Swarming Period Research Articles

Hybridization hotspots at bat swarming sites.

During late summer and early autumn in temperate zones of the Northern Hemisphere, thousands of bats gather at caves, mainly for the purpose of mating. We demonstrated that this swarming behavior most probably leads not only to breeding among bats of the same species but also interbreeding between different species. Using 14 nuclear microsatellites and three different methods (the Bayesian assignment approaches of STRUCTURE and NEWHYBRIDS and a principal coordinate analysis of pairwise genetic distances), we analyzed 375 individuals belonging to three species of whiskered bats (genus Myotis) at swarming sites across their sympatric range in southern Poland. The overall hybridization rate varied from 3.2 to 7.2%. At the species level, depending on the method used, these values ranged from 2.1–4.6% in M. mystacinus and 3.0–3.7% in M. brandtii to 6.5–30.4% in M. alcathoe. Hybrids occurred in about half of the caves we studied. In all three species, the sex ratio of hybrids was biased towards males but the observed differences did not differ statistically from those noted at the population level. In our opinion, factors leading to the formation of these admixed individuals and their relatively high frequency are: i) swarming behaviour at swarming sites, where high numbers of bats belonging to several species meet; ii) male-biased sex ratio during the swarming period; iii) the fact that all these bats are generally polygynous. The highly different population sizes of different species at swarming sites may also play some role. Swarming sites may represent unique hybrid hotspots, which, as there are at least 2,000 caves in the Polish Carpathians alone, may occur on a massive scale not previously observed for any group of mammal species in the wild. Evidently, these sites should be treated as focal points for the conservation of biodiversity and evolutionary processes.

First Sex Pheromone of the Order Strepsiptera: (3R,5R,9R)-3,5,9-Trimethyldodecanal in Stylops melittae Kirby, 1802

The twisted-wing parasites (Strepsiptera) are an unusual and small order of insects with about 600 known species. As obligate endoparasitoids, they develop and spend most of their lives living in other insects. Adults show an extreme sexual dimorphism: The free-living males have large eyes, branched antennae, reduced forewings, and well developed hind wings, while the neotenic females of most species lack all external characters that normally define an insect, remain endoparasitic, and only extrude the cephalothorax from the host. Due to the males' short life span of only a few hours, there must be an efficient means of mate finding. This is believed to be mediated by chemical cues released by virgin females. Here, we report the first identification and synthesis of a female-produced strepsipteran sex pheromone, (3R,5R,9R)-3,5,9-trimethyldodecanal, from Stylops melittae, a species parasitizing andrenid bees. We found this highly EAD-active compound to be present in cephalothoraxes of and released from unmated females, and synthetic samples proved to be extremely attractive when offered in the field during the swarming period of the males. The structural features of this new natural compound may further support the re-establishment of the Strepsiptera as the closest living relatives of the Coleoptera.

The relationship between potential solar radiation and spruce bark beetle catches in pheromone traps

We analysed the relationship between the amount of potential solar radiation and spruce bark beetle Ips typographus (L.) catches in pheromone traps in an unmanaged nature reserve in the Carpathians (middle Slovakia region), from 2006 through 2009. This relationship was analysed under outbreak conditions. The number of traps varied in different years from 70 to 92. The traps were installed in spruce-forest-dominated stands affected by a windstorm in 2004. A GPS device was used to mark the position of the pheromone traps. The potential solar radiation was calculated with GIS tools for three different time periods in each year: with entire year, for main flight season of the spruce bark beetle and the spring swarming period. The relationship between the amount of potential solar radiation and the spruce bark beetle catches was statistically significant for each year and each time period except for the spring warming in 2007, when the pheromone traps were not set up on time.

Genetic structure in three species of whiskered bats (genusMyotis) during swarming

We studied the population structure of 3 species in the Myotis mystacinus complex (M. mystacinus, M. brandtii, and M. alcathoe) during the swarming period by sampling bats at 27 caves in the Carpathian Mountains and adjacent areas in southern Poland using 14 or 15 nuclear microsatellite loci. None of the examined species were in Hardy-Weinberg equilibrium following the global test for heterozygote deficiency. Inbreeding coefficient (FIS) values at the species level also were statistically greater than zero. This may indicate some level of inbreeding in all examined taxa although it seems to be relatively low because significant Fis values were recorded in only 2 swarming sites of M. mystacinus, 2 sites of M. brandtii, and 3 sites of M. alcathoe. However, almost 10% of related individuals sharing 1 parent (i.e., half siblings) were found in the same swarming sites for M. mystacinus and M. brandtii, with fewer observed in M. alcathoe. At the population level, 4.1%, 5.9%, and 8.7% of individuals of M. mystacinus, M. brandtii, and M. alcathoe, respectively, were assigned as full siblings. These figures suggest the possibility of females mating selectively with the same male in more than 1 year.

Swarming and Migration of Apis dorsata and Apis laboriosa Honey Bees in India, Nepal and Bhutan

Swarming and Migration ofApis dorsataandApis laboriosaHoney Bees in India, Nepal and BhutanThe migratory open air nestingApis dorsataandApis laboriosahoneybees migrate at least twice a year. DNA genotyping showed that the same swarms return to their natal nesting sites. We examined 23 nesting sites in Nepal, India and Bhutan, on which 587 colonies ofA. dorsataandA. laboriosanested. The results showed that the frequency of the periodic mass flights (PMF) performed by the colonies is a good indicator of the status of current colony performance. During the swarming period, both,A. dorsataandA. laboriosaissue several swarms. In some colonies, so many bees swarmed out, that those remaining in the maternal colonies did not cover the combs. After the rest of the brood emerged, all the bees of such colonies abscond during the swarming period. Thus, absconding appeared in results of total out swarming. The swarms do not migrate directly to the seasonal alternative nesting sites, but establish new colonies in the areas around. After environmental conditions deteriorate, all the bees with their queens abscond and migrate to alternate seasonal nesting sites. The next season, the swarms do not return to their original reproductive natal sites, but to those sites they occupied the previous season lately, where from they absconded.

Multiple causes for non-eruptive seismic swarms at Mt. Martin, Katmai Volcanic Cluster, Alaska (2004–2008)

In January 2006, the Alaska Volcano Observatory recorded a non-eruptive swarm of over 778 volcano-tectonic (VT) earthquakes in the Katmai Volcanic Cluster (KVC), Alaska. The swarm earthquakes have estimated hypocentral depths of 0–3km below sea level (BSL) immediately below the summit of Mt. Martin volcano. In an effort to determine the cause of this swarm, we calculated 134 double-couple fault plane solutions (FPS) for a pre-swarm period (Jan 2004–Dec 2005), the swarm period (January 2006), and a post-swarm period (Feb 2006–Dec 2008), and examined temporal changes in FPS orientations using directional statistical analysis. Statistical tests of uniformity indicate that FPS orientations were heterogeneous during the pre-swarm period, and homogeneous during the 2006 swarm. 3D tests on P- and T-axis orientation data also indicate that FPS homogeneity may have persisted during the post-swarm period. Best-fit axial Von Mises distributions indicate a strongly preferred NNE–SSW P-axis azimuth for swarm FPS, and a NW–SE mean P-axis azimuth for the pre- and post-swarm period FPS. The NW–SE background P-axis azimuth is approximately parallel to regional maximum compressive stress along the northern segment of the Aleutian arc. Thus, we find evidence for a temporary ~90° reorientation of FPS P-axes during the 2006 swarm, and suggest that the change in FPS orientation may be indicative of intrusion of a shallow, small-volume magma- or fluid-filled dike beneath Mt. Martin in early 2006. In contrast, FPS for VT earthquakes comprising minor swarms in 2007 and 2008 indicate only normal faulting, suggesting a fundamentally different causative process from the major 2006 swarm. Our results demonstrate that the VT earthquake swarms which are a common occurrence in the KVC may be caused by several processes, including magma intrusion, tectonic activity, and hydrothermal fluid circulation; and highlight the need for increased monitoring and analysis during future periods of geophysical unrest.

Monitoring the West Bohemian earthquake swarm in 2008/2009 by a temporary small-aperture seismic array

The most recent intense earthquake swarm in West Bohemia lasted from 6 October 2008 to January 2009. Starting 12 days after the onset, the University of Potsdam monitored the swarm by a temporary small-aperture seismic array at 10 km epicentral distance. The purpose of the installation was a complete monitoring of the swarm including micro-earthquakes (ML 0.0). In the course of this work, the main temporal features (frequency–magnitude distribution, propagation of back azimuth and horizontal slowness, occurrence rate of aftershock sequences and interevent-time distribution) of the recent 2008/2009 earthquake swarm are presented and discussed. Temporal changes of the coefficient of variation (based on interevent times) suggest that the swarm earthquake activity of the 2008/2009 swarm terminates by 12 January 2009. During the main phase in our studied swarm period after 19 October, the b value of the Gutenberg–Richter relation decreases from 1.2 to 0.8. This trend is also reflected in the power-law behavior of the seismic moment release. The corresponding total seismic moment release of 1.02×1017 Nm is equivalent to ML,max = 5.4.

Seismic and geodetic investigation of the 1996-1998 earthquake swarm at Strandline Lake, Alaska

SUMMARY Microearthquake swarms occur frequently in volcanic environments, but do not always culminate in an eruption. Such non-eruptive swarms may be caused by stresses induced by magma intrusion, hydrothermal fluid circulation or possibly other tectonic processes such as slow-slip earthquakes. The Strandline Lake region of southcentral Alaska, located 30 km northeast of Mt Spurr volcano, experienced an intense earthquake swarm between 1996 August and 1998 September. A total of 2507 earthquakes were recorded by the Alaska Volcano Observatory's permanent seismic network during the swarm period, with a maximum magnitude of ML 3.6. The cumulative seismic moment of the swarm was 1.2 × 1015 N m, equivalent to that of a single Mw 4.0 earthquake. Because of the swarm's distance from the nearest Holocene volcanic vent, seismic monitoring was minimal and gas emission and GPS data do not exist for the swarm period. However, combined waveforms from a dense seismic network on Mt Spurr and from several regional seismic stations allow reanalysis of a representative set of swarm and background earthquakes. Swarm hypocentres calculated using a newly formulated 1-D velocity model and station corrections indicate a roughly circular swarm volume with dimensions of approximately 5 km, centred below 10 km below sea level (BSL). Composite fault-plane solutions for swarm earthquakes indicate oblique strike-slip faulting with a northeast-trending P-axis orientation. In contrast, a composite fault-plane solution for background earthquakes indicates a fundamentally different, though poorly constrained, local stress field orientation. Interferometric Synthetic Aperture Radar images spanning the swarm period unambiguously show no evidence of surface deformation, but do not rule out subcentimetre-scale deformation during the swarm. While a shallow ( 10 km BSL) magma intrusion representing an intrusive or protovolcanic segment of the Aleutian arc.

Swarming of Bats at Different Elevations in the Carpathian Mountains

Swarming bat activity was monitored at three caves at elevations ranging from 880 m to 1,907 m above sea level in the Carpathian Mountains, using an infrared light barrier with data-logger, a video camera with a night-scope system, and subsequently by mist netting. A total of 6,175 bats of 19 species was captured, and over 70,000 passes through cave openings were registered. Caves differed in bat species richness, sex ratio, abundance of particular species and species composition. Peak species richness was observed in the mid-elevation cave. Bat activity was high in all caves, but declined with increasing altitude. Swarming activity occurred earlier at high elevation than at lower elevations. Activity of boreal-alpine species, such as Eptesicus nilssonii, peaked at the start of the swarming period, that of species typical of lower elevations, such as Myotis emarginatus, peaked in the middle of the swarming season. In a few species, males showed a significant preference for higher altitude caves, in contrast to females. A similar pattern was observed in the proportion of adults to juveniles, which increased with increasing elevation. Our results also suggest that M. brandtii and M. alcathoe were more often encountered at lower elevations, M. mystacinus (sensu stricto) at higher ones.

Population dynamics in changing environments: the case of an eruptive forest pest species

In recent decades we have seen rapid and co-occurring changes in landscape structure, species distributions and even climate as consequences of human activity. Such changes affect the dynamics of the interaction between major forest pest species, such as bark beetles (Coleoptera: Curculionidae, Scolytinae), and their host trees. Normally breeding mostly in broken or severely stressed spruce; at high population densities some bark beetle species can colonise and kill healthy trees on scales ranging from single trees in a stand to multi-annual landscape-wide outbreaks. In Eurasia, the largest outbreaks are caused by the spruce bark beetle, Ips typographus (Linnaeus), which is common and shares a wide distribution with its main host, Norway spruce (Picea abies Karst.). A large literature is now available, from which this review aims to synthesize research relevant for the population dynamics of I. typographus and co-occurring species under changing conditions. We find that spruce bark beetle population dynamics tend to be metastable, but that mixed-species and age-heterogeneous forests with good site-matching tend to be less susceptible to large-scale outbreaks. While large accumulations of logs should be removed and/or debarked before the next swarming period, intensive removal of all coarse dead wood may be counterproductive, as it reduces the diversity of predators that in some areas may play a role in keeping I. typographus populations below the outbreak threshold, and sanitary logging frequently causes edge effects and root damage, reducing the resistance of remaining trees. It is very hard to predict the outcome of interspecific interactions due to invading beetle species or I. typographus establishing outside its current range, as they can be of varying sign and strength and may fluctuate depending on environmental factors and population phase. Most research indicates that beetle outbreaks will increase in frequency and magnitude as temperature, wind speed and precipitation variability increases, and that mitigating forestry practices should be adopted as soon as possible considering the time lags involved.

Myotis alcathoein Poland and Ukraine: new data on its status and habitat in Central Europe

Recently described Myotis alcathoe has been recorded in Ukraine for the first time and at eight new localities in three distant and geographically diverse areas of Poland (the mountains, the uplands and the lowlands). This data extends its distribution range in Central Europe and confirms it as a native breeding species in Poland. Specific identification of bats was confirmed by sequencing ND1 gene of mtDNA. In Poland its reproduction has been recorded in Łezczok reserve, Silesian Foothill and in Roztocze National Park. Bats were observed mostly within old broadleaved and mixed forests, near water, at altitude ranging from 112 to 736 m a.s.l. The only known roosting site for the species is a cave which is used both in spring and during the swarming period. In Poland the species co-occurred with both M. brandtii and M. mystacinus or with M. brandtii at all sites.

Seismohydrological effects related to the NW Bohemia earthquake swarms of 2000 and 2008: Similarities and distinctions

▶ 20 years seismohydrological investigations at Bad Brambach (Vogtland, Germany). ▶ For the first time, semi-quantitative comparison of two strong swarm earthquake series (2000 and 2008). ▶ Different migrations of the earthquake foci through time, horizontally and in depth. ▶ Fluid triggering not only the in earthquake initiating phases. ▶ Long-term gas flow increases at several gas outlets in the Vogtland/NW Bohemia region between 1998 and 2008 ▶ Systematic relationship between anomaly occurrence and seismic activity cannot be derived at present. For more than 20 years, seismohydrological investigations have been undertaken at the mineral aquifer system of Bad Brambach (Vogtland, Germany). Two strong swarm earthquake series in 2000–2001 and 2008–2009 at the Nový Kostel epicentre (Czech Republic, 10 km E of BB) have enabled for the first time a comparison of seismological and groundwater hydraulic features in a semi-quantitative way. In spite of their similar spatial distribution in 2001 and 2008, the earthquake foci of each swarm migrated differently through time, horizontally as well as in depth. The seismic energy of the 2008–2009 events was released predominantly within 1 month, in contrast to 2000–2001 when it occurred over 3 months. The main distinctive features of each are seen in the hydraulic pressure anomalies which accompanied the earthquake swarms: number, shape, and progression (duration) of the anomalies. The comprehensive hydraulic data, with high temporal resolution, suggest that fluid triggering dominated not only the earthquake initiating phases. In particular, the long-lasting seismicity of the 2008–2009 swarm can be attributed to a continued triggering of weak earthquakes by over-pressured deep fluids. Here, the remaining static strain was obviously not sufficient to generate strong earthquakes as at the beginning of the earthquake swarm periods. Furthermore, the enduring high fluid pressure in 2009 could also indicate a continuation of the long-term gas flow increase observed at several gas outlets in the Vogtland/NW Bohemia region between 1998 and 2008. However, it is not possible at present to derive a systematic relationship between anomaly occurrence and seismic activity, as generally proposed in the context of earthquake prediction discussion.

Form and growth of an embryonic continental rift: InSAR observations and modelling of the 2009 western Arabia rifting episode

SUMMARY A magma-driven rifting episode occurred at the Harrat Lunayyir (Harrat Al-shaqa) volcanic field, western Arabia, between 2009 April and July. It was accompanied by a swarm of more than 4000 M > 2 earthquakes, the largest ever documented in that region, with a peak Mw 5.7 shock on May 19. We combine Interferometric Synthetic Aperture Radar (InSAR) measurements and elastic modelling with seismic moment calculations to resolve the evolution of surface deformation associated with this event. Phase discontinuities and low-coherence lineaments are used to infer the location of the main active structures during the various deformation stages and descending-track interferograms that span the entire period are inverted to resolve the slip and opening distributions along two graben-bounding normal faults and a dyke, respectively. Assuming negligible rift-parallel displacements, we combine ascending- and descending-track interferograms to derive the vertical and rift-perpendicular deformation, which add up to a maximum surface extension of 1.5 m across the rift and subsidence of 0.8 m. The far-field deformation is dominated by the dyke opening, whereas the near-field displacements are mostly associated with movements along the faults. The cumulative seismic moment released during the entire swarm period accounts for about 14 per cent of the total geodetic moment, compared to about 55 per cent at the 2007 Gelai (Tanzania) and about 8 per cent at the 2005 Manda Hararo–Dabbahu (Afar) events. We propose that the differences in moment partitioning ratios are due to the different crustal and seismogenic layer thicknesses in the three regions and represent different stages in the evolution of a volcanic rift. The Gelai event represents the most juvenile stage of rifting, the Dabbahu event represents the most evolved and the Harrat Lunayyir event represents a rift that is intermediate between the two in its degree of maturity.

Spatio-temporal distribution of magma intrusions inducing repeated earthquake swarms off the east coast of the Izu peninsula, central Japan, for 1973–1998

A time-dependent model for magma intrusion associated with repeated earthquake swarm activities off the east coast of the Izu peninsula was developed from precise levelling, electronic distance measurements (EDM) and global positioning system (GPS), and sea level observation data for the period 1973–1998. Significant deformations were observed in the inter-swarm period, suggesting the existence of a preparatory process for repeated earthquake swarm events. The geometry of a dyke-like tensile crack at a depth of 13 km from deformation detected was estimated by precise levelling and GPS for the inter-swarm period, using a genetic algorithm. A large crack plane was considered; it had a combination of estimated deep tensile crack with a shallow dyke detected using the hypocentre distribution and calculated time-dependent inversion, which was carried out to estimate the temporal changes in the volumes of the crack in the period 1973–1998. Akaike's Bayesian information criterion (ABIC) was used to choose the smoothing hyperparameters for the inversion. The characteristics of the estimated temporal change in magma volume were as follows. A rapid and large magma intrusion occurred at a depth of about 15 km between 1976 and 1977. Despite a decrease in its expansion rate, a tensile crack at a depth greater than 8 km continued to inflate between 1978 and 1988. The swarm activity started just one year after the rapid and large intrusion around the 15 km depth. In addition, the tensile crack at the depth of about 15 km beneath the internal region of the eastern Izu peninsula expanded in the inter-swarm period and deflated in the swarm period. This result suggests that magma was temporarily reserved in a dyke-like body beneath the eastern part of the Izu peninsula at a depth of 15 km in a preparatory process for the earthquake swarm. These features strongly show that estimating the spatio-temporal distribution of this deep crack provides useful information for long-range predictions of volcanic earthquake swarms off the east coast of the Izu peninsula.

Effect of age on energy storage during prehibernation swarming in little brown bats (Myotis lucifugus)

At temperate latitudes insectivorous bats face substantial nutritional demands prior to hibernation. As temperature decreases and availability of insect prey declines, bats must deposit nutrient stores for hibernation. The use of torpor allows bats to limit energy expenditures resulting in a net energy gain despite decreased energy intake. However, subadult bats have lower initial fat stores than adults and may have greater difficulty depositing sufficient nutrient stores to survive the winter. We used plasma metabolite analysis to determine the fueling performance of little brown bats ( Myotis lucifugus (LeConte, 1831)) during swarming to see if subadults compensated for the increased challenges by increased feeding throughout the swarming period. During the period of our study (August and September), adult bats gained mass, while subadults lost mass. There was, however, no difference in nutrient intake of the age groups as indicated by plasma metabolite concentrations. The number of bats using torpor while roosting in the hibernaculum by day increased exponentially coincident with the onset of mating and a decrease in nutrient intake. The results are consistent with wild bats using torpor to minimize energy expenditure and compensate for lower nutrient intake. The difference in mass change for adults and subadults despite the same nutrient intake indicates that subadults incur greater energetic costs.

Food habits and niche overlap in three alpine passerine birds, South Norway

The three most common passerine birds in alpine habitats on the Hardangervidda plateau, South Norway, are the Meadow Pipit Anthus pratensis , the Wheatear Oenanthe oenanthe , and the Snow Bunting Plectrophenax nivalis . Analyses of stomach content showed that invertebrates are the main food of these species, but the Wheatear and the Snow Bunting also eat Empetrum berries, and the Snow Bunting becomes a seed eater in October when snow has arrived and the two other species have left the area. Seasonal changes in diet were related to phenological changes in the availability of food items. Adult Tipulidae were for instance taken by all species during the swarming period in July. Overlap in food choice was considered medium for all three species combinations. The Snow Bunting overlapped only slightly with the two other species in altitudinal distribution, habitat choice and territory. However, the Meadow Pipit and the Wheatear overlapped strongly in these parameters. Since their reproduction was successful, we assume that these two species can live together with strongly overlapping niches due to a surplus of available invertebrate food. All three species are able to change their food choice rapidly and are thus well adapted to manage unpredictable conditions in a harsh, high altitude ecosystem.

Monitoring of swarming sounds in bee hives for early detection of the swarming period

Beekeeping, known as one of the oldest forms of agriculture, in its complexity requires control for honey production with what modern technology can offer. Honey is included in animal production implying that farmers have interest in big productions according to the best blooming time, the presence of parasites, the genetic strain of his bees and the swarming periods of the honeybees (queen and her workers leaving the hive). This last fact has a big economic interest for the beekeeper as swarming means honey loss since bees start collecting the honey to migrate. Here for a method that enables the prediction of the swarming is required to prevent the queen from leaving the hives. In this experiment an acoustic method based on labelling of sounds is proposed to predict the swarming period. Three hives were monitored during 270 h. The microphones were sited inside the hives together with a temperature and humidity sensor. The sounds were recorded with a sample rate of 2 kHz, and analyzed via Matlab and Cool Edit Pro. During this period 9 swarming activities occurred. Swarming is indicated by an increase in the power spectral density at about 110 Hz; approaching to swarm the sound augmented in amplitude and frequency to 300 Hz, occasionally a rapid change occurred from 150 Hz to 500 Hz. Another finding indicating the initiation of a swarming period is the raise in temperature from 33 °C to 35 °C until the actual time of swarming when the temperature starts dropping to 32 °C. With more activity, ventilation from bee wings causes drop of temperature. Less information comes from the correlation between sound and humidity since this parameter is too much influenced by the external conditions and no significant variation occurred according to a swarm. This increase of temperature, together with the changes in acoustical features of the sound recorded in the hive, may be used as a predictor for swarming of the bees to reduce honey loss.

Impact of climate change on the population dynamics of Ips typographus in southern Sweden

Mass-propagation of the spruce bark beetle, Ips typographus, following windfalls and drought is a serious threat to mature spruce forests. Climate change will affect the population dynamics of I. typographus directly as the swarming activity and development rate are mainly controlled by temperature, and indirectly via changes in availability of brood trees. Today, I. typographus is in general univoltine in southern Sweden. In Denmark, however, the weather conditions usually support the production of a second generation. By modelling the temperature dependent population dynamics, we have evaluated the effect of regional climate change scenarios for the period of 2070–2099 on I. typographus in southern Sweden. Our results indicate that temperature increase will have a step-wise effect on the population dynamics. Earlier spring swarming and a faster development from egg to mature bark beetle increase the probability of a second swarming period during summer. A second swarming will be customary with an annual mean temperature increase of 2–3 °C. The thermal requirement for development of a second generation may, however, not be fulfilled every year with an annual mean temperature increase less than 5–6 °C. Winter is fatal for immature bark beetles, and the larger the temperature increase, the higher the probability that the second generation will complete development and survive hibernation. The temperature regime during autumn will therefore have a decisive impact on the size of the swarming population next spring.

Deep low-frequency earthquake swarm in the mid crust beneath Mount Fuji (Japan) in 2000 and 2001

Beneath Mount Fuji, the highest active volcano in Japan, deep low-frequency (DLF) earthquake activity has been monitored since the early 1980s. The DLF earthquakes occurred in the mid-crustal depth range, and burst-type activity lasting from several minutes to 30 min was detected 10 to 20 times in an ordinary year. The DLF earthquake activity increased sharply in the period from October 2000 to May 2001, showing swarm-like activity. The occurrence rate during the DLF earthquake swarm was approximately 20 times higher than the usual activity, and the wave energy released during the swarm period was twice as high as the total wave energy during the past 20 years. The DLF earthquakes in the period from 1987 to 2001 were relocated by estimating station corrections in order to reduce the effect of the change of seismic station distribution. The epicenters of most DLF earthquakes occurred in an elongated region with a long axis of about 5 km, whose center is located 2–3 km NE from the summit. A few percent of the DLF earthquakes, however, occurred around the summit area, significantly apart from the main epicenter region. The focal depths of well-located DLF events range from 10 to 20 km. During the high activity period in 2000 and 2001, most DLF events occurred within this main hypocenter area. The sharp increase of DLF earthquake activity at Mount Fuji started immediately after magma discharge and intrusion events in the Miyake-jima and Kozu-shima regions in July and August 2000. The tectonic and volcanic activity changes around the area suggest that the DLF earthquake swarm at Mount Fuji was triggered by the change of state of the deep magmatic system around Mount Fuji.

Seismically triggered microbial methane production relating to the Vogtland NW Bohemia earthquake swarm period 2000, Central Europe

Long-term radiometric and hydrological investigations at the Wettinquelle mineral spring in Bad Brambach demonstrated the existence of a fluidal connection to the currently most frequent earthquake-swarm hypocentre at Novy Kostel, 10 km east of Bad Brambach. The gas composition and δ13CCH4 values of this mineral spring were monitored from May 2000 until October 2003, i.e., before, during and after the protracted swarm earthquake period from late August until late December 2000. About eight weeks after the beginning of the seismically active period, we observed an increase in the methane concentration (from ≈40 up to ≈250 ppmv) accompanied by a decrease in the methane δ13C values from ≈-50 to ≈-70‰. For more than two years, such periods of variations were repeatedly observed before returning to the baseline signature. It is assumed that this additional methane was microbially produced in the granite-enclosed aquifer using H2, which was released (seismically triggered) from the fissured granite in which the Wettinquelle spring capture is located. The additional methane production might have started as a co-seismic event, with only the migration from the deep granite to the surface being responsible for the eight-week delay.