Closed‐form aftershock reliability of damage‐cumulating elastic‐perfectly‐plastic systems

Major earthquakes (i.e., mainshocks) typically trigger a sequence of lower magnitude events clustered both in time and space. Recent advances of seismic hazard analysis stochastically model aftershock occurrence (given the main event) as a nonhomogeneous Poisson process with rate that decays in time as a negative power law. Risk management in the post‐event emergency phase has to deal with this short‐term seismicity. In fact, because the structural systems of interest might have suffered some damage in the mainshock, possibly worsened by damaging aftershocks, the failure risk may be large until the intensity of the sequence reduces or the structure is repaired. At the state‐of‐the‐art, the quantitative assessment of aftershock risk is aimed at building tagging, that is, to regulate occupancy. The study, on the basis of age‐dependent stochastic processes, derived closed‐form approximations for the aftershock reliability of simple nonevolutionary elastic‐perfectly‐plastic damage‐cumulating systems, conditional on different information about the structure. Results show that, in the case hypotheses apply, the developed models may represent a basis for handy tools enabling risk‐informed tagging by stakeholders and decision makers. Copyright © 2013 John Wiley & Sons, Ltd.     

How does forest structure affect root reinforcement and susceptibility to shallow landslides?

Forests can decrease the risk of shallow landslides by mechanically reinforcing the soil and positively influencing its water balance. However, little is known about the effect of different forest structures on slope stability. In the study area in St Antönien, Switzerland, we applied statistical prediction models and a physically‐based model for spatial distribution of root reinforcement in order to quantify the influence of forest structure on slope stability. We designed a generalized linear regression model and a random forest model including variables describing forest structure along with terrain parameters for a set of landslide and control points facing similar slope angle and tree coverage. The root distribution measured at regular distances from seven trees in the same study area was used to calibrate a root distribution model. The root reinforcement was calculated as a function of tree dimension and distance from tree with the root bundle model (RBMw). Based on the modelled values of root reinforcement, we introduced a proxy‐variable for root reinforcement of the nearest tree using a gamma distribution. The results of the statistical analysis show that variables related to forest structure significantly influence landslide susceptibility along with terrain parameters. Significant effects were found for gap length, the distance to the nearest trees and the proxy‐variable for root reinforcement of the nearest tree. Gaps longer than 20 m critically increased the susceptibility to landslides. Root reinforcement decreased with increasing distance from trees and is smaller in landslide plots compared to control plots. Furthermore, the influence of forest structure strongly depends on geomorphological and hydrological conditions. Our results enhance the quantitative knowledge about the influence of forest structure on root reinforcement and landslide susceptibility and support existing management recommendations for protection against gravitational natural hazards. Copyright © 2015 John Wiley & Sons, Ltd.     

Vibrios in association with sedimentary crustaceans in three beaches of the northern Adriatic Sea (Italy)

In the marine environment, vibrios adhere to a number of substrates including chitin-rich organisms such as crustaceans. Their wide diffusion in coastal waters and pathogenic potential require knowledge of the lifestyle and environmental reservoirs of these bacteria. To test the presence of culturable vibrios in coastal areas and their association with benthic crustaceans, vibrios were isolated from water, sediments and crustaceans (copepods and anphipods) at three stations placed in front of heavily used tourist beaches of the Adriatic Sea. We observed significant correlations between vibrios and temperature. Benthic and planktonic copepods harboured vibrios in summer, while benthic amphipods harboured these bacteria in spring and autumn. Vibrio alginolyticus and Vibrio parahaemolyticus strains gave positive results using primers for Vibrio cholerae toxR and toxS. Sedimentary crustaceans may extend Vibrio persistence in seawater and may represent an additional aquatic reservoir of these bacteria.     

Construction dynamics of a lava channel

We use a kinematic GPS and laser range finder survey of a 200 m-long section of the Muliwai a Pele lava channel (Mauna Ulu, Kilauea) to examine the construction processes and flow dynamics responsible for the channel–levee structure. The levees comprise three packages. The basal package comprises an 80–150 m wide ′a′a flow in which a ∼2 m deep and ∼11 m wide channel became centred. This is capped by a second package of thin (<45 cm thick) sheets of pahoehoe extending no more than 50 m from the channel. The upper-most package comprises localised ′a′a overflows. The channel itself contains two blockages located 130 m apart and composed of levee chunks veneered with overflow lava. The channel was emplaced over 50 h, spanning 30 May–2 June, 1974, with the flow front arriving at our section (4.4 km from the vent) 8 h after the eruption began. The basal ′a′a flow thickness yields effusion rates of 35 m³ s⁻¹ for the opening phase, with the initial flow advancing across the mapped section at ∼10 m/min. Short-lived overflows of fluid pahoehoe then built the levee cap, increasing the apparent channel depth to 4.8 m. There were at least six pulses at 90–420 m³ s⁻¹, causing overflow of limited extent lasting no more than 5 min. Brim-full flow conditions were thus extremely short-lived. During a dominant period of below-bank flow, flow depth was ∼2 m with an effusion rate of ∼35 m³ s⁻¹, consistent with the mean output rate (obtained from the total flow bulk volume) of 23–54 m³ s⁻¹. During pulses, levee chunks were plucked and floated down channel to form blockages. In a final low effusion rate phase, lava ponded behind the lower blockage to form a syn-channel pond that fed ′a′a overflow. After the end of the eruption the roofed-over pond continued to drain through the lower blockage, causing the roof to founder. Drainage emplaced inflated flows on the channel floor below the lower blockage for a further ∼10 h. The complex processes involved in levee–channel construction of this short-lived case show that care must be taken when using channel dimensions to infer flow dynamics. In our case, the full channel depth is not exposed. Instead the channel floor morphology reflects late stage pond filling and drainage rather than true channel-contained flow. Components of the compound levee relate to different flow regimes operating at different times during the eruption and associated with different effusion rates, flow dynamics and time scales. For example, although high effusion rate, brim-full flow was maintained for a small fraction of the channel lifetime, it emplaced a pile of pahoehoe overflow units that account for 60% of the total levee height. We show how time-varying volume flux is an important parameter in controlling channel construction dynamics. Because the complex history of lava delivery to a channel system is recorded by the final channel morphology, time-varying flow dynamics can be determined from the channel morphology. Developing methods for quantifying detailed flux histories for effusive events from the evidence in outcrop is therefore highly valuable. We here achieve this by using high-resolution spatial data for a channel system at Kilauea. This study not only indicates those physical and dynamic characteristics that are typical for basaltic lava flows on Hawaiian volcanoes, but also a methodology that can be widely applied to effusive basaltic eruptions.     

Time-space variation of volcano-seismic events at La Fossa (Vulcano, Aeolian Islands, Italy): new insights into seismic sources in a hydrothermal system

We investigated the relationship between volcano-seismic events, recorded at La Fossa crater of Vulcano (Aeolian Islands, Italy) during 2004-2006, and the dynamics of the hydrothermal system. During the period of study, three episodes of increasing numbers of volcano-seismic events took place at the same time as geothermal and geochemical anomalies were observed. These geothermal and geochemical anomalies have been interpreted as resulting from an increasing deep magmatic component of the hydrothermal fluids. Three classes of seismic events (long period, high frequency and monochromatic events), characterised by different spectral content and various similarity of the waveforms, have been recognised. These events, clustered mainly below La Fossa crater area at depths of 0.5–1.1 km b.s.l., were space-distributed according to the classes. Based on their features, we can infer that such events at Vulcano are related to two different source mechanisms: (1) fracturing processes of rocks and (2) resonance of cracks (or conduits) filled with hydrothermal fluid. In the light of these source mechanisms, the increase in the number of events, at the same time as geochemical and geothermal anomalies were observed, was interpreted as the result of an increasing magmatic component of the hydrothermal fluids, implying an increase of their flux. Indeed, such variation caused an increase of both the pore pressure within the rocks of the volcanic system and the amount of ascending fluids. Increased pore pressures gave rise to fracturing processes, while the increased fluid flux favoured resonance and vibration processes in cracks and conduits. Finally, a gradual temporal variation of the waveform of the hybrid events (one of the subclasses of long period events) was observed, likely caused by heating and drying of the hydrothermal system.     

Detecting short-term evolution of Etnean scoria cones: a LIDAR-based approach

The 2001 and 2002–2003 flank eruptions on Mount Etna (Italy) were characterized by intense explosive activity which led to the formation of two large monogenetic scoria cones (one from each eruption) on the upper southern flank of the volcano. Continuous monitoring of Etna, especially during flank eruptions, has provided detailed information on the growth of these cones. They differ in genesis, shape, and size. A set of high resolution (1 m) digital elevation models (DEMs) derived from light detection and ranging (LIDAR) data collected during four different surveys (2004, 2005, 2006, and 2007) has been used to map morphology and to extract the morphometric parameters of the scoria cones. By comparing LIDAR-derived DEMs with a pre-eruption (1998) 10 m DEM, the volume of the two scoria cones was calculated for the first time. Comparison of the LIDAR-derived DEMs revealed in unprecedented detail morphological changes during scoria cone degradation. In particular, the morphologically more exposed and structurally weaker 2002–2003 cone was eroded rapidly during the first few years after its emplacement mainly due to gravitational instability of slopes and wind erosion.     

The distal segment of Etna’s 2001 basaltic lava flow

Etna’s 2001 basaltic lava flow provided a good example of the distal flow segment between the flow front and stable channel, across which the flow evolves from channel-contained to dispersed. This zone was mapped with meter precision using LIDAR data collected during 2004 and 2005. These data, supported by field mapping, show that the flow front comprised eight lobes each 10 to 20 m high. The flow front appears to have advanced not as a single unit, but as a series of lobes moving forward one lobe at a time. Primary lobes were centered on the channel axis and marginal lobes were off-axis. The lobes advanced as breakouts of low-yield-strength lava from the flow core of the stalled flow front. Marginal lobes were abandoned and contributed to marginal levees flanking the transitional channel. For Etna’s 2001 flow, the transitional channel is 140 m wide, 700 m long and fed a 240-m-long zone of dispersed flow; the change from stable to transitional channel occurred at a major reduction in slope. Above this, the stable channel is 5.2 km long, 55 to 105 m wide and bounded by 15- to 25-m-high levees, and the stable channel is located over a previous channel. In a final stage of activity, lava ponding at the break-in-slope that marks the terminus of the stable channel put pressure on the eastern levee, causing it to fail. Liberated lava then fed a final break-out to the east. Similar flow front-features occur at other volcanoes, indicating that similar processes are characteristic of dispersed flow zones.     

Relationship between cooling rate and cooling age of a mineral: Theory and applications to meteorites

Abstract— We reviewed here the recent development on the mathematical formulation of closure temperature of a cooling geochronological system, which permits direct retrieval of cooling rate from cooling age when the diffusion parameters, grain size and initial temperature are known. This formulation is used to show how the cooling rate can be retrieved by comparing the core and bulk age of a mineral determined by a single decay system. The cooling rates of seven H chondrites of the metamorphic types H4, H5 and H6 were retrieved from the available data on the Pb‐Pb model ages of the phosphates and the diffusion kinetic data of Pb in apatite. The results are in excellent agreement with the metallographic cooling rates and show an inverse relation with the metamorphic grade of these chondrites. We also addressed the problem of ～90 Ma younger Sm‐Nd mineral isochron age, defined by orthopyroxene, phosphate and plagioclase, of the Morristown mesosiderite compared to the Pb‐Pb age of the Estherville mesosiderite. It is shown that this younger age could have been a consequence of resetting during cooling instead of an “impulsive heating” event, as suggested earlier.     

An integrated approach to determine three‐dimensional accretion geometries of tidal point bars: Examples from the Venice Lagoon (Italy)

Low rates of lateral migration (centimetres to decimetres per year) combined with relatively high rates of vertical accretion (millimetres to centimetres per year) recorded in microtidal channels of the Venice Lagoon (Italy) give rise to point‐bar geometries and internal facies arrangements that differ substantially from widely accepted models of point‐bar sedimentary architecture. In this study, field data from the Venice Lagoon are combined with a three‐dimensional forward stratigraphic model, the ‘Point‐Bar Sedimentary Architecture Numerical Deduction’ (PB‐SAND), to predict the stratal geometries of point bars formed in aggradational settings. The PB‐SAND uses a combined geometric and stochastic modelling approach that can be constrained by field evidence. The model applied determines the geometry of four point bars generated by 9 to 11 m wide channels cutting through salt marshes. An iterative best‐fit modelling approach has been used to obtain multiple simulations for each case study, each of which fits the observations derived from the analysis of time‐series historical aerial photographs and 44 sedimentary cores. Results demonstrate how the geometry of the bars is determined by the development of two key stratal surfaces: the point‐bar brink and channel‐thalweg surfaces. These surfaces are defined by the progressive translation and vertical shift of the point‐bar brink (i.e. break of slope between bar top and bar slope) and the channel thalweg (i.e. deepest part of the channel) during bar evolution. The approach is used to: (i) reconstruct three‐dimensional point‐bar geometries; (ii) propose alternative reconstructions; (iii) provide insight to drive the acquisition of additional data to better constrain the proposed models; and (iv) provide insight into the mechanism of bar growth for slowly migrating channels in settings subject to relatively high rates of aggradation. This study highlights how interaction between styles of planform transformation and latero‐vertical shifts of meandering channels can determine the geometry of related sedimentary bodies.