Journal of Natural Disaster Science, Volume 25, Number 1, 2003, pp37-46

Microtremor Measurements and Building Damage during
the Changureh-Avaj, Iran Earthquake of June 2002
Abdolhossein FALLAHI
Doctoral Student, Graduate School of Natural Science and Technology, Kanazawa University, Japan
Reza ALAGHEBANDIAN
Assistant Professor, Earthquake Engineering Research Center, Department of Civil Engineering, The University of Tehran, Tehran, Iran
Masakatsu MIYAJIMA
Professor, Department of Civil Engineering, Faculty of Engineering, Kanazawa University, Japan(Received for 13. May., 2003 and in revised form 6 Aug., 2003)

ABSTRACT
A great many buildings were severely damaged or collapsed during the earthquake that struck the northwesternpart of Iran on June 22, 2002. We investigated the damage done to buildings and carried out microtremor measurements. Adobe buildings were completely destroyed or suffered heavy damage. Un-reinforced masonry(URM) structures commonly were damaged because of poor construction of their bearing walls. Absence of a lateral resistance system was one cause of failure of semi-steel structures. To examine the ground surface and structural seismic response characteristics in the stricken area, microtremor measurements were conducted on the
ground surface in several locations in Abdareh Village, in which almost 100% of the adobe dwellings were completely destroyed, and on an adobe house in Tablashkin Village. A Fourier analysis, based on the H/V spectral
ratio, was made to explore the relationship between the strong ground motion of the earthquake and the damage
done which may have been affected by local site conditions. Building damage could be explained by the goodcompatibility between ground microtremor findings and the dynamic characteristics of adobe buildings which collapsed in many places but survived in part of Abdareh.

1.

OUTLINE OF THE EARTHQUAKE AND ITS DAMAGE

The Changureh-Avaj earthquake of 22 June, 2002, Mw = 6.3(07:28 local time, 02:58 GMT) was strongly felt in most parts ofnorthwest Iran. The epicenter, about 250 km west of Tehran, wasin a roughly east-west oriented valley just west of Abegarm
Village (Fig. 1). The greatest damage occurred in the villages of

Abdareh and Changureh. The official report lists 230 fatalities,
1,466 injured, and more than 50,000 people left homeless. The
earthquake affected 4 cities and 50 villages (most of which werepartially damaged) in four provinces. The macroseismic intensityof VIII+ (EMS-98 scale) was assigned to Abdareh and Changurehin the epicentral area. Based on the length of the surface fissuresand estimated focal depth of 7 km, the magnitude Mw = 6.3 was

4B'E

49'E

50'E

36'N

35'N
4B'E

Fig. 1

36'N

35'N
49'E

Site map showing the earthquake affected area (IIEES website)

KEY WORDS: building damage, adobe houses, microtremors, H/V spectral ratio

50'E

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A. FALLAHI, R. ALAGHEBANDIAN AND M. MIYAJIMA

estimated for this event. Preliminary source parameter estimatesindicate a high stress drop and fast ground motion attenuation(Zare, 2002).
2. STRONG GROUND MOTION CHARACTERISTICS
The main shock of the earthquake was recorded by many digital accelerographs (Building and Housing Research Centre, BHRC
website; Farzaneghan et al., 2002). A scatter graph of the peakground acceleration, PGA (average of values in two horizontal
directions), of the recorded data versus epicentral distance isshown in Fig. 2. The regression curve for the recorded data wascalculated by a general equation for the attenuation relationship;log A＝a−log R＋bR
(1)
where A is the PGA (cm/s2), and R the epicentral distance (km). In
this case,
a＝3.41 and b＝−0.00047
Also shown in Fig. 2 is the attenuation curve proposed by
Fukushima et al. (1990). Because of lack of information, the epicentral distance was used instead of the shortest distance between
the site and fault rupture. Comparison of the two curves verifies
the fast attenuation characteristic of this earthquake.
A peak horizontal acceleration of nearly 0.5g was recorded at
the Avaj station about 28 km from the hypocenter. From the digitalrecords at this station (taken from the BHRC website), the acceleration time histories in the longitudinal (L), transverse (T), and vertical (V) directions, as well as their Fourier amplitude spectra andthe response spectra for a damping ratio of h = 5% are shown in
Fig. 3. Fig. 3-b indicates that the respective predominant frequencies in the L and T directions are 3.2 and 4.2 Hz. According to theazimuth values of 105 and 195, the L and T directions at that station roughly correspond to the EW and NS directions. Moreover,Fig. 3-c shows the maximum response acceleration for a period ofFig. 2

about 0.2 s.
3. BUILDING DAMAGE
3.1

Introduction
Many engineered as well as non-engineered structures wereseverely damaged and collapsed during the earthquake. Un-reinforced masonry (URM) structures commonly were damaged
because of poor construction of their bearing walls. The seismic
performance of the brick masonry infill in these URM structures
was catastrophic. Adobe buildings collapsed or suffered heavy
damage, the damage level depending on the distance from the ruptured surface fault.
Most of the destroyed villages were subjected to very intense
earthquake motion without attenuation because they were very
near the ruptured fault. The site effects amplified the earthquake
motion in some areas, causing excessive damage. In general, villages near the epicenter and ruptured fault suffered greater damage.
3.2

Characteristics of the building inventory
Traditionally, single-story adobe buildings have been the typeof construction used throughout the region. Adobe structures also
were used as warehouses or sheepfolds. The second most widelyused type of construction in the villages was masonry structures.
Newer construction materials, such as concrete and brick masonry,
were found widely throughout the region. The principal structural
system used in the villages consisted of un-reinforced brick masonry (URM) bearing walls that supported steel-masonry (Jack-arch)
floor slabs or a timber roof. This type of construction, with nearly
identical detailing, was seen in most residential units. Although the
majority of the buildings were single-story ones, there were sometwo- and even three-story buildings in relatively large villages.

PGA attenuation curves

MICROTREMOR MEASUREMENTS AND BUILDING DAMAGE DURING THE CHANGUREH-AVAJ, IRAN EARTHQUAKE OF JUNE 2002

Fig. 3 Strong earthquake motion characteristics at Avaj station: (a) acceleration time histories; (b) Fourier amplitude spectra; (c) acceleration, velocity, and displacement response spectra, h=5%

39

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A. FALLAHI, R. ALAGHEBANDIAN AND M. MIYAJIMA

Photo 1 Collapsed adobe buildings which had arched and /or timberroofs (Abdareh)

Photo 3 A typical collapsed URM building with a Jack-arch floorslab (Changureh)

Photo 2 A thatched timber roof bonded with asphalt (Tablashkin)Photo 4

Steel and reinforced concrete frames were rare in this region.
Adobe buildings
Old adobe buildings were used not only as residential units
but as warehouses and sheepfolds. The construction of these low
quality adobe buildings consisted of mud-brick bearing walls that
supported a flat slab or an arch floor slab. Stone was an alternative
material used for bearing walls in villages near mountains. Flatslabs were constructed with wooden beams thatched with mudstraw materials, whereas arch slabs built of mud-brick bonded with
mud-grout. The seismic performance of such non-engineered
dwellings is a function of wall thickness, internal subdivisions,roof mass, nature of the continuity with adjacent dwellings, distance to the fault, and site effects. Photo 1 shows failed adobebuildings with arch floor slabs and/or wooden flat floor slabs.
Un-reinforced masonry structures
Un-reinforced masonry (URM) structures suffered severe
damage during the earthquake. Unfortunately, masonry is the mostcommonly available and economical construction material, andindividual owners have used it throughout the region. Photo 2
shows a typical damaged URM building with a timber roof.
URM is commonly used in Iran in combination with steelmasonry (Jack-arch) floors poured on top of the URM bearingwalls. The high in-plane rigidity of the Jack-arch floor system produces good distribution of seismically induced forces on the bear-

Collapse of a URM building with a Jack-arch floor slab
(Changureh)

ing walls, but reliance on a floor with brittle URM bearing-walls toresist lateral forces ensures that once the strength threshold isexceeded, severe damage and/or dramatic collapse is likely tooccur. In-plane, out-of-plane, and combined in-plane and out-ofplane failures occurred, as is normally expected with URM structures. Photos 3 and 4 show collapsed URM structures with steelmasonry Jack-arch slabs.
Brick and mortar quality have a major influence on the inplane performance of URM structures. The use of solid bricks
bonded with mortar in URM bearing walls that have a thickness of
350-mm seems to be sufficient to prevent out-of-plane failure in
typical single-story URM structures. Out-of-plane failure of URM
bearing-walls was, however, the principal cause of the collapse of
URM structures. The use of poor mortar resulted in unstable walls
when buildings were subjected to out-of-plane seismic force.
Moreover, the absence of any anchorage at roof level between the
bearing walls and slabs compounded the problem, causing evengreater damage (Photo 4).
The Iranian Seismic Code (BHRC, 1988) has a chapter on
URM structures. As the design of newly constructed governmentbuildings, such as schools, conformed to the Iranian Seismic Codefor such constructions, although suffering heavy damage, this typeof building did not collapse. Unfortunately, it is not mandatory tofollow this code in the design and construction of buildings in rural
areas. One can erect any type of building as there is no specificcode for those areas.

MICROTREMOR MEASUREMENTS AND BUILDING DAMAGE DURING THE CHANGUREH-AVAJ, IRAN EARTHQUAKE OF JUNE 2002

Semi-steel structures
Because of its high cost, structural steel has rarely been used
for a lateral resistance system in structures in the region. The useof steel is limited to steel-masonry Jack-arch floors. A few semisteel structures had steel beams and columns. The exterior enclosures as well as the interior partitions, however, were constructedof non-bearing, un-reinforced brick masonry infill walls. Thesewalls contributed to the lateral stiffness of the buildings during theearthquake and, in many instances, controlled lateral drift andresisted seismic forces elastically. This was true only when the
bricks and grout were bonded sufficiently by the use of good construction practices. When the structural response and deformation
demands were very high, the masonry walls could not remain elastic. In such buildings, typical diagonal cracks in the exterior aswell as the interior brick walls were present. In most cases, out-ofplane failure of the infill walls occurred. Once the brick infill
failed, lateral strength and stiffness was provided by the framealone, which then experienced significant inelastic responses in thecritical regions of its members.
Photo 5 shows a two-story, semi-steel structure with steel
columns and beams. This building had a relatively light roofing
system. Steel beams and columns were used to support the gravity
dead load and live load. Obviously there was no lateral resistancesystem. The presence of reinforcement that tied the infill walls to

Photo 5 A surviving semi-steel structure with a Jack-arch floor slab
(Changureh)

Photo 6Out-of-plane failure of the infill of a semi-steel structure
house (Changureh)

41

the columns prevented out-of-plane failure of the infill walls,
resulting in a semi-lateral resistance system. This system together
with the light roof prevented complete collapse. The building,however, could no longer be occupied, and reconstruction was necessary because of severe deformation of its beam-to- column connections.
Out-of-plane failure of the infill walls was frequent in thesetypes of semi-steel structure systems. Photo 6 shows an example inwhich the horizontal and vertical steel ties were constructedaccording to the Iranian Seismic Code. The building itself survived, but failure of the heavy un-reinforced infill walls caused avast human damage.
4. MICROTREMOR MEASURMENTS
4.1

Introduction
Microtremor measurement is a very useful method for obtaining ground dynamic characteristics, including the predominantperiod and the site amplification factor (Enomoto et al, 2000). To
examine the ground surface and structural seismic response characteristics in the earthquake-stricken area, microtremor measurements were conducted in several locations in Abdareh Village, inwhich almost 100% of the adobe dwellings were destroyed, as wellas on an adobe house in Tablashkin Village. Moreover, a forced
vibration test that used human strength was conducted on an adobe
house. The dynamic characteristics of the house were determined

Photo 7 Microtremor measurement station sites in Abdareh(Photo: Majid Noorallahpoor, Iran)Photo 8 Recording ground tremors in Abdareh in front of a publicbath (station No. 4)

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A. FALLAHI, R. ALAGHEBANDIAN AND M. MIYAJIMA

Fig. 4

H/V Fourier spectral ratios of microtremors at stations Nos. 1-7

MICROTREMOR MEASUREMENTS AND BUILDING DAMAGE DURING THE CHANGUREH-AVAJ, IRAN EARTHQUAKE OF JUNE 2002

from the free vibration results. To make these measurements, highly sensitive SPC-35N (Tokyo Sokushin) seismometers with threeorthogonal components each, two horizontal (H) and one vertical(V), were used. Velocity amplitudes of microtremors at a sampling
frequency of 100 Hz with a 0.1 Hz high pass filter were measured.
Each record at a point had a time history duration of 300 seconds
which, after excising the visible noise, was divided into 5 segmentswith 1024 data each. A Fourier analysis, based on the H/V spectral
ratio (Nakamura, 1989), was made for each segment. The mean
spectrum was smoothed by a Parzen window of the 0.4 Hz band.
Because of the valley-shape topography of Abdareh, there theFourier spectra were considered separately for the EW and NS
directions and the geometric average of the two directions was cal-Fig. 5

Fig. 6

Fig. 743

culated to determine the predominant period of the ground surface
at each measurement site. Taking into account the natural period ofa typical adobe house, the relationship between the strong groundmotion of the earthquake and the damage done, which may beaffected by local site conditions, is discussed.
4.2

Microtremors in Abdareh
In Abdareh, tremors were measured on the ground surface at 7
stations, whose positions are roughly indicated in Photo 7.
Actually, station No. 1 is much further to the left than shown in the
Photo, and is behind a large, engineered-built house which sufferedslight damage to its masonry walls. No.4 is in front of an undamaged, stiff public bath (Photo 8), No. 7 is nearby some 50%-dam-

Predominant microtremor periods at staions 1-7

Horizontal (H) Fourier spectra of the 3-EW and 5-NS components

Predominent period of the ground at the microtremor measurement stations in Abdareh

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A. FALLAHI, R. ALAGHEBANDIAN AND M. MIYAJIMA

aged adobe houses. The other stations are in areas of completely
destroyed adobe houses. These stations were chosen because they
all are located approximately along the fault trace. In each measurement, two horizontal (H: EW and NS) and one vertical (V:UD) components of the tremors were recorded.
The smooth mean H/V Fourier spectral ratios of the noise-freesegments of the tremors at stations Nos. 1-7 in the two EW and NS
directions are shown in Fig. 4. This figure shows that amplitudes inthe EW direction, along the cross section of the valley and roughly
along the fault trace, are larger than those in the NS direction.
Considering that the predominant period range of ordinary groundis less than 1 s and according to the dynamic characteristics of thesensors, the predominant periods of the ground surface at the measurement sites, the highest peaks on the H/V graphs, were from0.05 s to about 1 s shown by the arrows in Fig. 4. These periods
are collated and shown in Fig. 5. Note that when the peak periodsof the H/V spectra are not clear, as in 3-EW and 5-NS, corresponding H spectra, such as those shown in Fig. 6, were used.
The Fourier spectra of the geometric average (square root of
the products) of the EW and NS components were used to determine the predominant periods of the ground surface at the measurement sites (Fig. 7). Based on the criteria in Kanai, 1983, theground conditions were evaluated as Class 2 (medium) at station
No. 2, and as Class 1 (good) at the others. As seen, station No. 7

Fig. 8Plan of the adobe test house

Fig. 9which had the lowest predominant period, 0.06 s, has the hardest
ground surface of all the sites, which is in good agreement with theearthquake-induced damage, because in that area of Abdareh
alone, some adobe houses survived (Photo 7). Another factor indamage interpretation is the taking into account of the dynamic
characteristics of structures. A forced vibration test therefore wasconducted on an adobe house in Tablashkin Village.
4.3

Microtremors in Tablashkin
Tablashkin Village is located far from the epicenter unlike thevillages of Abdareh and Changureh. Whereas in the latter villages
almost all the adobe dwellings were destroyed, Tablashkin Village
had a different appearance although there were some collapsed andmany damaged dwellings. Due to differences in house plans, it wasdifficult for the investigation team to identify a typical undamaged
adobe house in that village. A slightly damaged adobe house therefore was selected, and both microtremor measurements and aforced vibration test were made to obtain its natural period.
Microtremor and forced vibration tests on an adobe house
The plan and view of the test house respectively are shown in
Fig. 8 and Photo 9. The house height is 2.9 m, the lower 0.8 m ofwhich is stone wall and the upper 2.1 m adobe.
The first set of measurements made was for simultaneous
microtremors in five channels (Ch.1-5, Fig. 8): three on the groundsurface (two horizontal, one vertical), and two horizontal on the

Photo 9View of the adobe test house

H/V Fourier spectral ratios of the ground surface at the adobe test house

MICROTREMOR MEASUREMENTS AND BUILDING DAMAGE DURING THE CHANGUREH-AVAJ, IRAN EARTHQUAKE OF JUNE 2002

45

Fig. 10 Amplification spectra of the adobe test house

Fig. 11 Transverse recording of shaking by forced vibration on the roof of the adobe test house

Photo 10Forced vibration test on the adobe houseroof of the house in the transverse and longitudinal directions. The
two horizontal channels on the ground (Chs.1 and 2) were set parallel to those on the roof (Chs.4 and 5). In the two 300 s-duration
recordings (Recs.1 and 2), an analysis similar to that explained
previously was followed. The H/V Fourier spectra for the groundsurface were determined (Fig. 9). These findings show the predominant period of the ground to be 0.2 s. The spectral ratios of

Ch.4/Ch.1 and Ch.5/Ch.2 respectively give the amplification spectra in the transverse and longitudinal directions (Fig. 10), in which
the respective natural periods of the house in the transverse andlongitudinal directions are 0.2 s and about 0.08 s. The relatively
large difference, 2.5-fold, between these two periods may be due tothe large length-over-width ratio of the house; more than 3.7.
A forced vibration test was performed on the adobe house fortremors generated by human swaying on the roof in the transverse
direction (Photo 10) to distinguish the natural period and damping
ratio of the house in that direction from the ensuing free vibration.
After some trials the swaying period was adjusted to the natural
period of the house in order to record relatively large amplitudes.
Only two channels on the roof were recorded. The transverse
direction dealt with in Fig. 11, shows the recorded time history, inwhich about the last 15 s is that of free vibration. The Fourier spectrum of this part gives the natural period of the house as 0.2 s,
which is the same as the results of the microtremor experiment(Fig. 10-a). Furthermore, using that part, the damping ratio of thehouse was calculated to be about 3%.
5. CONCLUSIONSThe erection of non-ductile structural systems such as adobe
and un-reinforced masonry (URM) structures resulted in cata-46

A. FALLAHI, R. ALAGHEBANDIAN AND M. MIYAJIMA

strophic earthquake damage. URM structures which conformed to
the Iranian Seismic Code suffered heavy damage but survived collapse. Other URM structures readily collapsed due to out-of-plane
failure of their infill walls. The absence of anchorage, as well as
the use of poor mortar, was the cause of the out-of-plane failure of
the infill. Beyond the elastic limit, seismic survival of a building
depended heavily on the ductility of its structural components. The
use of slender columns and the absence of a lateral resistance system in semi-steel structures was another primary cause of structural failure.
Microtremor measurements were made on the ground surface
in Abdareh Village and on an adobe house in Tablashkin Village.
The measured microtremors in Abdareh were in good agreementwith the damage done by the earthquake. The focus there was on
adobe houses, because, although adobe houses are very vulnerable
even to mild earthquakes, the remarkable difference in ground conditions in one area of the village was the main reason for relatively
less damage being done to adobe houses there.
In rural areas, adobe houses are very similar in size, usage,and method of construction, and therefore in their dynamic characteristics. Because no undamaged adobe house survived in Abdareh,
microtremor and forced vibration tests were carried out on a typical adobe house in Tablashkin. From the findings for that house, its
natural period was 0.2 s and the damping ratio about 3%. Because
the predominant period of ground during an earthquake is larger
than during microtremors owing to the nonlinear effect, most of
the predominant periods of the ground surface at the measurement
stations in Abdareh (Fig. 7) are concluded to have shifted to 0.2 s
or more, and therefore amplification led to widespread severe damage.
ACKNOWLEDGMENTS
Site investigation and microtremor measurements were carried out as part of the work of the reconnaissance team that visited
the stricken Iranian areas from July 22 to August 1, 2002. We
greatly appreciate the funding by the Japan Society of Civil
Engineers (JSCE) which enabled the reconnaissance, and the aid ofProfessor Kazuo Konagai, team leader, and the members of theteam. We also are grateful for the provision of the strong groundmotion data on the earthquake obtained from the BHRC website,
the kind hospitality and support of the Earthquake Engineering
Research Center (EERC), Tehran University, as well as of thelocal people and officials of Iran. We thank Dr. T. Ikemoto,Research Associate, Kanazawa University, for his valuble comments on the microtremor analysis, and Mr. H. Nishikawa,
Doctoral Student, Kanazawa University, for his help with theattenuation curve analysis.
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