Volcaniclastic debris-flow occurrences in the Campania region (Southern Italy) and their relation to Holocene–Late Pleistocene pyroclastic fall deposits: implications for large-scale hazard mapping

The Campania Region (southern Italy) is characterized by the frequent occurrence of volcaniclastic debris flows that damage property and loss of life (more than 170 deaths between 1996 and 1999). Historical investigation allowed the identification of more than 500 events during the last four centuries; in particular, more than half of these occurred in the last 100 years, causing hundreds of deaths. The aim of this paper is to quantify debris-flow hazard potential in the Campania Region. To this end, we compared several elements such as the thickness distribution of pyroclastic fall deposits from the last 18 ka of the Vesuvius and Phlegrean Fields volcanoes, the slopes of relieves, and the historical record of volcaniclastic debris flows from A.D. 1500 to the present. Results show that flow occurrence is not only a function of the cumulative thickness of past pyroclastic fall deposits but also depends on the age of emplacement. Deposits younger than 10 ka (Holocene eruptions) apparently increase the risk of debris flows, while those older than 10 ka (Late Pleistocene eruptions) seem to play a less prominent role, which is probably due to different climatic conditions, and therefore different rates of erosion of pyroclastic falls between the Holocene and the Late Pleistocene. Based on the above considerations, we compiled a large-scale debris-flow hazard map of the study area in which five main hazard zones are identified: very low, low, moderate, high, and very high.     

Development mechanisms of an explosive cyclone over East Sea on 3–4 April 2012

The development mechanisms of the explosive cyclone that occurred during 3–4 April 2012 over East Sea (Sea of Japan) are examined through numerical simulation and sensitivity experiments using the Weather and Research Forecasting (WRF) model. The characteristics of this explosive cyclone are different from typical cyclonic features observed in this region, including its intensity, deepening rate, and formation time. Numerical simulation, reanalysis data, upper and surface weather charts, and satellite data indicate that the strong baroclinic instability and temperature advection associated with upper-level cut-off low and the interaction of potential vorticity (PV) anomalies between the lower- and upper-level are essential to explosive cyclogenesis.The sensitivity experiments of the explosive cyclone show that latent heat release (LHR) is an important factor in explosive cyclogenesis. The intensification, extent, and movement speed of the cyclone are amplified by LHR as well as the formation of an upper-level cut-off low. The role of LHR is primary important in the generation and evolution of the cyclone. Especially, the LHR contributes to roughly 50% of decrease in sea level pressure (SLP) and 50% of the central cyclone’s low-level PV generation in initial stage. During a 48-h simulation, the contributions of the LHR, surface heat flux, and their interaction on the decrease of SLP of the cyclone are found to be 40.6, −8.2, and 10.5%, respectively. These results reveal that the explosive cyclone has larger deepening rates than OJ cyclones, and develops with a large amount of LHR near the cyclone center.     

两例爆发性东北低压的对比诊断分析

该文选择了发展变化机制有一定差异的两例春季爆发性东北低压（分别是1983年4月25～26日气旋（简称A例）和1983年4月28～29日气旋（简称B例））,进行了对比诊断分析。结果表明：（1）非绝热加热和局地斜压不稳定对A例气旋发展来说是十分关键的因子,而空正IPV平流的显著增强及其与低层IPV分布中两个局地最大值的垂直耦合是B例气旋增强的一个重要原因;（2）两个风暴最大不同点在于非绝热加热效应在影响气旋增强的程度上有所不同。另外,B例事件中对流层中部产生的较强高空锋生可以在低压范围内导致深厚的上升运动并使高空锋向下游的正涡度平流得以加强,这对系统的发展是十分有利的。     

Numerical simulations on the explosive cyclogenesis over the kuroshio current

In this paper, the Pennsylvania State University-NCAR Mesoscale Model (MM4) is used to investigate the explosive oceanic cyclone of 14-15 March 1988 over the warm Kuroshio Current. A series of numerical simulations on this cyclogenesis indicates that the favorable weather condi-tions and strong baroclinity in the low- and middle-level are essential to its explosive development. The explosive cyclogenesis occurred over a wide range of sea surface temperatures (SST’s), which was then characterized by strong baroclinity, the low-level jet (LLJ) was initially formed under the favorable atmospheric circulation and then this LLJ advected the moisture and heat northward for the explosive development of the cyclone, the LLJ played an important role in the process of cyclogenesis. Sensitivity experiments show that the latent heating was a key factor to explosive cyclogenesis, the latent heating deepened the short-wave trough, which resulted in the rapid intensification of the cyclone; while in the explosive intensification stage and continuous de-velopment stage, there was less contribution of local surface processes for the explosion of the cy?clone.     

爆发性气旋发展中斜压强迫与潜热释放作用的数值试验

本文利用一个六层原始方程模式对五例爆发性气旋的发展进行了数值试验,试验结果表明：斜压强迫是气旋爆发性发展的基本因子,而潜热释放的重要性则有较大差异。潜热释放的作用主要集中在两个方面：一是对低层的直接增暖,加强了上升运动;二是对局地斜压性的加强,促进了锋面、槽脊系统的发展,而这两者均有利于气旋的发展。     

Rundle模式地震的分叉结构及其爆发判据

给出了Ｒｕｎｄｌｅ模式地震所满足的非线性微分方程,经过变换该方程变成标准Ｗｅｉｅｒｓｔｒａｓｓ方程,依据我们的发现,并被国际上确认的Ｗｅｉｅｒｓｔｒａｓｓ函数分叉结构理论,分析了该模式地震的三相结构及相变判据,并给出了强,弱地震自动爆发判据和弱小地震的准周期公式。     

Fluvial response to sudden input of pyroclastic sediments during the 2008–2009 eruption of the Chaitén Volcano (Chile): The role of logjams

The rhyolitic Plinian eruption of the Chilean Chaitén Volcano, initiated on May 2, 2008, suddenly introduced abundant pyroclastic sediments in the Blanco River catchment area, which experienced important modifications. Before May 2, the river was characterised by gravelly and moderate to low-sinuosity channels crossing a vegetated and locally urbanised (Chaitén City) floodplain. This river, limited by steep and densely forested highlands, was connected with the Pacific Ocean via a tidally-influenced delta plain. After heavy rains in May 11–20, the river discharge increased and triggered several responses including logjam formation and breakage, crevassing, avulsion (and channel abandonment), changes in the pattern and dimensions of channels, and construction of a new delta plain area. In this context, the goals of this contribution were: i) to document the sedimentological processes within a detailed geomorphic framework and ii) to understand the influence of logjams on fluvial dynamics. Upstream of the logjam zone, the deposits are mostly composed of ash and lapilli with abundant palaeovolcanic (epiclastic) sediments, which were produced by dilute currents and debris flows. Downstream of the logjam zone, deposits are composed by ash and lapilli, both pumice-rich and lacking important participation of older (epiclastic) sediments. The abandoned and filled palaeochannel, and the proximal part of crevasse splays experienced transient dilute flows with variable sediment concentration and, subordinately, hyperconcentrated flows. The distal sectors of crevasse splays mostly record settling from suspension. At the delta plain, tephra transported by the Blanco River was mixed with older sediments by tide and wave action (dilute flows). We conclude that immediately after eruption, both geomorphic and sedimentary processes of the river were mainly controlled by a combination of high availability of incoherent pyroclastic sediments on steep slopes, abundant rains, large logs that jammed the river and huge areas of devastated forest. Logjams played an important role in the river response to the volcanic eruption; they were responsible of the marked compositional change recorded upstream and downstream of the logjam zone and its breakage resulted in downstream flooding and avulsion. The likelihood of formation of logjams in rivers draining forested volcanic areas should be considered in the evaluation of volcanic hazards related to Plinian eruptions.     

Facies characteristics and magma–water interaction of the White Trachytic Tuffs (Roccamonfina Volcano, southern Italy)

 The Quaternary White Trachytic Tuffs Formation from Roccamonfina Volcano (southern Italy) comprises four non-welded, trachytic, pyroclastic sequences bounded by paleosols, each of which corresponds to small- to intermediate-volume explosive eruptions from central vents. From oldest to youngest they are: White Trachytic Tuff (WTT) Cupa, WTT Aulpi, WTT S. Clemente, and WTT Galluccio. The WTT Galluccio eruption was the largest and emplaced ∼ 4 km³ of magma. The internal stratigraphy of all four WTT eruptive units is a complex association of fallout, surge, and pyroclastic flow deposits. Each eruptive unit is organized into two facies associations, Facies Association A below Facies Association B. The emplacement of the two facies associations may have been separated by short time breaks allowing for limited reworking and erosion. Facies Association A consists of interbedded fallout deposits, surge deposits, and subordinate ignimbrites. This facies association involved the eruption of the most evolved trachytic magma, and pumice clasts are white and well vesiculated. The grain size coarsens upward in Facies Association A, with upward increases of dune bedform wavelengths and a decrease in the proportion of fine ash. These trends could reflect an increase in eruption column height from the onset of the eruption and possibly also in mass eruption rate. Facies Association B comprises massive ignimbrites that are progressively richer in lithic clast content. This association involved the eruption of more mafic magma, and pumice clasts are gray and poorly vesiculated. Facies Association B is interpreted to record the climax of the eruption. Phreatomagmatic deposits occur at different stratigraphic levels in the four WTT and have different facies characteristics. The deposits reflect the style and degree of magma–water interaction and the local hydrogeology. Very fine-grained, lithic-poor phreatomagmatic surge deposits found at the base of WTT Cupa and WTT Galluccio could record the interaction of the erupting magma with a lake that occupied the Roccamonfina summit depression. Renewed magma–water interaction later in the WTT Galluccio eruption is indicated by fine grained, lithic-bearing phreatomagmatic fall and surge deposits occurring at the top of Facies Association A. They could be interpreted to reflect shifts of the magma fragmentation level to highly transmissive, regional aquifers located beneath the Roccamonfina edifice, possibly heralding a caldera collapse event. Received: 26 August 1996 / Accepted: 27 February 1998     

Formation and development of normal-fault calderas and the initiation of large explosive eruptions

The ring fractures that form most collapse calderas are steeply inward-dipping shear fractures, i.e., normal faults. At the surface of the volcano within which the caldera fault forms, the tensile and shear stresses that generate the normal-fault caldera must peak at a certain radial distance from the surface point above the center of the source magma chamber of the volcano. Numerical results indicate that normal-fault calderas may initiate as a result of doming of an area containing a shallow sill-like magma chamber, provided that the area of doming is much larger than the cross-sectional area of the chamber and that the internal excess pressure in the chamber is smaller than that responsible for doming. This model is supported by the observation that many caldera collapses are preceded by a long period of doming over an area much larger than that of the subsequently formed caldera. When the caldera fault does not slip, eruptions from calderas are normally small. Nearly all large explosive eruptions, however, are associated with slip on caldera faults. During dip slip on, and doming of, a normal-fault caldera, the vertical stress on part of the underlying chamber suddenly decreases. This may lead to explosive bubble growth in this part of the magma chamber, provided its magma is gas rich. This bubble growth can generate an excess fluid pressure that is sufficiently high to drive a large fraction of the magma out of the chamber during an explosive eruption. Received: 2 January 1997 / Accepted: 22 April 1998