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Seismic site classification and site period mapping of Chennai City using geophysical and geotechnical data
Journal of Applied Geophysics 72 (2010) 152–168
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Journal of Applied Geophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j a p p g e o
Seismic site classification and site period mapping of Chennai City using geophysical and geotechnical data R. Uma Maheswari a, A. Boominathan a,⁎, G.R. Dodagoudar a a
Department of Civil Engineering, Indian Institute of Technology Madras, Chennai-600036, India
a r t i c l e
i n f o
Article history: Received 13 November 2009 Accepted 18 August 2010 Keywords: Multichannel Analysis of Surface Waves Shear wave velocity Site period Site classification Seismic microzonation
a b s t r a c t Subsurface conditions play a major role in the damage potential of earthquakes and the seismic soil amplification of a site which is a critical factor affecting the level of ground shaking. Shear wave velocity (Vs) of the soil layer is an important parameter influencing the amplification behaviour of the site. Site characterization in calculating seismic hazards is usually based on the near-surface shear wave velocity values. The average shear wave velocity of the top 30 m of the soil, referred to as (Vs)30 is commonly adopted by competent building codes to classify the sites for earthquake resistant design of structures and in general it is widely used in microzonation studies. In the present study, the shear wave velocity of soil layers was measured at 30 locations in Chennai City by Multichannel Analysis of Surface Waves (MASW) test. In addition, nearly 300 borehole data were used to estimate Vs based on the correlations between Vs and SPT-N values for Chennai developed by authors earlier. Merging of MASW test results with borehole data yields sufficient coverage of Vs to develop a new site classification map for Chennai based on the NEHRP standard. It is found that part of the city belong to site D category (stiff soil). The developed site period map reveals that the fundamental site period varies in the range of 0.03 to 0.6 s, thus the soil conditions in Chennai pose a potential threat during earthquakes to low rise buildings (less than 6 storeys) which are densely distributed throughout the city. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Local geological conditions generate significant amplification of the ground motion causing damage during earthquakes. Usually the younger and softer soils amplify the ground motion more strongly than older and more consolidated soils. There are numerous works that show strong correlations between the ground motion during earthquakes and the average shear wave velocity (e.g., Park and Elrick, 1998; Stewart et al., 2003). A simple way to characterize the site conditions is by estimating the shear wave velocity of the deposit. The average shear wave velocity of the top 30 m i.e., (Vs)30 is the most widely used parameter to predict the potential amplification of sites during ground shaking (Borcherdt 1994; Dobry et al., 2000; Holzer et al., 2005; Kanli et al., 2006; Anbazhagan and Sitharam, 2008). This parameter has been used in recent building codes (EC8, 2003; BSSC, 2009; IBC, 2009). (Vs)30 based site characterization is appropriate to sites having relatively shallow “seismic bedrock” or very firm soil conditions and flat stratigraphy (Pitilakis, 2004). In areas of low seismicity and lack of strong ground motion records, the correlation
⁎ Corresponding author. Postal address: Department of Civil Engineering, Indian Institute of Technology Madras, Chennai-600036, India. Tel.: + 91 44 22574273; fax: + 91 44 22570545. E-mail addresses: [email protected] (R.U. Maheswari), [email protected] (A. Boominathan), [email protected] (G.R. Dodagoudar). 0926-9851/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2010.08.002
between surface geology, borehole data and shear wave velocity measurements is one of the means to classify the distribution of seismic site effect. This approach was used to generate local and regional maps (Wills and Silva, 1998; Wills et al., 2000) according to site categories of the National Earthquake Hazards Reduction Program (NEHRP) and to predict amplification factors. The study area, Chennai is located in southeast of India, and is characterized by the coastal plains of Bay of Bengal. Southern Peninsular India is generally regarded as a stable continental region, characterized by medium seismicity with a lack of strong ground motion records. The major historical seismic events in and around Chennai were on 10 December 1807 (MMI VI), 29 January 1822 (MMI VI), 2 March 1823 (MMI VI), 3 July 1867 (MMI VII), 10 April 1966 (MMI VI) and 26 September 2001 (Mw 5.5). Other than these earthquakes, a few tremors have been felt in the city during the Pondicherry earthquake of magnitude, Mw 5.5 on 25 September 2001 centered 100 km from Chennai and Sumatra earthquake of magnitude, Mw 9.1 on 26 December 2004. The primary objective of this paper is to demonstrate the use of combined surface wave and borehole data for deriving subsurface characteristics in terms of average shear wave velocity within 30 m depth (Vs)30 for regions characterized by shallow sediments such as the Chennai City. The development of seismic site classification maps by (Vs)30 scheme and the fundamental site period map for earthquake hazard assessment of Chennai is also presented.
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2. Geological settings of the study area Chennai is the fourth largest city in India, covers an area of 172 km2 and is located between 12.75°–13.25° N and 80.0°–80.5° E on the southeastern coast of India. The base map of the study area prepared using ArcGIS 9.3 is presented in Fig. 1. The general geology of the city is composed mostly of sand, clay, shale and sandstone as shown in Fig. 2 (GSI, 1999). The surface geology of the study area and its surroundings are reported in Ballukraya and Ravi (1994); Seismotectonic Atlas of India. (2000) and Subramanian and Selvan (2001). Chennai is underlain by various geological formations from ancient Archeans to recent alluviums. They are grouped into three categories: (i) Archean crystalline metamorphic rocks (consolidated), (ii) upper Gondwanas composed of sandstones, siltstones and shales, tertiary (Eocene to Pliocene) sandstones (semi consolidated) and (iii) coastal and river alluvium (unconsolidated). The geology of the study area consists of shallow bedrock on the east and south, and Gondwanas below the alluvium in the north and west. Almost the entire area is covered by the pleistocene/recent alluvium, deposited by the Cooum and Adyar rivers. The coastal region of the city is fully covered by marine sediments. 3. Geomorphological settings Chennai located on the coastal plains has a flat topography with very gentle slope towards east. The altitudes of land surface vary
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from 10 m above MSL in the west to sea level in the east. The elevation data of the city were obtained from the USGS and Shuttle Radar Topography Mission (SRTM). The SRTM data were used to generate a digital topographic map of the Earth's land surface with data points spaced every 3 arc second for global coverage of latitude and longitude (approximately 90 m). The elevation data have a horizontal accuracy of 20 m and a vertical accuracy of ± 10 m and was used to assess the relative elevation of sites (USGS, 2000). The Digital Elevation Model (DEM) developed for the study area is shown in Fig. 3(a) which illustrates the topographic differences in the region. The 3D DEM was prepared from the SRTM (Shuttle Radar Topography Mission) data by: (a) converting a DEM ASCII file to an ArcGIS Grid format using the ArcTool box, (b) converting raster data into TIN by using raster to TIN in 3D analyst, (c) opening the newly converted raster file in ArcScene, (d) adding base heights to show 3D variation and (e) adding a vertical exaggeration to get the 3D topology. The 3D elevation model obtained for the study area using 3D analyst tool available in ArcMap 9.3 is shown in Fig. 3(b). It is evident from Fig. 3(a) and (b) that the city has an average elevation of 9 m with the lowest and highest points being 1 and 20 m from MSL respectively. Selected sample values from SRTM data presented in Table 1 are cross checked with elevation contour and bench marks provided in toposheets of GSI and it confirms the range of elevation values. The geomorphology map of the Chennai City shown in Fig. 4, indicates presence of alluvial and coastal plains, beach and beach
Fig. 1. Map of the study area.
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Fig. 2. Geological map of the study area.
Fig. 3. Elevation map: (a) 2D and (b) 3D.
R.U. Maheswari et al. / Journal of Applied Geophysics 72 (2010) 152–168 Table 1 Comparison of SRTM elevation contour values with GSI toposheets. Latitude
Longitude
SRTM data (m)
Toposheet contour data (m)
80.269 80.294 80.279 80.259
13.107 13.103 13.075 13.066
3–6 3–6 3–6 6–9
6.3 4.5 5.4 5.7
ridges, pediments, lagoon/back water, marsh, paleo channel, sedimentary high ground, tertiary plain and structural hills. Fluvial, marine and erosional landforms are also noticed in the study area. Marine transgression and neotectonic activity during the recent past have influenced the morphology resulting in various present landforms. Meandering streams with small sand bars are deposited along the course of Adyar river. Marina beach is the second longest natural beach in the world with width varying from 150 to 600 m from the coast and length of about 5.6 km. 4. Bedrock and subsurface profiling Nearly 300 borelogs from the reputed geotechnical agencies including deep borelogs from Public Works Department (PWD) of Tamil Nadu state, and Central Public Works Department (CPWD)
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were collected to establish the subsurface profile of the study region and their distribution in the study area is shown in Fig. 5. The depth to bedrock contour map established for the study area is presented in Fig. 6. The bedrock in the southern part is characterized by igneous and metamorphic rocks; marine sediments containing claysilt sands and charnockites are encountered in the eastern and northern parts, and the western parts are composed of sedimentary rocks. The depth to bedrock varies from 15 to 30 m along the eastern coast of the study area and it increases from east to west. The depth to bedrock decreases from north to south. Based on the collected borelogs, the overburden thickness of soil was mapped using the spatial analyst tool available in the ArcMap. The data were interpolated using natural neighborhood technique and are shown in Fig. 7. The thickness of the soil formation ranges from a few meters to 9 m in the southern part to as much as 31 m in the central part of the city. The thickness of the soil formation ranges from 12 to 18 m in the northern part. The detailed subsurface geological stratum at different crosssections of the study area was mapped using the deep borelogs and is shown in Fig. 8. The west–east oriented cross-sections A-A′ and B-B′ show more details including the variations of the geometry due to the intrusion of sandstone and shale as shown in Fig. 8(a) and (b) respectively. In these sections, the western part is predominantly dominated by clay/sandy clay and is followed by shale with intrusion
Fig. 4. Geomorphological map of the study area.
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Fig. 5. Borehole locations and MASW test locations.
Fig. 6. Depth to bedrock contour map.
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Fig. 7. Overburden thickness map.
of sandstone. However, the eastern part is dominated by sand/clayey sand and immediately followed by charnockite-granitic gneiss basement rock. The variation of thickness of the unconsolidated coastal/river sediments along the eastern coast is shown by the geological crosssections (C-C′) and (D-D′) constructed from the borehole data [Fig. 8(c) and (d)]. The section C-C′ is dominated by sand/clayey sand up to a depth of 25 to 30 m near to the coastal side. Similarly the section D-D′ is predominantly dominated by sand/clayey sand to a depth of 10 to 15 m with intrusion of clay/sandy clay layer which is observed away from the coastal side. The charnockitegranitic gneiss type of rock is immediately followed by alluvium sediments. Similarly, the west–south oriented cross-section E-E′ is shown in Fig. 8(e) with detailed subsurface geological stratum. 5. Evaluation of (Vs)30 Elastic properties of near-surface materials and their effects on seismic wave propagation are very important in earthquake engineering activities and in environmental and earth sciences studies. The increase of amplitudes in soft sediments is one of the most important factors responsible for the amplification of earthquake motions. Amplification is proportional to 1 pffiffiffiffiffiffiffiffiffiffi Vs :ρ
ð1Þ
where Vs is the shear wave velocity and ρ is the density of the investigated soil (Aki and Richards, 1980). Since density is relatively constant with depth, the Vs value can be used to represent site conditions. A thorough assessment of shallow shear wave velocity is crucial for earthquake hazard assessment studies (Wald and Mori, 2000). The average shear wave velocity of the upper 30 m (Vs)30 is computed in accordance with the following expression: ðVs Þ30 =
N
∑
i=1
30 .
ð2Þ
hi
Vi
where hi and Vi denote the thickness (in meters) and shear wave velocity (at a shear strain level of 10− 5 or less) of the ith formation or layer, in a total of N layers existing in the top 30 m of the deposit. In this study, (Vs)30 was determined from seismic surface wave test and based on the shear wave velocity estimated from SPT-N. 6. Geophysical investigations—surface wave method The most commonly used seismic method for shear wave velocity profiling is the surface wave method. In general for microzonation studies, the following two methods are preferred for the seismic site characterization: Spectral Analysis of Surface Waves (SASW) and
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Fig. 8. Cross-sectional profiles at (a) A-A′, (b) B-B′, (c) C-C′, (d) D-D′ and (e) E-E′.
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Multichannel Analysis of Surface Waves (MASW) tests (Nazarian et al., 1983; Stokoe et al., 1994; Park et al., 2002; Xu et al, 2006). Of all the types of seismic waves, surface waves have the strongest energy by virtue of which they have the highest signal to noise ratio (Park et al., 2002) making it a powerful tool for near-surface characterization. In the present study, MASW tests were performed to establish the shear wave velocity profile. In the MASW test, the motion generated by an impact source is detected simultaneously at several receiver locations and the corresponding signals are analyzed as a whole using double Fourier transform. Raw field data are transformed into the frequency–wave number (f–k) domain where phase velocities of Rayleigh waves are calculated to produce a dispersion curve. Then the calculated dispersion curve is inverted to estimate the Vs profile. The MASW tests were carried out at 30 sites in Chennai City (Fig. 5), so as to obtain representative regional estimates of average velocity-depth profiles throughout the city. The field configuration of MASW test is presented in Fig. 9. Tests were carried out using Geometrics make 24 channels Geode seismic recorder with single geode operating software (SGOS). The vertical geophones of natural frequency 4.5 Hz (24 numbers) were used to receive the wave fields generated by an active source of 8 kg sledgehammer. Although the acquisition of ground roll data seems to be an easy task but the field configurations need to be optimized for the requirement: geophone spacing and the offset range. The planar characteristics of surface waves evolve only after a certain distance from the source. In most cases, this distance needs to be greater than the half of the maximum desired wavelength (Stokoe et al., 1994). On the other hand, the amplitude of body wave and higher mode Rayleigh wave may dominate over the fundamental mode at high frequencies. The relationship between the energy of fundamental and higher modes is a complex function of layer parameters and offset but, as a rule of thumb, the maximum distance between the source and the furthest geophone should not exceed 100 m (Park et al., 2002). Twenty four geophones were deployed in a linear pattern with an equal receiver spacing in the range of 0.5 to 2 m interval with the nearest source to geophone offset in the range of 5 to 30 m to meet the requirement of different types of soil as suggested by Xu et al. (2006). These configurations were used depending on the characteristics of the site to obtain a realistic velocity structure, mainly at places where no information about thickness was available. The source and each receiver were connected to an individual recording channel as shown in Fig. 9. The test was repeated with applying source at the front, middle and end of the receivers to get the consistency of the field data. The main parameters of data acquisition are given in Table 2.
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Table 2 Data acquisition parameters. Parameter
Range
Number of channels Geophone spacing Sampling rate Record length Receiver Source Minimal source offset
24 0.5–2 m 0.25 ms 1s 4.5 Hz vertical (spikes or plates) 8 kg hammer (on metal plate) 5 to 30 m
A typical raw wiggle plot obtained from the test is shown in Fig. 10 (a). The data processing of wiggle plot consists of two main steps: (i) obtaining the dispersion curves of Rayleigh wave phase velocity from the records and (ii) determining the Vs profiles from which the (Vs)30 values are calculated. The data processing was carried out in accordance with the characteristics of field records. First, the records were muted to reduce the effect of random noise and the interference with other wave types [Fig. 10(b)]. After muting, only the surface wave component (jumping up) is used for the dispersion curve analysis. After careful analysis of the records only the fundamental mode of Rayleigh waves (from 4–5 to 30–50 Hz) was investigated. The acquired wave data were processed using the SurfSeis software (SurfSeis2, 2006) to develop experimental dispersion curve with signal to Noise (S/N) ratio as shown in Fig. 11. The dispersion curves before and after filtering are shown in Fig. 11(a) and (b) respectively. The wiggle plots and corresponding dispersion curves obtained for different source offsets such as 5, 10 and 20 m with S/N ratios are shown in Fig. 12(a), (b) and (c) respectively. Fig. 12 shows that the dispersion curve corresponds to source offset of 10 m and gives higher S/N ratio compared to other source offsets. The experimental dispersion curve was subjected to inversion analysis to develop one- and two-dimensional shear wave velocity profiles as shown in Fig. 13(a) and (b) respectively. At each location, MASW tests were repeated by varying the field parameters such as source offset and geophone spacing to arrive the shear wave velocity profile with greater accuracy with maximum signal to noise ratio. Based on these trials of tests carried out at various sites the optimum parameters for the MASW tests were arrived as: source offset of 10 to 15 m and geophone spacing of 1 m for a seismic array of 40 to 50 m. In case, where the MASW test conducted using optimum parameters were not able to procure data to the required depth of 30 m, the source offset was increased accordingly. By combining the analysis of different array length field data, it was possible to obtain shear wave velocity
Fig. 9. Field configuration of the MASW test.
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Fig. 10. Typical wiggle plots obtained from the MASW test: (a) raw and (b) after filtering.
Fig. 11. Typical dispersion curves with signal to noise (S/N) ratio: (a) raw and (b) after filtering.
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values up to the required depth with relatively better accurately at deeper depths of up to 30 m. 7. Estimation of shear wave velocity using borehole data It is preferable to determine shear wave velocity directly from field tests, but it is often not economically feasible to make Vs measurements at all locations. However to effectively utilize the available borelog in the study area, the shear wave velocity is also estimated
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using the following correlation between Vs (m/s) and uncorrected SPT-N values developed by the authors for three categories of soils in Chennai, the details of which can be found in the study of Uma Maheswari et al. (2010): For All Soils : Vs = 95:64N For Sand : Vs = 100:53N
0:301
0:265
2 ; r = 0:83
2 ; r = 0:84
Fig. 12. Typical wiggle plots and dispersion curves correspond to source offset of (a) 5 m, (b) 10 m and (c) 20 m.
ð3Þ ð4Þ
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Fig. 13. Typical shear wave velocity profile: (a) 1D and (b) 2D (Egmore site).
Fig. 14. (Vs)30 distribution map of Chennai.
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Fig. 15. Average shear wave velocity distribution maps down to (a) 5 m (Vs)5, (b) 10 m (Vs)10 and (c) 20 m (Vs)20 depths.
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Fig. 15 (continued ).
For Clay : Vs = 89:31N
0:358
2 ; r = 0:93
ð5Þ
Merging the seismic surface wave results with the borehole data yields sufficient coverage of (Vs)30 for the development of shear wave velocity distribution map for Chennai. If the number of data points is more, spatial variability will be less and accuracy will be more. The above correlation between SPT-N value and shear wave velocity is used to obtain the (Vs)30 where shear wave velocity profile is not available. Similar kind of merging MASW tests with the borehole data has been carried out for the development of the NEHRP maps for Orleans suburb of Ottawa, Ontario (Motazedian and Hunter, 2008). 8. Shear wave velocity distribution The shear wave velocity data obtained from the field experiments and that estimated from the empirical correlations developed by the authors are used for development of shear wave velocity distribution and site classification maps for Chennai City. Merging the seismic surface wave results with the borehole data yields sufficient coverage of Vs for the development of shear wave velocity distribution maps for Chennai (Fig. 14). It is noted that three different velocity zones exist in the study area. The high and medium shear wave values correspond to the shallow bedrock zone and outcrop zone while lower ones correspond to the alluvial basin. The northeastern and a few locations in the western part (blue coloured zone) are
characterized by medium shear wave velocity values (between 200 and 250 m/s). The transition zone (pink coloured) is characterized by a relatively high velocity zone (between 250 and 350 m/s) and spreads in the northwestern and central parts. The shallow bedrock zone in the southern part of the study area has the highest shear wave velocity values between 350 and 1000 m/s. In the (Vs)30 distribution map, the velocity zones coincide with the geotechnical, geological and geophysical investigations. Highest velocity is encountered in the southern part associated with the shallow bedrock geologically defined as Archean deposits. The zones which have medium and relatively high shear wave velocity represent the coastal/alluvium sediments. To understand variation in the shear wave velocity distribution with depth, the average velocity distribution maps were obtained at three different depths, 5, 10 and 20 m (Fig. 15). It can be observed from Fig. 15(a) that the average shear wave velocity for a depth of 5 m ranges from 100 to 450 m/s. The northeastern part of Chennai has an average shear wave velocity of 100–150 m/s, which indicates soft soil in the upper 5 m layer. The average shear wave velocity for the 10 m depth varies from 100 to 600 m/s [Fig. 15(b)]. In the 10 m average shear wave velocity map, very dense soil/soft rock with velocity ranging from 360 to 600 m/s is found in the southern part of the study area. In this location, the bedrock depth is found within 10 m. The 20 m depth map [Fig. 15(c)] shows a greater coverage of very dense soil/soft rock when compared to the 10 m depth map [Fig. 15(b)]. The northeastern and a few locations in the western parts have an average
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shear wave velocity less than 200 m/s indicating soft soil over a depth of 20 m. In general, there is an abrupt change in the shear wave velocity values at deeper depths because of intrusion of dense soil/soft rock. 9. Seismic site classification maps The seismic site classification map developed for Chennai based on NEHRP categorization (BSSC, 2009) is shown in Fig. 16. It shows contour lines for (Vs)30 with lines of 180, 360 and 760 m/s representing the boundaries of the NEHRP zones. The seismic site classification zones for the Chennai area, as shown in Fig. 16 are directly related to geological settings and soil thickness. High velocity areas are associated with shallow bedrock (zone B) which is encountered in the southernmost part of the city. The very dense soil and soft rock at relatively shallow bedrock areas are identified with zone C at southern part. The most part of the city characterized by stiff soil, belongs to the zone D category. The amplification rating based on NEHRP standard indicates that the city would be subjected to low to medium amplification. (Vs)30 based site classification map considers the shear wave velocity of both the soil and the rock below, thus does not represent the actual shear wave velocity variation of overburden soil. The classification map developed based on the average shear wave velocity for overburden soil is presented in Fig. 17. It shows that the study area has shear velocity of overburden ranging from 100 to
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350 m/s. Fifty percent of the city has shear wave velocity in the range of 100–200 m/s. The site classification maps based on (Vs)30 (Fig. 16) and (Vs)overburden (Fig. 17) are identical in the central region. Whereas, the northern part and the southern part of the city have lower average shear wave velocity in comparison to the NEHRP classification map. 10. Site period map The fundamental site period (Ts) corresponds to the first mode of vibration of the soil deposit which is used by several investigators as one of the parameters for seismic microzonation. It is governed by the thickness and shear wave velocity of the soil layer and is calculated using the following expression (Kramer, 1996): Ts =
4H
. Vs
ð6Þ
where H is the total thickness of alluvium sediments and Vs is the average shear wave velocity of the overburden soil. The developed site period map presented in Fig. 18 indicates that the southern part of the city has the lowest site period ranging from 0.03 to 0.2 s (corresponding to frequency of 5–33 Hz) due to lesser thickness of the overburden. The site period in other parts of the city ranges from 0.2 to 0.6 s (corresponding to frequency of 1.6–5 Hz) due to the presence of relatively thick alluvial deposits. Although the soft clay
Fig. 16. Seismic site classification map according to NEHRP based on (Vs)30.
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Fig. 17. Average shear wave velocity distribution map down to overburden.
and loose sandy deposits are distributed in most part of the city, the observed natural period is relatively low due to the fact that the thickness of these deposits is relatively small and is usually followed by relatively high velocity layers. The natural period of the sites less than 0.6 s is the typical period for shallow sediments (Dowrick, 2003; Pitilakis, 2004). Generally, the damage to the buildings is most severe if the fundamental site period approaches the natural period of the building. The natural period of the structure (T1) can be approximately calculated for steel and concrete moment resisting framed buildings having twelve storey or less in height with storey height of 3 m and above using the following formula (Di Julio, 2001): T1 = 0:1 N
ð7Þ
where N is the number of stories. The fundamental periods of buildings with different storeys were calculated based on the above formulae and are presented in Table 3. It is noted from Fig. 18 and Table 3 that 1–2 storey buildings are more vulnerable in the southern part of the city and 3–6 storey buildings are vulnerable in other parts of Chennai City due to matching of the periods leading to resonance. Hence, the soil conditions at Chennai pose a potential threat during an earthquake scenario mostly to low rise buildings (less than 6 storeys) which are densely distributed throughout the city.
11. Summary and conclusions Site classification and site period maps are the basic maps for seismic microzonation. In the present study these maps are developed for the Chennai City based on the shear wave velocity measured in the field using MASW tests and using the correlations between SPT-‘N’ and Vs developed by the authors utilizing 300 borelogs. MASW tests conducted at 30 locations in the city reveal the optimum field parameters to obtain the fundamental mode for a seismic array of 40 to 50 m as 10 to 15 m of source offset distance and 1 m geophone spacing. The average shear wave velocity maps developed for various depths indicate that the shear wave velocity zones match well with the geotechnical, geological and geophysical data. Highest velocity zones are associated with the southern part of the city characterized by the occurrence of rock at shallow depths. The zones with low to medium shear wave velocity correspond to the coastal/alluvium sediments in other parts of the city. The seismic site categorization based on the (Vs)30 approach as per the NEHRP guidelines shows three distinct zones in the study area: zone B (shallow bedrock) in southern most part, zone C (very dense soil at relatively shallow bedrock) in southern part and zone D (stiff soil) in all other parts of the city. The amplification rating based on NEHRP standard indicates that the city would be subjected to low to medium amplification. The site classification maps based on (Vs)30 and (Vs)overburden are identical in the central region. Whereas, the northern part and the southern
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Fig. 18. Site period map.
part of the city have lower shear wave velocity in comparison to the NEHRP site classification map. The developed site period map reveals that one to two storey buildings are more vulnerable in the southern part of the city whereas three to six storey buildings are vulnerable in the other parts of the city. Hence, the soil conditions at Chennai pose a potential threat during an earthquake scenario to the low rise buildings (less than 6 storeys) which are densely distributed throughout the city. Acknowledgements The authors wish to thank the Seismology Division, Ministry of Earth Sciences, Government of India for funding the sponsored research project titled “Seismological and geotechnical investigations for development of shake maps for Chennai City” (MoES/P.O (Seismo.) 223 (497)/2004). They extend their thanks to M/s. Geotechnical Solutions, Chennai, for providing assistance during field investigations.
Table 3 Fundamental period of typical buildings. Type of building
Fundamental period, T1(s)
1 Storey 2 Storey 3–4 Storey 5–6 Storey 10 Storey
0.1 0.2 0.3–0.4 0.5–0.6 1.0
References Aki, K., Richards, P.G., 1980. Quantitative Seismology. W.H. Freeman & Co., New York. Anbazhagan, P., Sitharam, T.G., 2008. Mapping of average shear wave velocity for Bangalore region: a case study. Journal of Environmental and Engineering Geophysics 13 (2), 69–84. Ballukraya, P.N., Ravi, R., 1994. Hydrogeological environment of Madras city aquifers. Journal of Applied Hydrology 87 (1 to 4), 75–82. Borcherdt, R.D., 1994. Estimates of site-dependent response spectra for design (methodology and justification). Earthquake Spectra 10 (4), 617–653. BSSC, 2009. NEHRP recommended provisions for the development of seismic regulations for new buildings and other structures 2009 edition. Part 1: Provisions, Report No. FEMAP-750, Building Seismic Safety Council for the Federal Emergency Management Agency, Washington, D.C., USA. Di Julio, R.M., 2001. Linear static seismic lateral force procedures, In: Naiem, F. (Ed.), The Seismic Design Handbook, 2nd ed. Kluwer Academic Publishers, Boston, pp. 247–274. Dobry, R., Borcherdt, R.D., Crouse, C.B., Idriss, I.M., Joyner, W.B., Martin, G.R., Power, M.S., Rinne, E.E., Seed, R.B., 2000. New site coefficients and site classification system used in recent building seismic code provisions. Earthquake Spectra 16 (1), 41–67. Dowrick, D.J., 2003. Earthquake Risk Reduction. Wiley, Chichester, England. EC8, 2003. Eurocode8. Design of structures for earthquake resistance. . GSI, 1999. Explanatory brochure on geological and mineral map of Tamilnadu and Pondicherry. Geological Survey of India. Holzer, T.L., Padovani, A.C., Bennett, M.J., Noce, T.E., Tinsley III, J.C., 2005. Mapping NEHRP V30 S site classes. Earthquake Spectra 21 (1), 161–177. IBC, 2009. International Building Code—2009. International Code Council: Inc, Falls Church, VA. Kanli, A.I., Tildy, P., Pronay, Z., Pinar, A., Hemann, L., 2006. V30 mapping and soil s classification for seismic site effect evaluation in Dinar region, SW Turkey. International Journal of Geophysics 165 (1), 223–235. Kramer, S.L., 1996. Geotechnical Earthquake Engineering. Prentice Hall, Upper Saddle River, NJ. Motazedian, D., Hunter, J., 2008. Development of an NEHRP map for the Orleans suburb of Ottawa, Ontario. Canadian Geotechnical Journal 45 (8), 1180–1188.
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Nazarian, S., Stokoe II, K.H., Hudson, W.R., 1983. Use of spectral analysis of surface waves method for determination of moduli and thickness of pavement systems. Transportation Research Record No. 930, 38–45. Park, S., Elrick, S., 1998. Predictions of shear wave velocities in southern California using surface geology. Bulletin of the Seismological Society of America 88 (3), 677–685. Park, C.B., Miller, R.D., Miura, H., 2002. Optimum Field Parameters of an MASW Survey [Exp. Abs.]. SEG-J, Tokyo. Pitilakis, K., 2004. Site effects. In: Ansal, A. (Ed.), Recent Advances in Earthquake Geotechnical Engineering and Microzonation. Kluwer Academic Publishers, Dordrecht, pp. 139–199. Seismotectonic Atlas of India., 2000. Geological Survey of India, New Delhi. Stewart, J.P., Liu, A.H., Choi, Y., 2003. Amplification factors for spectral acceleration in tectonically active regions. Bulletin of the Seismological Society of America 93 (1), 332–352. Stokoe II, K.H., Wright, G.W., James, A.B., Jose, M.R., 1994. Characterization of geotechnical sites by SASW method. In: Woods, R.D. (Ed.), Geophysical Characterization of Sites. ISSMGE Technical Committee #10. Oxford Publishers, New Delhi, pp. 15–25. Subramanian, K.S., Selvan, T.A., 2001. Geology of Tamil Nadu and Pondicherry. Geological Society of India, Bangalore.
Surfseis2, 2006. User's Manual. Kansas Geological Survey, The University of Kansas, Kansas. Uma Maheswari, R., Boominathan, A., Dodagoudar, G.R., 2010. Use of surface waves in statistical correlations of shear wave velocity and penetration resistance of Chennai soils. Geotechnical and Geological Engineering 28 (2), 119–137. USGS, 2000. USGS Geographic Data Download. . Website: http://edc.usgs.gov/doc/ edchome/ndcdb/ndcdb.html. Wald, L.A., Mori, J., 2000. Evaluation of methods for estimating linear site-response amplifications in the Los Angeles region. Bulletin of Seismological Society of America 90 (6B), S32–S42. Wills, C.J., Silva, W., 1998. Shear wave velocity characteristics of geologic units in California. Earthquake Spectra 14 (1), 533–556. Wills, J.C., Petersen, M., Bryant, W.A., Reichle, M., Saucedo, G.J., Tan, S., Taylor, G., Treiman, J., 2000. A site condition map for California based on geology and shear wave velocity. Bulletin of Seismological Society of America 90 (6B), 187–208. Xu, Y., Xia, J., Miller, R.D., 2006. Quantitative estimation of minimum offset for multichannel surface wave survey with actively exciting source. Journal of Applied Geophysics 59 (2), 117–125.
Contents lists available at ScienceDirect
Journal of Applied Geophysics j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / j a p p g e o
Seismic site classification and site period mapping of Chennai City using geophysical and geotechnical data R. Uma Maheswari a, A. Boominathan a,⁎, G.R. Dodagoudar a a
Department of Civil Engineering, Indian Institute of Technology Madras, Chennai-600036, India
a r t i c l e
i n f o
Article history: Received 13 November 2009 Accepted 18 August 2010 Keywords: Multichannel Analysis of Surface Waves Shear wave velocity Site period Site classification Seismic microzonation
a b s t r a c t Subsurface conditions play a major role in the damage potential of earthquakes and the seismic soil amplification of a site which is a critical factor affecting the level of ground shaking. Shear wave velocity (Vs) of the soil layer is an important parameter influencing the amplification behaviour of the site. Site characterization in calculating seismic hazards is usually based on the near-surface shear wave velocity values. The average shear wave velocity of the top 30 m of the soil, referred to as (Vs)30 is commonly adopted by competent building codes to classify the sites for earthquake resistant design of structures and in general it is widely used in microzonation studies. In the present study, the shear wave velocity of soil layers was measured at 30 locations in Chennai City by Multichannel Analysis of Surface Waves (MASW) test. In addition, nearly 300 borehole data were used to estimate Vs based on the correlations between Vs and SPT-N values for Chennai developed by authors earlier. Merging of MASW test results with borehole data yields sufficient coverage of Vs to develop a new site classification map for Chennai based on the NEHRP standard. It is found that part of the city belong to site D category (stiff soil). The developed site period map reveals that the fundamental site period varies in the range of 0.03 to 0.6 s, thus the soil conditions in Chennai pose a potential threat during earthquakes to low rise buildings (less than 6 storeys) which are densely distributed throughout the city. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Local geological conditions generate significant amplification of the ground motion causing damage during earthquakes. Usually the younger and softer soils amplify the ground motion more strongly than older and more consolidated soils. There are numerous works that show strong correlations between the ground motion during earthquakes and the average shear wave velocity (e.g., Park and Elrick, 1998; Stewart et al., 2003). A simple way to characterize the site conditions is by estimating the shear wave velocity of the deposit. The average shear wave velocity of the top 30 m i.e., (Vs)30 is the most widely used parameter to predict the potential amplification of sites during ground shaking (Borcherdt 1994; Dobry et al., 2000; Holzer et al., 2005; Kanli et al., 2006; Anbazhagan and Sitharam, 2008). This parameter has been used in recent building codes (EC8, 2003; BSSC, 2009; IBC, 2009). (Vs)30 based site characterization is appropriate to sites having relatively shallow “seismic bedrock” or very firm soil conditions and flat stratigraphy (Pitilakis, 2004). In areas of low seismicity and lack of strong ground motion records, the correlation
⁎ Corresponding author. Postal address: Department of Civil Engineering, Indian Institute of Technology Madras, Chennai-600036, India. Tel.: + 91 44 22574273; fax: + 91 44 22570545. E-mail addresses: [email protected] (R.U. Maheswari), [email protected] (A. Boominathan), [email protected] (G.R. Dodagoudar). 0926-9851/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.jappgeo.2010.08.002
between surface geology, borehole data and shear wave velocity measurements is one of the means to classify the distribution of seismic site effect. This approach was used to generate local and regional maps (Wills and Silva, 1998; Wills et al., 2000) according to site categories of the National Earthquake Hazards Reduction Program (NEHRP) and to predict amplification factors. The study area, Chennai is located in southeast of India, and is characterized by the coastal plains of Bay of Bengal. Southern Peninsular India is generally regarded as a stable continental region, characterized by medium seismicity with a lack of strong ground motion records. The major historical seismic events in and around Chennai were on 10 December 1807 (MMI VI), 29 January 1822 (MMI VI), 2 March 1823 (MMI VI), 3 July 1867 (MMI VII), 10 April 1966 (MMI VI) and 26 September 2001 (Mw 5.5). Other than these earthquakes, a few tremors have been felt in the city during the Pondicherry earthquake of magnitude, Mw 5.5 on 25 September 2001 centered 100 km from Chennai and Sumatra earthquake of magnitude, Mw 9.1 on 26 December 2004. The primary objective of this paper is to demonstrate the use of combined surface wave and borehole data for deriving subsurface characteristics in terms of average shear wave velocity within 30 m depth (Vs)30 for regions characterized by shallow sediments such as the Chennai City. The development of seismic site classification maps by (Vs)30 scheme and the fundamental site period map for earthquake hazard assessment of Chennai is also presented.
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2. Geological settings of the study area Chennai is the fourth largest city in India, covers an area of 172 km2 and is located between 12.75°–13.25° N and 80.0°–80.5° E on the southeastern coast of India. The base map of the study area prepared using ArcGIS 9.3 is presented in Fig. 1. The general geology of the city is composed mostly of sand, clay, shale and sandstone as shown in Fig. 2 (GSI, 1999). The surface geology of the study area and its surroundings are reported in Ballukraya and Ravi (1994); Seismotectonic Atlas of India. (2000) and Subramanian and Selvan (2001). Chennai is underlain by various geological formations from ancient Archeans to recent alluviums. They are grouped into three categories: (i) Archean crystalline metamorphic rocks (consolidated), (ii) upper Gondwanas composed of sandstones, siltstones and shales, tertiary (Eocene to Pliocene) sandstones (semi consolidated) and (iii) coastal and river alluvium (unconsolidated). The geology of the study area consists of shallow bedrock on the east and south, and Gondwanas below the alluvium in the north and west. Almost the entire area is covered by the pleistocene/recent alluvium, deposited by the Cooum and Adyar rivers. The coastal region of the city is fully covered by marine sediments. 3. Geomorphological settings Chennai located on the coastal plains has a flat topography with very gentle slope towards east. The altitudes of land surface vary
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from 10 m above MSL in the west to sea level in the east. The elevation data of the city were obtained from the USGS and Shuttle Radar Topography Mission (SRTM). The SRTM data were used to generate a digital topographic map of the Earth's land surface with data points spaced every 3 arc second for global coverage of latitude and longitude (approximately 90 m). The elevation data have a horizontal accuracy of 20 m and a vertical accuracy of ± 10 m and was used to assess the relative elevation of sites (USGS, 2000). The Digital Elevation Model (DEM) developed for the study area is shown in Fig. 3(a) which illustrates the topographic differences in the region. The 3D DEM was prepared from the SRTM (Shuttle Radar Topography Mission) data by: (a) converting a DEM ASCII file to an ArcGIS Grid format using the ArcTool box, (b) converting raster data into TIN by using raster to TIN in 3D analyst, (c) opening the newly converted raster file in ArcScene, (d) adding base heights to show 3D variation and (e) adding a vertical exaggeration to get the 3D topology. The 3D elevation model obtained for the study area using 3D analyst tool available in ArcMap 9.3 is shown in Fig. 3(b). It is evident from Fig. 3(a) and (b) that the city has an average elevation of 9 m with the lowest and highest points being 1 and 20 m from MSL respectively. Selected sample values from SRTM data presented in Table 1 are cross checked with elevation contour and bench marks provided in toposheets of GSI and it confirms the range of elevation values. The geomorphology map of the Chennai City shown in Fig. 4, indicates presence of alluvial and coastal plains, beach and beach
Fig. 1. Map of the study area.
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Fig. 2. Geological map of the study area.
Fig. 3. Elevation map: (a) 2D and (b) 3D.
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Longitude
SRTM data (m)
Toposheet contour data (m)
80.269 80.294 80.279 80.259
13.107 13.103 13.075 13.066
3–6 3–6 3–6 6–9
6.3 4.5 5.4 5.7
ridges, pediments, lagoon/back water, marsh, paleo channel, sedimentary high ground, tertiary plain and structural hills. Fluvial, marine and erosional landforms are also noticed in the study area. Marine transgression and neotectonic activity during the recent past have influenced the morphology resulting in various present landforms. Meandering streams with small sand bars are deposited along the course of Adyar river. Marina beach is the second longest natural beach in the world with width varying from 150 to 600 m from the coast and length of about 5.6 km. 4. Bedrock and subsurface profiling Nearly 300 borelogs from the reputed geotechnical agencies including deep borelogs from Public Works Department (PWD) of Tamil Nadu state, and Central Public Works Department (CPWD)
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were collected to establish the subsurface profile of the study region and their distribution in the study area is shown in Fig. 5. The depth to bedrock contour map established for the study area is presented in Fig. 6. The bedrock in the southern part is characterized by igneous and metamorphic rocks; marine sediments containing claysilt sands and charnockites are encountered in the eastern and northern parts, and the western parts are composed of sedimentary rocks. The depth to bedrock varies from 15 to 30 m along the eastern coast of the study area and it increases from east to west. The depth to bedrock decreases from north to south. Based on the collected borelogs, the overburden thickness of soil was mapped using the spatial analyst tool available in the ArcMap. The data were interpolated using natural neighborhood technique and are shown in Fig. 7. The thickness of the soil formation ranges from a few meters to 9 m in the southern part to as much as 31 m in the central part of the city. The thickness of the soil formation ranges from 12 to 18 m in the northern part. The detailed subsurface geological stratum at different crosssections of the study area was mapped using the deep borelogs and is shown in Fig. 8. The west–east oriented cross-sections A-A′ and B-B′ show more details including the variations of the geometry due to the intrusion of sandstone and shale as shown in Fig. 8(a) and (b) respectively. In these sections, the western part is predominantly dominated by clay/sandy clay and is followed by shale with intrusion
Fig. 4. Geomorphological map of the study area.
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Fig. 5. Borehole locations and MASW test locations.
Fig. 6. Depth to bedrock contour map.
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Fig. 7. Overburden thickness map.
of sandstone. However, the eastern part is dominated by sand/clayey sand and immediately followed by charnockite-granitic gneiss basement rock. The variation of thickness of the unconsolidated coastal/river sediments along the eastern coast is shown by the geological crosssections (C-C′) and (D-D′) constructed from the borehole data [Fig. 8(c) and (d)]. The section C-C′ is dominated by sand/clayey sand up to a depth of 25 to 30 m near to the coastal side. Similarly the section D-D′ is predominantly dominated by sand/clayey sand to a depth of 10 to 15 m with intrusion of clay/sandy clay layer which is observed away from the coastal side. The charnockitegranitic gneiss type of rock is immediately followed by alluvium sediments. Similarly, the west–south oriented cross-section E-E′ is shown in Fig. 8(e) with detailed subsurface geological stratum. 5. Evaluation of (Vs)30 Elastic properties of near-surface materials and their effects on seismic wave propagation are very important in earthquake engineering activities and in environmental and earth sciences studies. The increase of amplitudes in soft sediments is one of the most important factors responsible for the amplification of earthquake motions. Amplification is proportional to 1 pffiffiffiffiffiffiffiffiffiffi Vs :ρ
ð1Þ
where Vs is the shear wave velocity and ρ is the density of the investigated soil (Aki and Richards, 1980). Since density is relatively constant with depth, the Vs value can be used to represent site conditions. A thorough assessment of shallow shear wave velocity is crucial for earthquake hazard assessment studies (Wald and Mori, 2000). The average shear wave velocity of the upper 30 m (Vs)30 is computed in accordance with the following expression: ðVs Þ30 =
N
∑
i=1
30 .
ð2Þ
hi
Vi
where hi and Vi denote the thickness (in meters) and shear wave velocity (at a shear strain level of 10− 5 or less) of the ith formation or layer, in a total of N layers existing in the top 30 m of the deposit. In this study, (Vs)30 was determined from seismic surface wave test and based on the shear wave velocity estimated from SPT-N. 6. Geophysical investigations—surface wave method The most commonly used seismic method for shear wave velocity profiling is the surface wave method. In general for microzonation studies, the following two methods are preferred for the seismic site characterization: Spectral Analysis of Surface Waves (SASW) and
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Fig. 8. Cross-sectional profiles at (a) A-A′, (b) B-B′, (c) C-C′, (d) D-D′ and (e) E-E′.
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Multichannel Analysis of Surface Waves (MASW) tests (Nazarian et al., 1983; Stokoe et al., 1994; Park et al., 2002; Xu et al, 2006). Of all the types of seismic waves, surface waves have the strongest energy by virtue of which they have the highest signal to noise ratio (Park et al., 2002) making it a powerful tool for near-surface characterization. In the present study, MASW tests were performed to establish the shear wave velocity profile. In the MASW test, the motion generated by an impact source is detected simultaneously at several receiver locations and the corresponding signals are analyzed as a whole using double Fourier transform. Raw field data are transformed into the frequency–wave number (f–k) domain where phase velocities of Rayleigh waves are calculated to produce a dispersion curve. Then the calculated dispersion curve is inverted to estimate the Vs profile. The MASW tests were carried out at 30 sites in Chennai City (Fig. 5), so as to obtain representative regional estimates of average velocity-depth profiles throughout the city. The field configuration of MASW test is presented in Fig. 9. Tests were carried out using Geometrics make 24 channels Geode seismic recorder with single geode operating software (SGOS). The vertical geophones of natural frequency 4.5 Hz (24 numbers) were used to receive the wave fields generated by an active source of 8 kg sledgehammer. Although the acquisition of ground roll data seems to be an easy task but the field configurations need to be optimized for the requirement: geophone spacing and the offset range. The planar characteristics of surface waves evolve only after a certain distance from the source. In most cases, this distance needs to be greater than the half of the maximum desired wavelength (Stokoe et al., 1994). On the other hand, the amplitude of body wave and higher mode Rayleigh wave may dominate over the fundamental mode at high frequencies. The relationship between the energy of fundamental and higher modes is a complex function of layer parameters and offset but, as a rule of thumb, the maximum distance between the source and the furthest geophone should not exceed 100 m (Park et al., 2002). Twenty four geophones were deployed in a linear pattern with an equal receiver spacing in the range of 0.5 to 2 m interval with the nearest source to geophone offset in the range of 5 to 30 m to meet the requirement of different types of soil as suggested by Xu et al. (2006). These configurations were used depending on the characteristics of the site to obtain a realistic velocity structure, mainly at places where no information about thickness was available. The source and each receiver were connected to an individual recording channel as shown in Fig. 9. The test was repeated with applying source at the front, middle and end of the receivers to get the consistency of the field data. The main parameters of data acquisition are given in Table 2.
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Table 2 Data acquisition parameters. Parameter
Range
Number of channels Geophone spacing Sampling rate Record length Receiver Source Minimal source offset
24 0.5–2 m 0.25 ms 1s 4.5 Hz vertical (spikes or plates) 8 kg hammer (on metal plate) 5 to 30 m
A typical raw wiggle plot obtained from the test is shown in Fig. 10 (a). The data processing of wiggle plot consists of two main steps: (i) obtaining the dispersion curves of Rayleigh wave phase velocity from the records and (ii) determining the Vs profiles from which the (Vs)30 values are calculated. The data processing was carried out in accordance with the characteristics of field records. First, the records were muted to reduce the effect of random noise and the interference with other wave types [Fig. 10(b)]. After muting, only the surface wave component (jumping up) is used for the dispersion curve analysis. After careful analysis of the records only the fundamental mode of Rayleigh waves (from 4–5 to 30–50 Hz) was investigated. The acquired wave data were processed using the SurfSeis software (SurfSeis2, 2006) to develop experimental dispersion curve with signal to Noise (S/N) ratio as shown in Fig. 11. The dispersion curves before and after filtering are shown in Fig. 11(a) and (b) respectively. The wiggle plots and corresponding dispersion curves obtained for different source offsets such as 5, 10 and 20 m with S/N ratios are shown in Fig. 12(a), (b) and (c) respectively. Fig. 12 shows that the dispersion curve corresponds to source offset of 10 m and gives higher S/N ratio compared to other source offsets. The experimental dispersion curve was subjected to inversion analysis to develop one- and two-dimensional shear wave velocity profiles as shown in Fig. 13(a) and (b) respectively. At each location, MASW tests were repeated by varying the field parameters such as source offset and geophone spacing to arrive the shear wave velocity profile with greater accuracy with maximum signal to noise ratio. Based on these trials of tests carried out at various sites the optimum parameters for the MASW tests were arrived as: source offset of 10 to 15 m and geophone spacing of 1 m for a seismic array of 40 to 50 m. In case, where the MASW test conducted using optimum parameters were not able to procure data to the required depth of 30 m, the source offset was increased accordingly. By combining the analysis of different array length field data, it was possible to obtain shear wave velocity
Fig. 9. Field configuration of the MASW test.
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Fig. 10. Typical wiggle plots obtained from the MASW test: (a) raw and (b) after filtering.
Fig. 11. Typical dispersion curves with signal to noise (S/N) ratio: (a) raw and (b) after filtering.
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values up to the required depth with relatively better accurately at deeper depths of up to 30 m. 7. Estimation of shear wave velocity using borehole data It is preferable to determine shear wave velocity directly from field tests, but it is often not economically feasible to make Vs measurements at all locations. However to effectively utilize the available borelog in the study area, the shear wave velocity is also estimated
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using the following correlation between Vs (m/s) and uncorrected SPT-N values developed by the authors for three categories of soils in Chennai, the details of which can be found in the study of Uma Maheswari et al. (2010): For All Soils : Vs = 95:64N For Sand : Vs = 100:53N
0:301
0:265
2 ; r = 0:83
2 ; r = 0:84
Fig. 12. Typical wiggle plots and dispersion curves correspond to source offset of (a) 5 m, (b) 10 m and (c) 20 m.
ð3Þ ð4Þ
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Fig. 13. Typical shear wave velocity profile: (a) 1D and (b) 2D (Egmore site).
Fig. 14. (Vs)30 distribution map of Chennai.
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Fig. 15. Average shear wave velocity distribution maps down to (a) 5 m (Vs)5, (b) 10 m (Vs)10 and (c) 20 m (Vs)20 depths.
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Fig. 15 (continued ).
For Clay : Vs = 89:31N
0:358
2 ; r = 0:93
ð5Þ
Merging the seismic surface wave results with the borehole data yields sufficient coverage of (Vs)30 for the development of shear wave velocity distribution map for Chennai. If the number of data points is more, spatial variability will be less and accuracy will be more. The above correlation between SPT-N value and shear wave velocity is used to obtain the (Vs)30 where shear wave velocity profile is not available. Similar kind of merging MASW tests with the borehole data has been carried out for the development of the NEHRP maps for Orleans suburb of Ottawa, Ontario (Motazedian and Hunter, 2008). 8. Shear wave velocity distribution The shear wave velocity data obtained from the field experiments and that estimated from the empirical correlations developed by the authors are used for development of shear wave velocity distribution and site classification maps for Chennai City. Merging the seismic surface wave results with the borehole data yields sufficient coverage of Vs for the development of shear wave velocity distribution maps for Chennai (Fig. 14). It is noted that three different velocity zones exist in the study area. The high and medium shear wave values correspond to the shallow bedrock zone and outcrop zone while lower ones correspond to the alluvial basin. The northeastern and a few locations in the western part (blue coloured zone) are
characterized by medium shear wave velocity values (between 200 and 250 m/s). The transition zone (pink coloured) is characterized by a relatively high velocity zone (between 250 and 350 m/s) and spreads in the northwestern and central parts. The shallow bedrock zone in the southern part of the study area has the highest shear wave velocity values between 350 and 1000 m/s. In the (Vs)30 distribution map, the velocity zones coincide with the geotechnical, geological and geophysical investigations. Highest velocity is encountered in the southern part associated with the shallow bedrock geologically defined as Archean deposits. The zones which have medium and relatively high shear wave velocity represent the coastal/alluvium sediments. To understand variation in the shear wave velocity distribution with depth, the average velocity distribution maps were obtained at three different depths, 5, 10 and 20 m (Fig. 15). It can be observed from Fig. 15(a) that the average shear wave velocity for a depth of 5 m ranges from 100 to 450 m/s. The northeastern part of Chennai has an average shear wave velocity of 100–150 m/s, which indicates soft soil in the upper 5 m layer. The average shear wave velocity for the 10 m depth varies from 100 to 600 m/s [Fig. 15(b)]. In the 10 m average shear wave velocity map, very dense soil/soft rock with velocity ranging from 360 to 600 m/s is found in the southern part of the study area. In this location, the bedrock depth is found within 10 m. The 20 m depth map [Fig. 15(c)] shows a greater coverage of very dense soil/soft rock when compared to the 10 m depth map [Fig. 15(b)]. The northeastern and a few locations in the western parts have an average
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shear wave velocity less than 200 m/s indicating soft soil over a depth of 20 m. In general, there is an abrupt change in the shear wave velocity values at deeper depths because of intrusion of dense soil/soft rock. 9. Seismic site classification maps The seismic site classification map developed for Chennai based on NEHRP categorization (BSSC, 2009) is shown in Fig. 16. It shows contour lines for (Vs)30 with lines of 180, 360 and 760 m/s representing the boundaries of the NEHRP zones. The seismic site classification zones for the Chennai area, as shown in Fig. 16 are directly related to geological settings and soil thickness. High velocity areas are associated with shallow bedrock (zone B) which is encountered in the southernmost part of the city. The very dense soil and soft rock at relatively shallow bedrock areas are identified with zone C at southern part. The most part of the city characterized by stiff soil, belongs to the zone D category. The amplification rating based on NEHRP standard indicates that the city would be subjected to low to medium amplification. (Vs)30 based site classification map considers the shear wave velocity of both the soil and the rock below, thus does not represent the actual shear wave velocity variation of overburden soil. The classification map developed based on the average shear wave velocity for overburden soil is presented in Fig. 17. It shows that the study area has shear velocity of overburden ranging from 100 to
165
350 m/s. Fifty percent of the city has shear wave velocity in the range of 100–200 m/s. The site classification maps based on (Vs)30 (Fig. 16) and (Vs)overburden (Fig. 17) are identical in the central region. Whereas, the northern part and the southern part of the city have lower average shear wave velocity in comparison to the NEHRP classification map. 10. Site period map The fundamental site period (Ts) corresponds to the first mode of vibration of the soil deposit which is used by several investigators as one of the parameters for seismic microzonation. It is governed by the thickness and shear wave velocity of the soil layer and is calculated using the following expression (Kramer, 1996): Ts =
4H
. Vs
ð6Þ
where H is the total thickness of alluvium sediments and Vs is the average shear wave velocity of the overburden soil. The developed site period map presented in Fig. 18 indicates that the southern part of the city has the lowest site period ranging from 0.03 to 0.2 s (corresponding to frequency of 5–33 Hz) due to lesser thickness of the overburden. The site period in other parts of the city ranges from 0.2 to 0.6 s (corresponding to frequency of 1.6–5 Hz) due to the presence of relatively thick alluvial deposits. Although the soft clay
Fig. 16. Seismic site classification map according to NEHRP based on (Vs)30.
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Fig. 17. Average shear wave velocity distribution map down to overburden.
and loose sandy deposits are distributed in most part of the city, the observed natural period is relatively low due to the fact that the thickness of these deposits is relatively small and is usually followed by relatively high velocity layers. The natural period of the sites less than 0.6 s is the typical period for shallow sediments (Dowrick, 2003; Pitilakis, 2004). Generally, the damage to the buildings is most severe if the fundamental site period approaches the natural period of the building. The natural period of the structure (T1) can be approximately calculated for steel and concrete moment resisting framed buildings having twelve storey or less in height with storey height of 3 m and above using the following formula (Di Julio, 2001): T1 = 0:1 N
ð7Þ
where N is the number of stories. The fundamental periods of buildings with different storeys were calculated based on the above formulae and are presented in Table 3. It is noted from Fig. 18 and Table 3 that 1–2 storey buildings are more vulnerable in the southern part of the city and 3–6 storey buildings are vulnerable in other parts of Chennai City due to matching of the periods leading to resonance. Hence, the soil conditions at Chennai pose a potential threat during an earthquake scenario mostly to low rise buildings (less than 6 storeys) which are densely distributed throughout the city.
11. Summary and conclusions Site classification and site period maps are the basic maps for seismic microzonation. In the present study these maps are developed for the Chennai City based on the shear wave velocity measured in the field using MASW tests and using the correlations between SPT-‘N’ and Vs developed by the authors utilizing 300 borelogs. MASW tests conducted at 30 locations in the city reveal the optimum field parameters to obtain the fundamental mode for a seismic array of 40 to 50 m as 10 to 15 m of source offset distance and 1 m geophone spacing. The average shear wave velocity maps developed for various depths indicate that the shear wave velocity zones match well with the geotechnical, geological and geophysical data. Highest velocity zones are associated with the southern part of the city characterized by the occurrence of rock at shallow depths. The zones with low to medium shear wave velocity correspond to the coastal/alluvium sediments in other parts of the city. The seismic site categorization based on the (Vs)30 approach as per the NEHRP guidelines shows three distinct zones in the study area: zone B (shallow bedrock) in southern most part, zone C (very dense soil at relatively shallow bedrock) in southern part and zone D (stiff soil) in all other parts of the city. The amplification rating based on NEHRP standard indicates that the city would be subjected to low to medium amplification. The site classification maps based on (Vs)30 and (Vs)overburden are identical in the central region. Whereas, the northern part and the southern
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Fig. 18. Site period map.
part of the city have lower shear wave velocity in comparison to the NEHRP site classification map. The developed site period map reveals that one to two storey buildings are more vulnerable in the southern part of the city whereas three to six storey buildings are vulnerable in the other parts of the city. Hence, the soil conditions at Chennai pose a potential threat during an earthquake scenario to the low rise buildings (less than 6 storeys) which are densely distributed throughout the city. Acknowledgements The authors wish to thank the Seismology Division, Ministry of Earth Sciences, Government of India for funding the sponsored research project titled “Seismological and geotechnical investigations for development of shake maps for Chennai City” (MoES/P.O (Seismo.) 223 (497)/2004). They extend their thanks to M/s. Geotechnical Solutions, Chennai, for providing assistance during field investigations.
Table 3 Fundamental period of typical buildings. Type of building
Fundamental period, T1(s)
1 Storey 2 Storey 3–4 Storey 5–6 Storey 10 Storey
0.1 0.2 0.3–0.4 0.5–0.6 1.0
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