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Influence of Local Site Effects in the Ahmedabad Mega City on
the Damage due to Past Earthquakes in Northwestern India
by B. Sairam, A. P. Singh, Vandana Patel, Vasu Pancholi,
Sumer Chopra, V. K. Dwivedi, and M. Ravi Kumar*
Abstract The city of Ahmedabad of Gujarat in western India suffered severe dam-
age during the 2001 Mw7.7 Bhuj earthquake, despite being ∼250 km away from the
epicenter. Similar damage patterns were also reported during the 1819 Allah Bund
earthquake (Mw7.8). To investigate the probable causes, we employed an integrated
approach using multichannel analysis of surface waves (MASW), single and array mi-
crotremor measurements, broadband earthquake data, and geotechnical investigations.
Significant differences in site characteristics are observed in both the damaged and
undamaged areas. The investigations revealed shear-wave velocity values in excess
of 320 m=s and less than 220 m=s in the undamaged and damaged areas, respectively,
in the top 6 m of the subsurface. The refusal (N160) values are observed at 6–20 m and
20 m depth onward in the undamaged and damaged areas, respectively. The amplifi-
cation factor in the damaged areas varies from 3.3 to 6.6 in the 1.5–2.0 Hz frequency
range. On the other hand, the amplification factor in the undamaged areas varies from
1.0 to 3.0 in the 0.7–1.5 Hz frequency range. Nevertheless, the damages were mostly
restricted to mid-to-high-rise buildings, located in the western side of the Sabarmati
River, where the presence of a paleochannel is reported, and in some southeastern parts
of the city, along lakes and ponds. The low-rise buildings on the eastern side of the river
in the old city area remained almost intact. Our study confirms that local site effects
together with the poor quality of construction contributed to the damages.
Electronic Supplement: Table of shear-wave velocities, and figures of estimated
shear-wave velocity and comparison of standard penetration test (SPT) N160model at
the damaged and undamaged areas.
Introduction
The influence of local soil conditions on the damage pat-
terns is documented during the 1985 Mw8.1 Michoacán
earthquake that caused severe damage in the Mexico City,
located about 350 km away from the epicenter (Furumura
and Kennett, 1998). A similar destruction scenario was re-
ported in the city of Ahmedabad, western India, during the
26 January 2001 Mw7.7 Bhuj earthquake, located ∼250 km
away from the epicenter. This city is the most populated one
in Gujarat and is categorized as a mega city in the country,
because it has a population of more than six million. It is also
inscribed as India’s first World Heritage City by the United
Nations Educational, Scientific and Cultural Organization.
Incidentally, the damages observed due to the Bhuj earth-
quake were significantly different in various parts of the city
of Ahmedabad (e.g. Bhandari and Sharma, 2001;Goel,
2001;Earthquake Engineering Research Institute [EERI],
2002;Stewart et al., 2002;Narayan and Sharma, 2004;Saito
et al., 2004). Most of the damaged buildings were mainly
confined to the western and southeastern parts of the city and
the undamaged areas to eastern part of the city (Fig. 1). The
EERI team reported that the damage was primarily restricted
to mid-to-high-rise buildings (5–12 stories) along the pale-
ochannel of the Sabarmati River, lakes, and ponds (Fig. 1),
while the low-rise buildings were left intact (EERI, 2002).
Nonuniform damage patterns were also observed in the
pre- and postsatellite images (Saito et al., 2004). Interest-
ingly, no damages were reported in the eastern part of the
city, although the density of buildings was higher compared
to that in the damaged area (Goel, 2001). It is worthwhile to
mention that the eastern part of the city is older than the
western part that was founded in 1411 (Gupta et al., 2002).
*Now at Institute of Seismological Research, Raisan, Gandhinagar 382
007, Gujarat, India.
2170
Bulletin of the Seismological Society of America, Vol. 108, No. 4, pp. 2170–2182, August 2018, doi: 10.1785/0120170266
The old town located on the eastern side of the river
mostly contains 1–2- story load-bearing masonry brick wall
buildings, which virtually did not experience any damage,
except thin cracks in the walls (EERI, 2002). However,
the western and southeastern parts of the newly developed
area, mostly comprising decade old reinforced concrete
frame structures of 5 stories (ground + four) and a large num-
ber of 10-story buildings suffered severe damages (Goel,
2001;EERI, 2002;Murty et al., 2002). About 130 buildings
of 5–10 stories were collapsed, resulting in
∼750 causalities. Such extensive damages
were not expected in the city of Ahmeda-
bad because it is located far from the epi-
center. The level of accelerations at such a
large distance (Chandra et al., 2002)may
not be sufficient to cause severe damage to
even poorly constructed buildings. Simi-
lar, unexpected high damages were also
observed in the western parts of the city
of Ahmadabad during the 1819 Mw7.8
Allah Bund earthquake (Macmurdo, 1824;
Bilham, 1999). Locations of the collapsed
buildings (Fig. 1) clearly reveal that the
damaged pattern is not random. The dam-
age occurred in some groups toward the
western and the southeastern parts of the
city. All the clusters consist of 5–8 build-
ings, ∼1km in diameter, with the distance
between adjacent clusters varying between
1.5 and 2.5 km. Although the epicenter is
located about W13°N from Ahmedabad
(Fig. 1), the collapsed buildings are mostly
aligned along W40°N, as shown in
Figure 1. Furthermore, the EERI team did
not observe any geological evidence for a
subsurface connection that could give rise
to such a damaged pattern. We, therefore,
investigated the site response of the basin
on which the town of Ahmedabad is situ-
ated. Several researchers advocated that
local site effects contributed to the dam-
ages in Ahmedabad, besides poor quality
of building constructions (e.g., Bhandari
and Sharma, 2001;Goel, 2001;Murty
et al., 2002;Pande and Kayal, 2003;Go-
vindaraju et al., 2004;Parvez and Madhu-
kar, 2006;Parvez and Rosset, 2014).
However, to understand the causes of the
damage patterns in the city of Ahmedabad
no detailed studies have been carried out
so far, except qualitative postearthquake
investigations.
In the recent past, effects due to
several earthquakes have established the
fact that local site conditions have a
significant role in the amplification of ground motion, espe-
cially in those areas located on unconsolidated young sedi-
mentary materials (see Hunter et al., 2002;Rastogi et al.,
2011;Sairam et al., 2011;Singh, 2015). During strong earth-
quakes, the local site conditions can strongly influence the
ground shaking and the associated damage to nonengineered
structures (Campillo et al., 1989;Aki, 1993;Hough et al.,
2002;Rastogi et al., 2011;Verma et al., 2014;Singh,
2015). Detailed geophysical, seismological, and geotechni-
cal investigations can provide reliable estimates of the local
Figure 1. Geomorphological map of the study area with locations of geophysical
and geotechnical survey sites. The damaged area is marked based on the location of
buildings damaged during the 2001 Bhuj earthquake (modified after Bhandari and
Sharma, 2001;Govindaraju et al., 2004). The damaged buildings identified from sat-
ellite image (Saito et al., 2004) and Earthquake Engineering Research Institute (EERI)
report (EERI, 2002) are mainly confined to the western and southeastern parts of the city.
The curvilinear line shows the boundary of the city. Site numbers (1–5) are used for
detailed discussions. The sites B1–B12 show the location of broadband seismograph
stations. (Inset) Location in the Gujarat region. The star shows the epicenter of 26 Janu-
ary 2001 Mw7.7 Bhuj earthquake. The square shows the location of the city of Ahmeda-
bad in the Gujarat map. MASW, multichannel analysis of surface waves; MAM,
microtremor array measurement. The color version of this figure is available only in
the electronic edition.
Influence of Local Site Effects in the Ahmedabad Mega City on the Damage due to Past Earthquakes 2171
site effects (Andrus et al., 2006;Chopra et al., 2013;Singh
et al., 2014,2017). The average shear velocity (VS) of the top
30 m of soil (VS30) is an indicator of the strength of the
ground and is a critical parameter to evaluate the near-surface
stiffness and characterize a given site (Borcherdt, 1970;
Boore et al., 1997;Castro et al., 1997;Park and Elrick, 1998;
Garofalo et al., 2016;Foti et al., 2017). The National Earth-
quake Hazards Reduction Program (NEHRP) assigned site
classes (Building Seismic Safety Council [BSSC], 2004)
based on VS30 values only. Several studies have found a cor-
relation between amplifications and VS30 (e.g., Borcherdt,
1994;Sairam et al., 2011). The parameters describing the
site effects in seismic codes are expressed through soil char-
acterization and spectral amplification factor.
In this maiden study, we tried to elucidate the causes of
significant damage patterns in the city of Ahmedabad
through an integrated approach. In the damaged and undam-
aged areas, we employed geophysical approaches such as
multichannel analysis of surface waves (MASW), array
and single-station records of microtremors, and broadband
earthquake records. The estimated results are compared with
geotechnical data to quantify the site effects.
Geology of the Study Area
Ahmedabad is a part of mainland Gujarat that lies within
the Cambay basin, where the thickness of Quaternary sedi-
ments is about 350 m. The city is underlain by unconsoli-
dated recent alluvial sediments that are deposited down to
depths of 35–100 m, followed by ∼250–300mthick
Pleistocene sediments (Merh and Chamyal, 1993;Parvez
and Madhukar, 2006;Hasan et al., 2013;Parvez and Rosset,
2014). Our study area is distinctly divided into the eastern
and western parts by the Sabarmati River (Fig. 1). On the
western side of the river, a paleochannel was found ∼5:0km
away, which gradually shifted to the present path (Goel,
2001). There were several lakes and ponds in the western and
southeastern parts of the city. On the western side of the river,
the unconsolidated materials derived from the sediments
transported by the river are deposited in the top ∼6m.
Geomorphologically, the town is located on the banks of the
Sabarmati River. Therefore, floodplains cover the entire city
of Ahmedabad, where silty sediments to sands are exposed
on the surface. The alluvium mainly comprises paleodeltaic,
fluvial, and Aeolian sediments, consisting of alternate layers
of fine- to coarse-grained sand, gravel, and yellowish-brown
sticky clay. The upper layers are mostly silty fine sands of
Aeolian nature (Kumar et al., 1998;Parvez and Madhukar,
2006). Thick sequences of tertiary formations lie beneath the
alluvium layers (Kaila et al., 1990).
Geophysical Surveys
PS Logging
We carried out PS logging measurements at five sites
within the damaged and six in the undamaged areas (see
Fig. 1for location). In this method, initially, the probe is low-
ered into a fluid-filled borehole to a desired depth, to acquire
P- and S-wave data. Later, a pressure wave is generated in the
fluid using the seismic source in the probe. The pressure
waves are converted to Pand Swaves at the borehole walls,
which travel through the geological formations. Later, these
waves are converted back to the pressure wave at the bore-
hole wall. Consequently, the geophones in the probe detect P
and Swaves, which are recorded by a data acquisition system
(OYO Corporation, 2002). The VSdata were acquired at
every 1 m interval from the surface to the bottom of the bore-
hole. At each site, VSdown to a depth of 40 m was estimated.
Multichannel Analysis of Surface Waves
We performed MASW at 65 sites within the damaged
and undamaged areas in the city of Ahmedabad. This tech-
nique utilizes the dispersive properties of Rayleigh waves for
the estimation of a VSprofile of the subsurface. The entire
process of MASW consists of three major steps, namely
acquisition of multichannel seismic data, generation of the
dispersion curve, and estimation of the VSprofile. We fol-
lowed the recommendation of Park et al. (1999), to combine
the MASW with the common mid-point (CMP) roll-along
data acquisition procedure to obtain a laterally continuous
2D VSprofile. A 48-channel engineering seismograph was
used to acquire data in this current investigation. The data
were acquired using the standard CMP roll-along technique
to achieve a continuous shot gather. Vertically stacked 20
impacts of a 30-kg weight drop on a metal plate were used
as a source to generate seismic waves. These waves were
recorded by 24 vertical-component 4.5 Hz geophones/
receivers planted at every 2 m interval along the profile,
which responded to frequencies from 1 to 90 Hz in a
24-channel seismic shot gather. A typical record of the shot
gather is shown in Figure 2a. Acquisition parameters of the
geometry are selected to optimize the imaging of subsurface
layers down to more than 30 m depth. The signal-to-noise
ratio of the processed data was more than 85 at each site. In
the dispersion curve, the analyzed frequency was in the
4–50 Hz range. A typical dispersion curve is shown in
Figure 2b. Each dispersion curve was individually inverted
to obtain a VSprofile by setting the initial model and
performing a least-squares minimization (Xia et al., 1999),
inbuilt in SurfSeis (Kansas Geological Survey [KGS], 2010).
The initial velocity model is taken from the nearby PS log-
ging measurements. In situ density measurements in nearby
boreholes are also used in the initial model. All 1D VSpro-
files are gathered in a sequential order to generate a 2D VS
section. An example of a 2D VSmodel for site number 5 (see
Fig. 1for location) is shown in Figure 3.
At a few sites, where the MASW and PS logging points
are collocated, the VSvalues are estimated using MASW and
PS logging for validating our results. The VSvalues also cor-
relate well with the geotechnical lithologs. We found that all
the methods provide comparable results (Fig. 4). Finally, a
2172 B. Sairam, A. P. Singh, V. Patel, V. Pancholi, S. Chopra, V. K. Dwivedi, and M. Ravi Kumar
time-averaged velocity of the top 30 m (VS30) of each site is
estimated (ⒺFigs. S1 and S2 and Table S1, available in the
electronic supplement to this article), and the site is classified
following the NEHRP classes.
Geotechnical Investigations
In the city of Ahmedabad, a total of 16 boreholes (4 in
damaged and 12 in undamaged areas) are drilled for deter-
mining the lithological variations and decipher local geologi-
cal site conditions (Fig. 1). Different laboratory tests such as
specific gravity, field dry density, natural water content, sieve
analysis, hydrometer analysis, Atterberg limit, direct shear
test, and free swell index are performed on soil samples, fol-
lowing the Indian Standards (IS-2131, 1981) for determina-
tion of the index properties of soils. Finally, lithologs are
prepared for all the boreholes. Standard penetration tests
(SPTs) is conducted following IS-2131
(1981) in all the boreholes at every 3.0 m
interval (Fig. 4). The field SPT N-values
are corrected to obtain the standardized
N160values using the formula of Kayen
et al. (1992) and the correction factor of
Robertson and Wride (1997). The cor-
rected N-values are referred as N160be-
cause the original SPT hammer has about
60% efficiency (Skempton, 1986). The
overburden pressure correction is done
following the recommendations of Youd
et al. (2001). Sample N160models of the
top 30 m of the damaged and the undam-
aged areas are plotted in Figure 5. The
N160models of all borehole are provided
in ⒺFigure S3.
Microtremor Measurements
Single Station. We acquired single-
station microtremor measurements at 60
locations in and around the city, in the early
morning. The data were collected for
60 min at 100 samples=s. Three-component
Lennartz LE-3D (5 s resonance period)
seismometer coupled with a 24-bit City-
shark II system was used in this survey.
The sites 1–4(seeFig.1for locations),
which are closer to the boreholes, are con-
sidered for the analysis at undamaged and
damaged sites. The horizontal-to-vertical
(H/V) spectral ratio (HVSR) is obtained for
all the single-station microtremor measure-
ments (Nakamura, 1989). Although, a
similar survey has been done by Parvez and
Madhukar (2006) for seismic microzona-
tion of the city of Ahmedabad, the measure-
ment sites are slightly different than those in
the present study. We prepared resonant frequency maps of the
region to understand the local site effects and estimate the
thickness of soft sediments.
Microtremor Array Measurement. We have also conducted
microtremor array measurement (MAM) at 10 sites in the
city. The selection of MAM sites was made based on the
availability of open space closer to damaged and undamaged
areas (Fig. 1). The MAM at seven sites was performed using
a 15 and 30 m triangular geometry. Additionally, a slightly
larger aperture using 30 and 60 m was considered at three
sites to explore deeper structures. In MAM, all the stations
were preferably placed in a circular array (Garofalo et al.,
2016). We explored the VSstructure down to 100 m depth
at site number 1 in the damaged area and at site numbers 3
and 4 in the undamaged areas for understanding the deeper
seismic response attribute (Fig. 6). Seven sets of three
Figure 2. (a) A typical shot gather record of 24 channels, at site 5 (see Fig. 1for
location). The geophone interval is 2 m and the total spread length is about 50 m.
(b) Rayleigh-wave dispersion curve in the 4.5–50 Hz frequency range obtained by
processing the shot gather. The scale is shown at the bottom of the figure. The color
version of this figure is available only in the electronic edition.
Influence of Local Site Effects in the Ahmedabad Mega City on the Damage due to Past Earthquakes 2173
components Lennartz LE-3D seismometers each coupled
with a Cityshark II system were used in this survey. The sys-
tem has a master remote control that is used to trigger all the
seven stations at the same time, to avoid any phase shift
(Chatelain et al., 2000). For reliable experimental conditions,
the guidelines proposed by Koller et al. (2004) in the frame-
work of Site EffectS Assessment using AMbient Excitations
(SESAME; WP02 of SESAME Project), Garofalo et al.
(2016), and Foti et al. (2017) have been followed.
For all the MAM sites, the fundamental-mode
dispersion curves of Rayleigh waves were determined using
the frequency–wavenumber (f-k) analysis (Capon, 1969), as
implemented in the Geopsy software package (Wathelet
et al., 2008). This method is based on the picking of energy
maxima in the wavenumber plane for sliding time windows
and narrow frequency bands. The method provides azimuth
and slowness for the most coherent plane-wave arrivals. A
wavenumber grid search is performed using an iteratively re-
defined grid adjustment to allow for the accurate estimation
of local maxima. The central frequencies of the frequency
bands were sampled equidistantly on a logarithmic scale, and
the frequency bandwidth was defined to be 6% of the cor-
responding center frequencies. The time-window length was
chosen as 200 times the central period. The dispersion curves
are extracted for each of the subarrays considering the res-
olution limits arising from the array geometry and combining
the individual dispersion curves into one. In this study, we
gathered the energy maxima for all the array sizes into one
single histogram and picked the resulting dispersion curve,
the minimum and maximum wavenumber limits for each
subarray being determined from their theoretical array
response. The dispersion curve was selected based on the
approach of Wathelet (2008). The dispersion curves of
smaller and larger subarrays were merged into a single curve,
which was inverted using the Dinver software following the
recommendations of Wathelet et al. (2008). The advantages
and disadvantages of the f-kmethod are clearly described by
the Garofalo et al. (2016) and Foti et al. (2017).
The phase slowness at larger aperture (60 m) MAM sites
(sites 1, 3, and 4) shows a typical behavior. At all these sites,
reliable dispersion curves are obtained between 1.0 and
Figure 3. The 2D VSmodel at site 5 estimated by the MASW
method. At upper 8 m, VSis ∼200 m=s, while lower VS∼180 m=s
layer is observed at 5–7 m depth. The scale is shown at the bottom
of the figure. The color version of this figure is available only in the
electronic edition.
Figure 4. (a) A comparison of the shear-wave velocity (m=s)
model, obtained from the PS logger, MASW, and MAM models;
(b) lithologs down to depth of 30 m at site 5. The water table
>20 m is observed in the city. Codes in the legend column are given
following Indian Standard (IS-2002). The color version of this fig-
ure is available only in the electronic edition.
Figure 5. A close comparison of the SPT N160model at the
damaged and undamaged areas. The BH-1 and BH-2, and BH-3 and
BH-4 are represented damaged and undamaged areas, respectively.
The color version of this figure is available only in the electronic
edition.
2174 B. Sairam, A. P. Singh, V. Patel, V. Pancholi, S. Chopra, V. K. Dwivedi, and M. Ravi Kumar
6.0 Hz, and phase slowness between 0.0008 and 0:005 s=m.
At the damaged sites, the dispersion curves are obtained at
the 1.5–3.2 Hz frequency range with phase slowness between
0.0017 and 0:004 s=m. On the other hand, two sites that are
outside the damaged area show the lowest slowness, between
0.0006 and 0:0028 s=m, in 1.0–6.0 Hz frequency range. In
all the cases, the phase slowness increases as the frequency
increases (Fig. 6). The damaged sites are characterized by
very soft and thick sedimentary layers.
The inversion is performed using the neighborhood algo-
rithm (Sambridge, 1999a,b). This algorithm is based on sub-
dividing the solution space into Voronoi cells and refining the
sampling in regions of low misfit during each iteration. The
misfit is defined by the distance between the calculated and
observed dispersion curve, at every frequency sampling point.
When various modes are identified, inversion of all the modes
is performed by employing a multimodel misfit (Wat helet
et al., 2008). A simple two-layer model based on MASW
and PS logging observations at the respective sites is chosen
as the starting model. The density in each layer was kept con-
stant. The misfit values are indicated to judge the best-ob-
tained velocity model out of the several models used for
inversion. We compared the VSestimated from MAM with
those from MASW and PS logging, and found that they are
well correlated at shallower depths (Figs. 4and 7,and
ⒺFigs. S1 and S2). A spatial distribution map of VS30
was prepared, combining the MASW, MAM, and PS logging
measurements (Fig. 8).
Analysis of Broadband Earthquake Records
In the city of Ahmedabad, the Institute of Seismological
Research installed broadband velocity seismometers
Figure 6. Results of MAM analysis at (a) damaged area (site 5)
(b) undamaged area (site 3) and (c) undamaged area (site 4). The
left-side part of plots show the dispersion curves at the sites (see
Fig. 1for locations). The right-side plots show the VSprofiles ob-
tained from inversion of the dispersion curves. The observed
dispersion curves are represented by black dots. The scales indicate
the misfit of the models and the data. The MAM has revealed
VS≥400 m=s at 36 m depth in damaged; while VS≤600 m=sin
the undamaged sites. The color version of this figure is available
only in the electronic edition.
Figure 7. A close comparison of the VS(m=s) model at the dam-
aged and undamaged areas. VSmodels in the damaged areas (sites 1
and 2) are shown as dashed lines; whereas VSmodels in the undam-
aged areas (sites 3 and 4) are shown as thick lines. The color version
of this figure is available only in the electronic edition.
Influence of Local Site Effects in the Ahmedabad Mega City on the Damage due to Past Earthquakes 2175
(120 s–50 Hz frequency range) at 12 sites and recorded
earthquake signals over a minimum period of one month at
each site. The epicentral distances of the earthquakes
are ∼250 km. Recoding was continuous at the rate of
100 samples=s. We employed single-station estimates of
HVSR using 24 local earthquake records (M≥3:5). A time
window of 10.5 s beginning 0.5 s before the S-wave phase
arrival was used to compute the fast Fourier transform (FFT).
This time-window length was chosen such that it contains
most of the S-wave energy. The FFT was applied to all the
three components of earthquake records, and the amplitude
spectra were smoothened following the technique of Konno
and Ohmachi (1998). The estimated HVSR of 12 sites is
shown in Figure 9. We compared the response of damaged
(B1 and B2) and undamaged sites (B3 and B4) in de-
tail (Fig. 10).
Comparison between VSProfiles and Analysis of
Uncertainties
We compared profiles estimated using different geo-
physical and geotechnical observations (Figs. 4and 7, and
ⒺFigs. S1 and S2). These comparisons provided an oppor-
tunity to evaluate the accuracy of the employed approaches.
The Jacobian matrix is used to estimate uncertainties on the
dispersion curve, which is linked to the shear-wave velocity
profile (Lai et al., 2005;Foti et al., 2017). It is well known
that due to the data uncertainties in active and passive
methods and the nonlinearity of the problem itself, the sol-
ution of the observed dispersion curve inversion is generally
nonunique (Garofalo et al., 2016;Foti et al., 2017). Thus,
uncertainties in the determination of VSare directly linked
to the errors on the dispersion curves and to the frequency
range over which the curve is retrieved. Uncertainty in the
dispersion curve may also depend on the number of receiv-
ers. However, we have used recommended receiver configu-
rations in MASW and MAM that are important for spatial
sampling of the wavefield. On the other hand, in the MAM
inversion, considering the misfit ranked set of inverted
results, it is additionally possible to judge the uncertainty in
the modeled parameters, usually represented in terms of the
minimum misfit solution, often without any uncertainty
estimates. Parameter uncertainties are underestimated when
data error correlations are ignored and when VP,VS, and
density parameters are fixed in the inversion. The corre-
sponding depth curves in the MAM show layers at all depths
being resolved with confidence. Additionally, the standard
deviations for the HVSR peak frequencies deduced directly
from the curves are 1in all the cases.
It is found that uncertainty is lesser for the shallow
layers, in terms of the average relative difference in VSbeing
<3%. However, the VSvalues at deeper depths are signifi-
cantly overestimated with large scattering (15%). These de-
viations mostly appear at deeper depths where very stiff
formations with large impedance contrast do not allow the
penetration of sufficient amount of the source energy. We
noticed that the disparity among the different geophysical
and borehole data is generally 10% for PS logging, 12% for
MASW and MAM, and 13% for geotechnical approaches. A
maximum of 20% disparity could be adopted as a realistic
bound for the shear-wave velocity (Stephenson et al.,
2005). In addition, the uncertainties of the VS30 measure-
ments using MASW are 30 m=s; whereas uncertainties of
the VSmeasurements in the MAM are observed similar to the
MASW at shallower depths.
Results and Discussion
In this study, most of the HVSR peaks have more or less
a bent shape, which indicates lack of high impedance con-
trast with the formations changing gradually as semiconso-
lidated material overlying the beds. A spatial distribution of
the predominant frequency map was prepared, combining
the estimated HVSR results of the microtremor and earth-
quake records (Fig. 11). The results show that the predomi-
nant frequency mostly varies in the 0.7–2.2 Hz range in the
city. A few peaks are observed in the frequency bands of
4.0–10.0 Hz (Figs. 9and 10). Variations in the predominant
frequency confirm the differences in the nature of sediments.
We believe that the difference in thickness may be due to
erosion of soft sediments and deposition by rivers or lakes
(Parvez and Rosset, 2014).
Figure 8. Map showing the VS30 variation in the study area.
Squares are the locations of the damaged or collapsed buildings
due to the Bhuj earthquake (Bhandari and Sharma, 2001;Govindar-
aju et al., 2004). The color version of this figure is available only in
the electronic edition.
2176 B. Sairam, A. P. Singh, V. Patel, V. Pancholi, S. Chopra, V. K. Dwivedi, and M. Ravi Kumar
Both earthquake and microtremor observations show
high amplification factors (3.3–6.6 times) in the 1.5–2.0 Hz
frequency, especially in the damaged areas (Table 1). How-
ever, the amplifications below a factor of 2 are observed in
the 4–10 Hz frequency range. Amplification less than 2 may
not be significant, in accordance with the SESAME (2004)
guidelines. At such a large distance from the epicenter, the
energy of high-frequency waves attenuates and the most en-
ergetic spectral component of seismic waves is contained in
the lower frequencies, which are amplified due to thick sedi-
ments in the study region. Consequently, the high-rise build-
ings that have low-resonant frequencies are expected to
suffer damages. Because most of the collapsed buildings had
five and above stories, they might have been affected by the
resonant frequencies. A few researchers have estimated the
natural period of 5–11 story buildings in both the damaged
and undamaged areas and found them to vary between 0.3
and 1.0 s (Table 2; e.g., Kono and Tanaka, 2001;Govindraju
et al., 2004), closer to the natural period of the sites. The
natural period of a building can be calculated using an
elementary relationship Tn=10, in which Tis the period
and nis the number of stories (BSSC, 1995). This empirical
formula is restricted to buildings not exceeding 12 stories,
having a minimum story height of 3 m (BSSC, 1995).
The estimated natural frequencies of 5–12 and 1–2 story
buildings notionally correspond to the 0.8–2.0 Hz and
5–10 Hz ranges, respectively (Table 1), which is well cor-
roborated with the observed natural periods of the buildings
in the city of Ahmedabad. The observed and calculated data
of the buildings reveal that the natural periods of buildings in
the damaged area are different from those in the undam-
aged area.
The HVSRs show high amplification factors (3.0–6.6
times) in the 0.7–2.2 Hz frequency, which can resonate more
than five-floor buildings. Consequently, the building vibra-
tion may be increased many fold due to resonance phenom-
ena. We may also interpret that the mid-to-high-rise
buildings in the city of Ahmedabad can resonate strongly
Figure 9. Horizontal-to-vertical (H/V) spectral ratio (HVSR) from earthquake records of the study area. Sites B1, B2, and B5 are in the
damaged area, whereas sites B3 and B4 are in the undamaged area. Sites B6–B12 are at locations surrounding the damaged area. HVSRs
show high amplification factors (3.0–6.6 times) in the 1.5–2.0 Hz frequency, which can resonate more than five-floor buildings.
Influence of Local Site Effects in the Ahmedabad Mega City on the Damage due to Past Earthquakes 2177
with the ground vibrations and may experience more dam-
ages due to large earthquakes from the Kachchh region. One
and two story buildings are less prone to resonance effects in
the city due to far-field earthquakes.
The time-averaged velocity of the top 30 m (VS30) was
computed for each site (see details in ⒺFigs. S1 and S2, and
Table S1). Finally, a contour map of VS30 for the city of
Ahmedabad is presented (Fig. 8). In this study, the point data
are interpolated or extrapolated over the area of interest by
the Kriging method. The VS30 value in the study area gen-
erally varies in the 260–360 m=s range. The area falls in the
D-type soil according to the NEHRP classification. Addi-
tionally, geotechnical investigations at 16 sites down to 80 m
depth show that the sandy and clayey layers are present
alternatively from top to the termination depth (Fig. 4).
We noticed that the top 6 m contains very loose fine-grained
silty sand, followed by a layer from 6 to 26 m depth contain-
ing relatively compact silty sands with significant patches of
silts that overlie a 7–15-m-thick strong clay layer. The upper
6 m layer contains very recent deposits, the second layer
(6–26 m depth) has recent deposits, and the third (clayey
layer) may represent a formation of Pleistocene (Merh and
Chamyal, 1993). The depth to the water table is greater than
20 m in the city. Because the water table is slightly deeper,
liquefaction may not be expected. Also, no case of liquefac-
tion was reported during the past earthquakes in the city of
Ahmedabad (Govindaraju et al., 2004).
In the case of MAM analysis for larger aperture, we
observed a lower VS∼220 m=s down to 36 m depth in the
damaged area, whereas a higher VS∼400 m=s is seen in the
outside area. A VSvalue ≥400 m=s is encountered at 36 m
depth in the damaged area, whereas it varies between 16 and
20 m depth outside. Furthermore, the VSvalue is ≥800 m=s
at about 90 m depth in the damaged area, whereas this value
is reached between 40 and 56 m depth in the undamaged
areas. The results suggest that the damaged sites may consist
of softer soil deposits and/or filled sediments (e.g., Bhandari
and Sharma, 2001;Goel, 2001;Parvez and Rosset, 2014).
The results from MAM and MASW methods at shallower
depths are in agreement, both in terms of layer depth and
velocities (Figs. 4and 7). The results are not comparable
at deeper depths, because MASW has less probing potential
compared with the MAM (Singh, 2015). The reliability of
the inverted models has also been checked by means of
forward modeling. A compact soil may be present at deeper
depths beneath the damaged sites compared to the other sites.
Figure 11. Spatial distribution of fundamental resonance
frequency (Hz), the Sabarmati River is shown by curvilinear lines.
Triangles show the measurement points; while the squares show the
damage patterns. The frequency scale is shown in the bottom of
each map. The color version of this figure is available only in
the electronic edition.
Figure 10. Comparisons of HVSR in the (a) damaged area
(sites B1 and B2) and (b) undamaged area (sites B3 and B4).
The color version of this figure is available only in the electronic
edition.
2178 B. Sairam, A. P. Singh, V. Patel, V. Pancholi, S. Chopra, V. K. Dwivedi, and M. Ravi Kumar
The VSprofiles down to 100 m, at a damaged site A1 and
undamaged sites A2 and A3, resulting from the inversion of
MAM data are shown in Figure 6, with a scale indicating the
misfit value (see Fig. 1for site locations). These indicate a
remarkable difference in the soil structure. All the models are
fairly consistent with minimum error or misfit. The shallow
VSmay be statistically correlated with deeper VS(Boore
et al., 2011). Although a tertiary layer down to depth of about
1000 m is observed, the depth of the actual bedrock is not
very well known in the city. However, a study of the seismic-
ity and tectonics of the Gulf of Cambay indicated that the
graben comprises up to 5 km of post-Mesozoic clastic and
shallow marine deposits that overlie the Deccan Trap basalt
flows (Mukhopadhyay and Arora, 1999). They reported the
thickness of basin sediments near Ahmedabad to be
around 3 km.
The VS30 values in the damaged areas are in the
260–305 m=s range (Fig. 8), compared to 305–360 m=sin
the undamaged areas (Table 3). The old city area of Ahmeda-
bad has the highest VS30 values in the 340–360 m=s range.
Low VSvalues are observed in places covered by ancient
lakes, ponds, and paleochannels. Thus, the damage scenario
may be due to the presence of much younger and unconsoli-
dated Holocene sediments that were the key factors, in
addition to the resonance effect. In the damaged areas, the
major contrast in VSwas observed at a depth of 26 m. In the
undamaged areas, VSincreases gradually with depth and no
significant contrast is observed between the layers. In the
damaged areas, the average VSof the top 26 m layer is about
260 m=s. Our estimates of the predominant frequency (f)
due to the contrast at 26 m depth (h), estimated using the
formula fVS=4h, reveal a predominant frequency of
∼2:5Hz. More layers with contrast in shear velocity may
exist at deeper depths also, because our HVSR estimates
from broadband seismograph and microtremor data also
show predominant frequencies at 2.5 Hz and below (Fig. 11).
One of the reasons for the damages of buildings could be
amplification and resonant frequency due to layers at
26 m and deeper depths. In the damaged areas, the soil at
the top 30 m depth is D-type. However, in the undamaged
area, the top 15 m is of D-type, followed by a C-type soil.
The comparison of site attributes in the damaged and unad-
amged areas of Ahmedabad is summarized in Tables 1and 3.
The lithologs of the study area show significantly differ-
ent characteristics of the soil in the damaged and undamaged
areas. A comparison of the corrected SPT N-values, N160of
the damaged and undamaged areas, reveals that the N160
values of the soil strata in the damaged areas are less than
those in the undamaged areas (Fig. 5). In the damaged area,
the top 6 m layer shows N160values that are in the 10–20
Table 1
Comparison of Site Effects in the Damaged and Undamaged Areas of Ahmedabad
Parameters Damaged Area Undamaged Area
Area locations The distribution of collapsed buildings is confined to certain
pockets such as those along the western side of the
Sabarmati river and along lakes and ponds
The old town is located on the eastern side
of the river
Building types Most of the buildings were of 5 stories and a few of 10 stories Old city area has low-rise buildings
Natural frequencies of mid-to-high-rise buildings: mid-rise
buildings, 5–12 stories: 0.8–2.0 Hz; high-rise buildings, 5
and above stories: below 2.0 Hz
Natural frequencies of low-rise, 1–2-story
buildings may be in the 5.0–10.0 Hz
range
Soil strength; VS30=N160values VS30 ranges from ∼265 to 300 m=sVS30 ranges from ∼310 to 360 m=s
Lower N160values generally found down to 30 m depth Higher N160values generally found
down to 30 m depth
Refusal N160encountered at 20–30 m depth and downward Refusal N160encountered at 6–12 m
depth and downward
Top 2 0 –30 m soil is weaker Soil is stronger
Site-response estimates Amplifications in different frequency ranges: 5.6–6.6 in
0.6–2.0 Hz; 2.0–3.0 in 2.1–4.9 Hz; 1.0–2.0 in 5.0–10.0 Hz
Amplifications in different frequency
ranges: 3.0–3.3 in 0.6–2.0 Hz; 1.1–2.4
in 2.1–4.9 Hz; 1.0–1.4 in 5.0–10.0 Hz
Table 2
Natural Frequencies of Mid-to-High-Rise Damaged Buildings
Serial
Number Name of Building
Number
of Stories Height (m)
Natural
Frequency (Hz)
Damage Level
due to Earthquake
1 Akshar Deep 5 14.2 1.49 4
2 Siddhi Flat 5 15.6 1.79 3
3 Mansi Complex 11 30.7 1.02 3
4—4–11 —1.5–33
Source: Kono and Tanaka (2001) and Govindraju et al. (2004).
Influence of Local Site Effects in the Ahmedabad Mega City on the Damage due to Past Earthquakes 2179
range, whereas in the undamaged area, the N160values are
30–40. In the damaged area, the refusal N160values
(N160≥100) are encountered from 20 m onward (Fig. 5).
In contrast, in the undamaged areas, the refusal N160values
are encountered at 6 m depth onward. In the damaged area,
the density of soil in the top 20 m depth is in the
1400–1700 kg=m3range, whereas in the undamaged area,
the Bulk density is more than 2000 kg=m3. Lower N160
and density values indicate that the soil of the damaged area
is weaker in strength than the soils of the undamaged area.
The amplifications of ground motions are high in the areas of
low VSand N160, which may affect the built environ (Bend-
ick et al., 2001;Bhandari and Sharma, 2001;Goel, 2001;
Hough et al., 2002).
Some researchers reported that the damaged buildings
had several structural defects such as open ground story,
short column, poor quality of the construction, soft story, and
irregularities in the buildings that lead to large damage (Goel,
2001). However, such buildings are expected to be randomly
located and therefore cannot give rise to the observed
clustered damage. Our integrated study shows that the site
amplification at the predominant frequency coinciding with
the natural frequencies of the damaged buildings, in addition
to the loose unconsolidated soil in the damaged areas, con-
tributed to the heavy damage, besides the poor construction
practices.
Conclusions
We present results of integrated geophysical and seismo-
logical investigations together with geotechnical studies in
the city of Ahmedabad and attempt to understand the causa-
tive factors for the damage patterns due to past earthquakes.
HVSR curves reveal that the amplification factor is approx-
imately two times higher in the damaged area, with the
predominant frequency (0.7–2.2 Hz) matching with the
mid-to-high-rise buildings. Therefore, the mid-to-high-rise
buildings could have resonated with much-amplified ground
vibrations and experienced heavy damage during the 2001
Bhuj earthquake. The estimated shear-wave velocities show
VS∼320 and 220 m=s at 6 m in the undamaged and dam-
aged areas, respectively. Our estimates show a good corre-
lation with the site characteristics and damage pattern,
suggesting that the strength of soil layers at varying depths
is a dictating factor for damaged scenarios.
Data and Resources
We used the data acquired by the Institute of Seismo-
logical Research (ISR) Gandhinagar, Gujarat, India
(www.isr.gujarat.gov.in, last accessed February 2017).
Shear-wave velocities were estimated using multichannel
analysis of surface waves (MASW), PS logging, and micro-
tremor methods. Geometrics Geode Seismograph was used
to acquire the data for MASW. PS logger of OYO Inc., Japan,
was used for PS logging. A set of seven Lennartz LE-3D
seismographs with Cityshark II system was used in micro-
tremor surveys. Surfseis software of Kansas Geological sur-
vey, United States, was used for the processing of the
MASW data. Broadband seismographs of Güralp, United
States, were used to acquire the earthquake data. Many of
the figures are produced using the Generic Mapping Tools
software (Wessel and Smith, 1998).
Acknowledgments
The authors sincerely thank Associate Editor Thomas Brocher and two
anonymous reviewers for their constructive comments, which improved the
article significantly. The landowners are acknowledged for permission to
carry out the field survey on their property. The authors would like to thank
Pritesh Chauhan, Bharat Mevada, and Mahesh Valekar, Technical Assistants
of Institute of Seismological Research (ISR), Gandhinagar, for their help in
the field campaigns. The authors are thankful to the Ministry of Earth Sci-
ences (MoES) for funding the study of Microzonation of the Ahmedabad
City, Gujarat (India).
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Institute of Seismological Research (ISR)
Raisan
Gandhinagar 382007
Gujarat, India
apsingh07@gmail.com
(B.S., A.P.S., V.Pat., V.Pan., S.C., V.K.D.)
CSIR-National Geophysical Research Institute
Hyderabad 500 007
Telangana, India
(M.R.K.)
Manuscript received 14 September 2017;
Published Online 10 July 2018
2182 B. Sairam, A. P. Singh, V. Patel, V. Pancholi, S. Chopra, V. K. Dwivedi, and M. Ravi Kumar
