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Basement topography of the Kathmandu Basin using microtremor observation
Journal of Asian Earth Sciences 62 (2013) 627–637
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Basement topography of the Kathmandu Basin using microtremor observation Youb Raj Paudyal ⇑, Ryuichi Yatabe, Netra Prakash Bhandary, Ranjan Kumar Dahal Geo-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 2012 Received in revised form 22 October 2012 Accepted 5 November 2012 Available online 19 November 2012 Keywords: Kathmandu Valley Lacustrine sediments Microtremor Predominant frequency Basement topography
a b s t r a c t 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 172 sites spaced at a grid interval of 1 km. The results showed that the predominant frequency varies from 0.488 Hz to 8.9 Hz. 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. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Intensity of ground motion is a function of earthquake magnitude and distance from the seismic source, as well as local geological condition and topography of the area (Kramer, 1996). Although, geological structure of the area is an important factor, local site condition is known to have a great influence on the potential damage resulting from earthquakes (Seed and Idriss, 1969). Besides, geotechnical properties of the local soil site and behavior of the soil during earthquake depends upon the depth of the sedimentary column and its shear wave velocity. From geotechnical point of view, the shear wave velocity in the top 30 m column of soil is responsible for an unusual amplification of the ground motion (Finn, 1991). However, in the areas where the thickness of soft sediments is enormously large, like in the Kathmandu Basin, the amplification will rather depends on the extent of the soft sediment column and its elastic properties. Moreover, the fundamental phenomenon responsible for the amplification of seismic waves is due to the impedance contrast between sedimentary deposits and the underlying hard-strata or bedrock. Such site amplification can be estimated using microtremor measurement technique, which was first introduced by Kanai (1954). Several studies such as, Ohta et al. (1978), Lermo et al. (1988), Field et al. (1990), and Field and
⇑ Corresponding author. Address: Geo-disaster Research Laboratory, Graduate School of Science and Engineering, Ehime University, Bunkyo-3, Matsuyama 7908577, Japan. Tel./fax: +81 89 927 8566. E-mail address: [email protected] (Y.R. Paudyal). 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.11.011
Jacob (1993) have shown that microtremor analysis results revels the fundamental resonant frequency of sediments. One of the main challenges in determining the site amplification characteristics out of microtremor measurement is removing the source effects, which is often achieved by dividing the Fourier spectrum obtained on a soft ground point by that obtained on a nearby reference point on bedrock. For this, however, the microtremor source and path effects must be the same for both measurement points and the reference point or site must also have negligible site effects. To overcome this limitation, Nakamura (1989) has introduced a technique for estimating the site response by measuring solely the microtremor on the surface of the ground. According to him, the source effect can be removed by dividing the horizontal component of microtremor spectrum by the vertical component. Now, this technique has become widespread as a low-cost and effective tool to estimate the fundamental resonant frequency of sediments using Horizontal-to-Vertical (H/V) spectral ratio at a single-station. In his paper, Nakamura (1989) explains the use of this technique and gives a detailed explanation on the subsequent assumptions. In the last two decades, the H/V method has been widely used for various purposes, such as site effect evaluation, wave amplification estimation, liquefaction vulnerability assessment, sediment depth estimation, and microzonation studies in different geographical 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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an empirical relationship between fundamental resonant frequencies and sedimentary cover thickness has become practice since the work of Ibs-von Seht and Wohlenberg (1999) for Cologne area in Germany. They proposed that resonance frequency in H/V spectra correlate well with the overall soil thickness; ranging from tens of meters to more than 1000 m. Their developed relationships provide a practical means of sediment thickness estimation using a microtremor observation. Later, several studies, such as Delgado et al. (2000), Parolai et al. (2002), D’Amico et al. (2004), Hinzen et 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 microtremor H/V spectral ratio technique for measuring the thickness of soil cover over a hard stratum or bedrock. The present investigation is a first attempt to approximate the thickness 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 suffered great losses in the past earthquakes, such as in 1255, 1408, 1681, 1803, 1810, 1833, 1866, and 1934 (Rana, 1935; Chitrakar and Pandey, 1986; Bilham et al., 1995; Pandey et al., 1995; Upreti and Yoshida, 2009). The latest greatest earthquake of 1934 (Mw = 8.1) with a maximum intensity of X in Modified Mercally Intensity (MMI) Scale in the Kathmandu Basin reportedly killed about 4296 people and destroyed about 19% and damaged about 38% of the buildings in the basin alone (Rana, 1935; Pandey and Molnar, 1988). The observations of the damage pattern in the Kathmandu Basin during this earthquake, in particular, suggest that the spectral ground amplification due to fluvio-lacustrine sediments play a major role in intensifying the ground motion (Pandey and Molnar, 1988; Hough and Bilham, 2008). However, the valley basin still lacks adequate information on spatial variation of the lacustrine sediments. Moribayashi and Maruo (1980) estimated the basement topography of the central portion of the basin using gravitational method 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). In addition, the density contrast may not be constant everywhere for a basin like the Kathmandu Valley, where the thickness and properties of sediments vary significantly within the short distances. In this sense, the actual floor variation of the basement rock in a wider area of the Kathmandu Basin is still debatable. So, the objective of this study is to derive an empirical relationship between the resonance frequencies obtained from the H/V technique and the thickness of the lacustrine sediments, and then generate an approximate basement topography of the Kathmandu Basin. This information is critically important for earthquake ground motion simulation studies, especially because the densely populated urban area of the valley is under a great earthquake threat.
central part of the valley consist of weakly metamorphosed Phulchauki Group. The basin is filled with upper Pliocene to Quaternary clay, silt, sand and gravel (Moribayashi and Maruo, 1980; Yoshida and Gautam, 1988; Sakai, 2001) overlaying the Precambrian 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 sediments of lacustrine origin are extensively distributed within the Kathmandu Basin (Fig. 3). Kathmandu Valley has typical lacustrine sediments of its kind, which has attracted many geo-science researchers from various parts of the world. For example, Yoshida and Igarashi (1984), Dangol (1985), Fujii and Sakai (2002), and Sakai et al. (2002) have studied the depositional environment and stratigraphy of the sediment in the valley. Likewise, Katel et al. (1996), Dahal and Aryal (2002), and JICA (2002) have studied the engineering geological and geotechnical properties of the Kathmandu lacustrine sediment, and Rai et al. (2004) and Paudel (2010) have carried out the lithological and mineralogical evaluation of Kathmandu soils. More recently, Mugnier et al. (2011) have conducted a study on the seismic response of the Kathmandu Basin and they mention that the soft sediment deformation of the basin is mainly controlled by the fluidization of the silty layers during earthquake shaking. Most of these studies are based on the borehole data obtained by different agencies for various purposes, such as water supply project, and field observation. As these borehole cores were not recovered, the precise lithologic characteristics and stratigraphy were not confirmed (Sakai, 2001). Moreover, Sakai (2001) mentions that these previous studies faced several important problems of stratigraphic division and nomenclature of the formations, mainly because of lack of information on the subsurface geology and insufficient description on definition of each formation. To overcome this problem, Sakai et al. (2001) conducted a core drilling of the basin-fill sediments for the palaeoclimatic study of the Kathmandu Basin. This was the first large-scale drilling project in the valley with full core recovery, and solely dedicated to academic research purpose. Based on this study, they have divided the history of Palaeo-Kathmandu Lake into seven stages ranging from stage 1 – prior to the appearance of the lake to stage 7 – draining out of the lake-water (Sakai et al., 2001). Subsequent details of the different stages of changes of lithology and sedimentary facies of the sediment can be found in Sakai et al. (2001). Based on the available data from the previous study and paleoclimatic study of the Kathmandu Basin, Sakai (2001) has divided the sediments in the valley into three groups: (1) marginal fluvio-deltaic facies in the northern part, (2) open lacustrine facies in the central part, and (3) alluvial fan facies in the southern part, as shown in Fig. 1c.
3. Methodology 3.1. Field observations
2. Study area and geology Geologically, the Kathmandu Basin (Fig. 1) lies on the Kathmandu Nappe (Hagen, 1969; Upreti, 1999), which is located along the southern slopes of the Himalaya. It is one of the several tectonic intermontane basins developed in the Lesser Himalayan belt (Sakai et al., 2002) as shown in Fig. 2. The Kathmandu Nappe is composed of the Shivapuri Gneiss and marbles of the Bhimphedi Group (Stöcklin and Bhattarai, 1981). As illustrated in Fig. 2, the early Paleozoic Tethyan rocks, named as the Phulchauki Group, overlie the Bhimphedi Group in the Kathmandu region. Total thickness of both these groups attains 13 km (Stöcklin and Bhattarai, 1981). The northern slope of the Kathmandu Valley is mainly composed of gneiss, schist and granite, but the other slopes and the
Microtremor measurement survey was carried out at 172 1-km grid points in the study area (Fig. 1c) with the help of a portable velocity sensor. This sensor is capable of recording three components of vibration: two horizontal, i.e., east–west and north–south and one vertical (Fig. 4). At each survey point, the microtremor data were recorded for 300 s at a sampling frequency of 100 Hz (i.e., 30,000 samples at each point). Fourier analysis of each window (after removing unwanted noise) was carried out using Fast Fourier Transform (FFT) computer program, and the obtained spectra were smoothed using Parzen window of bandwidth 0.5 Hz. The average spectral ratio of the horizontal component of vibration to vertical (i.e., H/V) in each window was derived from the following equation (Delgado et al., 2000):
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(a) CHINA
(b) Nepal
INDIA
Kathmandu Valley
(c)
BH1 BH2
Major roads Microtremor observation points Main Rivers
Kathmandu Lalitpur Bhaktapur
Fluvio-deltaic facies Lacustrine facies
Borehole location
Fan depositions Talus deposits Basement Rocks Isolated basement rocks
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). A sediment distribution map of the Kathmandu Valley, microtremor measurement points and borehole location (BH1 and BH2) in the study area are shown (modified after Fujii and Sakai, 2002).
H=V ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðF 2NS þ F 2EW Þ=ð2F 2UD Þ
ð1Þ
Here, FNS, FEW and FUD are the Fourier amplitude spectra in the north–south (NS), east–west (EW) and vertical (UD) direction, respectively. After deriving H/V spectral ratios for all windows of a point, the H/V ratio for the particular point was obtained by averaging all
those spectral ratios. Based on the fact that the frequency corresponding to the first peak of the H/V spectrum plot represents fundamental resonant frequency of the site (Field and Jacob, 1993; SESAME, 2004; Bonnefoy-Claudet et al., 2006), the site specific fundamental frequency for each measurement point was obtained. Typical result of microtremor data analysis and calculation of predominant frequency of the sites in some of the location of
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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: Main Boundary 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 Katel et al., 1996; Sakai et al., 2002).
Vel. (cm/s)
0.004 0.002
(a)
Noise
0 - 0.002 - 0.004 0.00
20.48
40.96
61.44
81.92
102.40
122.88
143.36
163.84
184.32
204.80
225.28
245.76
266.24
286.72
307.20
Time (T) Sec
Vel. (cm/s)
(b)
Noise
Time (T) Sec
Vel. (cm/s)
(c)
Noise
Time (T) Sec 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).
Kathmandu Basin are shown in Fig. 5. In this study, the fundamental resonant frequency of the soil layer is used to calculate the thickness of the soft sediments in the Kathmandu Basin. 3.2. Theoretical calculation Ibs-von Seht and Wohlenberg (1999) showed that the fundamental resonant frequency of the soil layer is closely related to thickness of the soil layer as given in the following equation: b
h ¼ af r
ð2Þ
where ‘h’ and ‘fr’ are the depth of the Quaternary sediments and fundamental resonant frequency, and ‘a’ and ‘b’ are the standard errors of the correlation coefficients. Ibs-von Seht and Wohlenberg (1999) have studied both parameters (i.e, h and fr) and demonstrated that it is possible to establish a direct functional relationship between them without knowing the shear wave velocity (Vs). They estimated the value of ‘a’ and ‘b’ and proposed an empirical relationship (Eq. (3)) between the fundamental resonant frequency (fr) and the thickness of soft sediment cover (h) (Quaternary sediments), based on 34 boreholes ranging in depth from 15 m to 1257 m and data from 102 seismic
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10
10
P 40
H/V Ratio
H/V Ratio
P 10
1
0.1 0.1
1
1
0.1 0.1
10
1
Frequency (Hz) 10
10
P 84
H/V Ratio
H/V Ratio
P 54
1
0.1 0.1
1
1
0.1 0.1
10
1
Frequency (Hz)
10
Frequency (Hz) 10
10
P 134
H/V Ratio
P 100
H/V Ratio
10
Frequency (Hz)
1
0.1 0.1
1
1
0.1 0.1
10
1
10
Frequency (Hz)
Frequency (Hz) 100 10
P 163
H/V Ratio
H/V Ratio
P 144
1
0.1 0.1
1
10
10
1
0.1 0.1
1
10
Frequency (Hz)
Frequency (Hz)
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 is referred to the web version of this article.)
stations in the western Lower Rhine Embayment in Germany, which is covered with Tertiary and Quaternary sediments overlaying Palaeozoic bedrock. Similarly, Parolai et al. (2002) developed an empirical relationship (Eq. (4)) between thickness of sediment with resonant frequency for the Cologne area in Germany based on 32 boreholes with a depth range from 20 m to 402 m and 337 data from seismic stations. Recently, Birgören et al. (2009) have derived yet another empirical relationship (Eq. (5)) between the thickness of Tertiary–Quaternary sediments overlying Palaeozoic bedrock and their resonance frequencies for the Istanbul region based on the H/V ratio from 15 measurements at the borehole locations and velocity profile of two microtremor array measurements sites. The results obtained by Birgören et al. (2009) show a very strong relationship (R2 value: 0.995) between the resonant frequency and the thickness of the sediment which varies from 20 m to 449 m. Similarly, Özalaybey et al. (2011) have investigated 3-D basin structures and site response frequencies in the Izmit Bay area of Turkey by microtremor measurement in 239
stations and 405 – point gravity measurements and derived an equation (Eq. (6)) for sediment cover in Izmit Basin in Turkey which has the sedimentary cover thickness about 1200 m at the deepest part.
h ¼ 96fr1:388
ð3Þ
h ¼ 108fr1:551
ð4Þ
h ¼ 150:99fr1:1531
ð5Þ
h ¼ 141fr1:27
ð6Þ
So as to map the soft sediment thickness in the Kathmandu Basin, we adopt terrain specific equations given by the above researchers. A theoretical thickness for the sediments of the Kathmandu Basin is calculated using above equations, based on the fundamental frequency (fr) obtained for each station using
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400
400
350
350
300
300
Thickness (m)
Thickness (m)
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250 200 150 100 50 0
200 150 100 50
0
20
40
60
80
100
120
140
160
180
200
Microtremor observation points
microtremor observations. In this study, it is assumed that the H/V spectral ratio depends primarily on the source/site characteristics rather than geographical location. It has been observed that the variations of estimated depth, based on above four equations, were not much comparable with each other, and analysis clearly showed an average standard deviation of 41.88 m in thickness as shown in Fig. 6. In order to minimize the value of standard deviation and thereby to obtain the reliable results, the depth estimated based on the non-linear regression equation proposed by the four researchers were divided into two groups emphasizing the less variations of the estimated depth in each group. The standard deviation of each group is obtained as shown in Figs. 7 and 8. These figures show that the standard deviation (i.e., 48.55 m in thickness) of First Group (i.e. Ibsvon Seht and Wohlenberg, 1999; Parolai et al., 2002) (Fig. 7) is higher than (i.e., 7.44 m in thickness) the Second Group (Birgören et al., 2009; Özalaybey et al., 2011) (Fig. 8). The results also showed that 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 variations in the thickness due to the comparable geotechnical characteristics of the geological formation. Table 1 shows the comparative study of the geological characteristics of the geological formation of Kathmandu 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 the Kathmandu Basin as follows:
400 350 300 250 200 150 100 50 0
0
20
40
60
80
100
120
140
160
0
0
20
40
60
80
100
120
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Microtremor observation points
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 the length of the line suggests deviation from the average.
Thickness (m)
250
180
200
Microtremor observation points 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 indicates the average value whereas the length of the line suggests deviation from the average.
Fig. 8. Comparison between depths calculated using Birgören et al. (2009) and Özalaybey et al. (2011) relationships (Eqs. (5) and (6)). The circle indicates the average value whereas the length of the line suggests deviation from the average.
h ¼ 146:01fr1:2079
ð7Þ
The obtained equation (i.e. Eq. (7)) is further used for obtaining primary information on the relative depth variation (refer Supplementary Table 2) of the interface between the two physically contrasting layers of lacustrine sediment and the underlain hard strata (or bedrock) in the Kathmandu Basin. This observation is validated by comparing the results of the gravity contour map proposed by Moribayashi and Maruo (1980) and also with the depth of the bedrock based on the borehole drilled for academic purposes (Sakai et al., 2001) in the Kathmandu Basin.
4. Results and discussion The results of this study are expressed in terms of thickness of the lacustrine sediment and its variation in different areas of the Kathmandu Basin. The sediment depth in various locations in the basin is calculated using Eq. (7). The contour map of the estimated soft sediment thickness and a digital elevation model (DEM) for the study area (Kathmandu Basin) are shown in Figs. 9–11. The calculated values give a deep interface of soft sediment (unconsolidated) and basement layer in the center of the Kathmandu Basin and shallow in and around the outskirts of the valley. The sudden abrupt change in the sediment thickness is found at points A (i.e. thickness of sediment about 48 m) and B (i.e. thickness of sediment about 30 m) (Fig. 9), which are about 2 km and 3 km along north and east from the central part of the Kathmandu respectively, which indicates the presence of basement rock in the shallow depth in those areas. The calculated depth of the interface between two layers is used to plot the cross-profiles and digital elevation model (DEM) for the Kathmandu Basin. Fig. 11a and b shows the profiles along west to east and south to north direction respectively. The west to east profiles (Fig. 11a) along P167–P175, P157–P165, P144–P154, P133–P143, P54–P72, P35–P53, P16–P34, and P1–P15 show gentle slope of basement topography, whereas the profiles along P122– P132, P111–P121, P92–P110, and P73–P91 show steeper slope and increase in the soft sediment thickness mainly towards the center location. The variation of the soft sediment thickness in the valley can also be described using Fig. 11b in which the profiles along P3–P168, P4–P169, P5–P170, and P6–P171 give information about the distribution of sediment thickness along south–north direction, and they clearly indicate a steeper slope of the basement floor and increase of sediment thickness towards the center. The DEM of the hard stratum further reveal that the thicknesses of the sediment in depression (I) (refer Figs. 1c and 10 and Supplementary Table 2) at points P94, P95, P96, P114, P115, P116,
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Y.R. Paudyal et al. / Journal of Asian Earth Sciences 62 (2013) 627–637 Table 1 Comparative 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 and Maruo (1980) Lacustrine and fluvial in origin Katel et al. (1996); Sakai et al. (2001) 347 (current study)
Paleozoic rock Karkas and Coruk (2010) Quaternary alluvial and fluvial deposits Karkas and Coruk (2010) 1200 Özalaybey et al. (2011)
Palaeozoic rocks Ündül and Tugrul (2006) Halic and Bosphorus sediments Ündül and Tugrul (2006) 449 Birgören et al. (2009)
188–310 JICA (2002)
180–360 Zor et al. (2010)
80–375 Bozdag and Kocaoglu (2005)
0.448–8.89 (current study) 2–42 JICA (2002)
0.23–5 Özalaybey et al. (2011) 2–43 Karkas and Coruk (2010)
0.44–5 Birgören et al. (2009) 5–>50 Dalgic (2004)
2.34–2.77 Katel et al. (1996) 30–108 Katel et al. (1996) 5–43 Katel et al. (1996)
2.55–2.78 Sawicki and Swidzinski (2006) 33–66 Olgun et al. (2008) 10–37 Olgun et al. (2008)
2.42–2.79 Ündül and Tugrul (2006) 35–98 Ündül and Tugrul (2006) 7–50 Ündül and Tugrul (2006)
Sediment Maximum estimated depth of soft sediment (m) Shear wave velocity of sediment up to 30 m (m/s) Predominant frequency (Hz) Variation of SPT N value up to 30 m Specific gravity of soil Liquid limit (%) Plasticity index (%)
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 over Bedrock-soft sediment surface forming the sites of thickest deposit in the study area.
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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.
P117, P125, P126, P127, P128 show relative depth variations of 260 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 distribution in the basin is far from uniform and have an undulating topography with steep relief in many locations in the basin. Hagen (1969) mentioned that during the pre-lake formation in the Kathmandu Basin, drainage systems originating in the northern slope of the Shivapuri hill and termed as ‘‘Proto Bagmati River’’ were very active. These river systems were responsible for the deposition of coarse-grained sediments (gravels and coarse sand) below the lake deposits in the entire valley. The influence of Proto Bagmati river systems appear more in the northern and central part than in the other part of the valley. According to Yoshida and Igarashi (1984), this deposition took place some 2.5 Ma ago (i.e., during middle to late Pliocene period). After analyzing the digital elevation model of the basement topography in the Kathmandu Basin, there arise mainly two possible explanations. Firstly, the calculated depth of the sediment represents the total depth of lake deposit which is underlain by
basement rock in the Kathmandu Basin. The calculated depth may not necessarily indicate the presence of hard rock, rather it is a representation of the presence of basement layer at that depth, beyond which the sediment do/may not contribute to the amplification of the ground motion. From geotechnical point of view, this contrast corresponds to the bedrock. Secondly, although Figs. 9 and 10 show a number of small depressions in the whole study area, two large depressions are found. First depression at the central part of the Kathmandu City denoted by I in Fig. 10, which is wider and deeper represents the main ancient lake of the Kathmandu Basin while other is along the eastern part denoted by II which is relatively shallow and its catchments area elongated from northwest to southeast. Similarly, there are a number of buried ridges which separate/connect the depressions. The longest buried ridge, which separates/connects the central large and deep depression with the eastern shallow depression extends from northwest to southeast part of the valley (Fig. 10). In order to verify the estimated sediment thickness distribution map of the Kathmandu Basin, the thickness variation profile along south–north direction (Fig. 11b, profiles P5–P170) through the center of the Kathmandu Basin was compared with the borehole exploration-based ground profile (Fig. 3) proposed by Sakai et al.
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show that the depth of the basement rock (or hard sediment) estimated from the result of microtremor observation is at 196 m below the surface at borehole BH1 and 188 m at borehole BH2. The difference in depth of the bedrock with the estimated value may be due to the change in basement topography abruptly in nearby areas of these boreholes. Due to marginal area of the bowl shaped Kathmandu Basin, the basement contour values also vary abruptly within a short distance in and around these borehole locations. Borehole BH2 lies at the edge of the Kathmandu Basin and a steep slope of basement topography is observed in west, north and south directions (Fig. 9). Moreover, the calculated depth of the basement rock at microtremor observation points near the borehole BH1 is found 260 m and near the borehole BH2, it is 233 m (refer Figs. 1c and 9, and Supplementary Table 2). This indicates that the depth estimated in this study provides comparatively accurate result for the basement topography of the Kathmandu Basin and confirms the conclusions of previous studies (e.g., Ibs-von Seht and 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 encourages the use of microtremor observations for an approximate estimation of sediment depth over wide basin areas. Fig. 12. Gravity contour map in the Kathmandu Valley (redrawn after Moribayashi and Maruo (1980))
5. Conclusions (2002). These two sections correlate well and show that the distribution of the sediment thickness based on this study is in good agreement with the distribution of the soft sediment proposed by Sakai et al. (2002). We further compared the basement topography estimated in this study with the gravity contour map (Fig. 12) prepared by Moribayashi and Maruo (1980) for the central area of the Kathmandu Basin. The basement topography obtained from the microtremor observations in this study (Fig. 9) is found quite similar with the results of the gravity survey conducted by Moribayashi and Maruo (1980) because the low gravity is obtained in the center of the valley where the thickness of the lacustrine sediments is high and gradually increases towards the marginal area where the sediment thickness is low. Moreover, the first depression along north to south through central part of the valley and second depression along northwest to southeast proposed in this study matched quite well with the gravity data as mentioned by Moribayashi and Maruo (1980). In order to compare the calculated depth of the basement rock based on microtremor observation with the actual depth of the Kathmandu Basin, two boreholes (BH1 and BH2) used also by Sakai et al. (2001) for academic purposes were considered, as shown in Fig. 13. According to Stöcklin and Bhattarai (1981) and Sakai et al. (2001), the basement of the Kathmandu Valley consists of weakly metamorphosed Phulchauki Group (Fig. 2), which consists of Paleozoic sandstone, phyllite and weathered rocks. Moreover, Sakai et al. (2001) have mentioned that almost all sand and gravels at the lower part of the core are composed of detritus to weakly metamorphosed sedimentary rocks derives from the underlying Kathmandu Complex. There is always confusion in the type of lithological layer below the clay (unconsolidated) layer; hence, it is always difficult to differentiate the sediment of bedrock with the basal conglomerate or gravelly soil in the Kathmandu Basin from the borehole data. Fig. 13a and b shows the lithostratigraphy in two boreholes (BH1 and BH2) (Sakai, 2001, and Sakai et al., 2001), in which the depth of the basement rock is shown at about 252 m in borehole BH1 (Fig. 13a) and thick layer of the sand is shown below 232 m in borehole BH2 (Fig. 13b). These figures also
In order to explore the hazard level as well as to estimate the risk of next expected earthquake disaster in the Kathmandu Valley, a study on the floor variation of the lacustrine sediments in the Kathmandu Basin was done. Due to lack of adequate and precise scientific studies on the floor variation of sediments in the basin, however, it is always difficult to ascertain their characteristics during earthquakes, which ultimately leads to erroneous and assumed data for ground modeling as well as analysis and design of the infrastructures. This study attempts to fill this gap by proposing an approximate basement topography of the Kathmandu Basin using microtremor observation at 172 locations. This study also enables to estimate the soft sediment variation in the Kathmandu Basin using a non-linear regression equation (h ¼ 146:01fr1:2079 ), and provides the hidden basement topography of the Kathmandu Basin. The sediment/rock below this basement topography may not take part for the amplification of the ground motion during earthquake in the Kathmandu Valley. The distribution of sediment indicates that the deepest part of the lake existed mainly in the central part of Kathmandu, where the main core city exists at present, and is one of the oldest residential areas in the Kathmandu Valley. It also accommodates a number of departmental stores, Government Offices, historical monuments including UNESCO cultural world heritage sites. Moreover, due to an increasing population and developing as a greater commercial hub, the central part has seen a sharp rise in the number of mid-height to tall buildings, which are constructed without adequate geotechnical investigation. Depending upon the type and stories of buildings, the predominant frequencies are different. The thickness distribution map shows that the main lake part (i.e. central part) has a considerable thickness of the soft sediments, and hence, it is prone to higher amplification of seismic wave at the corresponding predominant frequencies. The map thus depicted shall not be meant as a detailed and highly constrained representation of the valley bedrock; however, it represents the first reliable reconstruction of the subsurface morphology of the Kathmandu Basin, which shows a good consistency with available geological/log data. This study represents a useful starting point for future research and investigation
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tem’ (Team Leader: Ryuichi Yatabe, Ehime University, AY2009– AY2011) and supported financially by the Government of Japan under 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. References
Fig. 13. (a) Litho-stratigraphic section in central part of the Kathmandu Basin, on the basis of drill-cores (BH1) (refer Figs. 1c and 9) collected by the PaleoKathmandu Lake Project in 2000; (b) Litho-stratigraphic section of borehole BH2 (refer Figs. 1c and 9). Downward black arrows show the depth of the basement as per present study.
activities, such as detailed surveys, numerical modeling, and seismic hazard or microzonation studies. Acknowledgements The authors are grateful to Mr. Ramhari Dahal (The Department of Education, Government of Nepal) and Mr. Prakash Poudyal (Kathmandu University) for their help during the extensive microtremor survey in the Kathmandu Valley. The help provided by Ms. Manita Timilsina (PhD candidate, Graduate School of Science and Engineering, Ehime University) in preparing a few GIS-based maps is greatly appreciated. The authors would also like to express their special appreciation to Dr. Shinichiro Mori (Associate Professor, Graduate School of Science and Engineering, Ehime University) for enabling the first author to analyze the microtremor survey data. The authors also appreciate the comments and suggestions provided by the anonymous reviewers, which help to modify the manuscript. This is a part of the study entitled ‘Integrated approach to studying rain- and earthquake-induced disasters in the Himalayan Watersheds and development of a strategic disaster education sys-
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Journal of Asian Earth Sciences journal homepage: www.elsevier.com/locate/jseaes
Basement topography of the Kathmandu Basin using microtremor observation Youb Raj Paudyal ⇑, Ryuichi Yatabe, Netra Prakash Bhandary, Ranjan Kumar Dahal Geo-disaster Research Laboratory, Graduate School of Science and Engineering, Ehime University, Ehime, Matsuyama, Japan
a r t i c l e
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Article history: Received 30 March 2012 Received in revised form 22 October 2012 Accepted 5 November 2012 Available online 19 November 2012 Keywords: Kathmandu Valley Lacustrine sediments Microtremor Predominant frequency Basement topography
a b s t r a c t 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 172 sites spaced at a grid interval of 1 km. The results showed that the predominant frequency varies from 0.488 Hz to 8.9 Hz. 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. Ó 2012 Elsevier Ltd. All rights reserved.
1. Introduction Intensity of ground motion is a function of earthquake magnitude and distance from the seismic source, as well as local geological condition and topography of the area (Kramer, 1996). Although, geological structure of the area is an important factor, local site condition is known to have a great influence on the potential damage resulting from earthquakes (Seed and Idriss, 1969). Besides, geotechnical properties of the local soil site and behavior of the soil during earthquake depends upon the depth of the sedimentary column and its shear wave velocity. From geotechnical point of view, the shear wave velocity in the top 30 m column of soil is responsible for an unusual amplification of the ground motion (Finn, 1991). However, in the areas where the thickness of soft sediments is enormously large, like in the Kathmandu Basin, the amplification will rather depends on the extent of the soft sediment column and its elastic properties. Moreover, the fundamental phenomenon responsible for the amplification of seismic waves is due to the impedance contrast between sedimentary deposits and the underlying hard-strata or bedrock. Such site amplification can be estimated using microtremor measurement technique, which was first introduced by Kanai (1954). Several studies such as, Ohta et al. (1978), Lermo et al. (1988), Field et al. (1990), and Field and
⇑ Corresponding author. Address: Geo-disaster Research Laboratory, Graduate School of Science and Engineering, Ehime University, Bunkyo-3, Matsuyama 7908577, Japan. Tel./fax: +81 89 927 8566. E-mail address: [email protected] (Y.R. Paudyal). 1367-9120/$ - see front matter Ó 2012 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.jseaes.2012.11.011
Jacob (1993) have shown that microtremor analysis results revels the fundamental resonant frequency of sediments. One of the main challenges in determining the site amplification characteristics out of microtremor measurement is removing the source effects, which is often achieved by dividing the Fourier spectrum obtained on a soft ground point by that obtained on a nearby reference point on bedrock. For this, however, the microtremor source and path effects must be the same for both measurement points and the reference point or site must also have negligible site effects. To overcome this limitation, Nakamura (1989) has introduced a technique for estimating the site response by measuring solely the microtremor on the surface of the ground. According to him, the source effect can be removed by dividing the horizontal component of microtremor spectrum by the vertical component. Now, this technique has become widespread as a low-cost and effective tool to estimate the fundamental resonant frequency of sediments using Horizontal-to-Vertical (H/V) spectral ratio at a single-station. In his paper, Nakamura (1989) explains the use of this technique and gives a detailed explanation on the subsequent assumptions. In the last two decades, the H/V method has been widely used for various purposes, such as site effect evaluation, wave amplification estimation, liquefaction vulnerability assessment, sediment depth estimation, and microzonation studies in different geographical 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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an empirical relationship between fundamental resonant frequencies and sedimentary cover thickness has become practice since the work of Ibs-von Seht and Wohlenberg (1999) for Cologne area in Germany. They proposed that resonance frequency in H/V spectra correlate well with the overall soil thickness; ranging from tens of meters to more than 1000 m. Their developed relationships provide a practical means of sediment thickness estimation using a microtremor observation. Later, several studies, such as Delgado et al. (2000), Parolai et al. (2002), D’Amico et al. (2004), Hinzen et 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 microtremor H/V spectral ratio technique for measuring the thickness of soil cover over a hard stratum or bedrock. The present investigation is a first attempt to approximate the thickness 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 suffered great losses in the past earthquakes, such as in 1255, 1408, 1681, 1803, 1810, 1833, 1866, and 1934 (Rana, 1935; Chitrakar and Pandey, 1986; Bilham et al., 1995; Pandey et al., 1995; Upreti and Yoshida, 2009). The latest greatest earthquake of 1934 (Mw = 8.1) with a maximum intensity of X in Modified Mercally Intensity (MMI) Scale in the Kathmandu Basin reportedly killed about 4296 people and destroyed about 19% and damaged about 38% of the buildings in the basin alone (Rana, 1935; Pandey and Molnar, 1988). The observations of the damage pattern in the Kathmandu Basin during this earthquake, in particular, suggest that the spectral ground amplification due to fluvio-lacustrine sediments play a major role in intensifying the ground motion (Pandey and Molnar, 1988; Hough and Bilham, 2008). However, the valley basin still lacks adequate information on spatial variation of the lacustrine sediments. Moribayashi and Maruo (1980) estimated the basement topography of the central portion of the basin using gravitational method 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). In addition, the density contrast may not be constant everywhere for a basin like the Kathmandu Valley, where the thickness and properties of sediments vary significantly within the short distances. In this sense, the actual floor variation of the basement rock in a wider area of the Kathmandu Basin is still debatable. So, the objective of this study is to derive an empirical relationship between the resonance frequencies obtained from the H/V technique and the thickness of the lacustrine sediments, and then generate an approximate basement topography of the Kathmandu Basin. This information is critically important for earthquake ground motion simulation studies, especially because the densely populated urban area of the valley is under a great earthquake threat.
central part of the valley consist of weakly metamorphosed Phulchauki Group. The basin is filled with upper Pliocene to Quaternary clay, silt, sand and gravel (Moribayashi and Maruo, 1980; Yoshida and Gautam, 1988; Sakai, 2001) overlaying the Precambrian 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 sediments of lacustrine origin are extensively distributed within the Kathmandu Basin (Fig. 3). Kathmandu Valley has typical lacustrine sediments of its kind, which has attracted many geo-science researchers from various parts of the world. For example, Yoshida and Igarashi (1984), Dangol (1985), Fujii and Sakai (2002), and Sakai et al. (2002) have studied the depositional environment and stratigraphy of the sediment in the valley. Likewise, Katel et al. (1996), Dahal and Aryal (2002), and JICA (2002) have studied the engineering geological and geotechnical properties of the Kathmandu lacustrine sediment, and Rai et al. (2004) and Paudel (2010) have carried out the lithological and mineralogical evaluation of Kathmandu soils. More recently, Mugnier et al. (2011) have conducted a study on the seismic response of the Kathmandu Basin and they mention that the soft sediment deformation of the basin is mainly controlled by the fluidization of the silty layers during earthquake shaking. Most of these studies are based on the borehole data obtained by different agencies for various purposes, such as water supply project, and field observation. As these borehole cores were not recovered, the precise lithologic characteristics and stratigraphy were not confirmed (Sakai, 2001). Moreover, Sakai (2001) mentions that these previous studies faced several important problems of stratigraphic division and nomenclature of the formations, mainly because of lack of information on the subsurface geology and insufficient description on definition of each formation. To overcome this problem, Sakai et al. (2001) conducted a core drilling of the basin-fill sediments for the palaeoclimatic study of the Kathmandu Basin. This was the first large-scale drilling project in the valley with full core recovery, and solely dedicated to academic research purpose. Based on this study, they have divided the history of Palaeo-Kathmandu Lake into seven stages ranging from stage 1 – prior to the appearance of the lake to stage 7 – draining out of the lake-water (Sakai et al., 2001). Subsequent details of the different stages of changes of lithology and sedimentary facies of the sediment can be found in Sakai et al. (2001). Based on the available data from the previous study and paleoclimatic study of the Kathmandu Basin, Sakai (2001) has divided the sediments in the valley into three groups: (1) marginal fluvio-deltaic facies in the northern part, (2) open lacustrine facies in the central part, and (3) alluvial fan facies in the southern part, as shown in Fig. 1c.
3. Methodology 3.1. Field observations
2. Study area and geology Geologically, the Kathmandu Basin (Fig. 1) lies on the Kathmandu Nappe (Hagen, 1969; Upreti, 1999), which is located along the southern slopes of the Himalaya. It is one of the several tectonic intermontane basins developed in the Lesser Himalayan belt (Sakai et al., 2002) as shown in Fig. 2. The Kathmandu Nappe is composed of the Shivapuri Gneiss and marbles of the Bhimphedi Group (Stöcklin and Bhattarai, 1981). As illustrated in Fig. 2, the early Paleozoic Tethyan rocks, named as the Phulchauki Group, overlie the Bhimphedi Group in the Kathmandu region. Total thickness of both these groups attains 13 km (Stöcklin and Bhattarai, 1981). The northern slope of the Kathmandu Valley is mainly composed of gneiss, schist and granite, but the other slopes and the
Microtremor measurement survey was carried out at 172 1-km grid points in the study area (Fig. 1c) with the help of a portable velocity sensor. This sensor is capable of recording three components of vibration: two horizontal, i.e., east–west and north–south and one vertical (Fig. 4). At each survey point, the microtremor data were recorded for 300 s at a sampling frequency of 100 Hz (i.e., 30,000 samples at each point). Fourier analysis of each window (after removing unwanted noise) was carried out using Fast Fourier Transform (FFT) computer program, and the obtained spectra were smoothed using Parzen window of bandwidth 0.5 Hz. The average spectral ratio of the horizontal component of vibration to vertical (i.e., H/V) in each window was derived from the following equation (Delgado et al., 2000):
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(a) CHINA
(b) Nepal
INDIA
Kathmandu Valley
(c)
BH1 BH2
Major roads Microtremor observation points Main Rivers
Kathmandu Lalitpur Bhaktapur
Fluvio-deltaic facies Lacustrine facies
Borehole location
Fan depositions Talus deposits Basement Rocks Isolated basement rocks
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). A sediment distribution map of the Kathmandu Valley, microtremor measurement points and borehole location (BH1 and BH2) in the study area are shown (modified after Fujii and Sakai, 2002).
H=V ¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi ðF 2NS þ F 2EW Þ=ð2F 2UD Þ
ð1Þ
Here, FNS, FEW and FUD are the Fourier amplitude spectra in the north–south (NS), east–west (EW) and vertical (UD) direction, respectively. After deriving H/V spectral ratios for all windows of a point, the H/V ratio for the particular point was obtained by averaging all
those spectral ratios. Based on the fact that the frequency corresponding to the first peak of the H/V spectrum plot represents fundamental resonant frequency of the site (Field and Jacob, 1993; SESAME, 2004; Bonnefoy-Claudet et al., 2006), the site specific fundamental frequency for each measurement point was obtained. Typical result of microtremor data analysis and calculation of predominant frequency of the sites in some of the location of
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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: Main Boundary 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 Katel et al., 1996; Sakai et al., 2002).
Vel. (cm/s)
0.004 0.002
(a)
Noise
0 - 0.002 - 0.004 0.00
20.48
40.96
61.44
81.92
102.40
122.88
143.36
163.84
184.32
204.80
225.28
245.76
266.24
286.72
307.20
Time (T) Sec
Vel. (cm/s)
(b)
Noise
Time (T) Sec
Vel. (cm/s)
(c)
Noise
Time (T) Sec 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).
Kathmandu Basin are shown in Fig. 5. In this study, the fundamental resonant frequency of the soil layer is used to calculate the thickness of the soft sediments in the Kathmandu Basin. 3.2. Theoretical calculation Ibs-von Seht and Wohlenberg (1999) showed that the fundamental resonant frequency of the soil layer is closely related to thickness of the soil layer as given in the following equation: b
h ¼ af r
ð2Þ
where ‘h’ and ‘fr’ are the depth of the Quaternary sediments and fundamental resonant frequency, and ‘a’ and ‘b’ are the standard errors of the correlation coefficients. Ibs-von Seht and Wohlenberg (1999) have studied both parameters (i.e, h and fr) and demonstrated that it is possible to establish a direct functional relationship between them without knowing the shear wave velocity (Vs). They estimated the value of ‘a’ and ‘b’ and proposed an empirical relationship (Eq. (3)) between the fundamental resonant frequency (fr) and the thickness of soft sediment cover (h) (Quaternary sediments), based on 34 boreholes ranging in depth from 15 m to 1257 m and data from 102 seismic
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10
10
P 40
H/V Ratio
H/V Ratio
P 10
1
0.1 0.1
1
1
0.1 0.1
10
1
Frequency (Hz) 10
10
P 84
H/V Ratio
H/V Ratio
P 54
1
0.1 0.1
1
1
0.1 0.1
10
1
Frequency (Hz)
10
Frequency (Hz) 10
10
P 134
H/V Ratio
P 100
H/V Ratio
10
Frequency (Hz)
1
0.1 0.1
1
1
0.1 0.1
10
1
10
Frequency (Hz)
Frequency (Hz) 100 10
P 163
H/V Ratio
H/V Ratio
P 144
1
0.1 0.1
1
10
10
1
0.1 0.1
1
10
Frequency (Hz)
Frequency (Hz)
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 is referred to the web version of this article.)
stations in the western Lower Rhine Embayment in Germany, which is covered with Tertiary and Quaternary sediments overlaying Palaeozoic bedrock. Similarly, Parolai et al. (2002) developed an empirical relationship (Eq. (4)) between thickness of sediment with resonant frequency for the Cologne area in Germany based on 32 boreholes with a depth range from 20 m to 402 m and 337 data from seismic stations. Recently, Birgören et al. (2009) have derived yet another empirical relationship (Eq. (5)) between the thickness of Tertiary–Quaternary sediments overlying Palaeozoic bedrock and their resonance frequencies for the Istanbul region based on the H/V ratio from 15 measurements at the borehole locations and velocity profile of two microtremor array measurements sites. The results obtained by Birgören et al. (2009) show a very strong relationship (R2 value: 0.995) between the resonant frequency and the thickness of the sediment which varies from 20 m to 449 m. Similarly, Özalaybey et al. (2011) have investigated 3-D basin structures and site response frequencies in the Izmit Bay area of Turkey by microtremor measurement in 239
stations and 405 – point gravity measurements and derived an equation (Eq. (6)) for sediment cover in Izmit Basin in Turkey which has the sedimentary cover thickness about 1200 m at the deepest part.
h ¼ 96fr1:388
ð3Þ
h ¼ 108fr1:551
ð4Þ
h ¼ 150:99fr1:1531
ð5Þ
h ¼ 141fr1:27
ð6Þ
So as to map the soft sediment thickness in the Kathmandu Basin, we adopt terrain specific equations given by the above researchers. A theoretical thickness for the sediments of the Kathmandu Basin is calculated using above equations, based on the fundamental frequency (fr) obtained for each station using
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400
400
350
350
300
300
Thickness (m)
Thickness (m)
632
250 200 150 100 50 0
200 150 100 50
0
20
40
60
80
100
120
140
160
180
200
Microtremor observation points
microtremor observations. In this study, it is assumed that the H/V spectral ratio depends primarily on the source/site characteristics rather than geographical location. It has been observed that the variations of estimated depth, based on above four equations, were not much comparable with each other, and analysis clearly showed an average standard deviation of 41.88 m in thickness as shown in Fig. 6. In order to minimize the value of standard deviation and thereby to obtain the reliable results, the depth estimated based on the non-linear regression equation proposed by the four researchers were divided into two groups emphasizing the less variations of the estimated depth in each group. The standard deviation of each group is obtained as shown in Figs. 7 and 8. These figures show that the standard deviation (i.e., 48.55 m in thickness) of First Group (i.e. Ibsvon Seht and Wohlenberg, 1999; Parolai et al., 2002) (Fig. 7) is higher than (i.e., 7.44 m in thickness) the Second Group (Birgören et al., 2009; Özalaybey et al., 2011) (Fig. 8). The results also showed that 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 variations in the thickness due to the comparable geotechnical characteristics of the geological formation. Table 1 shows the comparative study of the geological characteristics of the geological formation of Kathmandu 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 the Kathmandu Basin as follows:
400 350 300 250 200 150 100 50 0
0
20
40
60
80
100
120
140
160
0
0
20
40
60
80
100
120
140
160
180
200
Microtremor observation points
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 the length of the line suggests deviation from the average.
Thickness (m)
250
180
200
Microtremor observation points 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 indicates the average value whereas the length of the line suggests deviation from the average.
Fig. 8. Comparison between depths calculated using Birgören et al. (2009) and Özalaybey et al. (2011) relationships (Eqs. (5) and (6)). The circle indicates the average value whereas the length of the line suggests deviation from the average.
h ¼ 146:01fr1:2079
ð7Þ
The obtained equation (i.e. Eq. (7)) is further used for obtaining primary information on the relative depth variation (refer Supplementary Table 2) of the interface between the two physically contrasting layers of lacustrine sediment and the underlain hard strata (or bedrock) in the Kathmandu Basin. This observation is validated by comparing the results of the gravity contour map proposed by Moribayashi and Maruo (1980) and also with the depth of the bedrock based on the borehole drilled for academic purposes (Sakai et al., 2001) in the Kathmandu Basin.
4. Results and discussion The results of this study are expressed in terms of thickness of the lacustrine sediment and its variation in different areas of the Kathmandu Basin. The sediment depth in various locations in the basin is calculated using Eq. (7). The contour map of the estimated soft sediment thickness and a digital elevation model (DEM) for the study area (Kathmandu Basin) are shown in Figs. 9–11. The calculated values give a deep interface of soft sediment (unconsolidated) and basement layer in the center of the Kathmandu Basin and shallow in and around the outskirts of the valley. The sudden abrupt change in the sediment thickness is found at points A (i.e. thickness of sediment about 48 m) and B (i.e. thickness of sediment about 30 m) (Fig. 9), which are about 2 km and 3 km along north and east from the central part of the Kathmandu respectively, which indicates the presence of basement rock in the shallow depth in those areas. The calculated depth of the interface between two layers is used to plot the cross-profiles and digital elevation model (DEM) for the Kathmandu Basin. Fig. 11a and b shows the profiles along west to east and south to north direction respectively. The west to east profiles (Fig. 11a) along P167–P175, P157–P165, P144–P154, P133–P143, P54–P72, P35–P53, P16–P34, and P1–P15 show gentle slope of basement topography, whereas the profiles along P122– P132, P111–P121, P92–P110, and P73–P91 show steeper slope and increase in the soft sediment thickness mainly towards the center location. The variation of the soft sediment thickness in the valley can also be described using Fig. 11b in which the profiles along P3–P168, P4–P169, P5–P170, and P6–P171 give information about the distribution of sediment thickness along south–north direction, and they clearly indicate a steeper slope of the basement floor and increase of sediment thickness towards the center. The DEM of the hard stratum further reveal that the thicknesses of the sediment in depression (I) (refer Figs. 1c and 10 and Supplementary Table 2) at points P94, P95, P96, P114, P115, P116,
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Y.R. Paudyal et al. / Journal of Asian Earth Sciences 62 (2013) 627–637 Table 1 Comparative 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 and Maruo (1980) Lacustrine and fluvial in origin Katel et al. (1996); Sakai et al. (2001) 347 (current study)
Paleozoic rock Karkas and Coruk (2010) Quaternary alluvial and fluvial deposits Karkas and Coruk (2010) 1200 Özalaybey et al. (2011)
Palaeozoic rocks Ündül and Tugrul (2006) Halic and Bosphorus sediments Ündül and Tugrul (2006) 449 Birgören et al. (2009)
188–310 JICA (2002)
180–360 Zor et al. (2010)
80–375 Bozdag and Kocaoglu (2005)
0.448–8.89 (current study) 2–42 JICA (2002)
0.23–5 Özalaybey et al. (2011) 2–43 Karkas and Coruk (2010)
0.44–5 Birgören et al. (2009) 5–>50 Dalgic (2004)
2.34–2.77 Katel et al. (1996) 30–108 Katel et al. (1996) 5–43 Katel et al. (1996)
2.55–2.78 Sawicki and Swidzinski (2006) 33–66 Olgun et al. (2008) 10–37 Olgun et al. (2008)
2.42–2.79 Ündül and Tugrul (2006) 35–98 Ündül and Tugrul (2006) 7–50 Ündül and Tugrul (2006)
Sediment Maximum estimated depth of soft sediment (m) Shear wave velocity of sediment up to 30 m (m/s) Predominant frequency (Hz) Variation of SPT N value up to 30 m Specific gravity of soil Liquid limit (%) Plasticity index (%)
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 over Bedrock-soft sediment surface forming the sites of thickest deposit in the study area.
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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.
P117, P125, P126, P127, P128 show relative depth variations of 260 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 distribution in the basin is far from uniform and have an undulating topography with steep relief in many locations in the basin. Hagen (1969) mentioned that during the pre-lake formation in the Kathmandu Basin, drainage systems originating in the northern slope of the Shivapuri hill and termed as ‘‘Proto Bagmati River’’ were very active. These river systems were responsible for the deposition of coarse-grained sediments (gravels and coarse sand) below the lake deposits in the entire valley. The influence of Proto Bagmati river systems appear more in the northern and central part than in the other part of the valley. According to Yoshida and Igarashi (1984), this deposition took place some 2.5 Ma ago (i.e., during middle to late Pliocene period). After analyzing the digital elevation model of the basement topography in the Kathmandu Basin, there arise mainly two possible explanations. Firstly, the calculated depth of the sediment represents the total depth of lake deposit which is underlain by
basement rock in the Kathmandu Basin. The calculated depth may not necessarily indicate the presence of hard rock, rather it is a representation of the presence of basement layer at that depth, beyond which the sediment do/may not contribute to the amplification of the ground motion. From geotechnical point of view, this contrast corresponds to the bedrock. Secondly, although Figs. 9 and 10 show a number of small depressions in the whole study area, two large depressions are found. First depression at the central part of the Kathmandu City denoted by I in Fig. 10, which is wider and deeper represents the main ancient lake of the Kathmandu Basin while other is along the eastern part denoted by II which is relatively shallow and its catchments area elongated from northwest to southeast. Similarly, there are a number of buried ridges which separate/connect the depressions. The longest buried ridge, which separates/connects the central large and deep depression with the eastern shallow depression extends from northwest to southeast part of the valley (Fig. 10). In order to verify the estimated sediment thickness distribution map of the Kathmandu Basin, the thickness variation profile along south–north direction (Fig. 11b, profiles P5–P170) through the center of the Kathmandu Basin was compared with the borehole exploration-based ground profile (Fig. 3) proposed by Sakai et al.
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show that the depth of the basement rock (or hard sediment) estimated from the result of microtremor observation is at 196 m below the surface at borehole BH1 and 188 m at borehole BH2. The difference in depth of the bedrock with the estimated value may be due to the change in basement topography abruptly in nearby areas of these boreholes. Due to marginal area of the bowl shaped Kathmandu Basin, the basement contour values also vary abruptly within a short distance in and around these borehole locations. Borehole BH2 lies at the edge of the Kathmandu Basin and a steep slope of basement topography is observed in west, north and south directions (Fig. 9). Moreover, the calculated depth of the basement rock at microtremor observation points near the borehole BH1 is found 260 m and near the borehole BH2, it is 233 m (refer Figs. 1c and 9, and Supplementary Table 2). This indicates that the depth estimated in this study provides comparatively accurate result for the basement topography of the Kathmandu Basin and confirms the conclusions of previous studies (e.g., Ibs-von Seht and 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 encourages the use of microtremor observations for an approximate estimation of sediment depth over wide basin areas. Fig. 12. Gravity contour map in the Kathmandu Valley (redrawn after Moribayashi and Maruo (1980))
5. Conclusions (2002). These two sections correlate well and show that the distribution of the sediment thickness based on this study is in good agreement with the distribution of the soft sediment proposed by Sakai et al. (2002). We further compared the basement topography estimated in this study with the gravity contour map (Fig. 12) prepared by Moribayashi and Maruo (1980) for the central area of the Kathmandu Basin. The basement topography obtained from the microtremor observations in this study (Fig. 9) is found quite similar with the results of the gravity survey conducted by Moribayashi and Maruo (1980) because the low gravity is obtained in the center of the valley where the thickness of the lacustrine sediments is high and gradually increases towards the marginal area where the sediment thickness is low. Moreover, the first depression along north to south through central part of the valley and second depression along northwest to southeast proposed in this study matched quite well with the gravity data as mentioned by Moribayashi and Maruo (1980). In order to compare the calculated depth of the basement rock based on microtremor observation with the actual depth of the Kathmandu Basin, two boreholes (BH1 and BH2) used also by Sakai et al. (2001) for academic purposes were considered, as shown in Fig. 13. According to Stöcklin and Bhattarai (1981) and Sakai et al. (2001), the basement of the Kathmandu Valley consists of weakly metamorphosed Phulchauki Group (Fig. 2), which consists of Paleozoic sandstone, phyllite and weathered rocks. Moreover, Sakai et al. (2001) have mentioned that almost all sand and gravels at the lower part of the core are composed of detritus to weakly metamorphosed sedimentary rocks derives from the underlying Kathmandu Complex. There is always confusion in the type of lithological layer below the clay (unconsolidated) layer; hence, it is always difficult to differentiate the sediment of bedrock with the basal conglomerate or gravelly soil in the Kathmandu Basin from the borehole data. Fig. 13a and b shows the lithostratigraphy in two boreholes (BH1 and BH2) (Sakai, 2001, and Sakai et al., 2001), in which the depth of the basement rock is shown at about 252 m in borehole BH1 (Fig. 13a) and thick layer of the sand is shown below 232 m in borehole BH2 (Fig. 13b). These figures also
In order to explore the hazard level as well as to estimate the risk of next expected earthquake disaster in the Kathmandu Valley, a study on the floor variation of the lacustrine sediments in the Kathmandu Basin was done. Due to lack of adequate and precise scientific studies on the floor variation of sediments in the basin, however, it is always difficult to ascertain their characteristics during earthquakes, which ultimately leads to erroneous and assumed data for ground modeling as well as analysis and design of the infrastructures. This study attempts to fill this gap by proposing an approximate basement topography of the Kathmandu Basin using microtremor observation at 172 locations. This study also enables to estimate the soft sediment variation in the Kathmandu Basin using a non-linear regression equation (h ¼ 146:01fr1:2079 ), and provides the hidden basement topography of the Kathmandu Basin. The sediment/rock below this basement topography may not take part for the amplification of the ground motion during earthquake in the Kathmandu Valley. The distribution of sediment indicates that the deepest part of the lake existed mainly in the central part of Kathmandu, where the main core city exists at present, and is one of the oldest residential areas in the Kathmandu Valley. It also accommodates a number of departmental stores, Government Offices, historical monuments including UNESCO cultural world heritage sites. Moreover, due to an increasing population and developing as a greater commercial hub, the central part has seen a sharp rise in the number of mid-height to tall buildings, which are constructed without adequate geotechnical investigation. Depending upon the type and stories of buildings, the predominant frequencies are different. The thickness distribution map shows that the main lake part (i.e. central part) has a considerable thickness of the soft sediments, and hence, it is prone to higher amplification of seismic wave at the corresponding predominant frequencies. The map thus depicted shall not be meant as a detailed and highly constrained representation of the valley bedrock; however, it represents the first reliable reconstruction of the subsurface morphology of the Kathmandu Basin, which shows a good consistency with available geological/log data. This study represents a useful starting point for future research and investigation
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tem’ (Team Leader: Ryuichi Yatabe, Ehime University, AY2009– AY2011) and supported financially by the Government of Japan under 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. References
Fig. 13. (a) Litho-stratigraphic section in central part of the Kathmandu Basin, on the basis of drill-cores (BH1) (refer Figs. 1c and 9) collected by the PaleoKathmandu Lake Project in 2000; (b) Litho-stratigraphic section of borehole BH2 (refer Figs. 1c and 9). Downward black arrows show the depth of the basement as per present study.
activities, such as detailed surveys, numerical modeling, and seismic hazard or microzonation studies. Acknowledgements The authors are grateful to Mr. Ramhari Dahal (The Department of Education, Government of Nepal) and Mr. Prakash Poudyal (Kathmandu University) for their help during the extensive microtremor survey in the Kathmandu Valley. The help provided by Ms. Manita Timilsina (PhD candidate, Graduate School of Science and Engineering, Ehime University) in preparing a few GIS-based maps is greatly appreciated. The authors would also like to express their special appreciation to Dr. Shinichiro Mori (Associate Professor, Graduate School of Science and Engineering, Ehime University) for enabling the first author to analyze the microtremor survey data. The authors also appreciate the comments and suggestions provided by the anonymous reviewers, which help to modify the manuscript. This is a part of the study entitled ‘Integrated approach to studying rain- and earthquake-induced disasters in the Himalayan Watersheds and development of a strategic disaster education sys-
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