Journal of Volcanology and Seismology最新文献

This paper considers geological, mineralogical, and geochemical features of the epithermal Au‒Ag mineralization at the Televeem volcanic dome uplift (VDU) which complicates the Verkhne-Pykarvaam volcano-tectonic depression (VTD) in the Central Chukchi sector of the Okhotsk–Chukchi volcanogenic belt (OCVB). The structure of this ore occurrence is due to its localization within the eponymous VDU. The main vein zone (MVZ) of the Televeem ore occurrence, as wide as 500 m, extends for 2.5 km in the north–south direction. Along the MVZ direction, en-echelon proximal quartz–adularia veins are successively replaced with zones of thin streaks and brecciation in secondary quartzites and argillisites. The gold concentrations in these rocks vary between 1.4 and 17.3 g/t, that of gold is between 7.6 and 144.6 g/t. The ores contain abundant brecciated, framboidal lamellar, geode, and fine streaky structures. The more frequent ore minerals are pyrite, arsenopyrite, acanthite, grey ores of the freibergite–tetrahedrite series, stephanite, polybasite, low-fineness native gold (fineness varies in the range 249–532‰), and titanite. The amount of ore minerals in veins does not commonly exceed 0.5%, reaching 3% in some rare cases. Based on mineralogical evidence, this ore occurrence can be classified as belonging to low- or medium-eroded ones. The low erosion suggests a high probability of detecting buried ore bodies.

In the present analysis, the homogeneous, updated and complete seismic catalog of the Kohat-Potwar plateau was analyzed through dynamic time dependent statistical models. The study region earthquake interoccurrence times are assumed as a stochastic study variable and it follows one of the renewal models. A rich and updated class of statistical models was used to identify the most parsimonious one. The developed models will serve the purpose of describing, analyzing and forecasting the earthquake occurrence probabilities in the future. The model parameters were estimated through the method of maximum likelihood. Different statistical evaluation criteria were used to prioritize the estimated models according to their fitting performance to the observed data. We have found that the interoccurrence times of the study region can plausibly be described through the log-logistic model. Based on the log-logistic model, it has been found that there is a significantly high probability (>90%) of another earthquake occurrence of M_w ≥ 5.3 in the region in near future (2023‒2028). These probabilities increase with increasing time periods since the last event occurrence. The mean recurrence period of another earthquake of M_w ≥ 5.3, based on the most suitable log-logistic model, is almost three and half years with 1.80 years as standard error, i.e., 3.38 ± 1.80 years. The quantification of earthquake occurrence uncertainties in the study region provides many useful quantitative estimates that have crucial importance in seismic hazard and risk analysis studies of a region.

Iceland is a unique example where the rift zone of the Mid-Atlantic Ridge emerges at the surface, and whose morphology and tectonic structure differ considerably from typical rift zones of mid-oceanic ridges. The morphology and geodynamics of the western branch of Icelandic rifts are largely controlled by the thermal influence of the Iceland plume that has created the North Atlantic large igneous province. The western branch of Icelandic rifts is characterized by ceasing tectonic and magmatic activity. Overlapping the Eastern Rift Zone, it forms the rotating Hreppar microplate block, which results in a northward decrease of its tectono-magmatic activity. Based on a morphometric analysis of fault scarps, we identified the degree of present-day activity for individual areas of volcanic systems, and determined its variation during Late Quaternary for some areas. The inferences drawn here demonstrate distinct differences in the present-day tectonic structure and dynamics of rift zones and individual volcanic systems within them. The southernmost, transtensional Reykjanes Rift Zone shows tectono-magmatic activity decreasing to the east, which is due to a lower influence exerted by the Reykjanes Ridge that is adjacent to it to the southwest. We observed its gradual eastward diminution, which is probably due to an analogous southward movement of the most active Eastern Rift Zone and to the formation of a new transtensional zone that combines the present-day Reykjanes Rift Zone and South Iceland Seismic Zone. The Western Rift Zone is developing independently of the Reykjanes Rift Zone, having a major extension center in the area of Lake Thingvallavatn. The Holocene manifestations of tectono-magmatic activity in its northern part, as is the case in the Central Rift Zone, are very weak, being mostly due to glacio-isostatic reactivation of older structures. The identified structural inhomogeneities can also be traced in the morphological aspect of rift zones. As an example, the Western and Central Rift Zones typically contain well-developed shield volcanoes that are largely composed of hyaloclastites, while individual lava shield edifices are observed within fissure swarms. In contrast to this, the Reykjanes Rift Zone is characterized by an absence of central volcanoes expressed in topography, and chains of small volcanic vents are observed within fissure swarms.

Rawa Danau Volcanic Complex (RDVC) in the western part of Banten—Western part of Java Island, Indonesia, holds records and histories of past violent volcanic eruption. These past volcanic activities manifested as huge Rawa Danau caldera and massive Banten Tuff pyroclastic deposits that are distributed for more than ten kilometres away from the source and have played an important role in shaping the morphology of Banten. Previous studies suggest that Banten Tuff is composed of various pumiceous pyroclastic deposits presumably formed in Early Pleistocene. The deposits cover vast area of Banten, and their thickness exceeds 200 m. Despite their significance, there are no detailed studies that discuss the stratigraphic and eruption dynamics of the deposits. This paper provides new insight on the eruption dynamics based on fieldwork observation, volcanic stratigraphy, and petrologic interpretation. The study focuses on the eastern flank of RDVC, where the well-preserved stratified outcrops are located. Aligned with previous studies, the deposit can be divided into two major eruption sequences: 1) Lower Banten Tuff (LBT) and 2) Upper Banten Tuff (UBT). LBT is interpreted as lithic-bearing ignimbrite with accompanying surge deposit and considered as a single-phase eruption. Lithic-rich characteristics in this unit suggest that LBT might be related to the caldera-forming mechanism. On the other hand, UBT shows a more dynamic sequence. It is composed of two fallout units followed by an ignimbrite formed by the eruption column collapse. The appearance of banded pumice in UBT with contrasting andesitic and rhyolitic compositions suggests a mafic injection as the eruption trigger. Pumice and ash compositions are mostly in dacitic to rhyolitic range, with several samples in andesitic range. Componentry analysis suggests that all eruptions were magmatic in origin.

Sixteen morphometric relief parameters have been identified whose positive anomalies correspond with seismic areas in the Greater Caucasus. An analysis of four parameters which were considered to provide the most information using the γ-operator in fuzzy logic has enabled us to develop a scheme for an index of neotectonic activity that was used along with the results of computerized geodynamic simulation to identify zones of possible earthquake sources. The new approach does not require detailed information on present-day and paleo seismicity, hence can be used to deal with an analogous problem for territories whose seismotectonics is poorly known. We have demonstrated an interrelationship between recent deformations and regional seismicity, and the possibilities offered by the method of lineament analysis due to Yu.V. Nechaev (2010) for identification of active faults.

This study is assessing the tsunami hazard for a segment of the Kamchatka western coast around the Oktyabrsky Spit. The motivation is to ensure transportation access to the village of Oktyabrsky and to the fish processing facilities situated on the Spit. The hazard estimates were derived by the worst case method. An analysis of seismicity and historical data on tsunami occurrences in the Sea of Okhotsk resulted in identification of two tsunami-generating zones that constitute threat to the Oktyabrsky coast segment, with a set of model tsunami-generating earthquakes being determined for each zone. For this set of models we conducted numerical modeling for tsunami generation and propagation, resulting in a selection of model sources that would pose the greatest threat to the coastal strip of interest. The next step involved refining calculations for these sources using a sequence of nested grids to find the parameters of tsunami impact on the coast. The main results of the present study consist in identification of tsunami-generating zones that pose the highest threat to the Oktyabrsky coast, the selection of tsunami-generating model sources in these zones, and estimates of parameters that characterize extreme tsunami waves posing threat to this coast strip.

We have obtained the first data on the chemical composition of the eruptive materials from the explosive eruption of Bezymianny volcano on April 7, 2023. Our unique collection includes freshly sampled pumice lapilli from the eruption and juvenile blocks from pyroclastic flows. We have identified interesting patterns in both macro components and specific chalcophile elements, such as copper. The rocks we studied belong to medium-K two-pyroxene basaltic andesite (55.5‒57 wt % SiO₂), with mafic enclaves characterized by a slightly more primitive composition (53.7 wt % SiO₂). According to mineral geothermometry data, the phenocrysts of basaltic andesite crystallized at temperatures in the range from 940 to 960°C, while the formation of phenocryst rims and microlites occurred at 980°C, which corresponds to conditions immediately before the eruption. The composition of volcanic glass allows us to estimate the pressure at which the magma reached the last equilibrium with crystallizing phases before eruption (0.5‒0.6 kbar). Based on these findings, we have formulated hypotheses about the potential evolution of the shallow magma chamber of Bezymianny volcano during the period from 2017 to 2023.

The northwestern part of the Sevan–Shirak structural formation zone of the Lesser Caucasus contains ultra-high-K siliceous ignimbrites dating back to the Late Eocene–start of Early Oligocene (?), which associate with high-K volcanics of the calc-alkaline and shoshonite series. During Eocene–Oligocene time the Sevan–Shirak zone was an ensialic island arc with a Hercynian metamorphic basement. The formation of high-K and especially ultra-high-K rocks was due to the effects of mantle fluids on the continental crust.

This paper presents an analysis of recorded variations (anomalies) in the potential gradient of electrical field in the atmosphere caused by the propagation of eruption plumes discharged by eruptions of Shiveluch and Bezymianny volcanoes in Kamchatka. The anomalies were recorded at various distances from eruption centers and under different conditions of atmospheric stratification. These conditions have enabled us to show that the eruption plumes of Shiveluch and Bezymianny possessed a 3D electrostatic structure that is consistent with a known phenomenological model derived on the basis of surveys conducted on various volcanoes worldwide. According to this model, the top of an eruption plume contains a positive volumetric electrostatic charge, while the respective charges are negative in the middle, and positive in the lower part of the plume.

The period of more than twenty years in which recent movements in the Mountainous Altai were measured by methods of satellite geodesy includes various phases of the seismic process. A network consisting of 20 observing stations extends over a large area from Novosibirsk City in the north to the Mongolian border in the south, from the border with Kazakhstan in the west to the Sayan Mountains in the east. The Chuya earthquake of September 2003 divides, in a natural manner, the period of observation from 2000 to 2022 into several phases: the preseismic (2000‒2003), the coseismic (2003‒2004), the postseismic phase for the epicentral area of the Chuya earthquake (2004‒2013), and the interseismic phase for the other regions of the Mountainous Altai that have not been seriously affected by the earthquake (2000‒2022). The data supplied by multiyear measurements were analyzed using special programs of the most recent modifications. We have identified conspicuous features in the displacement fields during each phase: anomalous displacement rates before the Chuya earthquake, the coseismic displacements during the earthquake, postseismic effects in the epicentral zone, and slow tectonic movements. Our interpretation relied on 2D and 3D elastic and viscoelastic models of the crust. The depth of focus has been determined (14 km), as well as the two-meter relative right lateral displacement jump on the seismic fault. With a two-layered model, we obtained the value of viscosity in the lower crust for a variety of elastic modulus values η = 5 × 10¹⁹‒1.1 × 10²⁰ Pa s. The recent movements in that part of the Mountainous Altai which has not been affected by the Chuya earthquake were 0.8 mm/yr toward NNW. The rate of surface deformation in the southern mountainous part reached 2 × 10^–8/yr in the epoch 2000‒2022, which is an order of magnitude higher than that in the northern flat part of the area of study.