History of landslide susceptibility and a chorology of landslide-prone areas in the Western Ghats of Kerala,India

Kerala is the third most densely populated state in India. It is a narrow strip of land, of which 47% is occupied by the most prominent orographic feature of peninsular India, The Western Ghats mountain chain. The highlands of Kerala experience several types of landslides, of which debris flows are the most common. They are called “Urul Pottal” in the local vernacular. The west-facing Western Ghats scarps that runs the entire extent of the mountain system is the most prone physiographic unit for landslides. The highlands of the region experience an annual average rainfall as high as 500 cm through the South-West, North-East and Pre-Monsoon showers. A survey of ancient documents and early news papers indicates a reduced rate of slope instability in the past. The processes leading to landslides were accelerated by anthropogenic disturbances such as deforestation since the early 18th century, terracing and obstruction of ephemeral streams and cultivation of crops lacking capability to add root cohesion in steep slopes. The events have become more destructive given the increasing vulnerability of population and property. Majority of mass movements have occurred in hill slopes >20° along the Western Ghats scarps, the only exception being the coastal cliffs. Studies conducted in the state indicates that prolonged and intense rainfall or more particularly a combination of the two and the resultant pore pressure variations are the most important trigger of landslides. The initiation zone of most of the landslides was typical hollows generally having degraded natural vegetation. A survey of post-landslide investigation and news paper reports enabled the identification of 29 major landslide events in the state. All except one of the 14 districts in the state are prone to landslides. Wayanad and Kozhikode districts are prone to deep seated landslides, while Idukki and Kottayam are prone to shallow landslides.     

Digital signal processing for precision wide-swath bathymetry

A mathematical model is formulated which accurately represents the envelope function of bottom return signals received from a number of spatial directions comprising a wide swath. The bottom return signals are processed utilizing a digital nonrecursive matched filter whose coefficients are tapered using a Tukey window. High-speed convolution employing the fast Fourier transform is examined for implementation of the digital matched filtering operation. Computer simulation of the signal processing system indicates that, even in the presence of considerable background and fluctuation noises, the processor provides an output signal having a well-defined peak. The error in time of arrival is found to be less than 3 ms, corresponding to an error in depth of less than 0.1 percent, for an average signal-to-noise ratio of 15 dB and a vertical ocean depth of 12 000 ft (3.7 km). These performance figures apply to the most difficult case of mapping at angles ofpm 45degoff vertical.     

Depositional setting and vertebrate biostratigraphy of the Triassic Dockum Group of Texas

Triassic strata of the Dockum Group in Texas comprise two major upward-fining alluvial-lacustrine depositional sequences. The two sequences are represented by the (1) Santa Rosa-Tecovas, and (2) Trujillo-Cooper Canyon Formations. The second sequence is much thicker than the first, and occupies a greater geographic part of the Dockum basin. Each sequence of alluvial and lacustrine sediment accumulation is characterized by sediment derivation from a different source terrain. The unconformable relationship between the two depositional sequences, the change in mineralogical composition and presumed source areas between these units, differences in paleocurrent orientation between units, and evidence for intervening episodes of local deformation indicate that the sequences are of tectonic origin. These strata are not the product of a single sediment dispersal system, such as the centripetally-drained lacustrine delta complex previously envisioned for the Dockum basin. Both Dockum sequences are comprised largely of two typical alluvial facies associations; stream channel facies, and overbank flood-plain facies, that are similar to those described in nearly all fluvial deposits. In addition, the Dockum Group contains a peculiar lacustrine facies that accumulated in local flood-plain depressions, and probably resulted from subsidence over areas of subsurface salt dissolution. Vertebrate fossil assemblages are found in all three Dockum facies associations. Five fossiliferous sites in the Dockum are discussed in the context of these three depositional settings. The Dockum tetrapod diversity is reviewed in a hierarchical phylogeny with remarks on the history of collection, stratigraphic distribution of genera, and their taxonomic status. The stratigraphic ranges of tetrapod taxa do not support the recently proposed successive Otischalkian, Adamanian, Revueltian, and Apachean biochrons within the Dockum Group. Instead, a few index fossils provide a broad framework for correlation of Late Triassic nonmarine strata of the Dockum with the Carnian and Norian Alpine marine stages.     

Sapphirine + quartz assemblage in contrasting textural modes from the Eastern Ghats Belt, India: Implications for stability relations in UHT metamorphism and retrograde processes

Stability of the assemblage sapphirine + quartz in Mg–Al-rich granulites implies ultrahigh temperature (UHT) condition of metamorphism but their direct contact is rarely preserved in natural rocks. The present study shows contrasting textural relations between sapphirine and quartz in different parts of the same occurrence of a Mg–Al-rich granulite, Eastern Ghats Belt, India. Textural data suggest stabilization of the assemblage sapphirine + quartz with orthopyroxene and cordierite during the metamorphic peak. Thermometric estimates yield temperature exceeding 950 °C for the stability of this assemblage. Most of such sapphirine grains (Spr₁) are texturally separated from quartz and cordierite grains by double corona of sillimanite + orthopyroxene that results due to isobaric cooling during the post-peak stage. Sapphirine (Spr₂) also forms a symplectic intergrowth with quartz and orthopyroxene at the fringe of coarse orthopyroxene. This textural feature can be explained by the breakdown of (Fe, Mg)-Tschermak components of orthopyroxene during the same isobaric cooling episode from UHT peak condition. The preservation of grain contact of this intergrown sapphirine and quartz can be attributed to a problem in reaction kinetics. In the other mode, sapphirine (Spr₃) occurs with quartz with a thin skin of cordierite near a quartz vein. Such texture could result from isothermal decompression of the cooled crust. Alternatively and more possibly, cordierite could form from ingress of CO₂–H₂O rich fluid during terminal stage of cooling. Finally, sapphirine (Spr₄) and quartz show direct contact close to the quartz vein. Direct contact of such sapphirine and quartz represents textural disequilibrium as this particular quartz is introduced as a vein much later than the peak metamorphism but prior to the major foliation-forming deformation. Coarse sapphirine and vein quartz, therefore, accidentally came in contact with each other and persisted metastably. Therefore, though coexistence of sapphirine and quartz is considered to be a strong evidence for ultrahigh temperature condition, care should be taken to decipher their stable coexistence. Different types of textural relations involving this mineral pair could originate in a single rock, probably in different stages of its metamorphic history.     

Remote sensing GIS-based landslide susceptibility & risk modeling in Darjeeling–Sikkim Himalaya together with FEM-based slope stability analysis of the terrain

Natural Hazards - Landslide susceptibility (LSI) modeling of Darjeeling–Sikkim Himalaya is performed by integrating 28 causative factors on 28C28 combinations on Geographical Information...     

Long-term Variations in the Intensity of Plages and Networks as Observed in Kodaikanal Ca-K Digitized Data

In our previous article (Priyal et al. in Solar Phys. 289, 127, 2014) we have discussed the details of observations and methodology adopted to analyze the Ca-K spectroheliograms obtained at the Kodaikanal Observatory (KO) to study the variation of Ca-K plage areas, enhanced network (EN), and active network (AN) for Solar Cycles, namely 19, 20, and 21. Now, we have derived the areas of chromospheric features using KO Ca-K spectroheliograms to study the long-term variations of Solar Cycles 14 to 21. The comparison of the derived plage areas from the data obtained at the KO observatory for the period 1906?–?1985 with that of MWO, NSO for the period 1965?–?2002, earlier measurements made by Tlatov, Pevtsov, and Singh (Solar Phys. 255, 239, 2009) for KO data, and the SIDC sunspot numbers shows a good correlation. The uniformity of the data obtained with the same instrument remaining with the same specifications provided a unique opportunity to study long-term intensity variations in plages and network regions. Therefore, we have investigated the variation of the intensity contrast of these features with time at a temporal resolution of six months assuming that the quiet-background chromosphere remains unchanged during the period 1906?–?2005 and found that the average intensity of the AN, representing the changes in small-scale activity over solar surface, varies with solar cycle being less during the minimum phase. In addition, the average intensity of plages and EN varies with a very long period having a maximum value during Solar Cycle 19, which was the strongest solar cycle of twentieth century.     

Is an onset vortex important for monsoon onset over Kerala?

Inter-annual variability in the formation of the mini warm pool [sea-surface temperature (SST)>30°C] over the south eastern Arabian Sea (SEAS) and its role in the formation of the monsoon onset vortex (MOV) has been examined using two independent SST data sets. The role of SST, convection, integrated columnar water vapour and the low-level jet in the setting up of the monsoon onset over Kerala (MOK) is examined. It is found that the MOV which forms over the SEAS region upsets the delicate balance between convection, buildup of moisture and strengthening and deepening of the westerlies over the SEAS that is needed for the setting up of the MOK. Thus, the formation over the SEAS of an MOV is not necessarily conducive for MOK. Furthermore, it is shown that a mini warm pool over the southeastern Arabian Sea is not a sufficient condition for the formation of an MOV because such a warm pool is present over this region during most of the years, but an MOV does not necessarily form over there.     

The role of permeability evolution in fault zones on the structural and hydro-mechanical characteristics of shortening basins

We examine the role of basin-shortening on the development of structural compartments in passive margin basins. A coupled flow-deformation model is used to follow the evolution of an idealized prismatic basin during lateral shortening. This includes the deformation-induced generation (lateral compaction) and dissipation (hydraulic fracturing) of pore fluid pressures and the resulting natural evolution of an underlying décollement and subsidiary fault structures. This model is used to examine the influence of strata stiffnesses, strain softening, permeability-strain dependence, permeability contrast between layers, and deformation rate on the resulting basin structure and to infer fluid charge within these structures. For a geometry with a permeability contrast at the base of the basin a basal décollement forms as the basin initially shortens, excess pore pressures build from the impeded drainage and hydrofracturing releases fluid mass and resets effective stresses. As shortening continues, thrust faults form, nucleating at the décollement. Elevated pore pressures approaching the lithostat are localized at the hanging wall boundary of the faults. Faults extend to bound blocks that are vertically offset to yield graben-like structural highs and lows and evolve with distinctive surface topography and separate pore pressure signatures. Up-thrust blocks have elevated fluid pressures and reduced effective stresses at their core, and down-thrust blocks the converse. The development of increased permeability on localized fault structures is a necessary condition to yield this up-thrust and down-thrust geometry. In the anti-physical case where evolution of permeability with shear strain is artificially suppressed, pervasive shear develops throughout the basin depth as fluid pressures are stabilized everywhere to the lithostat. Correspondingly, permeability evolution with shear is an important, likely crucial, feedback in promoting localization.