Basement topography of the Kathmandu Basin using microtremor observation

Youb Raj Paudyal ⇑, Ryuichi Yatabe, Netra Prakash Bhandary, Ranjan Kumar DahalGeo-disaster Research Laboratory, Graduate School of Science and Engineering, Ehime University, Ehime, Matsuyama, Japan

a r t i c l e i n f o

Article history:Received 30 March 2012Received in revised form 22 October 2012Accepted 5 November 2012Available online xxxx

Keywords:Kathmandu ValleyLacustrine sedimentsMicrotremorPredominant frequencyBasement topography

a b s t r a c t

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

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1. Introduction

Intensity of ground motion is a function of earthquake magni-tude and distance from the seismic source, as well as local geolog-ical condition and topography of the area (Kramer, 1996).Although, geological structure of the area is an important factor, lo-cal site condition is known to have a great influence on the poten-tial damage resulting from earthquakes (Seed and Idriss, 1969).Besides, geotechnical properties of the local soil site and behaviorof the soil during earthquake depends upon the depth of the sedi-mentary column and its shear wave velocity. From geotechnicalpoint of view, the shear wave velocity in the top 30 m column ofsoil is responsible for an unusual amplification of the ground mo-tion (Finn, 1991). However, in the areas where the thickness of softsediments is enormously large, like in the Kathmandu Basin, theamplification will rather depends on the extent of the soft sedi-ment column and its elastic properties. Moreover, the fundamentalphenomenon responsible for the amplification of seismic waves isdue to the impedance contrast between sedimentary deposits andthe underlying hard-strata or bedrock. Such site amplification canbe estimated using microtremor measurement technique, whichwas first introduced by Kanai (1954). Several studies such as, Ohtaet al. (1978), Lermo et al. (1988), Field et al. (1990), and Field and

Jacob (1993) have shown that microtremor analysis results revelsthe fundamental resonant frequency of sediments. One of the mainchallenges in determining the site amplification characteristics outof microtremor measurement is removing the source effects, whichis often achieved by dividing the Fourier spectrum obtained on asoft ground point by that obtained on a nearby reference pointon bedrock. For this, however, the microtremor source and path ef-fects must be the same for both measurement points and the ref-erence point or site must also have negligible site effects. Toovercome this limitation, Nakamura (1989) has introduced a tech-nique for estimating the site response by measuring solely themicrotremor on the surface of the ground. According to him, thesource effect can be removed by dividing the horizontal compo-nent of microtremor spectrum by the vertical component. Now,this technique has become widespread as a low-cost and effectivetool to estimate the fundamental resonant frequency of sedimentsusing Horizontal-to-Vertical (H/V) spectral ratio at a single-station.In his paper, Nakamura (1989) explains the use of this techniqueand gives a detailed explanation on the subsequent assumptions.

In the last two decades, the H/V method has been widely usedfor various purposes, such as site effect evaluation, wave amplifica-tion estimation, liquefaction vulnerability assessment, sedimentdepth estimation, and microzonation studies in different geo-graphical and geological regions of the world (Field and Jacob,1993; Bour et al., 1998; Ibs-von Seht and Wohlenberg, 1999;Delgado et al., 2000; Tuladhar et al., 2004; Hasancebi and Ulusay,2006; Langston et al., 2009; Hardesty et al., 2010; Mucciarelli,2011; Paudyal et al., 2012a and Paudyal et al., 2012b). Estimating

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⇑ Corresponding author. Address: Geo-disaster Research Laboratory, GraduateSchool of Science and Engineering, Ehime University, Bunkyo-3, Matsuyama 790-8577, Japan. Tel./fax: +81 89 927 8566.

E-mail address: youbrajpaudyal@gmail.com (Y.R. Paudyal).

Journal of Asian Earth Sciences xxx (2012) xxx–xxx

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Please cite this article in press as: Paudyal, Y.R., et al. Basement topography of the Kathmandu Basin using microtremor observation. Journal of Asian EarthSciences (2012), http://dx.doi.org/10.1016/j.jseaes.2012.11.011

an empirical relationship between fundamental resonant frequen-cies and sedimentary cover thickness has become practice sincethe work of Ibs-von Seht and Wohlenberg (1999) for Cologne areain Germany. They proposed that resonance frequency in H/V spec-tra correlate well with the overall soil thickness; ranging from tensof meters to more than 1000 m. Their developed relationships pro-vide a practical means of sediment thickness estimation using amicrotremor observation. Later, several studies, such as Delgadoet al. (2000), Parolai et al. (2002), D’Amico et al. (2004), Hinzenet al. (2004), García-Jerez et al. (2006), Birgören et al. (2009),Dinesh et al. (2010), Gosar and Lenart (2010), Özalaybey et al.(2011), and Sukumaran et al. (2011) have applied microtremorH/V spectral ratio technique for measuring the thickness of soilcover over a hard stratum or bedrock.

The present investigation is a first attempt to approximate thethickness of the soft sediments of lacustrine origin (Sakai et al.,2002) in the Kathmandu Basin using microtremor observations.The basin falls under one of the active seismic regions, and has suf-fered great losses in the past earthquakes, such as in 1255, 1408,1681, 1803, 1810, 1833, 1866, and 1934 (Rana, 1935; Chitrakarand Pandey, 1986; Bilham et al., 1995; Pandey et al., 1995; Upretiand Yoshida, 2009). The latest greatest earthquake of 1934(Mw = 8.1) with a maximum intensity of X in Modified MercallyIntensity (MMI) Scale in the Kathmandu Basin reportedly killedabout 4296 people and destroyed about 19% and damaged about38% of the buildings in the basin alone (Rana, 1935; Pandey andMolnar, 1988). The observations of the damage pattern in the Kath-mandu Basin during this earthquake, in particular, suggest that thespectral ground amplification due to fluvio-lacustrine sedimentsplay a major role in intensifying the ground motion (Pandey andMolnar, 1988; Hough and Bilham, 2008). However, the valley basinstill lacks adequate information on spatial variation of the lacustrinesediments. Moribayashi and Maruo (1980) estimated the basementtopography of the central portion of the basin using gravitationalmethod by assuming the density contrast of 0.8 g/cm3; i.e., 2.67 g/cm3 for bedrock and 1.87 g/cm3 for lacustrine sediments. In reality,however, the density of the lacustrine sediments is found less (JICA,2002) than the value assumed by Moribayashi and Maruo (1980). Inaddition, the density contrast may not be constant everywhere for abasin like the Kathmandu Valley, where the thickness and propertiesof sediments vary significantly within the short distances. In thissense, the actual floor variation of the basement rock in a wider areaof the Kathmandu Basin is still debatable. So, the objective of thisstudy is to derive an empirical relationship between the resonancefrequencies obtained from the H/V technique and the thickness ofthe lacustrine sediments, and then generate an approximate base-ment topography of the Kathmandu Basin. This information is criti-cally important for earthquake ground motion simulation studies,especially because the densely populated urban area of the valleyis under a great earthquake threat.

2. Study area and geology

Geologically, the Kathmandu Basin (Fig. 1) lies on the Kath-mandu Nappe (Hagen, 1969; Upreti, 1999), which is located alongthe southern slopes of the Himalaya. It is one of the several tec-tonic intermontane basins developed in the Lesser Himalayan belt(Sakai et al., 2002) as shown in Fig. 2. The Kathmandu Nappe iscomposed of the Shivapuri Gneiss and marbles of the BhimphediGroup (Stöcklin and Bhattarai, 1981). As illustrated in Fig. 2, theearly Paleozoic Tethyan rocks, named as the Phulchauki Group,overlie the Bhimphedi Group in the Kathmandu region. Total thick-ness of both these groups attains 13 km (Stöcklin and Bhattarai,1981). The northern slope of the Kathmandu Valley is mainly com-posed of gneiss, schist and granite, but the other slopes and the

central part of the valley consist of weakly metamorphosed Phu-lchauki Group. The basin is filled with upper Pliocene to Quater-nary clay, silt, sand and gravel (Moribayashi and Maruo, 1980;Yoshida and Gautam, 1988; Sakai, 2001) overlaying the Precam-brian Bhimphedi Group and the lower Paleozoic Phulchauki Group(Stöcklin and Bhattarai, 1981). Katel et al. (1996) and Sakai et al.(2002) mention that more than 300 m thick muddy and sandy sed-iments of lacustrine origin are extensively distributed within theKathmandu Basin (Fig. 3).

Kathmandu Valley has typical lacustrine sediments of its kind,which has attracted many geo-science researchers from variousparts of the world. For example, Yoshida and Igarashi (1984),Dangol (1985), Fujii and Sakai (2002), and Sakai et al. (2002) havestudied the depositional environment and stratigraphy of the sed-iment in the valley. Likewise, Katel et al. (1996), Dahal and Aryal(2002), and JICA (2002) have studied the engineering geologicaland geotechnical properties of the Kathmandu lacustrine sediment,and Rai et al. (2004) and Paudel (2010) have carried out the litho-logical and mineralogical evaluation of Kathmandu soils. More re-cently, Mugnier et al. (2011) have conducted a study on the seismicresponse of the Kathmandu Basin and they mention that the softsediment deformation of the basin is mainly controlled by thefluidization of the silty layers during earthquake shaking. Most ofthese studies are based on the borehole data obtained by differentagencies for various purposes, such as water supply project, andfield observation. As these borehole cores were not recovered,the precise lithologic characteristics and stratigraphy were notconfirmed (Sakai, 2001). Moreover, Sakai (2001) mentions thatthese previous studies faced several important problems of strati-graphic division and nomenclature of the formations, mainly be-cause of lack of information on the subsurface geology andinsufficient description on definition of each formation. To over-come this problem, Sakai et al. (2001) conducted a core drillingof the basin-fill sediments for the palaeoclimatic study of the Kath-mandu Basin. This was the first large-scale drilling project in thevalley with full core recovery, and solely dedicated to academic re-search purpose. Based on this study, they have divided the historyof Palaeo-Kathmandu Lake into seven stages ranging from stage 1 –prior to the appearance of the lake to stage 7 – draining out of thelake-water (Sakai et al., 2001). Subsequent details of the differentstages of changes of lithology and sedimentary facies of the sedi-ment can be found in Sakai et al. (2001). Based on the availabledata from the previous study and paleoclimatic study of the Kath-mandu Basin, Sakai (2001) has divided the sediments in the valleyinto three groups: (1) marginal fluvio-deltaic facies in the northernpart, (2) open lacustrine facies in the central part, and (3) alluvialfan facies in the southern part, as shown in Fig. 1c.

3. Methodology

3.1. Field observations

Microtremor measurement survey was carried out at 172 1-kmgrid points in the study area (Fig. 1c) with the help of a portablevelocity sensor. This sensor is capable of recording three compo-nents of vibration: two horizontal, i.e., east–west and north–southand one vertical (Fig. 4). At each survey point, the microtremordata were recorded for 300 s at a sampling frequency of 100 Hz(i.e., 30,000 samples at each point). Fourier analysis of each win-dow (after removing unwanted noise) was carried out using FastFourier Transform (FFT) computer program, and the obtained spec-tra were smoothed using Parzen window of bandwidth 0.5 Hz. Theaverage spectral ratio of the horizontal component of vibration tovertical (i.e., H/V) in each window was derived from the followingequation (Delgado et al., 2000):

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H=V ¼ffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiðF2

NS þ F2EWÞ=ð2F2

UDÞq

ð1Þ

Here, FNS, FEW and FUD are the Fourier amplitude spectra in thenorth–south (NS), east–west (EW) and vertical (UD) direction,respectively.

After deriving H/V spectral ratios for all windows of a point, theH/V ratio for the particular point was obtained by averaging all

those spectral ratios. Based on the fact that the frequency corre-sponding to the first peak of the H/V spectrum plot represents fun-damental resonant frequency of the site (Field and Jacob, 1993;SESAME, 2004; Bonnefoy-Claudet et al., 2006), the site specific fun-damental frequency for each measurement point was obtained.Typical result of microtremor data analysis and calculation ofpredominant frequency of the sites in some of the location of

Nepal

Kathmandu Valley

CHINA

INDIA

BH2

BH1

Main Rivers

Fan depositions

Lacustrine facies

Basement RocksTalus deposits

Fluvio-deltaic facies

Isolated basement rocks

Microtremor observation points

Major roads

KathmanduLalitpurBhaktapur

Borehole location

(c)

(b)(a)

Fig. 1. Location map of the study area; (a) location map of Nepal in Asia; (b) location of the Kathmandu Valley in Nepal; and (c) map of Kathmandu Valley (study area). Asediment distribution map of the Kathmandu Valley, microtremor measurement points and borehole location (BH1 and BH2) in the study area are shown (modified after Fujiiand Sakai, 2002).

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Kathmandu Basin are shown in Fig. 5. In this study, the fundamen-tal resonant frequency of the soil layer is used to calculate thethickness of the soft sediments in the Kathmandu Basin.

3.2. Theoretical calculation

Ibs-von Seht and Wohlenberg (1999) showed that the funda-mental resonant frequency of the soil layer is closely related tothickness of the soil layer as given in the following equation:

h ¼ af br ð2Þ

where ‘h’ and ‘fr’ are the depth of the Quaternary sediments and fun-damental resonant frequency, and ‘a’ and ‘b’ are the standard errorsof the correlation coefficients.

Ibs-von Seht and Wohlenberg (1999) have studied both param-eters (i.e, h and fr) and demonstrated that it is possible to establisha direct functional relationship between them without knowingthe shear wave velocity (Vs). They estimated the value of ‘a’ and‘b’ and proposed an empirical relationship (Eq. (3)) between thefundamental resonant frequency (fr) and the thickness of soft sed-iment cover (h) (Quaternary sediments), based on 34 boreholesranging in depth from 15 m to 1257 m and data from 102 seismic

Fig. 2. A schematic geological cross-section through Central Nepal (after Sakai et al., 2002, and Stöcklin and Bhattarai, 1981). S: Siwalik Group, B: Bhimphedi Group, P:Phulchauki Group, N: Nawakot Complex, G: Granite, Gn: Gneiss Complex, K: Kathmandu Complex, MFT: Main Frontal Thrust, CCT: Central Churia Thrust, MBT: MainBoundary Thrust, MT: Mahabharat Thrust.

Black Clay

Sand and gravel bed

Fig. 3. A schematic geological cross-section of the Kathmandu Basin, showing north–south sediment distribution through the center of the Kathmandu Valley (after Katelet al., 1996; Sakai et al., 2002).

- 0.004

- 0.002

0

0.002

0.004

0.00 20.48 40.96 61.44 81.92 102.40 122.88 143.36 163.84 225.28 245.76 266.24 286.72 307.20

Time (T) Sec

Vel

. (cm

/s) Noise

Time (T) Sec

Vel

. (cm

/s) Noise

Time (T) Sec

Vel

. (cm

/s) Noise

204.80184.32

(a)

(b)

(c)

Fig. 4. Typical pattern of measured microtremor data; (a) in east–west direction (X-axis); (b) in north–south direction (Y-axis); (c) in up-down direction (Z-axis).

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stations in the western Lower Rhine Embayment in Germany,which is covered with Tertiary and Quaternary sediments overlay-ing Palaeozoic bedrock. Similarly, Parolai et al. (2002) developed anempirical relationship (Eq. (4)) between thickness of sedimentwith resonant frequency for the Cologne area in Germany basedon 32 boreholes with a depth range from 20 m to 402 m and337 data from seismic stations. Recently, Birgören et al. (2009)have derived yet another empirical relationship (Eq. (5)) betweenthe thickness of Tertiary–Quaternary sediments overlying Palaeo-zoic bedrock and their resonance frequencies for the Istanbul re-gion based on the H/V ratio from 15 measurements at theborehole locations and velocity profile of two microtremor arraymeasurements sites. The results obtained by Birgören et al.(2009) show a very strong relationship (R2 value: 0.995) betweenthe resonant frequency and the thickness of the sediment whichvaries from 20 m to 449 m. Similarly, Özalaybey et al. (2011) haveinvestigated 3-D basin structures and site response frequencies inthe Izmit Bay area of Turkey by microtremor measurement in 239

stations and 405 – point gravity measurements and derived anequation (Eq. (6)) for sediment cover in Izmit Basin in Turkeywhich has the sedimentary cover thickness about 1200 m at thedeepest part.

h ¼ 96f%1:388r ð3Þ

h ¼ 108f%1:551r ð4Þ

h ¼ 150:99f%1:1531r ð5Þ

h ¼ 141f%1:27r ð6Þ

So as to map the soft sediment thickness in the KathmanduBasin, we adopt terrain specific equations given by the aboveresearchers. A theoretical thickness for the sediments of theKathmandu Basin is calculated using above equations, based onthe fundamental frequency (fr) obtained for each station using

P 10

0.1

1

10

P 40

0.1

1

10

0.1 1 10

P 54

0.1

1

10P 84

0.1

1

10

0.1 1 101

P 100

0.1

1

10

P 144

0.1

1

10 P 163

0.1

1

10

100

0.1 1 10

P 134

0.1

1

10

0.1 1 10

Frequency (Hz)

H/V

Rat

io

Frequency (Hz)

H/V

Rat

io

Frequency (Hz)

H/V

Rat

io

Frequency (Hz)

H/V

Rat

io

Frequency (Hz)

H/V

Rat

io

Frequency (Hz)

H/V

Rat

io

Frequency (Hz)

H/V

Rat

io

Frequency (Hz)

H/V

Rat

io

0.1 1 10

0.1 1 10

0.1 1 10

0.1 1 10

Fig. 5. Typical H/V spectral ratio of some microtremor measurement points in the study area. Red line is the mean value and black and blue lines are ± standard deviation.Black pointed triangle represents the predominant frequency taken from H/V spectral ratio. (For interpretation of the references to colour in this figure legend, the reader isreferred to the web version of this article.)

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microtremor observations. In this study, it is assumed that the H/Vspectral ratio depends primarily on the source/site characteristicsrather than geographical location.

It has been observed that the variations of estimated depth,based on above four equations, were not much comparable witheach other, and analysis clearly showed an average standard devi-ation of 41.88 m in thickness as shown in Fig. 6. In order to mini-mize the value of standard deviation and thereby to obtain thereliable results, the depth estimated based on the non-linearregression equation proposed by the four researchers were dividedinto two groups emphasizing the less variations of the estimateddepth in each group. The standard deviation of each group is ob-tained as shown in Figs. 7 and 8. These figures show that the stan-dard deviation (i.e., 48.55 m in thickness) of First Group (i.e. Ibs-von Seht and Wohlenberg, 1999; Parolai et al., 2002) (Fig. 7) ishigher than (i.e., 7.44 m in thickness) the Second Group (Birgörenet al., 2009; Özalaybey et al., 2011) (Fig. 8). The results also showedthat the values of thickness obtained from Birgören et al. (2009)and Özalaybey et al. (2011) are more compatible with each other.In other words, the depths calculated using Birgören et al. (2009)and Özalaybey et al. (2011) show significantly smaller variationsin the thickness due to the comparable geotechnical characteristicsof the geological formation. Table 1 shows the comparative studyof the geological characteristics of the geological formation ofKathmandu Basin, Izmit Basin and Istanbul area.

We further averaged the values estimated using Eqs. (5) and (6)to obtain the best fit equation which we purposed for theKathmandu Basin as follows:

h ¼ 146:01f%1:2079r ð7Þ

The obtained equation (i.e. Eq. (7)) is further used for obtainingprimary information on the relative depth variation (refer Supple-mentary Table 2) of the interface between the two physically con-trasting layers of lacustrine sediment and the underlain hard strata(or bedrock) in the Kathmandu Basin.

This observation is validated by comparing the results of thegravity contour map proposed by Moribayashi and Maruo (1980)and also with the depth of the bedrock based on the boreholedrilled for academic purposes (Sakai et al., 2001) in the KathmanduBasin.

4. Results and discussion

The results of this study are expressed in terms of thickness ofthe lacustrine sediment and its variation in different areas of theKathmandu Basin. The sediment depth in various locations in thebasin is calculated using Eq. (7). The contour map of the estimatedsoft sediment thickness and a digital elevation model (DEM) for thestudy area (Kathmandu Basin) are shown in Figs. 9–11. The calcu-lated values give a deep interface of soft sediment (unconsolidated)and basement layer in the center of the Kathmandu Basin and shal-low in and around the outskirts of the valley. The sudden abruptchange in the sediment thickness is found at points A (i.e. thicknessof sediment about 48 m) and B (i.e. thickness of sediment about30 m) (Fig. 9), which are about 2 km and 3 km along north and eastfrom the central part of the Kathmandu respectively, which indi-cates the presence of basement rock in the shallow depth in thoseareas.

The calculated depth of the interface between two layers is usedto plot the cross-profiles and digital elevation model (DEM) for theKathmandu Basin. Fig. 11a and b shows the profiles along west toeast and south to north direction respectively. The west to eastprofiles (Fig. 11a) along P167–P175, P157–P165, P144–P154,P133–P143, P54–P72, P35–P53, P16–P34, and P1–P15 show gentleslope of basement topography, whereas the profiles along P122–P132, P111–P121, P92–P110, and P73–P91 show steeper slopeand increase in the soft sediment thickness mainly towards thecenter location. The variation of the soft sediment thickness inthe valley can also be described using Fig. 11b in which the profilesalong P3–P168, P4–P169, P5–P170, and P6–P171 give informationabout the distribution of sediment thickness along south–northdirection, and they clearly indicate a steeper slope of the basementfloor and increase of sediment thickness towards the center. TheDEM of the hard stratum further reveal that the thicknesses ofthe sediment in depression (I) (refer Figs. 1c and 10 and Supple-mentary Table 2) at points P94, P95, P96, P114, P115, P116,

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120 140 160 180 200Microtremor observation points

Thic

knes

s (m

)

Fig. 6. Comparison between depths calculated using Ibs-von Seht and Wohlenberg(1999), Parolai et al. (2002), Birgören et al. (2009) and Özalaybey et al. (2011)relationships (Eqs. (3)–(6)). The circle indicates the average value whereas thelength of the line suggests deviation from the average.

0

50

100

150200

250

300

350

400

0 20 40 60 80 100 120 140 160 180 200Microtremor observation points

Thic

knes

s (m

)

Fig. 7. Comparison between depths calculated using Ibs-von Seht and Wohlenberg(1999), and Parolai et al. (2002) relationships (Eqs. (3) and (4)). The circle indicatesthe average value whereas the length of the line suggests deviation from theaverage.

0

50

100

150

200

250

300

350

400

0 20 40 60 80 100 120 140 160 180 200Microtremor observation points

Thic

knes

s (m

)

Fig. 8. Comparison between depths calculated using Birgören et al. (2009) andÖzalaybey et al. (2011) relationships (Eqs. (5) and (6)). The circle indicates theaverage value whereas the length of the line suggests deviation from the average.

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Table 1Comparative study of the geotechnical characteristics of the geological formation of Kathmandu Basin, Izmit Basin and Istanbul area.

Descriptions Kathmandu Basin Izmit Basin Istanbul area

Basement rock Palaeozoic Tethyan rock Moribayashi andMaruo (1980)

Paleozoic rock Karkas and Coruk (2010) Palaeozoic rocks Ündül and Tugrul(2006)

Sediment Lacustrine and fluvial in origin Katel et al.(1996); Sakai et al. (2001)

Quaternary alluvial and fluvial depositsKarkas and Coruk (2010)

Halic and Bosphorus sedimentsÜndül and Tugrul (2006)

Maximum estimated depth ofsoft sediment (m)

347 (current study) 1200 Özalaybey et al. (2011) 449 Birgören et al. (2009)

Shear wave velocity of sedimentup to 30 m (m/s)

188–310 JICA (2002) 180–360 Zor et al. (2010) 80–375 Bozdag and Kocaoglu (2005)

Predominant frequency (Hz) 0.448–8.89 (current study) 0.23–5 Özalaybey et al. (2011) 0.44–5 Birgören et al. (2009)Variation of SPT N value up to

30 m2–42 JICA (2002) 2–43 Karkas and Coruk (2010) 5–>50 Dalgic (2004)

Specific gravity of soil 2.34–2.77 Katel et al. (1996) 2.55–2.78 Sawicki and Swidzinski (2006) 2.42–2.79 Ündül and Tugrul (2006)Liquid limit (%) 30–108 Katel et al. (1996) 33–66 Olgun et al. (2008) 35–98 Ündül and Tugrul (2006)Plasticity index (%) 5–43 Katel et al. (1996) 10–37 Olgun et al. (2008) 7–50 Ündül and Tugrul (2006)

Fig. 9. Contour map of the basement topography of the Kathmandu Basin.

Fig. 10. Bedrock-soft sediment palaeo-topography of the Kathmandu Valley Basin (the vertical scale is 15 times exaggerated). I and II are the depressions carved overBedrock-soft sediment surface forming the sites of thickest deposit in the study area.

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P117, P125, P126, P127, P128 show relative depth variations of260 m, 214 m, 233 m, 253 m, 279 m, 347 m, 278 m, 309 m,197 m, 166 m, 233 m and depression (II) at points P50, P86,P103, P104, P105, P132 show the depth variations of 194 m,214 m, 166 m, 173 m, 173 m, 166 m respectively.

The digital elevation model suggests that the sediment distribu-tion in the basin is far from uniform and have an undulating topog-raphy with steep relief in many locations in the basin. Hagen(1969) mentioned that during the pre-lake formation in the Kath-mandu Basin, drainage systems originating in the northern slope ofthe Shivapuri hill and termed as ‘‘Proto Bagmati River’’ were veryactive. These river systems were responsible for the deposition ofcoarse-grained sediments (gravels and coarse sand) below the lakedeposits in the entire valley. The influence of Proto Bagmati riversystems appear more in the northern and central part than in theother part of the valley. According to Yoshida and Igarashi(1984), this deposition took place some 2.5 Ma ago (i.e., duringmiddle to late Pliocene period).

After analyzing the digital elevation model of the basementtopography in the Kathmandu Basin, there arise mainly two possi-ble explanations. Firstly, the calculated depth of the sediment rep-resents the total depth of lake deposit which is underlain by

basement rock in the Kathmandu Basin. The calculated depthmay not necessarily indicate the presence of hard rock, rather itis a representation of the presence of basement layer at that depth,beyond which the sediment do/may not contribute to the amplifi-cation of the ground motion. From geotechnical point of view, thiscontrast corresponds to the bedrock. Secondly, although Figs. 9 and10 show a number of small depressions in the whole study area,two large depressions are found. First depression at the central partof the Kathmandu City denoted by I in Fig. 10, which is wider anddeeper represents the main ancient lake of the Kathmandu Basinwhile other is along the eastern part denoted by II which is rela-tively shallow and its catchments area elongated from northwestto southeast. Similarly, there are a number of buried ridges whichseparate/connect the depressions. The longest buried ridge, whichseparates/connects the central large and deep depression with theeastern shallow depression extends from northwest to southeastpart of the valley (Fig. 10).

In order to verify the estimated sediment thickness distributionmap of the Kathmandu Basin, the thickness variation profile alongsouth–north direction (Fig. 11b, profiles P5–P170) through the cen-ter of the Kathmandu Basin was compared with the boreholeexploration-based ground profile (Fig. 3) proposed by Sakai et al.

Fig. 11. Cross profiles showing the contact of soft sediment and bedrock, and variations of soft sediment (the vertical scale is 15 times exaggerated); (a) W–E profiles, and (b)S–N profiles.

8 Y.R. Paudyal et al. / Journal of Asian Earth Sciences xxx (2012) xxx–xxx

Please cite this article in press as: Paudyal, Y.R., et al. Basement topography of the Kathmandu Basin using microtremor observation. Journal of Asian EarthSciences (2012), http://dx.doi.org/10.1016/j.jseaes.2012.11.011

(2002). These two sections correlate well and show that the distri-bution of the sediment thickness based on this study is in goodagreement with the distribution of the soft sediment proposedby Sakai et al. (2002).

We further compared the basement topography estimated inthis study with the gravity contour map (Fig. 12) prepared by Mor-ibayashi and Maruo (1980) for the central area of the KathmanduBasin. The basement topography obtained from the microtremorobservations in this study (Fig. 9) is found quite similar with theresults of the gravity survey conducted by Moribayashi and Maruo(1980) because the low gravity is obtained in the center of the val-ley where the thickness of the lacustrine sediments is high andgradually increases towards the marginal area where the sedimentthickness is low. Moreover, the first depression along north tosouth through central part of the valley and second depressionalong northwest to southeast proposed in this study matched quitewell with the gravity data as mentioned by Moribayashi and Mar-uo (1980).

In order to compare the calculated depth of the basement rockbased on microtremor observation with the actual depth of theKathmandu Basin, two boreholes (BH1 and BH2) used also by Sakaiet al. (2001) for academic purposes were considered, as shown inFig. 13. According to Stöcklin and Bhattarai (1981) and Sakaiet al. (2001), the basement of the Kathmandu Valley consists ofweakly metamorphosed Phulchauki Group (Fig. 2), which consistsof Paleozoic sandstone, phyllite and weathered rocks. Moreover,Sakai et al. (2001) have mentioned that almost all sand and gravelsat the lower part of the core are composed of detritus to weaklymetamorphosed sedimentary rocks derives from the underlyingKathmandu Complex. There is always confusion in the type of lith-ological layer below the clay (unconsolidated) layer; hence, it is al-ways difficult to differentiate the sediment of bedrock with thebasal conglomerate or gravelly soil in the Kathmandu Basin fromthe borehole data. Fig. 13a and b shows the lithostratigraphy intwo boreholes (BH1 and BH2) (Sakai, 2001, and Sakai et al.,2001), in which the depth of the basement rock is shown at about252 m in borehole BH1 (Fig. 13a) and thick layer of the sand isshown below 232 m in borehole BH2 (Fig. 13b). These figures also

show that the depth of the basement rock (or hard sediment) esti-mated from the result of microtremor observation is at 196 m be-low the surface at borehole BH1 and 188 m at borehole BH2. Thedifference in depth of the bedrock with the estimated value maybe due to the change in basement topography abruptly in nearbyareas of these boreholes. Due to marginal area of the bowl shapedKathmandu Basin, the basement contour values also vary abruptlywithin a short distance in and around these borehole locations.Borehole BH2 lies at the edge of the Kathmandu Basin and a steepslope of basement topography is observed in west, north and southdirections (Fig. 9). Moreover, the calculated depth of the basementrock at microtremor observation points near the borehole BH1 isfound 260 m and near the borehole BH2, it is 233 m (referFigs. 1c and 9, and Supplementary Table 2). This indicates thatthe depth estimated in this study provides comparatively accurateresult for the basement topography of the Kathmandu Basin andconfirms the conclusions of previous studies (e.g., Ibs-von Sehtand Wohlenberg, 1999; Delgado et al., 2000; Parolai et al., 2002;Hinzen et al., 2004; García-Jerez et al., 2006; D’Amico et al.,2004; Birgören et al., 2009; Dinesh et al., 2010; Gosar and Lenart,2010; Özalaybey et al., 2011; Sukumaran et al., 2011) and encour-ages the use of microtremor observations for an approximate esti-mation of sediment depth over wide basin areas.

5. Conclusions

In order to explore the hazard level as well as to estimate therisk of next expected earthquake disaster in the Kathmandu Valley,a study on the floor variation of the lacustrine sediments in theKathmandu Basin was done. Due to lack of adequate and precisescientific studies on the floor variation of sediments in the basin,however, it is always difficult to ascertain their characteristics dur-ing earthquakes, which ultimately leads to erroneous and assumeddata for ground modeling as well as analysis and design of theinfrastructures. This study attempts to fill this gap by proposingan approximate basement topography of the Kathmandu Basinusing microtremor observation at 172 locations. This study alsoenables to estimate the soft sediment variation in the KathmanduBasin using a non-linear regression equation (h ¼ 146:01f%1:2079

r ),and provides the hidden basement topography of the KathmanduBasin. The sediment/rock below this basement topography maynot take part for the amplification of the ground motion duringearthquake in the Kathmandu Valley.

The distribution of sediment indicates that the deepest part ofthe lake existed mainly in the central part of Kathmandu, wherethe main core city exists at present, and is one of the oldest resi-dential areas in the Kathmandu Valley. It also accommodates anumber of departmental stores, Government Offices, historicalmonuments including UNESCO cultural world heritage sites. More-over, due to an increasing population and developing as a greatercommercial hub, the central part has seen a sharp rise in the num-ber of mid-height to tall buildings, which are constructed withoutadequate geotechnical investigation. Depending upon the type andstories of buildings, the predominant frequencies are different. Thethickness distribution map shows that the main lake part (i.e. cen-tral part) has a considerable thickness of the soft sediments, andhence, it is prone to higher amplification of seismic wave at thecorresponding predominant frequencies.

The map thus depicted shall not be meant as a detailed andhighly constrained representation of the valley bedrock; however,it represents the first reliable reconstruction of the subsurfacemorphology of the Kathmandu Basin, which shows a good consis-tency with available geological/log data. This study represents auseful starting point for future research and investigation

Fig. 12. Gravity contour map in the Kathmandu Valley (redrawn after Moribayashiand Maruo (1980))

Y.R. Paudyal et al. / Journal of Asian Earth Sciences xxx (2012) xxx–xxx 9

Please cite this article in press as: Paudyal, Y.R., et al. Basement topography of the Kathmandu Basin using microtremor observation. Journal of Asian EarthSciences (2012), http://dx.doi.org/10.1016/j.jseaes.2012.11.011

activities, such as detailed surveys, numerical modeling, and seis-mic hazard or microzonation studies.

Acknowledgements

The authors are grateful to Mr. Ramhari Dahal (The Departmentof Education, Government of Nepal) and Mr. Prakash Poudyal(Kathmandu University) for their help during the extensive micro-tremor survey in the Kathmandu Valley. The help provided by Ms.Manita Timilsina (PhD candidate, Graduate School of Science andEngineering, Ehime University) in preparing a few GIS-based mapsis greatly appreciated. The authors would also like to express theirspecial appreciation to Dr. Shinichiro Mori (Associate Professor,Graduate School of Science and Engineering, Ehime University)for enabling the first author to analyze the microtremor surveydata. The authors also appreciate the comments and suggestionsprovided by the anonymous reviewers, which help to modify themanuscript.

This is a part of the study entitled ‘Integrated approach tostudying rain- and earthquake-induced disasters in the HimalayanWatersheds and development of a strategic disaster education sys-

tem’ (Team Leader: Ryuichi Yatabe, Ehime University, AY2009–AY2011) and supported financially by the Government of Japan un-der Grant-in-Aid for Overseas Scientific Research and Investigation.

Appendix A. Supplementary material

Supplementary data associated with this article can be found,in the online version, at http://dx.doi.org/10.1016/j.jseaes.2012.11.011.

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