Geology and hazard implications of the Maraunot notch in the Pinatubo Caldera,Philippines

The 1991 Pinatubo eruption left 5–6 km³ of debris on the volcano slopes, much of which has been mobilized into large lahars in the following rainy seasons. Also during the eruption, collapse, localized in part along preexisting faults, left a caldera 2.5 km in diameter that almost immediately began to accumulate a 1.6 × 10⁸ m³ lake. By 2001, the water had risen to the fault-controlled Maraunot Notch, the lowest, northwestern portion of the caldera rim comprising the physiographic sill of the Caldera Lake. That year, a narrow artificial canal dug into an old volcanic breccia underlying the outlet channel failed to induce a deliberate lake breakout, but discharge from heavy rains in July 2002 rapidly deepened the notch by 23 m, releasing an estimated 6.5 × 10⁷ m³ of lake water that bulked up into lahars with a volume well in excess of 1.6 × 10⁸ m³. Lakes in other volcanoes have experienced multiple breakouts, providing practical motivation for this study. Fieldwork and high-resolution digital elevation models reveal andesites and ancient lacustrine deposits, strongly fractured and deformed along a segment of the Maraunot Fault, a prominent, steeply dipping, left-lateral fault zone that trends N35°–40°W within and parallel to the notch. Seismicity in 1991 demonstrated that the Maraunot Fault is still active. The fault zone appears to have previously been the erosional locus for a large channel, filled with avalanche or landslide deposits of an earlier eruption that were exhumed by the 2002 breakout floods. The deformed lacustrine sediments, with an uncalibrated ¹⁴C age of 14,760 ± 40 year BP from a single charcoal sample, attest to the existence of an earlier lake, possibly within the Tayawan Caldera, rim remnants of which survive as arcuate escarpments. That lake may well have experienced one or more ancient breakouts as well. The 2002 event greatly reduced the possibility of another such event by scouring away the erodible breccia, leaving less erodible fractured andesites and lacustrine rocks, and by enlarging the outlet channel and its discharge capacity. Several lines of evidence indicate, however, that future lahar-generating lake breakouts at the notch may keep populations of Botolan municipality downstream at risk: (1) a volume of 9.5 × 10⁷ m³ of lake water remains perched 0.8 km above sea level; (2) seismicity in 1991 demonstrated that the Maraunot Fault is still active and movements of sufficient magnitude could enlarge the outlet and the discharge through it; (3) more likely, however, with or without earthquake activity, landslides from the steep to overhanging channel walls could block the channel again, and a major rainstorm could then cause a rise in lake level and sudden breakouts; (4) intrusion of a new dome into the bottom of the lake, possibly accompanied by phreatic explosions, could expel large volumes of lahar-generating water.     

Dual models of pore spaces

We present a model for pore spaces that consists of two parts related by duality: (1) a decomposition of an open polyhedral pore space into open contractible pore bodies separated by relatively open interfaces and (2) a pore network that is homotopy equivalent to the pore space. The dual model is unique and free of parameters, but it relies on regularity conditions for the pore space. We show how to approximate any pore space by the interior of a polyhedral complex such that the regularity conditions are fulfilled. Thus, we are able to calculate the dual model from synthetic porous media and images of real porous media. The pore bodies are unions of relatively open Delaunay cells with respect to the corners of the pore boundary, and the pore network consists of certain at most two-dimensional (2D) Voronoi cells with respect to the corners of the pore boundary. The pore network describes the neighborhood relations between the pore bodies. In particular, any relatively open 2D Delaunay face f separating two pore bodies has a unique (relatively open) dual network edge. In our model, f is a pore throat only if it is hit by its dual network edge. Thus, as opposed to widespread intuition, any pore throat is convex, and adjacent pore bodies are not necessarily separated by pore throats. Due to the duality between the pore network and the decomposition of the pore space into pore bodies it is straightforward to store the geometrical properties of the pore bodies [pore throats] as attributes of the dual network vertices [edges]. Such an attributed network is used to perform 2D drainage simulations. The results agree very well with those from a pore-morphology based modeling approach performed directly on the digital image of a porous medium. Contractibility of the pore bodies and homotopy equivalence of the pore space and the pore network is proven using discrete Morse theory and the nerve theorem from combinatorial topology.     

GIS‐assisted modelling for debris flow hazard assessment based on the events of May 1998 in the area of Sarno,Southern Italy: Part I. Maximum run‐out

Based on the debris flow events that occurred in May 1998 in the area of Sarno, Southern Italy, this paper presents an approach to simulate debris flow maximum run‐out. On the basis of the flow source areas and an average thickness of 1·2 m of the scarps, we estimated debris flow volumes of the order of 10⁴ and 10⁵ m³. Flow mobility ratios (ΔH/L) derived from the x, y, z coordinates of the lower‐most limit of the source areas (i.e. apex of the alluvial fan) and the distal limit of the flows ranged between 0·27 and 0·09. We performed regression analyses that showed a good correlation between the estimated flow volumes and mobility ratios. This paper presents a methodology for predicting maximum run‐out of future debris flow events, based on the developed empirical relationship. We implemented the equation that resulted from the calibration as a set of GIS macros written in Visual Basic for Applications (VBA) and running within ArcGIS. We carried out sensitivity analyses and observed that hazard mapping with this methodology should attempt to delineate hazard zones with a minimum horizontal resolution of 0·4 km. The developed procedure enables the rapid delineation of debris flow maximum extent within reasonable levels of uncertainty, it incorporates sensitivities and it facilitates hazard assessments via graphic user interfaces and with modest computing resources. Copyright © 2007 John Wiley & Sons, Ltd.     

Ekman motion in shallow open sea in the presence of time-harmonic variation of water depth

The time variation of the wind-induced flow in a homogeneous unbounded sea region has been analytically investigated. The time-dependent Ekman solution in a homogeneous, shallow open sea has been further extended by taking into account the time variation in water depth which might be caused by either tidal motion or change in the mixed layer thickness. The solution approach taken in this study is based on the Galerkin-eigenfunction method originally developed by Heaps [1972. On the numerical solution of three-dimensional hydrodynamical equations for tides and storm surges. Memoires de la Societe Royale des Sciences de Liege Serie 6(2), 143–180]. A series of calculations have been made with emphasis on the influence of the time variation in water depth upon the build-up of Ekman spirals in the presence of oscillatory variations in water depth. It has been found that two oscillations contribute to the wind drift current, the inertial oscillation and the depth-variation-induced oscillation; the inertial oscillation decays with time, but the depth-variation-induced oscillation remains undamped despite the presence of bottom friction. The presence of time-harmonic variation produces peculiar forms of hodograph with a curled or circular pattern according to the angular frequency of the water depth variation.     

The development of an intervening opportunities model with spatial dominance effects

In this paper an intervening opportunities model with spatial dominance is developed. The usual assumption in spatial theory is that decision makers are influenced not just by the size of a destination or distance but by these two factors in combination, that is, spatial dominance. Decision-makers will have more knowledge about, and clearly perceive destinations that exert the greatest amount of spatial dominance on their origins, just as they would primate cities. Thus destinations are ranked in terms of the spatial dominance calculated for each destination. Empirical verification of the model utilizes state-to-state migration flow data for the US. Calibration results compare favourably with the traditional intervening opportunities model and the production constrained gravity model. Received: 7 October 1999/Accepted: 15 June 2000     

Long internal waves in shear flows: topographic resonance and wave-induced global instability

Issues pertaining to a mechanism whereby long internal waves in shallow seas may give rise to enhanced rates of resuspension of sedimentary material are addressed. The proposed mechanism is intimately related to the creation of conditions in the bottom boundary layer which are favorable for flow separation and spontaneous onset of global instability. It is shown that long waves generated by topographic resonance and propagating upstream against the oncoming current, especially a sheared current, have a strong tendency to release a coherent, pulsating dynamics in their footprint. The passage-through-resonance problem for a sheared, stratified flow is considered, conditions for topographic resonance in the flow model are defined, and preliminary results for the unsteady dynamics in the boundary layer under the footprint of a long wave packet are presented.     

金沙江流域（云南段）砂金成因类型及其找矿前景

根据成矿作用、成矿环境及矿床特征，金沙江流域（云南段）砂金矿床，可划分为：残坡积、洪积、冲洪积、冲积和冰碛等五种类型。成矿主要受碰撞带、深大断裂、大断裂及其派生断裂、基底岩石、地貌、水质和水化学环境等综合控制。以巨甸－大具及中江街－金江街两片区最具有找矿前景。     

TAKING INTO ACCOUNT THE EFFECT OF SECONDARY FLOWS IN DEPTH AVERAGED MORPHODYNAMIC SIMULATIONS

1 INTRODUCTIONThe numerical simulation of morphological changes in rivers, estuaries and coastal areas is because ofits econondcal merit an outstanding scientific task. The comPlexity of the problem is connected with theheterogeneity of the sediment material, an adequate rePresentation of the involved hydrOdynndc stressesand the large comPutational effort fOr long term simulations.Although the aPpllcation of three dimensional models tO engineering problems is increasing and wellvalldate…     

Geochemistry and magnetostratigraphy of deccan flows at Anjar, Kutch

Chemical analysis of nine Deccan flow basalts at Anjar, Kutch, western India, indicates that all, except the uppermost flow F-9, are alkaline. In their major and trace element composition, the alkali basalts resemble Ocean island basalts (OIB). Similarities of many diagnostic trace element ratios (e.g. Sm/Nd, Ba/Nb,Y/Nb and Zr/Nb) are similar to those found in the Réunion Island basalts. The uppermost basalt is tholeiitic and chemically resembles the least contaminated Deccan basalt (Ambenali type). The Anjar basalts have iridium concentration ranging between 2 and 178 pg/g. Some of these values are higher by about an order of magnitude compared to the Ir concentration in other basalts of the Deccan. A synthesis of chemical, palaeomagnetic and geochronologic data enables us to construct a chemical and magnetic stratigraphy for these flows. The three flows below the iridium enriched intertrappean bed (IT III) show normal magnetic polarity whereas all except one of the upper basalts show reversed magnetic polarity. The sequence seems to have started in polarity zones 31N and probably continued up to 28R or 27R. The results presented here support the view that Deccan volcanism in Kutch occurred on a time span of a few million years.     

In situ formation of welded tuff-like textures in the carapace of a voluminous silicic lava flow,Owyhee County,SW Idaho

 The Badlands rhyolite, on the Owyhee Plateau of southwestern Idaho, can be demonstrated to be a large lava flow on the basis of its geometry of large and small flow lobes, its well-exposed near-vent features, and its response to pre-existing topography. However, samples of the dense upper vitrophyre of the unit reveal a range of annealed fragmental textures, including material which closely resembles the compressed, welded glass shards which are characteristic of ignimbrites. Formation of these tuff-like textures involved processes probably common to emplacement of most silicic lava flow units. Decompression upon extrusion causes inflation of pumice at the surface of the lava flow; some of this pumice is subsequently comminuted, producing loose bubble-wall shards, bits of pumice, chips of dense glass, and fragments of phenocrysts. This debris sifts down around loose blocks and into open fractures deeper in the flow, where it can be reheated, compressed, and annealed to varying degrees. The end result is a dense vitrophyre layer (beneath the true upper, non-welded carapace breccia) which can be extremely texturally heterogeneous, with areas of flow-foliated lava occurring very near lava which in many aspects looks like welded ignimbrite, complete with flattened pumices. Identical textures in other silicic units have been cited by previous workers as evidence that those units erupted as pyroclastic flows which then underwent sufficient rheomorphism to create a flow-foliated rock which otherwise appears to be lava. The textures described herein indicate that lava flows can come to mimic rheomorphic ignimbrites, at least at scales ranging from thin sections to outcrops. Voluminous silicic units with scattered fragmental textures, but with otherwise lava-like features, are probably true effusive lava flows. Received: January 30, 1995 / Accepted: January 22, 1996