determined and is found to be 42–44 km. The analysis has confirmed the unique pattern of seismicity in the territory of the Commander segment of the Aleutian arc, which differs from that observed in the Northern and Southern segments.

Analysis of Spatial Clustering of Seismic Events by the DPS Topological Filtering Algorithm: Lake Baikal Region

The paper presents the results of applying the Discrete Perfect Set (DPS) topological filtering algorithm to analyze the spatial clustering of seismic epicenters in the Lake Baikal region. The study utilizes earthquake data recorded by the seismic network of the Baikal Branch of the Geophysical Survey of the Russian Academy of Sciences within the latitude range 48°N, 58°N and longitude range of 99°E, 122°E for the period from 1964 to 2018. Clustering characteristics are obtained for (i) the recording period from 1989 to 2018 with varying parameters of the DPS algorithm and four levels of the minimum energy class KР of seismic events and (ii) six non-overlapping time intervals from 1964 to 2018 and seismic events of energy class KР ≥ 8.6 with fixed parameters of the DPS algorithm. The dynamics of the clustering parameters from 1964 to 2018 may characterise the variability of the seismic regime of the region. Specifically, the decrease in the linear size of the areas of identified epicenter groups from about a thousand km to tens km may indicate a significant change in the seismic regime of the Lake Baikal region at the end of the 1990s and the beginning of the 2000s compared to the period between 1964 and 1997.

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

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

Quasilinear Chains of Earthquakes in Clusters of Seismic Events: The Baikal Rift System

Quasilinear Chains of Earthquakes in Clusters of Seismic Events: The Baikal Rift System

Identification of the mining accidents by a two-step clustering method for the mining-induced seismicity

Clustering methods aim to categorize data or samples into distinct groups based on their similarity. When applying clustering methods to earthquake events, it is crucial to establish a metric for quantifying the similarity between these events. Directly applying this clustering method to a catalog of mining-induced seismicity may lead to clustering earthquake events induced by different mining activities or accidents into the same group. To address this issue, a two-step clustering method has been proposed and applied for analyzing a catalog of mining-induced seismicity. The first step involves spatial distance-based clustering of seismic events, while the second step focuses on moment tensor analysis-based clustering of these events. The results obtained from the MT-based clustering method are visualized using Hudson Graphs, and box plots serve as an evaluation tool for assessing the quality of MT clustering. Most box plots demonstrate desirable quality in terms of MT cluster results, indicating successful outcomes. By the proposed two-step clustering method combined with actual mining activities, the potential accident locations and categories can be hypothesized while valuable recommendations provided for mining operations.

Subsurface deformation monitoring with InSAR and elastic inversion modeling in west Texas

Recent Interferometric Synthetic Aperture Radar (InSAR) observations depict centimeter-level surface uplift/subsidence signals in west Texas that correlate spatially and temporally with increased fluid extraction and injection activities over the past decade. Linear subsidence features from InSAR data near the Pecos area, that align well with maximum horizontal stress orientations and recent clusters of seismic events, suggest fault motion is also associated with this deformation. In this study, we investigate the relationship between observed surface displacement and inferred subsurface fault slip and reservoir compaction/inflation by inversely solving a combined model of elastic dislocation (Okada, 1985, 1992) and poroelastic reservoir compaction/inflation model (Geertsma, 1973).Our geomechanical analysis coupled with InSAR observations are consistent with the idea that both fault slip and reservoir inflation/compaction are occurring associated with fluid injection and withdrawal in the Pecos area. Numerical results show that the linear surface displacement patterns can be accurately reproduced using clusters of finite slip on multiple elastic dislocations in the subsurface, but also matching the broader areal subsidence and uplift signals requires the addition of poroelastic subsurface inflation and depletion. The inverted areal reservoir pressure distribution correlates well with known injection and withdrawal operations in the area. The best-fit surface deformation locates most fault slips patches at depths of 1–3 km, which is shallower than most reported earthquakes in this area. The difference in the estimation of depth suggests there could be aseismic fault slippages.

Seismic activity at the eastern Pyrenean termination

Although the seismic activity at the eastern Pyrenees is nowadays moderate and sparse, with events usually not exceeding magnitudes 4.5, this area has been affected in the past by the most destructive event occurred in the Pyrenees, reaching an intensity of IX, whose seismogenic structure is not well understood and still under debate. In order to progress in the knowledge of these structures, we present here the results derived from a 14 months-long broad-band seismic deployment focused on the Cerdanya Basin, but encompassing the eastern termination of the Pyrenees. The dense station coverage has allowed us to obtain accurate hypocentral locations, as well as up to 23 focal mechanisms from local low-magnitude earthquakes. In addition to a relatively sparse seismicity, several clusters of seismic events located in well-defined, small areas and depth ranges have been identified. The results show a few low-magnitude seismic events located in the southern limit of the Cerdanya Basin that could be related to oblique secondary faults within the footwall of the Têt Fault, the major tectonic structure in the area. Our data shows also that the Capcir Fault has associated seismicity, with some of the events located out of the fault plane, perhaps on secondary fault branches. To the east, a cluster of low-magnitude events is detected in the epicentral area of two relatively large earthquakes occurred recently. Further west, the Maladeta Massif has a sustained seismic activity, although its origin does not seem to be related to the most relevant structure in the area, the North Maladeta Fault. Regarding focal mechanisms, most of them show normal fault solutions with nodal planes NW-SE oriented, which agree with the extensional regime perpendicular to the axis of the chain derived from the seismic and geodetic data.

Impact of past mining on public safety: seismicity in area of flooded abandoned coal Gardanne mine, France

This paper focuses on the impact of past mining on public safety. It emphasizes the need to understand the induced seismic hazard and consequently improve the post-mining management procedures and legislations, as many mining sites are located in proximity to populated areas. Due to many challenges and complexity of the post-mining environments, induced seismic hazard nowadays remains largely unknown. However, the return experience of several post-mining sites in recent decades have shown us that the mine flooding and/or degradation of mining works can lead to the stress perturbations, inducing the seismicity and the reactivation of the surrounding geological faults. Hence, it is important to advance the seismic monitoring and research of seismicity in flooded post-mining districts. As the number of mine closures worldwide is rising, it can be expected that flooding induced reactivation of the surrounding faults becomes a more often observed phenomenon. We present in this paper the experience of the abandoned flooded coal mine of Gardanne in France, which has been experiencing post-mining seismicity problems since its closure in 2010. We show the results of a recent study of seismic multiplets and clustering of seismic events, as well as their spatio-temporal activity compared to meteorological conditions. These results provide us new insights as well as lead to raising new questions on seismic sources and triggering mechanisms.

A Global Catalog of Calibrated Earthquake Locations

Abstract We produced a globally distributed catalog of earthquakes and nuclear explosions with calibrated hypocenters, referred to as the Global Catalog of Calibrated Earthquake Locations (GCCEL). This dataset currently contains 18,782 events in 289 clusters with >3.2 million arrival times observed at 19,258 stations. The term “calibrated” refers to the property that the hypocenters are minimally biased by unknown Earth structure. In addition, we calculate uncertainties using empirically determined variability of the arrival-time data itself, specific to each calibrated cluster of hypocenters. Outliers in the arrival-time dataset are removed based on measured variability of the data. In each cluster, we estimate the empirically determined uncertainty for each set of station-phase arrival times. We use a version of the hypocentroidal decomposition multiple event relocation algorithm specifically adapted for calibrated relocations of clusters of seismic events. Most clusters are calibrated by fitting the subset of direct crustal first arrivals (Pg and Sg) with a locally appropriate travel-time model to estimate the cluster hypocentroid. A few clusters are calibrated by aligning the pattern of relative locations in space and time with one or more events for which a ground-truth hypocenter is available from an independent source with known uncertainty, such as a nuclear explosion. Epicentral uncertainties in GCCEL typically range from 1 to 5 km with a 90% confidence interval. Most events have depth constraint from one or more sources, usually with an uncertainty of ≤5 km. GCCEL is a significant resource for research at local, regional, and global scales because it provides minimally biased absolute hypocenters, meaningful associated error estimates, and curated arrival times as a reference dataset that can be used as prior constraints in the development of new regional, national, and global earthquake catalogs; validation of new location techniques; and the generation of advanced Earth models.

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

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

An Approach for Clustering of Seismic Events using Unsupervised Machine Learning

An Approach for Clustering of Seismic Events using Unsupervised Machine Learning

“Noise” during long-term continuous magnetotelluric monitoring of RN-15/IDDP-2 well engineering (Reykjanes peninsular, Iceland): A geogenic origin?

To date, magnetotelluric monitoring of processes during reservoir engineering of geothermal systems have been carried out only at three sites world-wide. Here, we add a new survey at the Reykjanes peninsular (Iceland). The MT data acquisition at RN-15/IDDP-2 covered the last third of the drilling period and thermo-hydraulic stimulation. Drilling was accompanied by temporal total fluid losses of up to 60 L/s as well as randomly distributed induced seismicity. Our experimental results of a two-months magnetotelluric monitoring during the deepening of the RN-15/IDDP-2 well on the Reykjanes peninsular (Iceland) to 4’659 m are in line with earlier observations on decreasing resistivities at periods of a few s to about 20 s (or up to about 40 s) in conjunction with fluid injection. Simple models indicate however that it is not the fluid volume itself that causes the anomaly. Moreover, here, temporal decreases in the electric resistivities occur at 0.2-20 s with minima at periods of about 0.4 s 1-2 days ahead of clusters of seismic events with magnitudes up to ML < 2. Unlike in the previous studies, where resistivity decreases occur on the component that is parallel to Shmin, here they in both components (XY and YX) of the MT data. At the Reykjanes peninsular, Shmin is oriented N120°E off the XY component (N0°E). Therefore, if we rotate our data in Shmin direction, the difference between both components would be slightly smaller than that for the non-rotated ones. The decrease in resistivity (on both components) extends over > 1 order of magnitudes over a short period range and is thus considered noise induced in the subsurface. This study aims at adding an essential dataset to the general discussion on MT monitoring of reservoir processes. The results show a temporal relation between decreasing apparent resistivity and (i) the geomagnetic field activity, (ii) the fluid losses up to 60 L/s, as well as (iii) mechanic processes occurring in the reservoir before clusters of induced seismicity on the other hand.

Прибайкалье и Забайкалье

The seismicity of Pribaikalye and Transbaikalia in 2013 is reviewed. 6706 earthquakes with КР≥5.6 were recorded there during that year. Most of them (92 %) are located in the Baikal rift zone. The high degree of clustering of seismic events is observed in South Baikal and Baikal-Muja areas. The largest earthquake with Mw=4.3 was in the Baikal-Muja area. Focal mechanisms of 47 earthquakes (КР9.6) were determined in 2013. Most of them correspond to normal faults, with strike-slip component contribution in some cases. Overall, the rate of seismic activity observed in Pribaikalye and Transbaikalia in 2013 was low.

Якутия

. The paper presents the results of monitoring seismicity in Yakutia in 2013 based on the data from 24 digital seismic stations. A total of 5197 seismic events with Кp=6–15 are detected. A map of earthquake epicenters is compiled showing their distribution in different areas. Two major clusters of seismic events are recognized. The first one corresponds to the Arctic-Asian seismic belt (AASB) extending across the Laptev Sea shelf and the north eastern Asian continent towards the Sea of Okhotsk. The second Baikal–Stanovoy belt in the south of the region is traced from Lake Baikal through the Stanovoy Highlands and the Stanovoy Ridge to the Sea of Okhotsk. In South Yakutia, the Olekma–Stanovoy zone (OSZ), which makes the eastern part of the belt, is considered. Both belts form boundaries of major lithospheric plates. The first belt separates the Eurasian and North American plates, while the other divides the Eurasian plate from the Amurian one. The minimum seismic level is recorded within the OCZ, with no more than 0.04 % of the total seismic energy re-leased during 2013. The maximum seismicity is noted in the AASB with released energy making up 99.9 % of the total amount. That abrupt change in seismicity is due to the occurrence here of three strong earth-quakes, Ulakhan-Chistay, Ilin-Tas (Abyi), and Tas-Khayakhtakh with the intensities VII-IX in the epicenters (MS=4.9–6.9). The epicentral zone of the Tas-Khayakhtakh event (MS=4.9) that occurred on May, 2013 is considered in more detail. The motion in the source of this event indicate thrusting along the Dogdo fault. All the shakes took place within the Chersky Range, in the zone of influence of major faults of the Kolyma–Indigirka system (Ulakhan, Dogdo, Ilin–Tas) under conditions of NE compression (regime of interplate collision and transpression ). Seismotectonic relations, macroseismic data, and earthquake focal mechanism pa-rameters are discussed.

Declustering of Seismicity Flow: Statistical Analysis

The known methods for identifying the clusters of seismic events that are mainly formed by the aftershocks frequently include debatable initial assumptions or a complicated system of successive approximations. One of the most recent and the most logically consistent method for identifying the clusters of aftershocks is the nearest neighbor distance (NND) method which is however algorithmically most challenging. In this paper, we propose a new declustering method based on a generalized distance (GD) metric, which employs some assumptions of the NDD method but is as simple in practical implementation as the window methods previously proposed for this purpose. In analyzing and substantiating this new method, a procedure of random shuffling of seismic events with respect to time is used for generating a real catalog which is however free of genetic relations between different events. The efficiency of the existing window methods, the GD method, and the NND methods is compared by a number of the criteria for 17 regions. It is shown that the GD method is, on average, noticeably more efficient than the standard window methods and compares very favorably with the NND method. In the conclusions, a certain speculativeness of separating the events into main and dependent shocks is discussed.

COMPLEX ELECTROMAGNETIC MONITORING OF GEODYNAMIC PROCESSES IN THE NORTHERN TIEN SHAN (BISHKEK GEODYNAMIC TEST AREA)

At the Bishkek geodynamic test area in the Northern Tien Shan, complex electromagnetic monitoring was conducted, including a wide frequency range by magnetotelluric sounding in various modifications and induction sounding with a controlled impulse source (method of sounding in the far zone, SFZ). The geoelectrical sections of the lithosphere were constructed for the study area. Based on the geophysical monitoring data and the profile MT moni‐ toring data on the Baitik depression, the main faults belonging to the Northern Tien Shan fault system were detected: Shamsi‐Tyundyuk (Predkirgizsky), Baitik, Chonkurchak, and Issyk‐Ata. From the geoelectrical data, we obtained new information that is independent of other geophysical methods used for studying the deep structure of the Earth – the hidden fault structures and the geoelectrical segmentation of the study area were revealed. The latter reflects the main elements of the block structure of the junction zone of the Chuya basin and the Kirghiz ridge. This information needs to be taken into account when constructing a comprehensive model showing the geological, geophysical and geodynamical features of the development of the Tien Shan, a vivid example of an intracontinental orogen. The MTS, FS and SFZ data from the Ak‐Suu and Chon‐Kurchak electromagnetic monitoring stations were interpreted. Based on the representative array of experimental electromagnetic data, we analyzed the effective depths of field penetration, which are most sensitive to changes in the electromagnetic parameters of the medium for the stationary and routine observation sites with respect to the clusters of seismic events. This paper discusses the development of the azimuth MT monitoring technique and the analysis of the time series of electromagnetic parameters, which were used for de‐ termining the contribution of each component of the impedance tensor to the informativeness of the monitoring stu‐ dies. The data from the 2004–2016 KNET Catalogue of the Research Station of RAS were analyzed in order to investi‐ gate the relationship between the space‐time seismicity distribution and the electrical conductivity variations that correspond to the features of the seismicity pattern at depth. Examples discussed are the time‐frequency series of MT monitoring in 2007 and 2016 at the Ak‐Suu and Chon‐Kurchak stations that identified anomalous electromagnetic parameters corresponding to the fluid redistribution model for the pore‐cracked medium: syn‐phase decreasing and increasing values of the parameter by the orthogonal azimuths. Thus, a phenomenological model has been tested. It relates the change in the stress‐strain state of the medium with the redistribution of fluids between the systems of fractures, which causes variations in the active and reactive components of the electrical resistance.

Compositional change of seismic event sequences in focal zones during preparation of strong earthquakes

Abstract: The change of the structure of seismic event sequences in focal area in preparation period of strong earthquakes has been studied. Findings show that weak earthquake clustering in time increases in earlier stage of forming of focal area. However, spatial clustering of seismic events rises at the latest stage.

Earth Surface Subsidence in the Kuznetsk Coal Basin Caused by Manmade and Natural Seismic Activity According to ALOS PALSAR Interferometry

This paper presents results of a spaceborne radar interferometry technique application for land subsidence observations in a coal mining area in Kuzbass, Russia. Joint analysis of radar interferometry measurements with simultaneous seismic observations shows that the land subsidence is triggered by seismic events, both natural and caused by human underground activity. Surface displacements are linked typically to the boundaries of block structures and correlate with the location of clusters of seismic events.

Formation of higher stress zones and clusters of seismic events in deep mining in Tashtagol

Based on the numerical modeling and experimental stress-strain state assessment in rocks, the authors have determined the regularity patterns in the formation of highest stress zones and clusters of seismic events in the course of sequential to downward ore extraction with caving in the condition of high horizontal stresses.

Inferences on active faults at the Southern Alps–Liguria basin junction from accurate analysis of low energy seismicity

Seismotectonic studies concern themselves with understanding the distribution of earthquakes in space, time, size and style. Therefore, the better these parameters are known, the most correct the association of any seismic event with the faulting structure that caused it will result. The use of accurate location methods is especially required when dealing with very complex areas, where several faulting systems or relatively small seismogenic structures exist. In fact, even though routinely determined epicentres are capable of revealing the rough picture of the seismicity, they are not suitable for studies of the fine structure of the causative fault, as their location uncertainties are often larger than the source dimension itself. In this work the probabilistic approach of the “Non Linear Localization” has been used to compute precise locations for earthquakes occurred in the last twenty years nearby the Saorge–Taggia line, a complex fault system situated in Western Liguria, close to the border between Italy and France. Together with the Breil–Sospel–Monaco and the Peille–Laghet faults, this line is responsible for the seismic activity of the area. The seismotectonic study is completed through a local tomographic study and the analysis of the focal mechanisms computed for an enlarged area. The results show that the seismicity associated with this fault system is confined within the first 10 km depth. Many clusters of seismic events are identified along the Saorge–Taggia line. The existence of a not previously mapped branch perpendicular to the Saorge–Taggia line is also recognized. Although its position may suggest it to be the continuation of the Breil–Sospel–Monaco fault system towards NE, our finding would rather suggest no association with the fault. The overall results confirm the complexity of the area; in particular the hypothesis that the Saorge–Taggia system may represent the eastward limit of a subalpine crustal block comprised within the Nice Arc, the named fault and a thrust front which is supposed to be located 20 km offshore find a confirm in the shallow depth of the seismic events. In addition we propose that the western limit of the block, located along the Nice arc, could be instead shifted where the Peille–Laghet fault lays.