Earthquake activity related to the 1991 eruption of the Hekla volcano,Iceland

The 1991 eruption of the Hekla volcano started unexpectedly on 17 January. No long-term precursory seismicity was observed. The first related activity was a swarm of small earthquakes that began approximately half an hour before the eruption. Intensive seismicity, both earthquakes and volcanic tremor, accompanied the violent onset of the eruption. Almost 400 events up to M_L magnitude 2.5 were recorded during the first few hours. During the later phases of the eruption, the earthquake activity was modest and the main volcano-related seismic signal was the persistent volcanic tremor. The tremor died away, together with the eruption on 11 March, and Hekla was seismically quiet until the beginning of June 1991, when a sudden swarm of numerous small shallow earthquakes occurred. This activity is atypical for Hekla and is interpreted to be a failed attempt to resume the eruption.     

The Krafla fissure swarm, Iceland, and its formation by rifting events

Fissure swarms at divergent plate boundaries are activated in rifting events, during which intense fracturing occurs in the fissure swarm accompanied by intrusion of magma to form dikes that sometimes lead to eruptions. To study the evolution of fissure swarms and the behaviour of rifting events, detailed mapping was carried out on fractures and eruptive fissures within the Krafla fissure swarm (KFS). Fracture densities of dated lava flows ranging from 10,000?years bp to ~30?years old were studied, and the fracture pattern was compared with data on the historical Myvatn rifting episode (1724–1729) and the instrumentally recorded Krafla rifting episode (1975–1984). Additionally, the interaction of transform faults and fissure swarms was studied by analysing the influence of the Húsavík transform faults on the KFS. During the historical rifting episodes, eruptions on the fissure swarm occurred within ~7?km from the Krafla central volcano, although faults and fractures were formed or activated at up to 60–70?km distance. This is consistent with earlier rifting patterns, as Holocene eruptive fissures within the KFS are most common closer to the central volcano. Most fractures within the central Krafla caldera are parallel to the overall orientation of the fissure swarm. This suggests that the regional stress field is governing in the Krafla central volcano, while the local stress field of the volcano is generally weak. A sudden widening of the graben in the northern KFS and a local maximum of fracture density at the junction of the KFS and the extrapolation of the Húsavík transform fault zone indicates possible buried continuation of the Húsavík transform fault zone which extends to the KFS. Eruptive fissures are found farther away from the Krafla central volcano in the southern KFS than in the northern KFS. This is either due to an additional magma source in the southern KFS (the Heiearsporeur volcanic system) or caused by the Húsavík transform faults, transferring some of the plate extension in the northern part. Fracture density within particular lava flow fields increases with field age, indicating that repeated rifting events have occurred in the fissure swarm during the last 10,000?years bp. The fracture density in the KFS is also generally higher closer to the Krafla central volcano than at the ends of the fissure swarm. This suggests that rifting events are more common in the parts of the fissure swarm closer to the Krafla central volcano.     

An alternative method for estimating densification point velocity based on back propagation artificial neural networks

The establishment of Turkish National Fundamental GPS Network (TNFGN) was completed in 2001 and Large Scale Map and Map Information Production Regulation (LSMMIPR) came into force in 2005 in parallel with the establishment of TNFGN and the increase in the use of GPS applications. TNFGN has been designed as first order GPS network and it comprises second-, third- and fourth-order GPS densification networks. LSMMIPR has required determining the positions of first-, second- and third-order GPS densification (C1, C2 and C3) points with the reference epoch besides the measurement epoch. Therefore, it is necessary to estimate the velocity vectors of the densification points. In practise, the velocity vectors of C1, C2 and C3 points are estimated from TNFGN points or higher-order densification points velocity vectors by interpolation methods but LSMMIPR did not specify the interpolation method for this procedure. The objective of this study is to use a back propagation artificial neural network (BPANN) that has been more widely applied in engineering among all other neural network models for estimating the velocity of the densification point as an alternative to the interpolation methods. BPANN and selected ten interpolation methods are evaluated over a test area, in terms of root mean square error (RMSE). The results showed that the employment of BPANN estimated the densification point velocity (V_X,Y,Z) with a better accuracy (±5.0 mm, ±4.0 mm, ±3.9 mm, respectively) than the interpolation methods in the test area and indicated that BPANN can be a useful tool for estimating point velocity in the densification networks as a real alternative to the interpolation methods.     

Seismicity around the Hekla and Torfajökull volcanoes, Iceland, during a volcanically quiet period, 1991–1995

The volcano Hekla in south Iceland had its latest eruption in January–March 1991. The eruption was accompanied and followed by considerable seismic activity. This study examines the seismicity in the Hekla region (63°42′–64°18′N, 18°30′–20°12′W) during a period when the high activity related to the eruption had ceased, from July 1991 to October 1995. The aim is to define the level of the normal background seismicity of the area that can be compared to the eruption-related activity. The Hekla Volcano proper was generally aseismic during the study period. The most prominent earthquake cluster is in the neighbouring Torfajökull Volcano. The epicentres are concentrated in the western part of the caldera and west of it. The hypocentres are located at all depths from the surface down to 14?km, with highest activity at 5–12?km. Inside this cluster, in the northwest part of the caldera, is a spherical volume void of earthquakes, approximately 4?km in diameter and centred at 8?km depth. This is interpreted as a cooling magma body. Small, low-frequency events of volcanic origin were occasionally recorded at Torfajökull. This activity has mainly occurred in swarms and was most abundant during the first year of the study period, presumably reflecting some kind of connection to the 1991 Hekla eruption. Our study area also includes the easternmost section of the South Iceland seismic zone, a transform zone characterized by bookshelf faulting on transverse faults. Two lineaments of epicentres were identified, roughly corresponding to mapped faults of the South Iceland seismic zone. The hypocentres are relatively deep, mainly at 6–12?km, matching the general trend of hypocentral depth increasing toward the east. The seismicity is highest in the area of the mapped faults. However, the epicentres extend beyond them and indicate greater width of the South Iceland seismic zone, or 20–30?km rather than approximately 10?km as indicated by the length of the surface faults. The seismicity in the volcanic systems of Hekla and Vatnafjöll shows some characteristics of the South Iceland seismic zone. Epicentres are concentrated into two N–S lineaments, one of which coincides with the location of the 1987 Vatnafjöll earthquake (M_w=5.9), a strike-slip event on a N- to S-trending fault. The hypocentres of the Hekla–Vatnafjöll events are mainly at 8–13?km depth, which indicates a continuation of the depth trend of the earthquakes of the South Iceland seismic zone. The events located at Hekla proper and immediately north of it are all of low-frequency character, which can be held as an indication of volcanic origin. On the other hand, they show clear S arrivals at observing stations like normal high-frequency tectonic earthquakes.     

The KOSMA large-scale CO survey of clouds in the Galactic Molecular Ring

We present the status of the KOSMA large-scale observations of clouds in the Galactic Molecular Ring (GMR) in CO J=3-2, 2-1 and ¹³CO 2-1 following up on the BU-FCRAO ¹³CO 1-0 survey. The GMR is one of the prominent large scale structures in the Milky Way and an interesting laboratory to studystar formation. Whereas the ¹³CO data yield a good measureof the overall column density structure, the new observations of CO 2-1provide a picture of the thin extended gas and the CO 3-2 data show thedistribution of the dense and warm molecular gas. The combination of observations of these lines thus allows to distinguish between extended,quiescent gas, dense cold parts, and warm regions, influenced by star formation. In performing large scale observations we are able to analysethe structure of this material.     

Spectral investigation of the chromosphere

The results of the analysis of 16 H spectra taken with high spatial resolution are presented. The properties of the line profile in the boundary regions of supergranular cells (SGB) are described by the statistical distribution functions and correlation coefficients of four parameters which determine the shape of a suitable fitting curve. Although some of the line profile properties of both H and H in the SGB can be interpreted in terms of a model which was recently proposed by Durrant (1975) the agreement between that theory and our observations is not entirely satisfactory.Mitteilungen aus dem Fraunhofer Institut Nr. 137.     

Spectral investigation of the chromosphere

Highly resolved H spectra and filtergrams obtained at the Fraunhofer Observatory on Capri were analysed by a method whose principles have been described before. As a result the tentative conclusion of our previous work has been confirmed: The mottles of the chromospheric fine structure are clouds superimposed on the low chromosphere. Furthermore, it is proposed that the latter is identical with the interior of the supergranular cells whose spatial averages lend themselves to an interpretation in terms of a spherically symmetric model. On the other hand, we present evidence that the boundary regions which are formed by the mottles of the fine structure pattern and which constitute the upper chromosphere are not adequately described by spatial averages. Instead, the properties of the individual structural elements should be taken into account by an appropriate theory, if only in a statistical fashion. Some of these properties, viz. source function, optical thickness, Doppler width and velocity, were measured for a large number of elements. On the basis of indirect evidence we suggest that the spicules seen on limb photographs of chromospheric lines are identical with the elements of the supergranule boundary structure.Mitteilung aus dem Fraunhofer Institut Nr. 116.     

Der südwestliche Randbereich der Espinhaço-Zone,Minas Gerais,Brasilien

Zusammenfassung Die vorliegende Arbeit umfaßt eine Kartierung in präkambrischen Metamorphiten und Metasedimenten am SE-Rand des São-Francisco-Beckens. Als älteste Einheit finden sich die Präminas-Gesteine, die von der miogeosynklinalen Quarzitfazies der MinasSerie überlagert werden. Die jüngere São-Francisco-Serie liegt diskordant über den älteren Einheiten. Sie gliedert sich in eine grobklastische Randfazies, das Macaúbas, und eine feinklastische, karbonatische Beckenfazies, das Bambuí. Die São-Francisco-Serie wurde noch in die ausklingende Minas-Orogenese einbezogen. Die Minas- und Macaúbas-Quarzite wurden tektonisch intensiv verschuppt und über das Bambuí überschoben. Im Bambuí entwickelte sich nur ein flacher Faltenbau. Aufgrund der tektonischen und sedimentologischen Beziehungen wird angenommen, daß die São-Francisco-Serie die Molasse des Minas-Orogens darstellt.

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This paper is based on geological mapping in Precambrian metamorphic rocks at the SE border of the São Francisco Basin. The oldest unit are the Preminas rocks, which are overlain by the miogeosynclinal, quartzitic facies of the Minas Series. These units are unconformably overlain by the younger São Francisco Series. The later is subdivided in a marginal facies with coarse-grained clastics, the Macaúbas, and a fine clastic and calcareous basin facies, the Bambuí. The São Francisco Series was involved in the final phase of the Minas orogeny. The quartzites of the Minas and Macaúbas are intensively thrust-faulted now overlying the Bambuí. In the Bambuí only large amplitude folding developed. Based on the tectonical and the sedimentological relations it is assumed that the São Francisco Series is the molasse of the Minas geosyncline.

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Resumo É apresentado um mapeamento geológico em metamorfitos e metasedimentos précambrianos na borda SE da Bacia do São Francisco. A unidade mais antiga é constituida por rochas da Série Préminas. Sobrepõem-se quartzitos da Série Minas em fácies miogeosmclinal. A Série São Francisco, como unidade mais recente, jaz em discordância sôbre as séries mais antigas. Divide-se em duas fácies: uma marginal, o Macaúbas, com sedimentos clásticos grosseiros, e outra com sedimentos clásticos finos e carbonates, o Bambuí. A Série São Francisco foi afetada pela fase final da orogenese Minas. Os quartzitos da Série Minas e do Grupo Macaúbas mostram intenso falhamento de empurrão e foram acavalados sôbre o Bambuí. No Bambuí formaram-se apenas dobras amplas. Conclui-se pelas relações tectônicas e sedimentológicas, que a Série São Francisco constitue a molassa da geossinclinal Minas.

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