Swarm Events Research Articles

Related spatio-temporal changes in hypocenters and the b value in the 2017 Kagoshima Bay swarm activity indicating a rise of hot fluids

Spatio-temporal characteristics of the b value in relation to changes in hypocentral distribution and the occurrence rate of swarm earthquakes in the 2017 Kagoshima Bay activity were investigated. Noting stepwise changes in the area as well as the occurrence rate of swarm events, we divided the activity from December 2016 to October 2017 into six stages. Characteristics of the activity in each stage were clearly grasped when swarm area was divided into several rectangular zones in a NNW-SSE direction and when hypocenters and the b value in each zone were examined in detail. After the largest shock with a magnitude (M) of 5.3 on July 11, 2017, the swarm area expanded conspicuously. However, the dominant factor that played a leading role in the development of swarm activity was not the occurrence of the M 5.3 shock, but activity in the zone (zone B) just to the west of the focal region. The number of events in zone B was more than two times the whole number of events that occurred in the entire swarm area, except for zone B. The hypocentral distribution in zone B was relatively shallow from early stages, but the most notable feature was the abrupt diffusion of events to a shallower layer at the time of the M 4.4 shock on August 24, 2017. The b value in the central part of zone B was noticeably high from earlier stages, but remarkably high for the shallow events of the burst-like activity after the M 4.4 shock. From these spatio-temporal characteristics of hypocenters and the b value, we propose that hot fluids that had existed beneath the central part of zone B ascended at the time of the M 4.4 shock. Moreover, hot fluids were the major cause of the 2017 Kagoshima Bay swarm activity.

Sendai-Okura earthquake swarm induced by the 2011 Tohoku-Oki earthquake in the stress shadow of NE Japan: Detailed fault structure and hypocenter migration

We investigated the distribution and migration of hypocenters of an earthquake swarm that occurred in Sendai-Okura (NE Japan) 15days after the 2011 M9.0 Tohoku-Oki earthquake, despite the decrease in shear stress due to the static stress change. Hypocenters of 2476 events listed in the JMA catalogue were relocated based on the JMA unified catalogue data in conjunction with data obtained by waveform cross correlation. Hypocenter relocation was successful in delineating several thin planar structures, although the original hypocenters presented a cloud-like distribution. The hypocenters of this swarm event migrated along several planes from deeper to shallower levels rather than diffusing three-dimensionally. One of the nodal planes of the focal mechanisms was nearly parallel to the planar structure of the hypocenters, supporting the idea that each earthquake occurred by causing slip on parts of the same plane. The overall migration velocity of the hypocenters could be explained by the fluid diffusion model with a typical value of hydraulic diffusivity (0.15m2/s); however, the occurrence of some burst-like activity with much higher migration velocity suggests the possibility that aseismic slip also contributed to triggering the earthquakes. We suggest that the 2011 Sendai-Okura earthquake swarm was generated as follows. (1) The 2011 Tohoku-Oki earthquake caused WNW–ESE extension in the focal region of the swarm, which accordingly reduced shear stress on the fault planes. However, the WNW–ESE extension allowed fluids to move upward from the S-wave reflectors in the mid-crust immediately beneath the focal region. (2) The fluids rising from the mid-crust intruded into several existing planes, which reduced their frictional strengths and caused the observed earthquake swarm. (3) The fluids, and accordingly, the hypocenters of the triggered earthquakes, migrated upward along the fault planes. It is possible that the fluids also triggered aseismic slip, which caused intermittent burst-like activity.

Clues on the origin of post-2000 earthquakes at Campi Flegrei caldera (Italy)

The inter-arrival times of the post 2000 seismicity at Campi Flegrei caldera are statistically distributed into different populations. The low inter-arrival times population represents swarm events, while the high inter-arrival times population marks background seismicity. Here, we show that the background seismicity is increasing at the same rate of (1) the ground uplift and (2) the concentration of the fumarolic gas specie more sensitive to temperature. The seismic temporal increase is strongly correlated with the results of recent simulations, modelling injection of magmatic fluids in the Campi Flegrei hydrothermal system. These concurrent variations point to a unique process of temperature-pressure increase of the hydrothermal system controlling geophysical and geochemical signals at the caldera. Our results thus show that the occurrence of background seismicity is an excellent parameter to monitor the current unrest of the caldera.

Source characteristics and geological implications of the January 2016 induced earthquake swarm near Crooked Lake, Alberta

On 2016 January 12, an intraplate earthquake with an initial reported local magnitude (ML) of 4.8 shook the town of Fox Creek, Alberta. While there were no reported damages, this earthquake was widely felt by the local residents and suspected to be induced by the nearby hydraulic-fracturing (HF) operations. In this study, we determine the earthquake source parameters using moment tensor inversions, and then detect and locate the associated swarm using a waveform cross-correlation based method. The broad-band seismic recordings from regional arrays suggest a moment magnitude (M) 4.1 for this event, which is the largest in Alberta in the past decade. Similar to other recent M ∼ 3 earthquakes near Fox Creek, the 2016 January 12 earthquake exhibits a dominant strike-slip (strike = 184°) mechanism with limited non-double-couple components (∼22 per cent). This resolved focal mechanism, which is also supported by forward modelling and P-wave first motion analysis, indicates an NE–SW oriented compressional axis consistent with the maximum compressive horizontal stress orientations delineated from borehole breakouts. Further detection analysis on industry-contributed recordings unveils 1108 smaller events within 3 km radius of the epicentre of the main event, showing a close spatial-temporal relation to a nearby HF well. The majority of the detected events are located above the basement, comparable to the injection depth (3.5 km) on the Duvernay shale Formation. The spatial distribution of this earthquake cluster further suggests that (1) the source of the sequence is an N–S-striking fault system and (2) these earthquakes were induced by an HF well close to but different from the well that triggered a previous (January 2015) earthquake swarm. Reactivation of pre-existing, N−S oriented faults analogous to the Pine Creek fault zone, which was reported by earlier studies of active source seismic and aeromagnetic data, are likely responsible for the occurrence of the January 2016 earthquake swarm and other recent events in the Crooked Lake area.

Hypocenter Relocation of Earthquake Swarm in West Halmahera, North Molucca Region, Indonesia by using Double-Difference Method and 3D Seismic Velocity Structure

The earthquake swarm events sequence occurred in west Halmahera, north Molucca, Indonesia for the period of October 2015 to February 2016 as reported by Meteorological, Climatological, and Geophysical Agency (BMKG) of Indonesia. There were tenths swarm events with Magnitude larger than four in the region during the period. In this study, we used the earthquake catalog data compiled by BMKG to improve the location of swarms event in west Halmahera, north Molucca, Indonesia. We relocated 86 swarm events by applying teleseismic double-difference method and 3D seismic velocity model. The focus depth of swarm events mainly concentrated at depth of 5 to 12 km at south-east of Jailolo volcano. Our preliminary interpretation the earthquake swarms may be related to the stress change around the deep magma region of the volcano.

Field Investigation of the November to December 2015 Earthquake Swarm in West Halmahera, Indonesia

The West Halmahera Swarm (WHS) event occurred from November to December 2015. A seismic station located in Ternate City detected at least 1000 shocks of magnitude (M) ≥ 1 and 11 shocks of M ≥ 4.5. Our estimation yields a cumulative seismic moment of 3.87 × 1021 N m, which is equivalent to a single M 8.3 earthquake. We conducted a field survey after the swarm event and found that roads and residents’ houses experienced cracks, while some parts of buildings were destroyed during the event. We find that the 2015 WHS was located just below the Jailolo volcano and was strongly associated with volcanic activity.

The 2014–2015 earthquake series in the northern Upper Rhine Graben, Central Europe

Since March 2014, an unusually large amount of earthquakes occur southeast of the city of Darmstadt in the northern Upper Rhine Graben. During the period, until April 2015, we have recorded 356 earthquakes with magnitudes ranging from ML = −0.6 to 4.2. We identified two source clusters separated laterally by about 5 km. The hypocentres within these clusters are aligned vertically extending over a depth range from 1 to 8 km with a lateral extent of about 1 to 2 km. Focal mechanisms show left-lateral strike-slip movements; b values are changing with time between b = 0.6 and b = 0.9. This is the first time in almost 150 years that such high earthquake rates have been observed in the region. Historical accounts dating back to the nineteenth century report of over 2000 felt earthquakes over a time span from 1869 to 1871. From these, maximum intensities of VII have been estimated. Other seismic activities in the region were reported in the 1970s. The observations of the 2014–2015 earthquake series do not completely match a typical main shock–aftershock sequence or a typical earthquake swarm. Especially the activity at the beginning of the earthquake series may be considered as a mixture of a main shock–aftershock sequence and a short-lasting swarm event. Whether or not the time gap between the current seismic activity, which actually takes place at the same locations as parts of the seismic swarm in 1869–1871, and the seismic activity in the nineteenth century or the seismic activity in the 1970s can be interpreted as a seismic cycle remains unclear.

Mid-crustal fluid related to the Matsushiro earthquake swarm (1965–1967) in northern Central Japan: Geochemical reproduction

The Matsushiro district in the northern part of Ngano Prefecture, Central Japan, experienced a severe earthquake swarm associated with ground uplift and groundwater outflow during a period from 1965 to 1967. Geochemistry and isotope geochemistry of surface water and groundwater in the swarm area have been investigated. Water samples for the analyses were obtained from saline springs and streams, fresh water from streams and a shallow well, and, from seven wells of up to 600m deep drilled in the Matsushiro fault zone. The results of analyses of water from the deep wells in the fault zone indicate a linear increase in dissolved Na, K, Al, Cl, Br, B and SiO4, and isotopic ratios of δD, δ18O and δ13C. The hydrogen and oxygen isotopic ratios of the well water indicate a mixing between the surface water and isotopically heavy water exemplified by the “andesitic water” of Giggenbach (1992). Assuming that the isotopically heavy water has δD similar to the “andesitic water”, the geochemistry of deep-seated brine is reproduced from the analytical data of well water. The reproduced composition of deep-seated brine is enriched in Na, Ca, Cl and HCO3 and has total dissolved solids comparable to seawater. The water of the reproduced composition is estimated to occupy a deep part (>3km below surface) of the fault zone. Available data show that the geochemical regime in the Matsushiro fault zone has remained unchanged since the swarm event. The reproduced composition of water is concluded to be similar to the brine triggering the swarm of earthquakes.The composition of the reproduced water is not as saline as the deep-seated fluid derived from dehydration of a subducting oceanic plate which results in water having a Cl content about twice that of the seawater. The relatively low-salinity nature comparable to seawater is one of the characteristics of mid-crustal fluid in volcanic fields.

Swarms: Spatiotemporal grouping across domains

This paper presents cross-domain evidence that natural language makes use of (at least) two ways of individuating collective entities that differ in terms of how they cohere. The first kind, which I call swarm reference, picks out higher-order collective entities defined in terms of the spatial and temporal configuration of their constituent individuals. The second, which corresponds to canonical cases of group reference (e.g. committee, team, etc.), makes use of non-spatiotemporal notions. To motivate this distinction, I present systematic differences in how these two types of collective reference behave linguistically, both in the individual and event domains. These differences support two primary results. First, they are used as tests to isolate a new class of collective nouns that denote swarm individuals, both in English, as well as other languages like Romanian. I then consider a crosslinguistically common type of pluractionality, called event-internal in the previous literature (Cusic 1981, Wood 2007), and show that its properties are best explained if the relevant verbs denote swarm events. By reducing event-internal pluractionality to a type of collective reference also available for nouns, this work generates a new strong argument that pluractionality involves the same varieties of plural reference in the event domain that are seen in the individual domain.

Waveform inversion and analysis of an unusual earthquake swarm in the Boshan mining area, Shandong Province, China

Moment tensor solutions were retrieved for the earthquake swarm that occurred during November and December 2010 in the Boshan mining area, Shandong Province, China. The results showed that the double-couple components in the source mechanisms were higher at the beginning of the swarm and consisted mainly of shear faulting controlled by tectonic stress. The subsequent events had significant non-double-couple components, indicating tensile faulting. The double-couple components predominately presented as normal faulting and the P axes were orientated almost vertically. The slip vectors of the swarm events were relatively stable. With reference to the tectonic features near the epicenter, we concluded that the swarm was a result of subordinate fault motion related to the Wangmu Mountain fault and that high-pressure pore fluids played a crucial role in the activity of the earthquake swarm.

An Enigmatic Little Earthquake Swarm near Searsport, Maine

A swarm of 21 small earthquakes, with the largest being MLg 1.7, was recorded by regional seismic network monitoring from near Searsport, Maine, in April and May 2011. An additional five events were detected by two portable seismic instruments that were installed in the Searsport area for the later part of the swarm. Relative locations of the larger events of the swarm, computed in relation to a selected master event, showed that the swarm events extended for a distance of about 2.5 km and migrated from northeast to southwest. The events also became shallower toward the southwest. If the area of the swarm had ruptured in a single earthquake, the magnitude of the event would have been about M 5.1–5.5. The S‐P time of only about 0.34 s at one of the portable seismic stations for the detected events from the swarm indicates that the station was located about 2.7 km from the hypocenters, thus constraining the location of the southwest end of the swarm. The events took place within the Devonian Mount Waldo pluton, a granitic body that locally cuts northeast–couthwest‐oriented thrust faults that parallel the Norumbega fault zone. The trend of the swarm events is parallel to and on‐strike with the trend of a thrust fault mapped to the southwest of the Mount Waldo pluton. The seismic data suggest that the fault might be seismically active, although the modern seismotectonic relationship of the fault and the pluton is far from clear.

Relationships among seismic velocity, metamorphism, and seismic and aseismic fault slip in the Salton Sea Geothermal Field region

AbstractThe Salton Sea Geothermal Field is one of the most geothermally and seismically active areas in California and presents an opportunity to study the effect of high‐temperature metamorphism on the properties of seismogenic faults. The area includes numerous active tectonic faults that have recently been imaged with active source seismic reflection and refraction. We utilize the active source surveys, along with the abundant microseismicity data from a dense borehole seismic network, to image the 3‐D variations in seismic velocity in the upper 5 km of the crust. There are strong velocity variations, up to ~30%, that correlate spatially with the distribution of shallow heat flow patterns. The combination of hydrothermal circulation and high‐temperature contact metamorphism has significantly altered the shallow sandstone sedimentary layers within the geothermal field to denser, more feldspathic, rock with higher P wave velocity, as is seen in the numerous exploration wells within the field. This alteration appears to have a first‐order effect on the frictional stability of shallow faults. In 2005, a large earthquake swarm and deformation event occurred. Analysis of interferometric synthetic aperture radar data and earthquake relocations indicates that the shallow aseismic fault creep that occurred in 2005 was localized on the Kalin fault system that lies just outside the region of high‐temperature metamorphism. In contrast, the earthquake swarm, which includes all of the M > 4 earthquakes to have occurred within the Salton Sea Geothermal Field in the last 15 years, ruptured the Main Central Fault (MCF) system that is localized in the heart of the geothermal anomaly. The background microseismicity induced by the geothermal operations is also concentrated in the high‐temperature regions in the vicinity of operational wells. However, while this microseismicity occurs over a few kilometer scale region, much of it is clustered in earthquake swarms that last from hours to a few days and are localized near the MCF system.

Seismic detection of an active subglacial magmatic complex in Marie Byrd Land, Antarctica

Numerous volcanoes exist in Marie Byrd Land, a highland region of West Antarctica. High heat flow through the crust in this region may influence the stability of the West Antarctic Ice Sheet 1‐4 . Volcanic activity progressed from north to south in the Executive Committee mountain range between the Miocene and Holocene epochs, but there has been no evidence for recent magmatic activity 5‐7 . Here we use a recently deployed seismic network to show that in 2010 and 2011, two swarms of seismic activity occurred at 25‐40 km depth beneath subglacial topographic and magnetic highs, located 55 km south of the youngest subaerial volcano in the Executive Committee Range. We interpret the swarm events as deep long-period earthquakes based on their unusual frequency content. Such earthquakes occur beneath active volcanoes, are caused by deep magmatic activity and, in some cases, precede eruptions 8‐11 . We also use radar profiles to identify a prominent ash layer in the ice overlying the seismic swarm. Located at 1,400 m depth, the ash layer is about 8,000 years old and was probably sourced from the nearby Mount Waesche volcano. Together, these observations provide strong evidence for ongoing magmatic activity and demonstrate that volcanism continues to migrate southwards along the Executive Committee Range. Eruptions at this site are unlikely to penetrate the 1.2 to 2-km-thick overlying ice, but would generate large volumes of melt water that could significantly affect ice stream flow.

Seismic swarms, fluid flow and hydraulic conductivity in the forearc offshore North Costa Rica and Nicaragua

At the continental margin of north Costa Rica and Nicaragua, the strongly hydrated Cocos Plate subducts beneath the Caribbean Plate. From the downgoing Cocos plate fluids are released through extensional fractures in the overriding plate. At the seafloor, they form fluid seeps, mounds and other types of fluid expulsion. Using an offshore temporary seismic network, we investigated seismicity possibly related to these processes and observed several swarms of earthquakes located on the continental slope trenchward of the seismogenic zone of S Nicaragua. The seismicity occurred within the downgoing plate, near the plate interface and in the overriding plate. We interpret these swarm events as an expression of pore pressure propagation under critical stress conditions driven by fluid release from the downgoing plate. In order to estimate hydraulic diffusivity and permeability values, we applied a theory developed for injection test interpretation to the spatio-temporal development of the swarms. The resulting diffusivity and permeability values are in the ranges of 28–305 m²/s and 3.2 × 10−14 m²–35.1 × 10−14 m², respectively, applying to the continental and oceanic crust near the plate interface. These values are somewhat larger than observed in drill logs on the margin wedge off north Costa Rica, but of comparable magnitude to values estimated for the Antofagasta 1995 earthquake aftershock sequence.

Evaluation of seismogenesis behavior in Himalayan belt using data mining tools for forecasting

AbstractIn the proposed study, non-linear behavioral patterns in the seismic regime for earthquakes in the Himalayan basin have been studied using a complete, verified EQ catalogue comprised of all major events and their aftershock sequences in the Himalayan basin for the past 110 years [1900–2010]. The dataset has been analyzed to give better decision making criteria for impending earthquakes. A series of statistical tests based on multi-dimensional rigorous statistical studies, inter-event distance analyses, and statistical time analyses have been used to obtain correlation dimensions. The time intervals of earthquakes within a seismic regime have been used to train the neural network to analyze the nature of earthquake patterns in the different clusters. The results obtained from descriptive statistics show high correlation with previously conducted gravity studies and radon anomaly variation. A study of the time of recurrence of the numerical properties of the regime for 60 years from 1950 to 2010 for the Himalayan belt for analysis of significant EQ failure events has been done to find the best fit for an empirical data probability distribution. The distribution of waiting time of swarm events occurring in the Himalayan basin follows a power-law model, while independent events do not fit the power-law distribution. This suggests that probability of the occurrence of swarm events [M ⩽ 6.0] with frequent shaking may be more frequent than that of the occurrence of independent events of magnitude [M >6.0] in the Himalayan belt. We propose a three-layer feed forward neural network model to identify factors, with the actual occurrence of the maximum earthquake level M as input and target vectors in Himalayan basin area. We infer through a series of statistical results and evaluations that probabilistic forecasting of earthquakes can be achieved by finding the meta-stable cluster zones of the Himalayan clusters for the spatio-temporal distribution of earthquakes in the area.

Meteorological Influences on Swarm Emergence in Honey Bees (Hymenoptera: Apidae) as Detected by Crowdsourcing

A crowdsourced dataset of 1,335 honey bee (Apis mellifera L.) swarm events in Germany in 2011 was created by beekeepers, public institutions, and members of the public and analyzed with respect to prevailing weather. The emergence of swarms appeared to be influenced by temperature and rainfall. On successive warm days in May the number of swarming events increased noticeably, but during a mid-month frost event the number of swarming events dropped markedly. Swarming events also occurred only rarely on rainy days. This study showed how crowdsourcing can be used to generate large, useful, phenological datasets.

Fluid-triggered earthquake swarms in the Rwenzori region, East African Rift—Evidence for rift initiation

The Rwenzori Mountains are located within the Albertine Rift Valley in western Uganda. To monitor the microseismic activity in the area we have deployed a seismic network of up to 35 stations for a period of about 20months. The analysis of the recordings revealed several earthquake clusters within a restricted area NE of the mountain block. The clusters form elongated pipes with 1–2km diameter and vertical extensions of 3–5km. Most of them are located in 5–16km depths; however one cluster reaches down to 22km. Each cluster is composed of a series of single earthquake swarms with durations between a few days and more than a week, interrupted by intervals of inactivity of up to several months. Some of the swarm events exhibit vertical migration tendencies with estimated velocities between 0.3 and 1km/day. Local magnitudes range from ML=0.5 to ML=4.0 with b-values between 0.96 and 1.2. The source mechanisms of the swarm earthquakes are dominated by normal faulting with tension-axes orientations perpendicular to the rift axis. There are only few strike-slip events and no reverse mechanisms. From petrological considerations we presume that the earthquake swarms are triggered by fluids and gases which originate from a magmatic source below the crust. Melt and/or CO2 are guided along the intersection lines of two steep fault sets that were identified by shear-wave splitting analysis and fault mapping in the Rwenzori area. The existence of a magmatic source within the lithosphere is supported by the detection of a shear-wave velocity reduction in 55–80km depth from receiver-function analysis and the location of mantle earthquakes at about 60km. We interpret these observations as indication for an initial rifting process that may eventually lead to the complete detachment of the Rwenzori block from the surrounding rift flanks.

Mass estimates of the scalar seismic moments of small earthquake foci on southern Sakhalin

The frequency dependence of the function of the seismic wave attenuation was determined for the first time for southern Sakhalin on the basis of seismic coda of local earthquakes using the model of single scattering. The algorithm of the automated definition of the scalar seismic moments was realized for small earthquake foci. Mass estimates of the scalar seismic moments were obtained as exemplified by the after-shocks of the August 17, 2006, Gornozavodsk earthquake (MW 5.6) and the May–June 2004 Kostromskoe earthquake swarm events, which occurred in South Sakhalin. The dynamic parameters of the earthquake foci were determined from the SH-wave spectra adjusted for absorption and geometrical spreading. The loglinear relationship determined between the seismic moment and the local magnitude is in good agreement with the estimates obtained for other regions and, in a certain sense, does not contradict the average world dependence.

Intraplate earthquake swarms in West Bohemia/Vogtland (Central Europe)

West Bohemia (Czech Republic) and Vogtland (Germany) are among the most active intraplate earthquake-swarm areas in Europe with the largest events mostly of magnitudes $M_L$ \< 4.0. The principal char-acteristics of the West Bohemia/Vogtland earthquake swarms are derived on the basis of local observations from the network WEBNET during the period between 1991 and 2009. Swarm microearthquakes clustered in number of small focal areas; however, about 90\% of the total seismic moment was released in the Nový Kostel (NK) focal zone, which was formed by an NNW striking and steeply dipping fault plane. The focal depths ranged between 5 and 22 km in the whole region and between 4.5 and 11 km in the NK zone, while most of the swarm events clustered at depths from 8.5 to 9.5 km. All larger earthquake swarms took place in the NK zone; though they were located close, they differed significantly in their evolution. Two swarms, the 2000 ($M_L$ ≤ 3.3) and 2008 ($M_L$ ≤ 3.8) swarms were located on the same portion of the NK focal zone which implies reactivation of the same fault segment. Attention was paid to source mechanisms of the 1997 and 2000 swarms, particularly to the non-shear components in the 1997-swarm earthquakes. All the 1997 sources were of dipole character: the slightly compressive dipoles dominating in the first swarm phase were replaced by tensile dipoles in the second phase. On the contrary, the 2000 swarm events possessed pure double-couple sources which resulted in pure shears along the NK fault. We infer that the pure-shear rupturing in the 2000 swarm was on account of the favourably oriented NK fault plane with respect to the local tectonic stress field whereas the additional tensile forces were needed for rupturing due to the unfavourably oriented 1997-swarm fault segments. Further, we analyzed statistic characteristics and we show that the magnitude-frequency distribution with the b-value ≈ 1.0 is typical for the West Bohemia/Vogtland earthquake swarms and that scalar moments $M_0$ and the WEBNET magnitudes $M_L$ are related according to power-law $M_0$ ∝ 10$^{M_L}$, which is inconsistent with the definition of the moment magnitude $M_0$ ∝ 10$^{1.5 M_w}$ given by Kanamori (1977). We also show that this inconsistency results in a discrepancy in b-value of the magnitude-frequency distribution. Taking into account total seismic moment of the 2000 and 2008 swarms we infer that $M_{L_{MAX}}$ ∼ 5.0 to 5.3 is the maximum expected magnitude in the main NK focal zone and thus in the whole region. There is still an open question concerning the internal and external triggering of the West Bohemia/Vogtland earthquake swarms. We refer to the results of our recent study and infer that the earthquake swarm probably represents gradually propagating rupture along the fault and that both static and dynamic Coulomb stress changes along the fault plane due to co-seismic slip contribute significantly to triggering of the swarm events. Repeatedly observed almost simultaneous occurrence of seismic activity in different focal zones suggests that common triggering forces act in a broader area of West Bohemia/Vogtland.

Characteristics of earthquake sequence in northern Himalayan region of South Central Tibet – Precursor search and location of potential area of future earthquake

This paper reports that three earthquake sequence occurred in 1996 ( m b 5.9), 1998 ( m b 5.8) and 2004–2005 ( m b 6.2, 6.3) were observed preceding the anomalous seismic activity. The nature of anomalous seismicity associated with these earthquakes have been investigated considering earthquakes with m b ⩾ 4.1 (cut off magnitude for this region) in four seismic episodes (NAGM): normal/background (N); anomalous/swarm (A); precursory gap (G) and mainshock sequence (M), respectively. It is established here that the anomalous seismicity/swarm patterns follow episodes of relatively very low seismic activity and it is an important finding to visualize that an area might be preparing for the occurrence of a forthcoming mainshock. Such anomalous seismic patterns were observed prior to medium size mainshocks that occurred from 1963 to 2006 in South Central Tibet (SCT) region to the north of Central Himalaya. From these observations it may be inferred here that the patterns of anomalous seismicity/earthquake swarms may be considered as an important parameter (precursor) for the forecasting of long-range earthquake hazards in the SCT region. In consideration of spatial and the temporal clustering of swarm events that are prominent and confined in a vertical column of 10–45 km, enabled to locate potential area enclosed by 29.6–30.1°N and 87.8–88.1°E where future earthquake of magnitude 6.0 and above in the depth range 25 ± 15 km may be seated. In view of this multi-parameter short term precursory signals monitoring is suggested for the delineated preparatory area to make earthquake prediction/forecasting programs more meaningful.