Intensity map of Mw 6.9 2011 Sikkim–Nepal border earthquake and its relationships with PGA: distance and magnitude

We compiled available information of damages and other effects caused by the September 18, 2011, Sikkim–Nepal border earthquake from the print and electronic media, and interpreted them to obtain Modified Mercalli Intensity (MMI) at over 142 locations. These values are used to prepare the intensity map of the Sikkim earthquake. The map reveals several interesting features. Within the meizoseismal area, the most heavily damaged villages are concentrated toward the eastern edge of the inferred fault, consistent with eastern directivity. The intensities are amplified significantly in areas located along rivers, within deltas or on coastal alluvium such as mud flats and salt pans. We have also derived empirical relation between MMI and ground motion parameters using least square regression technique and compared it with the available relationships available for other regions of the world. Further, seismic intensity information available for historical earthquakes which have occurred in NE Himalayas along with present intensity has been utilized for developing attenuation relationship for NE India using two-step regression analyses. The derived attenuation relation is useful for assessing damage of a potential future earthquake (earthquake scenario-based planning purposes) for the northeast Himalaya region.

     

Seismic hazard assessment and mitigation in India: an overview

The Indian subcontinent is characterized by various tectonic units viz., Himalayan collision zone in North, Indo-Burmese arc in north-east, failed rift zones in its interior in Peninsular Indian shield and Andaman Sumatra trench in south-east Indian Territory. During the last about 100 years, the country has witnessed four great and several major earthquakes. Soon after the occurrence of the first great earthquake, the Shillong earthquake (M _w: 8.1) in 1897, efforts were started to assess the seismic hazard in the country. The first such attempt was made by Geological Survey of India in 1898 and since then considerable progress has been made. The current seismic zonation map prepared and published by Bureau of Indian Standards, broadly places seismic risk in different parts of the country in four major zones. However, this map is not sufficient for the assessment of area-specific seismic risks, necessitating detailed seismic zoning, that is, microzonation for earthquake disaster mitigation and management. Recently, seismic microzonation studies are being introduced in India, and the first level seismic microzonation has already been completed for selected urban centres including, Jabalpur, Guwahati, Delhi, Bangalore, Ahmadabad, Dehradun, etc. The maps prepared for these cities are being further refined on larger scales as per the requirements, and a plan has also been firmed up for taking up microzonation of 30 selected cities, which lie in seismic zones V and IV and have a population density of half a million. The paper highlights the efforts made in India so far towards seismic hazard assessment as well as the future road map for such studies.     

A source and ground motion study of earthquakes in and near Delhi (the National Capital Region), India

Natural Hazards - We study source parameters of 10 local earthquakes (2.7 ≤  $$M_{w}$$ ≤ 4.5) that have occurred in the National Capital Region (NCR) since 2001 and...     

Performance of Scrap Tire Shreds as a Potential Leachate Collection Medium

The use of scrap-tire-shreds as leachate collection layer in landfills would reduce the magnitude of the current tire disposal problem by converting a waste into a beneficial material. Laboratory studies were conducted to investigate scrap-tire-shreds as an attractive potential alternative to conventional gravel in the drainage layer of leachate collection system by comparative analysis of various physico-chemical parameters. Gravel and scrap-tire-shreds in combination were used as leachate collection layer. Laboratory Test Cells consisting of different combinations of scrap-tire-shreds (size range length = 25.4 mm to 76.1 mm and width = 5 mm) and gravel (size range 10 mm to 20 mm) beds as leachate collection layer with total bed thickness of 500 mm were constructed. Performance study of Test Cells- 1 to 7, having different combinations of scrap-tire-shreds and gravel bed thickness, were studied to work out the best combination. Combined beds of scrap-tire-shreds and gravel gave better results as compared to conventional gravel or scrap- tire- shreds bed when used singly. Test Cell -3 having scrap-tire-shreds layer (200 mm) and gravel layer (300 mm) gave the best results in terms of percentage reduction in various physico-chemical parameters of leachate. The reduction was as high as 68.8 and 79.6% in case of Biological Oxygen Demand (BOD) and Chemical Oxygen Demand (COD) values respectively. Further, three more laboratory Test Cells-8, 9 and 10 were constructed having scrap-tire-shreds and gravel layer ratio same as that of Test Cell- 3 (best combination) but having scrap-tire-shreds of different widths 10 mm, 15 mm and 20 mm to find out the most suitable size. Bed of smaller size scrap-tire-shreds (5 mm) gave better results in comparison to bigger sizes. Leachate sample after passing through combined beds of scrap-tire-shreds and gravel gave better results in percentage reduction in various physico-chemical parameters of leachate as compared to conventional gravel or scrap-tire-shreds bed when used singly.     

The 2007 Bengkulu earthquake, its rupture model and implications for seismic hazard

The 12 September 2007 great Bengkulu earthquake (M _w 8.4) occurred on the west coast of Sumatra about 130 km SW of Bengkulu. The earthquake was followed by two strong aftershocks of M _w 7.9 and 7.0. We estimate coseismic offsets due to the mainshock, derived from near-field Global Positioning System (GPS) measurements from nine continuous SuGAr sites operated by the California Institute of Technology (Caltech) group. Using a forward modelling approach, we estimated slip distribution on the causative rupture of the 2007 Bengkulu earthquake and found two patches of large slip, one located north of the mainshock epicenter and the other, under the Pagai Islands. Both patches of large slip on the rupture occurred under the island belt and shallow water. Thus, despite its great magnitude, this earthquake did not generate a major tsunami. Further, we suggest that the occurrence of great earthquakes in the subduction zone on either side of the Siberut Island region, might have led to the increase in static stress in the region, where the last great earthquake occurred in 1797 and where there is evidence of strain accumulation.     

Isotopic and REE studies of lunar basalt 12038: implications for petrogenesis of aluminous mare basalts

We report Sr, Nd, and Sm isotopic studies of lunar basalt 12038, one of the so-called aluminous mare basalts. A precise internal Rb-Sr isochron yields a crystallization age of 3.35±0.09 AE and initial⁸⁷Sr/⁸⁶Sr=0.69922?2 (2σ error limits, 1AE=10⁹ years, λ(⁸⁷Rb)=0.0139AE^?1). An internal Sm-Nd isochron yields an age of 3.28±0.23AE and initial¹⁴³Nd/¹⁴⁴Nd=0.50764?28. Present-day¹⁴³Nd/¹⁴⁴Nd is less than the “chondritic” value, i.e. ?(Nd, 0)=?2.3±0.4 where ?(Nd) is the deviation of¹⁴³Nd/¹⁴⁴Nd from chondritic evolution, expressed as parts in 10⁴. At the time of crystallization ?(Nd, 3.2AE)=1.5±0.6.We have successfully modeled the evolution of the Sr and Nd isotopic compositions and the REE abundances within the framework of our earlier model for Apollo 12 olivine-pigeonite and ilmenite basalts. The isotopic and trace element features of 12038 can be modeled as produced by partial melting of a cumulate mantle source which crystallized from a lunar magma ocean with a chondrite-normalized REE pattern of constant negative slope. Chondrite-normalized La/Yb=2.2 for this hypothetical magma ocean pattern. A plot of I(Sr) versus ?(Nd) for the Apollo 12 basalts clearly shows the influence of varying proportions of olivine, clinopyroxene, orthopyroxene, and plagioclase in the basalt source regions. A small percentage of plagioclase (～5%) in the 12038 source apparently is responsible for low I(Sr) and ?(Nd) in this basalt. Aluminous mare basalts from Mare Crisium (Luna 24) and by inference Mare Fecunditatis (Luna 16) occupy locations on the I(Sr)-?(Nd) plot similar to that of 12038, implying that some basalts from three widely separated lunar regions came from plagioclase-bearing source regions. A summary of model calculations for mare basalts shows a record of lunar mantle solidification during the period when REE abundances in the lunar magma ocean increased from ～20× chondritic to >100× chondritic. Although there is a general trend from olivine to clinopyroxene-dominated source regions with progressive magma ocean evolution, significant mineralogical heterogeneities in mantle composition apparently formed at any given stage of evolution, as evidenced in particular by the three Apollo 12 magma types.     

Depth Estimation from Gravity Data Using the Maximum Entropy Method (MEM) and the Multi Taper Method (MTM)

The depth determination from the gravity data in frequency domain is carried out using the classical fast Fourier transform (FFT) method utilizing scaling properties of ensemble of anomalous source. The problem of calculating power spectrum from the FFT is well described in the literature. Here, the application of other high-resolution methods of power spectrum calculation, such as maximum entropy method (MEM) and multi-taper method (MTM) are explored to estimate depth to anomalous sources. At the outset, the FFT, the MEM and the MTM are tested on synthetic gravity data, generated for different types of synthetic models and then all these methods are applied to the field gravity data of the Bengal basin. The MTM with scaling is found to be superior for providing the detailed subsurface information rather than the MEM and the FFT methods in the case of synthetic as well as field examples.     

Bearing capacity tests of strip footings on reinforced layered soil

The ultimate bearing capacity of strip footings resting on subsoil consisting of a strong sand layer (reinforced/unreinforced) overlying a low bearing capacity sand deposit has been investigated. Three principal problems were analysed based on results obtained from the model tests as follows: (1) the effect of stratified subsoil on the foundations bearing capacity; (2) the effect of reinforcing the top layer with horizontal layers of geogrid reinforcement on the bearing capacity; (3) effect of reinforcing stratified subsoil (reinforced and unreinforced) on the settlement of the foundation. It has been observed that reinforcing the subsoil after replacing the top layer of soil with a well-graded soil is beneficial as the mobilization of soil-reinforcement frictional resistance will increase.     

The Mw 7.5 2009 Coco earthquake, north Andaman region

The recent 10 August 2009 Coco earthquake (Mw 7.5), the largest aftershock of the giant 2004 Sumatra Andaman earthquake, occurred within the subducting India plate under the Burma plate. The Coco earthquake nucleated near the northwestern edge of the 2004 Sumatra-Andaman earthquake rupture under the unruptured updip segment of the plate boundary interface. The earthquake with predominant normal motion on approximately north-south to northeast-southwest oriented plane is very similar to the 27 June 2008 Little Andaman earthquake which occurred in the South Andaman region near the trench. We provide the only available estimate of coseismic offset due to the 2009 Coco earthquake at a survey-mode GPS site in the north Andaman, located about 60 km south of the Coco earthquake epicentre. The not so large coseismic displacement of about 2 cm in the ESE direction is consistent with the earthquake focal mechanism and its magnitude. We suggest that, like the 2008 Little Andaman earthquake, this earthquake too occurred on one of the approximately north-south to northeast-southwest oriented steep planes of the obliquely subducting 90°E ridge which was reactivated in normal motion after subduction, under the favourable influence of coseismic and ongoing postseismic deformation due to the 2004 Sumatra-Andaman earthquake. Another notable feature of this earthquake is its relatively low aftershock productivity. We suggest that the earthquake occurred very close to the aseismic region of the Irrawaddy frontal arc of very low seismicity where pre-existing faults are not so critically stressed and because of which the earthquake could trigger only a few aftershocks in its immediate vicinity.