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Gerhard Jentzsch - One of the best experts on this subject based on the ideXlab platform.

  • Comparative geodynamic modelling about Earthquake Swarm areas and processes.
    2006
    Co-Authors: Marco Naujoks, Gerhard Jentzsch, Thomas Jahr, Jochen H. Kurz
    Abstract:

    An Earthquake Swarm is a temporary and regional sequence of Earthquakes with several 1000 events within a period lasting from hours to days. Usually it occurs in an area of just a few square kilometers without any prevailing single event. Earthquake Swarms are observed worldwide, especially in connection with fluid movement and volcanism. To clarify possible physical mechanisms that lead to the phenomenon of Earthquake Swarms, two prominent regions are compared by numerical investigations using the finite element analysis software ABAQUS: The Vogtland/NW-Bohemia area situated at the border of Germany and the Czech Republic and the Magadi region in the Kenya Rift. Geodynamic models for the two regions were constructed which take into account the regional stress field and thermal stresses as well as creep and plasticity with a porous elastic rheology. The investigations are focussed on the interaction between pore pressure variations, temperature changes, fluid movement, stress accumulation and deformations. It is suspected that these processes play an essential role in the generation of Earthquake Swarms. The results of the modelling for the Magadi and Vogtland areas were compared with each other and with information from other Earthquake Swarm areas. Conclusions were drawn to general as well as to area-specific mechanisms. An essential result of the modelling is that the existence of the regional stress field alone neither explains the occurrence of the Earthquake Swarms in the Vogtland area nor in the Magadi area. It reveals that physical processes in different regions of the earth's crust are coupled. Temperature changes and periodic pore pressure variations in the earth's crust are most important for the geodynamic processes. In combination with the local geology and heterogenous distributed material parameters these processes have a crucial influence on the generation of Earthquake Swarms, even though they are weighed differently in each focal area. Consequently the modelling leads to an improved understanding of the processes and interactions that contribute to the occurrence of Earthquake Swarms.

  • Earthquake Swarm examples and a look at the generation mechanism of the vogtland western bohemia Earthquake Swarms
    Physics of the Earth and Planetary Interiors, 2004
    Co-Authors: Jochen H. Kurz, Thomas Jahr, Gerhard Jentzsch
    Abstract:

    Earthquake Swarms occur at many locations all over the world. In most cases, they do not occur just once but happen rather repeatedly at a particular location. We compared intraplate Earthquake Swarm regions, especially the Vogtland/Western Bohemia area, with plate margins, leading to the conclusion of a fluid triggered Earthquake Swarm mechanism which is independent of the global tectonics. These observations were brought together with theoretical models concerning hydro fracturing with respect to pore pressure changes due to fluid movements in seismically active areas. This concept was applied to a finite element model of the Vogtland/Western Bohemia Earthquake Swarm region. The results show that it is not possible to get high enough stress accumulations for the generation of Earthquake Swarms within relatively short timespans by the regional stress field alone. But periodic pore pressure variations and linear temperature changes caused realistic deformations and stress accumulations which coincide with the measured ones of the Vogtland/Western Bohemia Earthquake Swarm region.

  • Earthquake Swarm examples and a look at the generation mechanism of the Vogtland/Western Bohemia Earthquake Swarms
    Physics of the Earth and Planetary Interiors, 2004
    Co-Authors: Jochen H. Kurz, Thomas Jahr, Gerhard Jentzsch
    Abstract:

    Earthquake Swarms occur at many locations all over the world. In most cases, they do not occur just once but happen rather repeatedly at a particular location. We compared intraplate Earthquake Swarm regions, especially the Vogtland/Western Bohemia area, with plate margins, leading to the conclusion of a fluid triggered Earthquake Swarm mechanism which is independent of the global tectonics. These observations were brought together with theoretical models concerning hydro fracturing with respect to pore pressure changes due to fluid movements in seismically active areas. This concept was applied to a finite element model of the Vogtland/Western Bohemia Earthquake Swarm region. The results show that it is not possible to get high enough stress accumulations for the generation of Earthquake Swarms within relatively short timespans by the regional stress field alone. But periodic pore pressure variations and linear temperature changes caused realistic deformations and stress accumulations which coincide with the measured ones of the Vogtland/Western Bohemia Earthquake Swarm region.

Josef Horálek - One of the best experts on this subject based on the ideXlab platform.

  • From Earthquake Swarm to a main shock–aftershocks: the 2018 activity in West Bohemia/Vogtland
    Geophysical Journal International, 2020
    Co-Authors: M. Bachura, Tomas Fischer, Jana Doubravová, Josef Horálek
    Abstract:

    SUMMARY In Earthquake Swarms, seismic energy is released gradually by many Earthquakes without a dominant event, which offers detailed insight into the processes on activated faults. The Swarm of May 2018 that occurred in West Bohemia/Vogtland region included more than 4000 Earthquakes with ML = 〈0.5, 3.8〉 and its character showed significant changes during the two weeks duration: what started as a pure Earthquake Swarm ended as a typical main shock–aftershock sequence. Based on precise double-difference relocations, four fault segments differing in strikes and dips were identified with similar dimensions. First, two segments of typical Earthquake Swarm character took place, and at the end a fault segment hosting a main shock–aftershock sequence was activated. The differences were observable in the Earthquakes spatio-temporal evolutions (systematic versus disordered migration of the hypocentres), b-values (>1.3 for the Swarm, <1 for the main shock–aftershocks), or the smoothness of seismic moment spatial distribution along the fault plane. Our findings can be interpreted by local variations of fault rheology, differential stress and/or smoothness of the faults surface, possibly related to the crustal fluids circulating along the fault plane and their interplay with the seismic cycle.

  • the 2011 west bohemia central europe Earthquake Swarm compared with the previous Swarms of 2000 and 2008
    Journal of Seismology, 2015
    Co-Authors: Hana Cermakova, Josef Horálek
    Abstract:

    This paper presents the basic characteristics of the 2011 West Bohemia/Vogtland Earthquake Swarm and compares it with the Swarms in 2000 and 2008. All these Swarms occurred in the Nový Kostel focal zone. Up to 25,000 ML≤3.7 events with depths between 6 and 10 km were detected in the 2011 Swarm. Utilizing WEBNET data, we analysed the cumulative seismic moment, magnitude-frequency and interevent time distributions, space-time distribution of foci and typical focal mechanisms. For this purpose, we improved the formula for estimating the local magnitude ML used by WEBNET. The 2011 Swarm exhibited much higher rapidity than the Swarms of 2000 and 2008. The magnitude-frequency distributions of all the three Swarms are similar, having the b-value close to 1.0. However, the events of higher magnitudes, roughly ML∼3.0+, depart markedly from the general trend of the weaker events. The probability density functions of the interevent times of all the Swarms comply with power law ∝T−1.4, which points to Omori law-like mainshock-aftershock activity. All Swarms exhibit a pronounced focal migration; however, no regularity was found. The spatial distribution of the 2011 foci indicates two active fault segments which differ from the segment triggered in the Swarms of 2000 and 2008. Furthermore, we analysed the spatial distribution of the mini-Swarm of 2013 and found that it complements the Swarm of 2011. The prevailing focal mechanisms in the 2011 Swarm are of both oblique-normal and oblique-thrust types and correspond closely to the geometry of the activated fault segments. Our analyses indicate that the Nový Kostel area is more complex than was believed to be.

  • Possible role of fluids in the process of Earthquake Swarm generation in the West Bohemia/Vogtland seismoactive region
    Tectonophysics, 2001
    Co-Authors: Aleš Špičák, Josef Horálek
    Abstract:

    Abstract The West Bohemia/Vogtland region in the western part of the Bohemian Massif, known for periodic occurrence of Earthquake Swarms, is also characterised by 0.2–0.5 Ma old Quaternary volcanoes, rich CO2 emissions, anomalies of mantle-derived He, mineral springs and moffets. The detailed analysis of the time-space pattern of seismicity during the January 1997 Swarm demonstrates the gradual and ordered migration of micro-seismic activity, suggesting the step-by-step penetration of fluids into a small fractured volume. This scenario is supported by the similarity of the source mechanisms of the natural events of the January 1997 Swarm with artificial injection-induced micro-Earthquakes during the German deep drilling project (KTB) in neighbouring NE Bavaria. Also the comparison of the West Bohemia/Vogtland Earthquake Swarm region with other intraplate Earthquake Swarm regions and with Earthquake Swarms induced by current volcanic processes indicates a role of intrusions of magma and related fluids in generation process of Earthquake Swarms.

  • possible role of fluids in the process of Earthquake Swarm generation in the west bohemia vogtland seismoactive region
    Tectonophysics, 2001
    Co-Authors: Aleš Špičák, Josef Horálek
    Abstract:

    Abstract The West Bohemia/Vogtland region in the western part of the Bohemian Massif, known for periodic occurrence of Earthquake Swarms, is also characterised by 0.2–0.5 Ma old Quaternary volcanoes, rich CO2 emissions, anomalies of mantle-derived He, mineral springs and moffets. The detailed analysis of the time-space pattern of seismicity during the January 1997 Swarm demonstrates the gradual and ordered migration of micro-seismic activity, suggesting the step-by-step penetration of fluids into a small fractured volume. This scenario is supported by the similarity of the source mechanisms of the natural events of the January 1997 Swarm with artificial injection-induced micro-Earthquakes during the German deep drilling project (KTB) in neighbouring NE Bavaria. Also the comparison of the West Bohemia/Vogtland Earthquake Swarm region with other intraplate Earthquake Swarm regions and with Earthquake Swarms induced by current volcanic processes indicates a role of intrusions of magma and related fluids in generation process of Earthquake Swarms.

  • scenario of the january 1997 west bohemia Earthquake Swarm
    Studia Geophysica Et Geodaetica, 2000
    Co-Authors: Josef Horálek, Jan Silený, Tomas Fischer, Alice Slancova, Alena Bouskova
    Abstract:

    In order to learn more about the nature of the dynamic processes taking place in the West Bohemia/Vogtland Earthquake Swarm region, we investigated the temporal and spatial variations of the source mechanisms of the January 1997 Swarm beneath Nový Kostel (NKC). Visual analyses of WEBNET seismograms of over 800 events revealed that a specific feature of this Swarm was the occurrence of eight classes of multiplet events. The result of single-source, absolute moment tensor inversion of the P and SH peak amplitudes of a subset of 70 events representing all multiplet classes indicated that eight statistically significant types of mechanisms occurred during the Swarm. Two of them, typesAandBin our denotation, comprised all ML≥ 1.3 events and predominated in the Swarm. TypeAwere pure strike-slip mechanisms or strike-slip mechanisms containing a small normal component, with a nearly pure double-couple source. For classBevents, oblique-thrust faulting and non-double-couple components significant at a fairly high confidence level were typical. TypeAevents predominated in the southern subcluster of the Swarm, whereas most of typeBevents occurred in the subcluster northwards from NKC. This indicates that two major seismogenic planes were active during the Swarm. The Swarm essentially developed in four phases: in the first, typeAevents prevailed and the southern plane was active; during the second, characterised by the occurrence of both typeAandBevents (the former in the southern, the latter predominantly in the northern subcluster), the activity of the Swarm culminated; in the third and fourth, the occurrence of typeBevents in the northern plane predominated, and only weak single events occurred southwards from NKC. Mechanisms of typesAB,C,D,E,FandG, which were typical for ML≤1.2 events, occurred randomly throughout the Swarm. TypeABevents were identified in both the southern and northern clusters, typeC,E,FandGmechanisms only southwards from NKC. TypeDevents exhibited a large scatter of hypocentres which fell in neither the southern nor the northern cluster. Focal mechanisms like those reported in this study and with analogous temporal and spatial variations were observed by other authors already fifteen years ago in the 1985/86 Earthquake Swarm and may, therefore, be typical for the region under study.

Jochen H. Kurz - One of the best experts on this subject based on the ideXlab platform.

  • Comparative geodynamic modelling about Earthquake Swarm areas and processes.
    2006
    Co-Authors: Marco Naujoks, Gerhard Jentzsch, Thomas Jahr, Jochen H. Kurz
    Abstract:

    An Earthquake Swarm is a temporary and regional sequence of Earthquakes with several 1000 events within a period lasting from hours to days. Usually it occurs in an area of just a few square kilometers without any prevailing single event. Earthquake Swarms are observed worldwide, especially in connection with fluid movement and volcanism. To clarify possible physical mechanisms that lead to the phenomenon of Earthquake Swarms, two prominent regions are compared by numerical investigations using the finite element analysis software ABAQUS: The Vogtland/NW-Bohemia area situated at the border of Germany and the Czech Republic and the Magadi region in the Kenya Rift. Geodynamic models for the two regions were constructed which take into account the regional stress field and thermal stresses as well as creep and plasticity with a porous elastic rheology. The investigations are focussed on the interaction between pore pressure variations, temperature changes, fluid movement, stress accumulation and deformations. It is suspected that these processes play an essential role in the generation of Earthquake Swarms. The results of the modelling for the Magadi and Vogtland areas were compared with each other and with information from other Earthquake Swarm areas. Conclusions were drawn to general as well as to area-specific mechanisms. An essential result of the modelling is that the existence of the regional stress field alone neither explains the occurrence of the Earthquake Swarms in the Vogtland area nor in the Magadi area. It reveals that physical processes in different regions of the earth's crust are coupled. Temperature changes and periodic pore pressure variations in the earth's crust are most important for the geodynamic processes. In combination with the local geology and heterogenous distributed material parameters these processes have a crucial influence on the generation of Earthquake Swarms, even though they are weighed differently in each focal area. Consequently the modelling leads to an improved understanding of the processes and interactions that contribute to the occurrence of Earthquake Swarms.

  • Earthquake Swarm examples and a look at the generation mechanism of the vogtland western bohemia Earthquake Swarms
    Physics of the Earth and Planetary Interiors, 2004
    Co-Authors: Jochen H. Kurz, Thomas Jahr, Gerhard Jentzsch
    Abstract:

    Earthquake Swarms occur at many locations all over the world. In most cases, they do not occur just once but happen rather repeatedly at a particular location. We compared intraplate Earthquake Swarm regions, especially the Vogtland/Western Bohemia area, with plate margins, leading to the conclusion of a fluid triggered Earthquake Swarm mechanism which is independent of the global tectonics. These observations were brought together with theoretical models concerning hydro fracturing with respect to pore pressure changes due to fluid movements in seismically active areas. This concept was applied to a finite element model of the Vogtland/Western Bohemia Earthquake Swarm region. The results show that it is not possible to get high enough stress accumulations for the generation of Earthquake Swarms within relatively short timespans by the regional stress field alone. But periodic pore pressure variations and linear temperature changes caused realistic deformations and stress accumulations which coincide with the measured ones of the Vogtland/Western Bohemia Earthquake Swarm region.

  • Earthquake Swarm examples and a look at the generation mechanism of the Vogtland/Western Bohemia Earthquake Swarms
    Physics of the Earth and Planetary Interiors, 2004
    Co-Authors: Jochen H. Kurz, Thomas Jahr, Gerhard Jentzsch
    Abstract:

    Earthquake Swarms occur at many locations all over the world. In most cases, they do not occur just once but happen rather repeatedly at a particular location. We compared intraplate Earthquake Swarm regions, especially the Vogtland/Western Bohemia area, with plate margins, leading to the conclusion of a fluid triggered Earthquake Swarm mechanism which is independent of the global tectonics. These observations were brought together with theoretical models concerning hydro fracturing with respect to pore pressure changes due to fluid movements in seismically active areas. This concept was applied to a finite element model of the Vogtland/Western Bohemia Earthquake Swarm region. The results show that it is not possible to get high enough stress accumulations for the generation of Earthquake Swarms within relatively short timespans by the regional stress field alone. But periodic pore pressure variations and linear temperature changes caused realistic deformations and stress accumulations which coincide with the measured ones of the Vogtland/Western Bohemia Earthquake Swarm region.

Thomas Jahr - One of the best experts on this subject based on the ideXlab platform.

  • Comparative geodynamic modelling about Earthquake Swarm areas and processes.
    2006
    Co-Authors: Marco Naujoks, Gerhard Jentzsch, Thomas Jahr, Jochen H. Kurz
    Abstract:

    An Earthquake Swarm is a temporary and regional sequence of Earthquakes with several 1000 events within a period lasting from hours to days. Usually it occurs in an area of just a few square kilometers without any prevailing single event. Earthquake Swarms are observed worldwide, especially in connection with fluid movement and volcanism. To clarify possible physical mechanisms that lead to the phenomenon of Earthquake Swarms, two prominent regions are compared by numerical investigations using the finite element analysis software ABAQUS: The Vogtland/NW-Bohemia area situated at the border of Germany and the Czech Republic and the Magadi region in the Kenya Rift. Geodynamic models for the two regions were constructed which take into account the regional stress field and thermal stresses as well as creep and plasticity with a porous elastic rheology. The investigations are focussed on the interaction between pore pressure variations, temperature changes, fluid movement, stress accumulation and deformations. It is suspected that these processes play an essential role in the generation of Earthquake Swarms. The results of the modelling for the Magadi and Vogtland areas were compared with each other and with information from other Earthquake Swarm areas. Conclusions were drawn to general as well as to area-specific mechanisms. An essential result of the modelling is that the existence of the regional stress field alone neither explains the occurrence of the Earthquake Swarms in the Vogtland area nor in the Magadi area. It reveals that physical processes in different regions of the earth's crust are coupled. Temperature changes and periodic pore pressure variations in the earth's crust are most important for the geodynamic processes. In combination with the local geology and heterogenous distributed material parameters these processes have a crucial influence on the generation of Earthquake Swarms, even though they are weighed differently in each focal area. Consequently the modelling leads to an improved understanding of the processes and interactions that contribute to the occurrence of Earthquake Swarms.

  • Earthquake Swarm examples and a look at the generation mechanism of the vogtland western bohemia Earthquake Swarms
    Physics of the Earth and Planetary Interiors, 2004
    Co-Authors: Jochen H. Kurz, Thomas Jahr, Gerhard Jentzsch
    Abstract:

    Earthquake Swarms occur at many locations all over the world. In most cases, they do not occur just once but happen rather repeatedly at a particular location. We compared intraplate Earthquake Swarm regions, especially the Vogtland/Western Bohemia area, with plate margins, leading to the conclusion of a fluid triggered Earthquake Swarm mechanism which is independent of the global tectonics. These observations were brought together with theoretical models concerning hydro fracturing with respect to pore pressure changes due to fluid movements in seismically active areas. This concept was applied to a finite element model of the Vogtland/Western Bohemia Earthquake Swarm region. The results show that it is not possible to get high enough stress accumulations for the generation of Earthquake Swarms within relatively short timespans by the regional stress field alone. But periodic pore pressure variations and linear temperature changes caused realistic deformations and stress accumulations which coincide with the measured ones of the Vogtland/Western Bohemia Earthquake Swarm region.

  • Earthquake Swarm examples and a look at the generation mechanism of the Vogtland/Western Bohemia Earthquake Swarms
    Physics of the Earth and Planetary Interiors, 2004
    Co-Authors: Jochen H. Kurz, Thomas Jahr, Gerhard Jentzsch
    Abstract:

    Earthquake Swarms occur at many locations all over the world. In most cases, they do not occur just once but happen rather repeatedly at a particular location. We compared intraplate Earthquake Swarm regions, especially the Vogtland/Western Bohemia area, with plate margins, leading to the conclusion of a fluid triggered Earthquake Swarm mechanism which is independent of the global tectonics. These observations were brought together with theoretical models concerning hydro fracturing with respect to pore pressure changes due to fluid movements in seismically active areas. This concept was applied to a finite element model of the Vogtland/Western Bohemia Earthquake Swarm region. The results show that it is not possible to get high enough stress accumulations for the generation of Earthquake Swarms within relatively short timespans by the regional stress field alone. But periodic pore pressure variations and linear temperature changes caused realistic deformations and stress accumulations which coincide with the measured ones of the Vogtland/Western Bohemia Earthquake Swarm region.

Tomas Fischer - One of the best experts on this subject based on the ideXlab platform.

  • From Earthquake Swarm to a main shock–aftershocks: the 2018 activity in West Bohemia/Vogtland
    Geophysical Journal International, 2020
    Co-Authors: M. Bachura, Tomas Fischer, Jana Doubravová, Josef Horálek
    Abstract:

    SUMMARY In Earthquake Swarms, seismic energy is released gradually by many Earthquakes without a dominant event, which offers detailed insight into the processes on activated faults. The Swarm of May 2018 that occurred in West Bohemia/Vogtland region included more than 4000 Earthquakes with ML = 〈0.5, 3.8〉 and its character showed significant changes during the two weeks duration: what started as a pure Earthquake Swarm ended as a typical main shock–aftershock sequence. Based on precise double-difference relocations, four fault segments differing in strikes and dips were identified with similar dimensions. First, two segments of typical Earthquake Swarm character took place, and at the end a fault segment hosting a main shock–aftershock sequence was activated. The differences were observable in the Earthquakes spatio-temporal evolutions (systematic versus disordered migration of the hypocentres), b-values (>1.3 for the Swarm, <1 for the main shock–aftershocks), or the smoothness of seismic moment spatial distribution along the fault plane. Our findings can be interpreted by local variations of fault rheology, differential stress and/or smoothness of the faults surface, possibly related to the crustal fluids circulating along the fault plane and their interplay with the seismic cycle.

  • the august december 2000 Earthquake Swarm in nw bohemia the first results based on automatic processing of seismograms
    Journal of Geodynamics, 2003
    Co-Authors: Tomas Fischer
    Abstract:

    Main features of the August–December 2000 Earthquake Swarm which occurred in the major focal area of the North-West Bohemia / Vogtland Swarm region are presented. Seismograms from four stations of WEBNET were automatically processed to get arrival times, first motion amplitudes and hypocentre coordinates of a representative set of events. Altogether 7017 microEarthquakes in the magnitude range of ML=0–3.3 were identified. It is shown the decay of activity of individual Swarm phases followed the modified Omori law, which points to a partial similarity with aftershock sequences of tectonic Earthquakes. The space-time distribution of a subset of 2913 events with low location residuals shows a strong space clustering of the Earthquake hypocentres and their pronounced migration between individual Swarm phases. Most of the activity took place along an elliptical, nearly vertically dipping, 6 km long N-S oriented fault plane in depths ranging from 10.5 to 6.5 km. The P and T axes were estimated by FOCMEC for the 782 strong events and three groups of Earthquakes with similar faulting type were distinguished. In contrast to the normal and strike-slip faulting events that created the prevailing portion of the Swarm and were distributed uniformly within the focal area, the reverse events were clustered in time and space.

  • indications for a successively triggered rupture growth underlying the 2000 Earthquake Swarm in vogtland nw bohemia
    Journal of Geophysical Research, 2002
    Co-Authors: Sebastian Hainzl, Tomas Fischer
    Abstract:

    [1] The characteristics of Earthquake Swarms can neither be described by simple laws nor are the underlying mechanisms presently understood. Swarm activity is often assumed to be caused by an intrusion of fluids into the seismogenic zone. We have studied the Earthquake catalog of the large Earthquake Swarm that occurred in the year 2000 in Vogtland, SE-Germany and NW-Bohemia, an area well known for its episodic Swarm generation. We observe a significant decrease of the Gutenberg–Richter b value during the Swarm evolution as well as a fractal temporal clustering of the Earthquakes. The spatial spreading of the Swarm's activity, which is approximately confined to one plane, cannot simply be explained by a process of fluid diffusion. Instead, we observe a simple relationship between the spatial spreading and the seismic moment release, which is in good agreement with empirical relationships derived for tectonically driven Earthquakes and theoretical crack growth models. This observation points to a progressively growing main fracture underlying the Swarm activity. In addition, we find that the Swarm Earthquakes themselves trigger aftershocks near the border of their rupture area. The stick-slip behavior of the rupture propagation can be explained by stress transfers and induced fluid flows due to Earthquakes in a fluid-permeated critically loaded fault zone. However, during the first phase, the temporal behavior is found to be different, pointing to intrusion of fluids initiating the Swarm activity.

  • scenario of the january 1997 west bohemia Earthquake Swarm
    Studia Geophysica Et Geodaetica, 2000
    Co-Authors: Josef Horálek, Jan Silený, Tomas Fischer, Alice Slancova, Alena Bouskova
    Abstract:

    In order to learn more about the nature of the dynamic processes taking place in the West Bohemia/Vogtland Earthquake Swarm region, we investigated the temporal and spatial variations of the source mechanisms of the January 1997 Swarm beneath Nový Kostel (NKC). Visual analyses of WEBNET seismograms of over 800 events revealed that a specific feature of this Swarm was the occurrence of eight classes of multiplet events. The result of single-source, absolute moment tensor inversion of the P and SH peak amplitudes of a subset of 70 events representing all multiplet classes indicated that eight statistically significant types of mechanisms occurred during the Swarm. Two of them, typesAandBin our denotation, comprised all ML≥ 1.3 events and predominated in the Swarm. TypeAwere pure strike-slip mechanisms or strike-slip mechanisms containing a small normal component, with a nearly pure double-couple source. For classBevents, oblique-thrust faulting and non-double-couple components significant at a fairly high confidence level were typical. TypeAevents predominated in the southern subcluster of the Swarm, whereas most of typeBevents occurred in the subcluster northwards from NKC. This indicates that two major seismogenic planes were active during the Swarm. The Swarm essentially developed in four phases: in the first, typeAevents prevailed and the southern plane was active; during the second, characterised by the occurrence of both typeAandBevents (the former in the southern, the latter predominantly in the northern subcluster), the activity of the Swarm culminated; in the third and fourth, the occurrence of typeBevents in the northern plane predominated, and only weak single events occurred southwards from NKC. Mechanisms of typesAB,C,D,E,FandG, which were typical for ML≤1.2 events, occurred randomly throughout the Swarm. TypeABevents were identified in both the southern and northern clusters, typeC,E,FandGmechanisms only southwards from NKC. TypeDevents exhibited a large scatter of hypocentres which fell in neither the southern nor the northern cluster. Focal mechanisms like those reported in this study and with analogous temporal and spatial variations were observed by other authors already fifteen years ago in the 1985/86 Earthquake Swarm and may, therefore, be typical for the region under study.