The flow dynamics of an extremely large volume pyroclastic flow,the 2.08-Ma Cerro Galán Ignimbrite,NW Argentina,and comparison with other flow types

The 2.08-Ma Cerro Galán Ignimbrite (CGI) represents a >630-km³ dense rock equivalent (VEI 8) eruption from the long-lived Cerro Galán magma system (∼6 Ma). It is a crystal-rich (35–60%), pumice (<10% generally) and lithic-poor (<5% generally) rhyodacitic ignimbrite, lacking a preceding plinian fallout deposit. The CGI is preserved up to 80 km from the structural margins of the caldera, but almost certainly was deposited up to 100 km from the caldera in some places. Only one emplacement unit is preserved in proximal to medial settings and in most distal settings, suggesting constant flow conditions, but where the pyroclastic flow moved into a palaeotopography of substantial valleys and ridges, it interacted with valley walls, resulting in flow instabilities that generated multiple depositional units, often separated by pyroclastic surge deposits. The CGI preserves a widespread sub-horizontal fabric, defined by aligned elongate pumice and lithic clasts, and minerals (e.g. biotite). A sub-horizontal anisotropy of magnetic susceptibility fabric is defined by minute magnetic minerals in all localities where it has been analysed. The CGI is poor in both vent-derived (‘accessory’) lithics and locally derived lithics from the ground surface (‘accidental’) lithics. Locally derived lithics are small (<20 cm) and were not transported far from source points. All data suggest that the pyroclastic flow system producing the CGI was characterised throughout by high sedimentation rates, resulting from high particle concentration and suppressed turbulence at the depositional boundary layer, despite being a low aspect ratio ignimbrite. Based on these features, we question whether high velocity and momentum are necessary to account for extensive flow mobility. It is proposed that the CGI was deposited by a pyroclastic flow system that developed a substantial, high particle concentration granular under-flow, which flowed with suppressed turbulence. High particle concentration and fine-ash content hindered gas loss and maintained flow mobility. In order to explain the contemporaneous maintenance of high particle concentration, high sedimentation rate at the depositional boundary layer and a high level of mobility, it is also proposed that the flow(s) was continuously supplied at a high mass feeding rate. It is also proposed that internal gas pressure within the flow, directed downwards onto the substrate over which the flow was passing, reduced the friction between the flow and the substrate and also enhanced its mobility. The pervasive sub-horizontal fabric of aligned pumice, lithic and even biotite crystals indicates a consistent horizontal shear force existed during transport and deposition in the basal granular flow, consistent with the existence of a laminar, shearing, granular flow regime during the final stages of transport and deposition.     

Composition and geochronology of the deep-seated xenoliths from the southeastern margin of the North China Craton

A variety of deep-seated xenoliths occur within the Mesozoic Jiagou dioritic porphyry in the southeastern margin of the North China Craton (NCC). In this study we present a combined petrologic, geochronological, Hf isotope and geochemical study on the different types of xenoliths and use these data to better constrain the composition and age of the deep crust beneath the area. Most of the xenoliths are mafic meta-igneous rocks, among which garnet-bearing lithologies are common. The xenoliths can be classified into three broad petrographic groups: spinel-bearing garnet clinopyroxenite/phlogopite clinopyroxenite/spinel pyroxenite (Group 1), garnet amphibolite or hornblendite/garnet granulite/mafic gneiss lacking pyroxene (Group 2), and garnet-bearing felsic (intermediate-acid) gneiss (Group 3). Among these, the mafic–ultramafic rocks constitute the dominant category. The protoliths of the studied xenoliths range from basalt through andesite to dacite. Geochemical and Hf-isotope data indicate that most xenoliths belonging to Groups 2 and 3 resemble magmatic rocks formed at convergent continental margin arc setting. A few of them (mostly belonging to Group 1) represent mantle-derived products. Multiple metasomatic imprints, with contribution from subduction-related or mantle-derived fluids or melts have been recognized from the multistage mineral assemblages and ages.SHRIMP zircon U–Pb dating, Hf isotope and geochemical data offer evidence for subduction-related adakite-like and arc-related rocks in the southeastern margin of the NCC at ca. 2.5 Ga and 2.1 Ga, and confirm the occurrence of high-pressure granulite-facies metamorphism at ca. 1.8 Ga. These data suggest an episodic growth of Precambrian lower crust beneath this region in response to two stages of subduction–accretion and one vertical accretion of mantle-derived basaltic magma at the base of the lower crust. Additionally, a previously unknown late mantle-derived basaltic magmatism at 393 ± 7 Ma has also been recognized. The data presented in this paper demonstrate that the deep crust beneath the southeastern margin of the NCC is composed of hybrid protoliths derived from Paleozoic, Paleoproterozoic and late Neoarchean sources.     

Hazard assessment of far-range volcanic ash dispersal from a violent Strombolian eruption at Somma-Vesuvius volcano, Naples, Italy: implications on civil aviation

Long-range dispersal of volcanic ash can disrupt civil aviation over large areas, as occurred during the 2010 eruption of Eyjafjallaj?kull volcano in Iceland. Here we assess the hazard for civil aviation posed by volcanic ash from a potential violent Strombolian eruption of Somma-Vesuvius, the most likely scenario if eruptive activity resumed at this volcano. A Somma-Vesuvius eruption is of concern for two main reasons: (1) there is a high probability (38?%) that the eruption will be violent Strombolian, as this activity has been common in the most recent period of activity (between AD 1631 and 1944); and (2) violent Strombolian eruptions typically last longer than higher-magnitude events (from 3 to 7?days for the climactic phases) and, consequently, are likely to cause prolonged air traffic disruption (even at large distances if a substantial amount of fine ash is produced such as is typical during Vesuvius eruptions). We compute probabilistic hazard maps for airborne ash concentration at relevant flight levels using the FALL3D ash dispersal model and a statistically representative set of meteorological conditions. Probabilistic hazard maps are computed for two different ash concentration thresholds, 2 and 0.2?mg/m³, which correspond, respectively, to the no-fly and enhanced procedure conditions defined in Europe during the Eyjafjallaj?kull eruption. The seasonal influence of ash dispersal is also analysed by computing seasonal maps. We define the persistence of ash in the atmosphere as the time that a concentration threshold is exceeded divided by the total duration of the eruption (here the eruption phase producing a sustained eruption column). The maps of averaged persistence give additional information on the expected duration of the conditions leading to flight disruption at a given location. We assess the impact that a violent Strombolian eruption would have on the main airports and aerial corridors of the Central Mediterranean area, and this assessment can help those who devise procedures to minimise the impact of these long-lasting low-intensity volcanic events on civil aviation.     

Urban scenarios modifications due to the earthquake: ruins formation criteria and interactions with pedestrians’ evacuation

One of the most influencing elements in inhabitants’ earthquake safety definition is represented by the interactions between people and post-event environment in urban scenarios. Understanding and simulating rules for pedestrians’ motion in earthquake evacuation could be useful to inquire the risk assessment introducing the “human” factor influence: integrated “risk maps” could be realized by combining results of similar analyses with the traditional site hazard, buildings vulnerability and exposition indices. This work proposes an innovative approach based on the analysis of these interactions. Two experimentally-based activities are required: an analysis of human behaviors towards the post-earthquake environment; a relation for defining environmental modifications. Results firstly show a summary of man-environment interactions in earthquake evacuations. A possible criterion for path choice in evacuation is also numerically defined. A theoretical agent-based model is developed on these bases and summarizes phases, motion rules and man-environment interactions in earthquake pedestrians’ evacuation in urban scenarios. Secondly, quick criteria for scenario modifications involving ruins formation are proposed and evaluated: for each building, the percentages of internal and external ruins area is a function of its vulnerability and the expected earthquake Richter magnitude. Moreover, the external ruins formation criterion is validated by comparing predicted and effective values of ruins area depth in real cases. The model could be proposed as a tool for evaluating probable pedestrians’ choices in post-event scenarios, in order to reduce the interferences between the built environment and the evacuation process through interventions on buildings, urban fabric and strategies for emergency management.     

Numerical study on basin-edge effects in the seismic response of the Gubbio valley,Central Italy

The Gubbio basin in Central Italy is a intermountain basin of extensional tectonic origin, typical of Central and Southern Apennines, characterized by moderate seismicity. The strongest recorded event within the area is a magnitude 5.7 earthquake which occurred on 29 April 1984 along the Gubbio fault, bordering the eastern side of the basin. The main objective of this study is to analyze the features of earthquake ground motion as related to basin-edge effects, by performing physics-based numerical simulations of the 1984 earthquake through a high-performance spectral element code. The simulated ground motions are found in reasonable agreement with the recorded motions when using the kinematic source model developed by Ameri et al. (Bull Seismol Soc Am 99:647–663, 2009), with a rise-time equal to 1 s and a nucleation point located in the middle of the fault. Pronounced differences were noted between records from the basin and adjacent sites at outcropping bedrock, owing to both the strong impedance contrast between soft alluvial sites and bedrock formations (lithostratigraphic amplification), as well as lateral discontinuities related to the 2D/3D geometry of the basin (generation of surface waves). Since the fault was located beneath the basin, 1D amplification effects were found to be more relevant than those associated with the generation of surface waves from the basin edge. Finally, an envelope delay spectrum was computed for the simulated ground motions, showing that surface waves are excited in the frequency band of 0.2–0.8 Hz with a significant increase of ground motion duration within the basin.     

Protecting the hand that feeds us: Seagrass (Zostera marina) serves as commercial juvenile fish habitat

Although fisheries are of major economic and food security importance we still know little about specific juvenile habitats that support such production. This is a major issue given the degradation to and lack of protection afforded to potential juvenile habitats such as seagrass meadows. In the present study we investigate the role of seagrass in supporting juvenile fish of commercial value. By assessing seagrass relative to adjacent sand we determined the presence of abundant juvenile fish. Nine commercial species were recorded and the most abundant of these were Plaice, Pollock and Herring. We provide the first quantitative evidence of the presence of juvenile fish of commercial value in seagrass surrounding Great Britain. Although the species that we found in seagrass as juveniles are not obligate seagrass users the resources that seagrass meadows offer to these fish provide significant long-term fitness benefits, potentially enhancing the whole population.     

The variability of European floods since AD 1500

The paper presents a qualitative and quantitative analysis of flood variability and forcing of major European rivers since AD 1500. We compile and investigate flood reconstructions which are based on documentary evidence for twelve Central European rivers and for eight Mediterranean rivers. Flood variability and underlying climatological causes are reconstructed by using hermeneutic approaches including critical source analysis and by applying a semi-quantitative classification scheme. The paper describes the driving climatic causes, seasonality and variability of observed flood events within the different river catchments covering the European mainland. Historical flood data are presented and recent research in the field of historical flood reconstructions is highlighted. Additionally, the character of the different flood series is discussed. A comparison of the historical flood seasonality in relation to modern distribution is given and aspects of the spatial coherence are presented. The comparative analysis points to the fact that the number of flood events is predominately triggered by regional climatic forcing, with at most only minor influence on neighbouring catchments. The only exceptions are extreme, supra-regional climatic events and conditions such as anomalous cold winters, similar to that of 1784, which affected large parts of Europe and triggered flood events in several catchments as a result of ice-break at the beginning of the annual thaw. Four periods of increased occurrence of flooding, mostly affecting Central European Rivers, have been identified; 1540–1600, 1640–1700, 1730–1790, 1790–1840. The reconstruction, compilation and analysis of European-wide flood data over the last five centuries reveal the complexity of the underlying climatological causes and the high variability of flood events in temporal and spatial dimension.     

The contribution of satellite SAR-derived displacement measurements in landslide risk management practices

Natural Hazards - Landslides are common phenomena that occur worldwide and are a main cause of loss of life and damage to property. The hazards associated with landslides are a challenging concern...