Kathmandu Basin Research Articles

Analysis of strong ground motions and site effects at Kantipath, Kathmandu, from 2015 Mw 7.8 Gorkha, Nepal, earthquake and its aftershocks

Strong ground motions from the 2015 Mw 7.8 Gorkha, Nepal, earthquake and its eight aftershocks recorded by a strong-motion seismograph at Kantipath (KATNP), Kathmandu, were analyzed to assess the ground-motion characteristics and site effects at this location. Remarkably large elastic pseudo-velocity responses exceeding 300 cm/s at 5 % critical damping were calculated for the horizontal components of the mainshock recordings at peak periods of 4–5 s. Conversely, the short-period ground motions of the mainshock were relatively weak despite the proximity of the site to the source fault. The horizontal components of all large-magnitude (Mw ≥ 6.3) aftershock recordings showed peak pseudo-velocity responses at periods of 3–4 s. Ground-motion prediction equations (GMPEs) describing the Nepal Himalaya region have not yet been developed. A comparison of the observational data with GMPEs for Japan showed that with the exception of the peak ground acceleration (PGA) of the mainshock, the observed PGAs and peak ground velocities at the KATNP site are generally well described by the GMPEs for crustal and plate interface events. A comparison of the horizontal-to-vertical (H/V) spectral ratios for the S-waves of the mainshock and aftershock recordings suggested that the KATNP site experienced a considerable nonlinear site response, which resulted in the reduced amplitudes of short-period ground motions. The GMPEs were found to underestimate the response values at the peak periods (approximately 4–5 s) of the large-magnitude events. The deep subsurface velocity model of the Kathmandu basin has not been well investigated. Therefore, a one-dimensional velocity model was constructed for the deep sediments beneath the recording station based on an analysis of the H/V spectral ratios for S-wave coda from aftershock recordings, and it was revealed that the basin sediments strongly amplified the long-period components of the ground motions of the mainshock and large-magnitude aftershocks.

Two times lowering of lake water at around 48 and 38 ka, caused by possible earthquakes, recorded in the Paleo-Kathmandu lake, central Nepal Himalaya

Sedimentary facies and micro-fossil analyses, and AMS14C dating were performed in order to reveal the water-level fall events and draining process of the lake (Paleo-Kathmandu Lake) that existed in the past in the Central Nepal Himalaya. The sedimentary facies change from the lacustrine Kalimati Formation to the deltaic Sunakothi Formation in the southern and central Kathmandu basin, and the abrupt and prominent increase of phytoliths Bambusoideae and Pediastrum, and contemporaneous decrease of sponge spicule and charcoal grains around 48 and 38 ka support the lowering of water level at these times. According to the pollen analysis, both events occurred under rather warm and wet climate, thus supporting that they were triggered by tectonic cause and not by climate change. The first event might be linked to a possible occurrence of a large earthquake with an epicenter in the vicinity of the Paleo-Kathmandu Lake. The occurrence of a mega landslide in Langtang area close to the north of the Kathmandu Valley producing pseudotachylite dated at 51 ± 13 ka could be linked to this earthquake. Finally, the water was completely drained out from the remnant lake at the central part of the Kathmandu basin by ca.12 ka.

Estimation of the source process of the 2015 Gorkha, Nepal, earthquake and simulation of long-period ground motions in the Kathmandu basin using a one-dimensional basin structure model

The source rupture process of the 2015 Gorkha, Nepal, earthquake was estimated by the joint kinematic source inversion with near-field waveforms, teleseismic waveforms, and geodetic data. The estimated seismic moment and maximum slip are 7.5 × 1020 Nm (M w 7.9) and 7.3 m, respectively. The total source duration is approximately 50 s. The derived source model has a unilateral rupture toward the east and a large-slip area north of Kathmandu with the maximum slip. Using the estimated source model together with a one-dimensional (1-D) velocity basin structure model, long-period (> 4 s) ground motions were simulated at a site located in the Kathmandu basin, where strong ground motions with predominant components in a 4–5s period were observed during the 2015 Gorkha earthquake. This simulation demonstrated that the major features of the observed waveforms can be reproduced by our source model and the 1-D basin structure model.

Paleoclimatic significance of the late Quaternary plant macrofossils from the Gokarna Formation, Kathmandu Valley, Nepal

Late Quaternary plant macrofossils from fluvial, fluvio-deltaic and lacustrine sediments of the Gokarna Formation in the Besigaon, Mulpani, and Dhapasi sections in the Kathmandu Valley, Nepal were analyzed. The obtained plant macrofossils allowed us to reconstruct the late Pleistocene paleoenvironment in central Nepal, which was influenced strongly by the SW Indian monsoon system and tectonic movements of the Himalayas. In total, 102 taxa from 52 families were identified to genus or species levels. The dominance of Eurya, Ficus, Morus, Zizyphus, Stephania, Rubus, Quercus subgen. Cyclobalanopsis, Pyracantha, and Carpinus in the lower and middle plant macrofossil assemblages indicates influence of a warm climate. In the upper plant macrofossil assemblages, the higher contents of cooler climatic taxa such as Abies, Pinus, Picea smithiana, Tsuga dumosa, Taxus wallichiana, Quercus subgen. Lepidobalanus, and Betula indicate a climatic deterioration. The higher diversity and common occurrence of aquatic plants indicate rich wetland vegetation in the Kathmandu basin. The plant macrofossil analysis shows at least three cycles of climate fluctuation between warm and cool phases during the deposition of the Gokarna Formation between 53,170±820yrs. BP and 49,300±2100/1700yrs. BP. These cycles seem to be fairly local or regional, but this can also be correlated with the overall global cooling context in GISP2 oxygen isotope curve in the late Pleistocene.

Himalayan megathrust geometry and relation to topography revealed by the Gorkha earthquake

The Himalayan mountain range has been the locus of some of the largest continental earthquakes, including the 2015 magnitude 7.8 Gorkha earthquake. Competing hypotheses suggest that Himalayan topography is sustained and plate convergence is accommodated either predominantly on the main plate boundary fault, or more broadly across multiple smaller thrust faults. Here we use geodetic measurements of surface displacement to show that the Gorkha earthquake ruptured the Main Himalayan Thrust fault. The earthquake generated about 1 m of uplift in the Kathmandu Basin, yet caused the high Himalaya farther north to subside by about 0.6 m. We use the geodetic data, combined with geologic, geomorphological and geophysical analyses, to constrain the geometry of the Main Himalayan Thrust in the Kathmandu area. Structural analyses together with interseismic and coseismic displacements are best explained by a steep, shallow thrust fault flattening at depth between 5 and 15 km and connecting to a mid-crustal, steeper thrust. We suggest that present-day convergence across the Himalaya is mostly accommodated by this fault—no significant motion on smaller thrust faults is required. Furthermore, given that the Gorkha earthquake caused the high Himalayan mountains to subside and that our fault geometry explains measured interseismic displacements, we propose that growth of Himalayan topography may largely occur during the ongoing post-seismic phase.

Soft sediments deformation structures: Implication for draining of Paleo-Kathmandu Lake

Kathmandu basin is one of the Quaternary intermontane basins in the central Nepal Himalaya. It is bounded by several faults on both southern and northern margins. The basin is filled with Plio-Pliestocene terrestrial sediments and their characteristics indicate four types of unconsolidated successions within the basin. These are before lake succession, during lake succession, draining stage lake succession and fluvial succession. Late Pleistocene aged Sunakothi Formation crops out along the southern part of the basin. It is a typical fluviolacustrine delta succession that extended from 1390 m in the southern margin to nearly 1300 m toward center of the basin. It is composed by poorly consolidated sand, gravelly sand, silt and mud beds. Various soft-sediment deformation structures occur in the formation, especially in fine- to medium –grained sands, silts and mud: load structures, flame structures, clastic dikes (sand dike), disturbed layers, convolute beds, slumps and synsedimentary faulting. The deformation mechanism and driving force for the soft-sediment deformation are related, essentially, to gravitational instability, dewatering, liquefaction and brittle deformation. Field data and the wide lateral extent of the structures as well as regional geological data show that most of the deformation is related to seismicity and the structures are interpreted as seismites. In addition, there have also been experimental studies undertaken by various authors within the different sedimentary basin.
 Soft-sediments deformation structure in Kathmandu basin are mainly considered to be part of the initial diagenetic changes of the sediments and include: Slump structure which occurred on the slope like delta-front area, dewatering structures which occurred by the processes of upward escape of water commonly due to loading, load structures which occurred due to density contrasts between sand and underlying wet mud. The existence of seismites in the Sunakothi Formation is evidence of continuing tectonic activity in the study area during the late Pleistocene and is a main factor for draining of the Paleo- Kathmandu lake water.

Vegetation and climate from Late Quaternary Thimi Formation (Phaidhoka Section), Bhaktapur, Nepal

The Thimi Formation is fluvio-deltaic deposit that constitutes the uppermost part of the sedimentary sequence in the Kathmandu Basin, and is featured by carbonaceous and diatomaceous clay, silty clay, silt, fine to medium grained sand beds, and thin to medium lignite beds. The Phaidhoka Section is located on the way to Nala from Chyamasingh, and is one of the major exposures of the Thimi Formation. Forty four samples were collected from 25 m thick surface exposure for palynological study. The study revealed the dominance of gymnosperm over the angiosperm and herbaceous members. The pollen diagram suggested Pinus, Picea and Quercus as the most dominant trees whereas Poaceae is other dominant among the grasses. Three major pollen assemblage zones were marked in the Thimi Formation. Zone P-I indicated warm temperate climate, whereas zone P-II and P-III indicated cold temperate climate. Molluscan operculum in the upper part indicated shallow water condition. The Bovid molars, limb and pelvic bones from the middle part of the section confirm the early findings of molar bones in this area.

New Discovery of Late Pleistocene Vertebrate Fossils from the Thimi Formation, Bhaktapur, Nepal

The fluvio-deltaic deposit of the Thimi Formation constitutes the uppermost part of the sedimentary sequence in the Kathmandu Basin, and is consists of carbonaceous clay, diatomaceous clay, silty-clay, silt, fine to medium grained sand, and thin to medium lignite beds. A 25 m thick fresh surface exposure of this formation at Phaidhoka, Bhaktapur yielded significant number of vertebrate fossils. The vertebrate fossils recovered from a sand bed at 14 m from the bottom of the sequence consist of six teeth. The teeth were identified as Bovid teeth. Along with Bovid molars fragments of limb and pelvic bones were also found belonging to the same individual.Journal of Institute of Science and Technology, 2015, 20(2): 73-75

Plio-Pleistocene Plant Megafossils from the Lukundol Formation (Nakkhu Khola, Lalitpur District, Central Nepal)-First Results

The lacustrine sediments of the Kathmandu Basin are rich in plant mega and micro fossils. The Lukundol Formation which is distributed in the southern part of the Kathmandu Basin yielded nicely preserved plant megafossils from different locations. A 5.5 m thick carbonaceous clay bed at the left bank of the Nakkhu Khola, near Chyasikot village, Lalitpur contains abundant plant debris, leaf impressions, seed and fruits, gymnosperm needles and cones. The plant fossils discovered from this location are identified as Berberis asiatica (leaf), Rhododendron arboreum (leaf), Rhododendron barbatum (leaf), Litsea elongata (leaf), Pinus roxburghii (needles and cones) Trapa bispinosa and Trapa quadrispinosa (fruits). Many leaf impressions are also collected from Bungmati and Chovar area. This paper deals with the first reporting of plant fossils from Nakkhu Khola section. The findings from other sections will be reported separately.Journal of Institute of Science and Technology, 2015, 20(1): 141-144

Relationship of Mineralogy and Paleoenvironments in the Late Pleistocene Kalimati to Sunakothi and Thimi Formations, Kathmandu Basin, Central, Nepal

The investigation was carried out on mineralogical composition of the three different stratigraphic units of the Kathmandu Basin-fill sediments. On the basis of the variation of the non-clay and clay fraction, here the investigation was interpreted to establish the paleoenvironmental and paleoclimatic condition during the deposition from the lacustrine facies of the Kalimati Formation to the fluvio-lacustrine facies of the Sunakothi Formation and probable caused of these change. Both Kalimati and Sunakothi sediments contain same mineral composition while Thimi sediments did not contain carbonate (calcite) minerals and amount of mica is higher than the Sunakothi and Kalimati sediments. Higher amount of smectite within the clay fraction and presence of calcite within both clay and non-clay fraction in the basal part of the Sunakothi Formation indicates seasonal and prolong dry climatic condition occurred during the deposition of the Kalimati Formation to Sunakothi Formation. On the other hand, lower amount of kaolinite/smectite ratio, and excess amount of carbonate mineral within the basal part of the Sunakothi Formation shows depositional environmental changes from deep to shallow, and lake water became more alkaline. Gradually lacustrine condition changes into the fluvio-lacustrine condition. On the other hand, higher ratio of the kaolinite/smectite within the Kalimati Formation to lower ratio of the kaolinite/smectite within the basal part of the Sunakothi Formation indicates that climatic condition was more seasonal and cold/ dry than the Kalimati Formation. Both kaolinite/smectite and smectite/illite value indicate paleoprecipitation of the Kathmandu valley, was higher during the Kalimati phase than the Sunakothi.Journal of Institute of Science and Technology, 2014, 19(1): 86-95

The 2015 Gorkha earthquake investigated from radar satellites: slip and stress modeling along the MHT

The active collision at the Himalayas combines crustal shortening and thickening, associated with the development of hazardous seismogenic faults. The 2015 Kathmandu earthquake largely affected Kathmandu city and partially ruptured a previously identified seismic gap. With a magnitude of Mw 7.8 as determined by the GEOFON seismic network, the 25 April 2015 earthquake displays uplift of the Kathmandu basin constrained by interferometrically processed ALOS-2, RADARSAT-2 and Sentinel-1 satellite radar data. An area of about 7,000 km² in the basin showed ground uplift locally exceeding 2 m, and a similarly large area (approx. 9000 km2) showed subsidence in the north, both of which could be simulated with a fault that is localized beneath the Kathmandu basin at a shallow depth of 5-15 km. Coulomb stress calculations reveal that the same fault adjacent to the Kathmandu basin experienced stress increase, similar as at sub-parallel faults of the thin skinned nappes, exactly at the location where the largest aftershock occurred (Mw 7.3 on 12. May, 2015). Therefore this study provides insights into the shortening and uplift tectonics of the Himalayas and shows the stress redistribution associated with the earthquake.

Overview of the Large 25 April 2015 Gorkha, Nepal, Earthquake from Accelerometric Perspectives

Central Nepal was struck on 25 April 2015 by the M w 7.8 ( M L 7.6) Gorkha earthquake, which initiated about 80 km northwest of Kathmandu and ruptured toward the east along a 140-km-long west-northwest–east-southeast fault segment. Kathmandu basin, located about halfway along the ruptured segment a few kilometers from the southernmost fault tip, was strongly affected. In this article, we present a preliminary analysis of the acceleration-time histories of the 25 April 2015 M w 7.8 mainshock and the 12 May 2015 M w 7.3 ( M L 6.8) aftershock recorded at the Department of Mines and Geology office building in central Kathmandu valley. We analyze their frequency content using Stockwell et al. (1996) time–frequency decomposition and a polarization analysis (Pinnegar, 2006). We then compute their strong-motion parameters and finally compare their spectral accelerations with the Boore and Atkinson (2008) ground-motion prediction equation.

Slip pulse and resonance of the Kathmandu basin during the 2015 Gorkha earthquake, Nepal.

Detailed geodetic imaging of earthquake ruptures enhances our understanding of earthquake physics and associated ground shaking. The 25 April 2015 moment magnitude 7.8 earthquake in Gorkha, Nepal was the first large continental megathrust rupture to have occurred beneath a high-rate (5-hertz) Global Positioning System (GPS) network. We used GPS and interferometric synthetic aperture radar data to model the earthquake rupture as a slip pulse ~20 kilometers in width, ~6 seconds in duration, and with a peak sliding velocity of 1.1 meters per second, which propagated toward the Kathmandu basin at ~3.3 kilometers per second over ~140 kilometers. The smooth slip onset, indicating a large (~5-meter) slip-weakening distance, caused moderate ground shaking at high frequencies (>1 hertz; peak ground acceleration, ~16% of Earth's gravity) and minimized damage to vernacular dwellings. Whole-basin resonance at a period of 4 to 5 seconds caused the collapse of tall structures, including cultural artifacts.

Riverbank erosion potential and channel stability status of the Kodku River, southern Kathmandu Basin, Central Nepal

The Kodku River is a southern tributary of the Manahara River and extends for about 15.86 km with 35.67 sq. km of watershed area. It is quite a potential linkage between the hilly, southern Kathmandu and the urban, inner Kathmandu. The river corridors are frequently subject to bank erosion, slope movements and flash flooding. Riverbank erosion is an important cause of toe erosion of slopes causing landslides and also posing threat on the infrastructures. Stream channel stability is crucial to understand overall river stability. Recognition of existing stability condition of river is to understand nature and behavior of the river, and is important in many ways: (a) to recognize the bank erosion and lateral instability hazard, (b) to develop infrastructure along or nearby the river corridor, (c) to start on where to restore the river, (d) to develop reservoir and exploit natural resources, and (e) to develop safe settlement areas. The Kodku River is a gravelly mixed-load meandering river. Level II classification distinguishes the Badikhel Segment as a ‘B4c’ type stream, the Taukhel Segment as a ‘C6c’ type, and the fifth order segments such as the Arubot, Thaiba and Harisidhi Segments as ‘C4c’ type streams. The ‘B4c’ type stream is entrenched and somewhat laterally confined by steep valley slopes and terrace landforms. It has the highest unit stream power (16.64 Nm/ s/m2), high potential of bed material scouring and tendency of vertical instability. The ‘C6c’ type stream is a meandering stream with shallow channel and wide valley. The ‘C4c’ type streams have shallow and wide meandering channels with well developed flood plains and lateral bars, and have the least unit stream power (in Harishiddi Segment 0.11N-m/s/m2), low potential of river bed material erosion but have tendency of lateral instabilities. The bank erosion hazard map indicates that the upper third order stretch and few downstream stretches lie in low hazard zone, but the overall areas of the Harisidhi Segment, Gwarko, Imadol and some other areas lie in high to very high hazard zone because of devegetation, modification of channels and other anthropogenic activities in addition to the weak nature of the bank materials.DOI: http://dx.doi.org/10.3126/bdg.v17i0.12723Bulletin of the Department of Geology, Vol. 17, 2014, pp. 1-41

Identification of major factors affecting spatial and temporal variation of water quality in Kathmandu Basin, Nepal, using multivariate statistical analysis

Statistical techniques were applied to analyse the water quality datasets of Kathmandu Basin, Nepal, to identify the major factors affecting water quality and to investigate the spatial and seasonal variations. Spatial variation was investigated using cluster analysis in which a total of 40 river monitoring sites were divided into two major clusters: low and high polluted zones. Discriminant analysis (DA), principal component analysis (PCA) and factor analysis (FA) were subjected to water quality parameters to identify the pollution sources. DA confirmed the seasonal variation of water quality data into three seasons providing 92.5% correct classification. Principal components (PCs) were extracted to distinguish the anthropogenic processes and the symmetrical correlation matrix computed with ten variables for winter, monsoon and post monsoon seasons. The significant components extracted have eigenvalues greater than 1, and account for 79%, 68% and 77% of the total variance in the winter, monsoon and post monsoon data respectively. Non-parametric tests were applied for each parameter to identify their significance differences spatially and temporally.

Facies analysis of Sunakothi Formation, Kathmandu basin, Nepal and its significance

This study decipher facies characteristic of Sunakothi Formation at southern part of Kathmandu Basin. Thick sandy and muddy sequence is found on an open lacustrine facies of the Kalimati Formation. Five facies associations have been recognized within the sandy and muddy facies. These are: (a) muddy rhythmites and silt and laminated to ripple sand bed of the prodeltaic origin (pd), (b) association of cross-stratification, rippledrift and parallel lamination in the lacustrine delta front origin (df), (c) muddy flood-plain and alteration of the fine and coarse sediments of delta-plain origin (dp), (d) sandy to silty rhythmites of the marginal shallow lacustrine origin above the delta-plain (ml), and (e) association of fluvial origin (fl ). Former three associations are interbeded by the thick gravel deposits, which is gravelly braided river origin. Transition from lacustrine to alluvial system is characterized by fluvial and deltaic system in the south. Sedimentology of the Sunakothi Formation indicates deposition during rapid lake level rise and also the thick channelized fluvial gravel beds within the sandy and muddy sequence indicate lake level fall. The cause could be climatic as well as activity of the basin margin tectonics. Sunakothi Formation is the southern counterpart of the Thimi-Gokarna Formations distributed in the northern part of the basin.

Late Pleistocene paleoenvironmental changes of the Kathmandu Basin-fill sediments revealed by the minerals composition

The amounts and their variation of minerals in the present river sediments (PR) and basement rocks (BR) of the Kathmandu Valley were estimated by using the X-ray diffraction (XRD) method, in order to estimate the source of the Kathmandu Basin sediments and to interpret paleoenvironmental and paleoclimatic condition of the past Kathamdu Valley. XRD result of the BR and PR samples roughly correspond along respective parts of the Kathmandu Valley, except calcite (carbonate rocks in the eastern, southern, and western parts). The non-clay minerals are quartz, K-feldspar, plagioclase, mic a, chlorite, calcite, and illite, chlorite, kaolinite, smectite, mixed layer illite-chlorite are detected within the clay fraction of the PR sediments. The clay fraction is < 5 wt % within the PR-sediments. Among non-clay fraction, BR and PR from the north contains more or less equal amount of mica, feldspars and quartz, while those from south, east and west are rich in quartz. Therefore, the amount of silicate minerals in the sediments within the Kathmandu Basin is the key to grasping the provenance of the sediments about northern and the other origin. Chlorite in the PR-North is likely a weathering product of mica because chlorite was not included in the BR-North samples. The chlorite and mica were greater in the lower course of the river from the north than in the upper course of the river. Plagioclase is tends to be poor and K-feldspar is rich in the PR-samples, while the amount of the two feldspar are similar to each other in the BR-samples. This tendency may be due to the difference in stability to weathering between the two feldspars. Calcite was probably dissolved during weathering, erosion, and transportation processes, because calcite included in the BR samples was hardly detected in the PR samples. This mineralogical information is applied to the mineralogical studies in the past basin-fill sediments of the Kathmandu Basin (RB-drill core) and to estimate the source of the sediments and paleoenvironmental and paleoclimatic information on this region. Calcite detected in the past basin-fill sediments of the Kathmandu Basin is an important key mineral showing cold and dry climate within the RB drill core. It can also be used to correlate the strata.

Lateral uniformity of India Plate strength over central and eastern Nepal

The current understanding of the Himalayan lithosphere stems mostly from cross-sections through the range at the longitude of the Kathmandu Basin. In this paper we laterally extend the analyses of structures and rheology along the Nepal Himalayas between the Pokhara valley and the Arun river. We take advantage of available information and a new data set including gravity measurements and a receiver function profile. It appears that the geometry of the Moho inferred from seismological profiles and long-wavelength gravity anomalies does not exhibit major East–West variations within the 350-km-wide study area. Using thermomechanical modelling, we show that the northward deepening of the Moho observed along profiles perpendicular to the main thrust faults can be interpreted simply as the bending of a strong India Plate. This result suggests a gradual mechanical decoupling between the crust and the mantle, leading to a northward decrease of the effective elastic thickness of the Indian lithosphere from ∼75 km to ∼25 km beneath the Ganga Basin and the Tibetan Plateau, respectively. Our results also confirm (partially) eclogitized lower Indian crust beneath southern Tibet. At shorter wavelengths, the observed gravity profiles exhibit some small lateral variations that can be interpreted in terms of east–west variations of the thickness of subsurface geological structures such as the Ganga Basin and the Tethyan Sedimentary Sequence.

Basinal and planform characteristics of the Kodku and the Godavari Rivers, Kathmandu, Central Nepal

The rivers of the Kathmandu Basin are vulnerable to flash floods and disturbances caused by anthropogenic as well as climatic changes. Two southern tributaries of the Bagmati River: the Kodku and the Godavari Rivers, have been considered for their (i) watershed-scale geomorphic parameters such as relative relief, drainage texture and stream order, (ii) stretchscale planform parameters such as sinuosity (K), meander belt width (Wblt), meander wavelength (Lm) and radius of curvature (Rc), and (ii) longitudinal profiles and slopes. Both Kodku and the Godavari Rivers are elongate basins with wide ranges of the watershed-scale parameters. The Godavari River is longer, larger and more sinuous compared to the Kodku River. The development of the patterns of the fifth order main stem stretches of both rivers with respect to the stream slopes, and asymmetric patterns of the meander loops indicate anomalous growth of the river stretches. DOI: http://dx.doi.org/10.3126/bdg.v15i0.7414 Bulletin of the Department of Geology, Vol. 15, 2012, pp. 15-22

Basement topography of the Kathmandu Basin using microtremor observation

Kathmandu Valley, an intermontane basin of the Himalaya, has experienced many destructive earthquakes in the past. The observations of the damage pattern during the 1934 Earthquake (Mw=8.1), in particular, suggest that the spectral ground amplification due to fluvio-lacustrine sediments plays a major role in intensifying the ground motion in the basin. It is, therefore, imperative to conduct a detailed study about the floor variation of sediments in the basin. In this paper, a preliminary attempt was made to estimate the thickness of soft sediment in the Kathmandu Basin using microtremor observations. The measurements of microtremors were carried out at 172sites spaced at a grid interval of 1km. The results showed that the predominant frequency varies from 0.488Hz to 8.9Hz. A non-linear regression relationship between resonance frequency and sediment depth was proposed for the Kathmandu Basin. The thickness of lacustrine sediments at various points in the basin was estimated using the proposed equation, and then the estimated thickness was used to plot a digital elevation model of the basement topography and cross profiles of the sediment distribution in the basin. The results were validated by correlating the estimated sediment thickness with geology and geomorphology of the study area.