10th Asian Regional Conference of IAEG (2015)

Estimation of subsurface structure using microtremor in Karaj city, Iran
Nastaran EHSANI (1), Mohammad Reza GHAEMGHAMIAN (1), Mohsen FAZLAVI (2)and Ebrahim HAGHSHENAS (1)(1) International Institute of Earthquake Engineering and Seismology, Tehran, IranE-mail: n.ehsani@iiees.ac.ir(2) Imam Khomeini International University, Qazvin, Iran
Abstract
In order to estimate the site effects in Karaj city, microtremor H/V spectral ratios
were used in 37 locations of the city along the north-south-west profile. The results showthat the dominant frequency changes in the range of 0.4 to 2 Hz. 1-D numerical simulation
was performed using Deepsoil software based on the linear method at low strain levels andresults of theoretical response were compared with fundamental frequencies obtained frommicrotremor measurements. The obtained modelling results are indicative of an effective
contrast at depth of 200 to 300 meters and deep contrast at a depth of about 2 kilometers
due to difference in bedrock type. Finally, using the results and according to the geologicalconditions of the region, subsurface structures in Karaj city were examined in
two-dimension along the studied profiles.
Keywords: Site effects, Microtremor, spectral ratio H/V, Dominant frequency, 1D sitemodelling
1.

Introduction
The earthquake is one of the most devastating
natural disasters that always threats human societies
in terms of health and financial issues. Iran is one of
the most seismic prone countries of the world due tolocating on Alpine- Himalayan Orogenic belt. On theother hand, growing population and increased
construction of tall buildings, increases the damages
caused by large earthquakes, especially in large cities.
Karaj is one of the most populous cities in Iran which
there has been considerable industrial and economic
development in recent years.
When an earthquake occurs, seismic waves
radiate away from source and travel rapidly throughthe earth crust. When these waves reach the groundsurface, they produce shaking that may last fromseveral seconds to a few minutes. During earthquakes,
different alluviums with different structures show
various reactions. It is well-accepted that, besides theearthquake magnitude and fault distance, local
geologic conditions, known as site effects, can also
exert significant influences on characteristics of theseismic waves such as amplitude, frequency content
and duration of strong ground motion at a given
location (Kramer, 1996). The seismic ground motion
at any site is influenced by the type of soil in that
region. Younger and softer soils usually amplify

ground motion more than older soils or bedrocks(Purnachandra Rao et al., 2011).
There are theoretical and experimental methods toevaluate the site response. In the present study, theNakamura's H/V spectral ratio method has been used
to evaluate the resonance frequency in 37 locations at
Karaj site. In addition, a preliminary 1-D site
response modelling has been conducted using
Deepsoil program according to downhole, array andgeology data. Site frequencies obtained frommodelling are presented and compared with site
frequencies
obtainedthroughmicrotremor
measurements.
2.

Geology and Seismicity of Karaj
Karaj city is situated 20 km west of Tehran, at thefoothills of the Alborz Mountains. It is the fourthlargest and most populous city in Iran which has beendeveloped on disaster-prone areas and tectonically
active region according to the seismic hazard map of
Iran.
The tectonics of the Alborz Mountain ischaracterized by boundary conditions, due toconvergence between Arabia and Eurasia, which
probably began in the Cretaceous simultaneouslywith establishment of the Alpine-Himalayan orogeny.
Andesitic and clastic rocks of the Karaj Formation1

10th Asian Regional Conference of IAEG (2015)

were composed as the result of the Pyrenees phase in
the Eocene with a thickness of more than 3300 m.
Continental sediments of the Red Formation was
deposited between middle-upper Miocene, which
consists of conglomerate, sandstone and siltstone andhas been located with disconformity on older rocks,
particularly on Tuffites of the Karaj Formation. The
Red Formation is considered as the bedrock in south
and west of the Karaj site. The last dominant orogenic
movements in Alborz occurred during the late
Pliocene-early Pleistocene due to the Pasadenianphase which lead to faulting and mild thrusting in the
Alborz Mountain. Conglomerate tall hills of
Hezardareh Formation (or A Formation) werecomposed as the result of this orogeny (Rieben, 1966).
The lower part of this Formation has a low porosity(Berberian et al., 1985) which can be considered as
the bedrock in north of Karaj site.
The Alborz Mountains have been eroded due tothe activities of Karaj and Kordan rivers as well as
the rivers and seasonal floods originating from thevalleys since the late Pliocene until now. Therefore,
Quaternary alluvial sediments were deposited on thesouthern slopes of the Alborz Mountains with an areaof 210 km2 and the Karaj city has been developing onthese soft and young deposits. Stratigraphy andlithology features as well as age of the Quaternary
alluvial deposits were studied by Rieben (1955) as
Kahrizak formations (B), alluvial sediments of
Tehran (C) and recent sediments (D).
Subsidence of the Caspian Sea floor and erosionof sediments at one hand, and subduction of oceanicof Caspian Sea crust beneath the Iran crust on theother hand, increases activity of faults in northern

Iran. There are many faults, fractures and lineamentsin the seismotectonic zones of Alborz. Historical andinstrumental studies of earthquakes in Alborz
indicated that many areas such as Rasht, Fasham,
Damavand, Tehran, Rey and Qazvin were destroyed
by large earthquakes. Therefore, the existence ofactive faults near Karaj city (Figure 1) is a threatening
factor in Karaj site (Karaj-Kordan, Mahdasht, North
Tehran thrust, Mosha-Fasham, Eshtehard, Kahrizak,
and Rey).
Therefore, evaluation of site response andassessment of natural hazard risk is very important inyoung alluvial sediments with respect to the geology
and seismicity at the Karaj site.
3.

Microtremor data collection and analysis
procedures
Single station microtremor measurements at the
Karaj site were carried out by the International
Institute of Earthquake Engineering and Seismology(IIEES) in 2012 with a three-component broadband
seismometer (Guralp CMG-6TD). In the present
study, we have used 37 microtremor data along thenorth-southwest profile because at this profile,
geological section was available and these stations
contained geotechnical boreholes data. Figure 2
shows location of microtremor recording stations andgeotechnical boreholes in nine sites.
Dynamic range of sensor changes between 0.033-50 Hz and has a natural period of 1 second. 24-bit
analog-to-digital (A/D) converter digitized therecorded data. The recording system was operated
continuously for about 30 minutes with sampling
frequency of 100 Hz.

Fig.1 Active faults map of Karaj site.

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10th Asian Regional Conference of IAEG (2015)

Fig.2 Karaj engineering geology map and location of microtremor recordings stations, array and downhole data.

The use of ambient vibrations for analysis of thelocal site effects has been studied in detail in theframework of the European research project
SESAME (Site Effects Assessment Using Ambient
Excitations). The recommended guidelines on the
H/V spectral ratio technique are the result of thecomprehensive and detailed analysis performed by
the SESAME participants during three years ofinvestigations (2001-2004).
H/V spectral ratio was carried out by the Geopsy
software (Wathelet, 2007). The process starts by
converting data from binary format to ASCII format.
After DC offset removal, eighth order Butterworth
band pass filter used within the range of 0.1 Hz to 50
Hz. The Anti-triggering algorithm STA/LTA has
been selected to reject energetic transients fromambient vibration recordings, so STA and LTA wereconsidered respectively 1 and 30 second. Minimum
and maximum STA/LTA thresholds were selected
between 0.2 and 2.5. For each station, the time-seriesof the record is divided into windows of 40 to 100
seconds in three components with an overlap of 50%.
Also, a cosine taper with the length of 5% of the total
window length was used at each end.
The amplitude spectra of each selected window iscomputed with a fast Fourier transform (FFT) andsmoothed using the Konno-Ohmachi function(Bandwidth=40). Then, two horizontal components
are merged by squared average. Finally, the H/Vspectral ratio of Nakamura is applied for eachindividual window, and the final predominantfrequency is obtained by averaging the H/V spectral
ratio of all windows (Nakamura, 1989).
The presence of clear peak on H/V spectral ratio
curve is indicative of the impedance contrast between
the uppermost surface soil and the underlying hardrock, where large peak values are generally
associated with sharp velocity contrasts, and is likely
to amplify the ground motion (Purnachandra Rao etal., 2011). The H/V spectral ratio in some stations
shows a clear peak and at the others might show twoor multiple peaks which represents the geologicallycomplex areas. The H/V analysis results as well as
their average for six stations in site 8 have beenshown in figure 3. Calculated dominant frequencychanges between 0.4 and 2 Hz. These low values
indicate the existence of basement at greater depths
and large thickness of sediments on basement(Parolai et al., 2002).

Fig. 3 The H/V results at six stations of site 8
with their mean.

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10th Asian Regional Conference of IAEG (2015)

4.

Site modelling
The results of H/V spectral ratio are affected by
the local geologic structure. Based on this assumption,
we can produce theoretical H/V curve withknowledge of the geologic structure in the area
(Harutoonian et al., 2010). One-dimensionalmodelling is a suitable method to evaluation of thesite response due to the local geology which requires
geotechnical and geophysical data. In the
one-dimensional modelling, it is assumed that allboundaries are horizontal in the infinite media and the
response of a soil deposit is predominantly caused by
SH-wave propagating vertically from the underlying
bedrock (Kramer, 1996).
In this present study, one-dimensional modelling
was carried out using Deepsoil software (Hashash etal., 2009). Due to the very small deformations in soils
by microtremor and producing a low levels of strain,
we applied the linear method to evaluate the groundseismic response during mild earthquake shakes. In
this software, homogeneous and isotropic soil profile
is considered as N horizontal layers. The site response(transfer function) is evaluated by parameters such as
layer thickness (m), density (ρ), shear modulus (G),and damping factor of layers (β), which are obtained
from available geotechnical boreholes.
Usually, engineering bedrock is considered forthe purpose of numerical modelling. According to
TC4 (1994), the seismic bedrock was defined as alayer with a shear wave velocity of more than 600
m/s. Shima (1978) recommended that the upper crustwith a shear wave velocity of about 3000 m/s, isadopted as bedrock when large scale structures withlonger vibration period are being considered.
International building code (ICC2000) has defined
the seismic bedrock by a shear wave velocity of morethan 760 m/s. According to Unified Building Code(UBC97), bedrock is defined into two groups: A(very hard rock with a speed of more than 1500 m/s)
and B (rock with a speed of 760 to 1500 m/s).
Therefore, the proposed values of the shear wave
velocity are different for considering seismic bedrock.
In order to consider the uncertainty of the shear wave
velocity in the present one-dimensional modelling,
three scenarios for the bedrock, were performed withthree speeds of 760 m/s (based on engineering
bedrock), 1300 m/s (bedrock geology), and 2500 m/s(corresponding to tuff-andesite of the Karajbasement) at different depths, according to theregional geological map. Then, three scenarios of thenumerical modelling were compared withmicrotremor transfer function.

were used in nine sites (Fig. 2) which were available
up to the maximum depth of 50 meters at 20
boreholes and 96 meters at A09 borehole (Figure 4).
Therefore, low thickness of alluvium (about 17-85
meters) was considered with engineering bedrock(>760 m/s) for numerical modelling. The results
represent higher frequency range compared with themicrotremor data (Figure 5).
In some previous studies where engineering
bedrock had been defined by shear wave velocity
values between 700 to 800 m/s in 1-D modelling, theresults of the theoretical model is incompatible withexperimental results (Kamalian et al., 2008; Jafari etal., 2005). Thus, it seems that it is not suitable toconsider the engineering bedrock in 1-D modelling.

Fig.4 Shear wave velocity profile obtained fromdownhole test at A09 borehole (site 8).

Fig.5 Comparison of empirical and theoreticaltransfer functions at site 8, by considering theengineering bedrock (>760 m/s) at depth of 85
4.2 One-dimensional modelling at Karaj site using
microtremor array data for geology bedrock (>1300 m/s)
By considering the seismic bedrock (>760 m/s)
at depths of 17 to 85 meters and calculating the
one-dimensional transfer function, the peaks in higher
frequency compared with the experimental method isobserved. According to reliability of experimental
H/V results which has been proved by researchers
around the world (Haghshenas et al., 2008), thedifference between the transfer function results in

4.1 One-dimensional modelling at the Karaj siteusing downhole data for engineering bedrock (>760 m/s)In order to access the shear wave velocity profile
for 1-D modelling, downhole data from 21 boreholes

410th Asian Regional Conference of IAEG (2015)

experimental and theoretical methods indicates thattwo variables of shear wave velocity or depth of
bedrock and alluvium thickness have not been
properly modeled. It seems that in order to get better
results, it’s necessary to analysis by considering thegeology bedrock at greater depth. Tchalenko, et al.,(1974) considered lower part of Plio-Quaternary
sediments of Hezardareh Formation and Miocene
marl-limestone of Upper Red Formation as the
bedrock in the Karaj plain. Shafiee and Azadi (2006)
computed shear wave velocity characteristics of these
geological units throughout Tehran city (table 1).
Therefore, a mean velocity of 1300 m/s wasconsidered for the geology bedrock during themodelling.

according to lithology of the region. The result of thismodelling has shown a peak at frequency range of
0.87 Hz that is compatible with the microtremor
peaks at this site. The results and input data for thismodelling are presented in table 2 and figure 7.

Fig.7 Comparison of empirical and theoretical transfer
functions at site 8, by considering the geology bedrock(>1300 m/s) at depth of 230 m.

Table 1. The mean shear wave velocity ofgeological units throughout Tehran city
(Shafiee and Azadi, 2006).

In other site this modelling was performed usingarray and downhole data. The results indicated thatthe first effective contrast occurs at depth of 200 to
300 meters.
Table 2. Input data in order to numericalmodeling at site 8.

In order to access the shear wave velocity
profiles at greater depths, microtremor array stations
were designed by seven seismometer with 100 m
radius at A09 (site 8) borehole. The results have been
illustrated in Figure 6. As it can bee seen, a clear
contrast at a depth of about 230 m is observed.
Therefore, the modelling was carried out by taking
230 m alluvial thickness on geology bedrock

4.3 One-dimensional modelling at the Karaj site
for basement (> 2500 m/s)
Transfer functions obtained from the previous
model, did not cover low frequency peaks in theexperimental methods. Therefore, the presence ofother low-frequency peaks is either due to thegeometry of the sedimentary basin or deep contrast. It
seems that due to the geology of the region, tuffandesite of the Karaj Formation as basement plays animportant role in the creation of low-frequency peaks.
Therefore, to obtain a better model, deep contrast wasconsidered about 2 kilometers due to differences inthe type of bedrock with a shear wave velocity of2500 m/s (Table 1).
For this purpose, according to the properties of
the Upper Red Formation, an average constant speedof 1400 (m/s) was considered in modelling and bychanging the thickness of this layer, the modelling
was continued in a trial and error manner until thenumerical model is consistent with microtremor

510th Asian Regional Conference of IAEG (2015)

peaks. The modelling results in nine site indicate that
there is basement at the depth of 2000 to 2250 meters.
Comparison of empirical and theoretical transfer
functions at site 8, are presented at figure 8, byconsidering the Karaj basement (>2500 m/s) at depthof 2200 m.

1-D modelling, thickness of this layer is about 200 to
300 m which has been deposited on geology bedrock.
As mentioned before, lower parts of Hezardareh
Formation at the north of Karaj and Upper red
Formation in the south west of Karaj are considered
as geology bedrock. Upper Red Formation was
deposited with unconformity on tuff-andesite of the
Karaj basement at depths of 2000 to 2250 meters.
6.

Conclusions
The use of empirical methods based on
microtremor is an efficient way to estimate the site
effects in Karaj city, although the use of earthquake
records could provide better evidence of the depth
and geometry of basement. One-dimensionalmodelling of shear wave velocity profiles obtained
from downhole data and considering the engineering
bedrock (> 760 m/s) at depths of 17 to 85 meters, is
not a good way to estimate the dominant frequency of
alluvium. By considering the greater depth of
alluvium and using shear wave velocity profiles
obtained from microtremor array, 1-D modelling was
carried out for geology bedrock (1300 m/s).
Therefore, peak frequency in transfer function at therange of 0.87 Hz has been associated with effective
contrast at depths of 200 to 300 meters. It seems that
Karaj basement (> 2500 m/s) with about 2 kilometers
depth plays an important role in the production of
low-frequency peaks in transfer function.

Fig.8 Comparison of empirical and theoreticaltransfer functions at site 8, by considering the Karajbasement (>2500 m/s) at depth of 2200 m.
5.

Two-dimensional model of the Karaj site
Using the one-dimensional analysis andevaluation of the geological map of the area, twodimensional geological structure was rebuilt instudied profiles which is shown in Figure 9. Greenand gray tuffs and igneous rocks of Karaj Formation
outcrops in north of Karaj and constitute the Alborz
Mountains. This Mountains eroded by the action ofrivers and were deposited in the form of large alluvial
fans. Coarse sandy sediments were deposited near
mountains wherein energies of rivers and streams
were extremely high (site 1 to 4). Furthermore,fine-grained sediments were deposited at far distancesby decreasing in the energy of streams (site 5 to 9).
Berberian et al (1985) divided B Formation in two
parts: heterogeneous deposits of sand, gravel, rockand clay in north of Tehran (Qbn) and silts and clays
of Kahrizak (Qbs) in south of Tehran. According toReferences
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Fig. 9 Two-dimensional model of Karaj subsurface structure (Derived from the geological map with changes)
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