Bam (SE Iran) earthquake of 26 December 2003, Mw6.5:
Dr. Mehdi Zaré
A Preliminary Seismological Overview
International Institute of Earthquake Engineering and
Seismology, Tehran, P.O.Box: 19395/3913, IRAN
The Bam earthquake of December 26, 2003 (Mw6.5) occurred at 01:56:56 GMT (05:26:26 local time,
USGS, December 2003) under
the city of Bam in the southeast of Iran (Figure 1). The earthquake
occured very early in the morning when most of the inhabitants were
sleeping, which can be one of the causes of the great life losses. The
number of victims was declared officially to be more than 40,000 at the
time of the preparation of this article (19/01/2004). More than 50,000
people were injured and about 100,000 people became homeless.
Figure 1. The location
map of the epicenter and the reactivated Bam fault. The surface fissures
created after the earthquake are shown with dashed lines.
The Kerman province is one of the largest provinces in Iran, with an
area of 186,422 km2 and located in the southeast of country.
At the time of the earthquake the population of Bam and its suburbs was
about 120,000, while the suburban area of Bam had a population of about 80,000.
The earthquake damages were limited to the city of Bam and a smaller city, Baravat,
located to the east of Bam. The inhabitants of the villages nearby Bam left
their houses after the earthquake, for fear of more earthquakes
(aftershocks) and due to extensive to moderate damages to their buildings.
The city of Bam is well-known for its historical citadel of Arg-e-Bam,
which is about 2000 years old and the largest mud-brick complex in the world.
This citadel was, unfortunately, almost totally destroyed in the earthquake
(Figure 2). This historical monument is located on an igneous rock hill along
the Silk Road, has an area of some 240,000 square meters and was constructed
mainly from un-dried brick and clay.
There exists no information about the exact date of its construction,
but according to Persian history it goes back to 2000 years ago. It has
been repaired several times throughout history and was residential up till
150 years ago. Since there is no mention of earthquake occurrence for the city
of Bam in the Iranian historical earthquake catalogue, it seems that it was the first
time during the last 2000 years that a disastrous earthquake has taken
place due to the reactivation of the Bam fault (Ambraseys and Melville, 1982).
Date(s) of older previous earthquake(s) may be determined in the paleoseismological
studies on the BAM fault that have started just after earthquake.
The historic site of Arg-e-Bam before and after the earthquake. (Click for
The Bam earthquake prone area in SE Iran is an
active seismic zone (Table 1, Figure 3). Although the city itself had no reported
large historical earthquakes prior to the event of 26/12/2003, towards the
northwest of Bam, 4 major earthquakes with magnitudes greater than 5.6
have stricken the cities and villages between 1981 and 1998. The trends of
the main faults (including the Bam fault) in this region is North-South,
and NW-SE (Figure 4). These two systems intersect in the western Lut area.
The NW-SE faults (Kuhbanan and Ravar faults) and the N-S faults (Nayband,
Chahar-Farsakh, Anduhjerd, Gowk, Sarvestan and Bam faults) have determined
the border of the north-south structures in the Lut area with the NW-SE
structures. These intersecting zones have been the main sources for the
disastrous earthquakes. The Gowk fault system is known for the
surface ruptures during the 1981, 1989 and 1998 earthquakes as well as a
hot spring system. In the west of Golbaf-Sirch valley is the Lut
depression, where a vertical offset of more than 4000 meters has been measured.
Four large earthquakes hit the region during recent years:
the Golbaf earthquake of 11 June 1981, Ms=6.6,
the Sirch earthquake of 28 July 1981, Ms=7.0,
the South Golbaf earthquake of 20 November 1989, mb=5.6 and
the North Golbaf (Fandogha) earthquake of 14 March 1998, Mw=6.6.
The Golbaf earthquake, in which 1071 persons were killed, occurred in the
southern parts of the Golbaf valley (strike = N5-15E) could be associated
with a fault rupture along the Gowk fault and caused great damages in the
The Sirch earthquake occurred 49 days after the Golbaf earthquake and caused the
loss of life of 877 person. It may have occured on a secondary fault along the
Gowk fault (N-S trend) or it may have been the hidden continuation of the Kuhbanan
fault (NW-SE trend). Both faults are intersecting.
A similar mechanism may have caused the large earthquakes around Sirch in 1877 and
1981 (both with a magnitude larger than 7.0).
In the South Golbaf earthquake 4 people were killed, 45 injured. It caused also
damage in Golbaf. Some surface faulting and folding have been reported that
could be related to this event.
In the North Golbaf earthquake 5 people were killed and 50 injured. This
earthquake was associated with surface faulting (about 20km length) in
northern Golbaf. The focal mechanism of all those earthquakes show the
compressional and strike slip mechanisms along the Gowk and Kuhbanan fault systems.
Figure 3. The
seismicity map of the Bam region in SE of Iran (base map is from USGS
world Digital map, 2001).
Date Time Coordinates
yyyy mm dd hh mm ss Lat.N Lon.E FD mb Ms Mw Efa Ref Region
1948 07 05 13 53 29.460 57.780 0 5.9 6.0 .0 Amb Gowk
1962 09 29 06 54 00 28.290 57.480 83 5.5 .0 .0 N.US
1964 05 11 06 07 38 28.220 57.390 73 5.3 .0 .0 N.IS
1964 08 27 11 58 39 28.160 58.830 50 5.1 .0 .0 N.IS
1976 11 13 10 12 36 28.250 57.340 62 5.0 .0 .0 ISC
1981 06 11 07 24 25 29.895 57.718 30 6.6 6.0 .0 * ISC Golbaf
1981 07 28 17 22 23 29.987 57.770 11 5.9 7.0 .0 * ISC Sirch
1981 10 14 09 12 39 29.900 57.758 43 5.2 .0 .0 ISC
1982 10 15 02 53 55 28.280 57.398 83 5.0 .0 .0 ISC
1983 01 31 18 56 53 28.919 57.318 133 5.0 .0 .0 ISC
1984 10 11 05 09 27 29.539 58.030 48 5.1 .0 .0 ISC
1986 07 25 10 08 09 28.068 57.303 69 5.2 .0 .0 ISC
1989 11 20 04 19 04 29.880 57.721 18 5.5 5.7 5.9 ISC South Golbaf
1998 06 10 08 30 16 28.227 58.507 113 5.0 .0 .0 ISC
Table 1. Known recent earthquakes in the region as reported by the
NEIC and the
Focal mechanismThe focal mechanism of the Bam earthquake shows
a strike slip fault with a small reverse component
University, Department of Seismology, December 2003, Figure 4). This
corresponds well with the surface evidences of right-lateral strike slip
movement of the Bam fault. The reactivated fault plane had a near
north-south direction with a dip to the west.
The Harvard Moment tensors of most of the earthquakes between 1975 and 2003
in the Bam region indicate a strike-slip and compressional mechanism (Figure 4).
Figure 4. The fault map of the Bam
earthquake prone area of the 26/12/2003 earthquake. The focal mechanisms
Harvard CMT catalog) are plotted as well.
Using the accelerogram recorded in station Bam (discussed later in this article),
the seismic moment of the main shock is estimated to be Mo=0.92x1019 (NM).
from which we obtain the moment magnitude Mw = 6.5. The focal depth was estimated
to be 8 km (based on a S-P estimation from the accelerogram obtained in Bam at a
hypocentral distance of about 12 km). Assuming a circular source shape, the source
radius is calculated as 6.2 km and the stress drop was 48 MPa during the mainshock.
This is in agreement with the moment magnitude estimated by
Dr. Yagi (at ERI, University of Tokyo, 2003)
Mw = 6.5 (Figure 5). His slip model of the Bam earthquake
(Figure 5) shows slip vectors oriented towards the surface with
trends towards the north. Such slip could indicate a vertical directivity, which may have
caused amplification of the low frequency signals perpendicular to the fault.
Figure 5. The slip model of the Bam earthquake, as from
Preliminary Results of Rupture Process by Dr. Yagi
Surface fault ruptures
The Bam fault lies with a near north-south direction in the vicinity of
the city of Bam (Figures 6 and 7). It runs at less than 1 km
distance to the east of Bam, between the cities of Bam and Baravat
Figure 6 shows the near north-south strike of Bam and Sarvestan faults on
a satellite image. They form a parallel fault system interrupting the
NW-SE trend of the Bam plain and Barez mountains (in southern Bam).
The Bam fault consists of at least 3 different segments in the vicinity
(east) of the city of Bam (Figure 7).
The Bam fault has an old escarpment with a topographic displacement of
about 5 to 20 meters that can easily be distinguished as a major change in
the topography of the plain in the east of Bam towards Baravat (Figures
7, 8 and 9).
The surface fissures created by the Bam earthquake are observed around
the Bam fault between the cities of Bam and Baravat (Figure 10).
These fissures consists of separate segments of 50 to 2000 meters length,
in different directions (N40E to N30W).
Nearby some of the wells of Qanats located nearby the Bam fault scarp,
some old Sinkholes (with diameters of 10 to 15 meters) are observed.
Qanat is the traditional Persian irrigation system in the form of horizontal
underground canals - that conduct the water stream - and the consecutive
wells dug each 10 to 100 meters. Some of the new sinkholes are found
nearby in the Bam fault scarp in the west of the city of Baravat (Figure
12). It seems that these sinkholes are created nearby the Qanats,
according to some local collapses in the roofs and walls of the Qanats
during the earthquake.
Satellite image taken by NASA the day after the mainshock (27/12/2003)
from the Bam region. The traces of the Bam and Sarvestan faults are
plotted by the author on the image (reference:
http://www.spaceimaging.com, December 2003).
(Click for larger image)
Figure 7. Zoom from the NASA satellite
image taken from the Bam region, focused on the the city of Bam and its
suburban region. The image is interpreted by the author. The traces of the
Bam fault segments and the localities of New city of Bam (Arg-e Jadid),
Airport and the city of Baravat are shown on the image.
Figure 8. 1:55000 aerial photo of the Bam fault (near
Baravat). The image is taken in 1936.
Figure 9. The Bam fault scarp, in the east of Bam.
Figure 10. A surface fissure created after
the Bam earthquake nearby the Bam fault scarp.
Figure 11. Old sinkholes created nearby the Bam fault scarp around the Qanat of Bam.
Figure 12. New sinkholes created in Baravat (east of the Bam fault scarp).
Seismic GapThe existing records on historical seismicity indicate
no major earthquakes in Bam since the historic time. It seems that the
earthquake of 26/12/2003 has ended a seismic gap along the Bam fault. This
seismic gap seems corroborated with the existence of the citadel of Arg-e
Bam, which is constructed about 2000 years ago and has not been demolished
by earthquakes untill this recent earthquake (historical catalog of Persia
published by Ambraseys and Melville, 1982).
Strong motions for
this event have been recorded in 23 stations of the national Iranian strong
motion network (according to the Building and Housing Research Center, BHRC, December
2003). The record obtained in station Bam (Figure 13) has been processed
and bandpass filtered between 0.11Hz and 40Hz.
The horizontal Peak Ground Acceleration (PGA) was found to be 775 and 623 cm/sec2
for the horizontal fault-normal (east-west) and fault parallel (north-south)
components, respectively. The vertical PGA is 992 cm/sec2.
The relatively large PGA of the vertical component and a relatively low frequency
amplification on the east-west (fault-normal) direction suggest a vertical
directivity source effect (Zaré et al 1999).
The accelerations and its spectra are shown in Figure 13.
The preliminary observations of the damages in Bam may be explained by
this vertical directivity effect.
The demolished walls and building of Bam (mostly in the east-west
direction) are representative for such effects, as well as the great
vertical (up-down) and east-west shakings that have been reported by the
people that survived the mainshock. The Bam residents that survived the
earthquake explained to the reconnaissance team members that they felt
strong up-down displacements during the mainshock. These strong motions
were attenuated very rapidly with distance, specially towards the
east-and west (fault normal) direction.
The acceleration response spectra for the Bam earthquake record in
station Bam are shown in Figure 14 for the 5% of damping values. Note
that the largest spectral ordinates belong to the vertical component
with a predominant period of 0.1 second. The fault normal and fault
parallel horizontal components indicate the lesser spectral ordinates,
respectively, with the predominant period of 0.2 second.
13. The accelerogram of Bam earthquake obtained in the Bam station (within
1 km to the fault). up-left: the band-pass filtered record at 0.11 and
40Hz; the upper traces in horizontal-fault normal component, middle trace
is the vertical component and lower trace in the horizontal- fault
parallel component, up-right: the signal to noise ratio, down-left: the
acceleration spectra for the 3 components; the vertical spectral component is
traced with dashed lines, the down-right: the H/V ratio. (Click for larger
Figure 14. Response spectra of acceleration
for 5% of damping for the vertical and horizontal-fault normal and
horizontal-fault parallel components.
The city of Bam has been largely
damaged in the 26/12/2003 earthquake (Figure 15). The historical citadel
of Arg-e Bam has been nearly completely demolished in this event (Figure 16).
However some parts of the Arg-e Bam citadel seems to be only partially
destroyed and archeologists believe that it can be reconstructed, based
on the previous documentation of the Arg-e Bam and recent studies
after the earthquake. The macroseismic intensity of the earthquake is
estimated to be I0=IX (EMS98 scale), where the strong motions and damaging
effects seems to be attenuated very fast specially in the fault-normal
direction (Figure 17). The intensity levels are estimated to be VIII in
Baravat, VII in New-Arg (Arg-e Jadid) and the airport area. The intensity
level was estimated to be around IV-V in Kerman and Mahan.
Figure 15. A damaged building in Bam.
Figure 16. The damage to the citadel of Arg-e Bam satellite image (taken by spaceimaging.com) and interpreted by the author. The rough estimation of the damages is based on the site visit the day after the event and
synthesis of the damage reports from the citadel of Arg-e Bam within the
first two weeks after the earthquake.
Macroseismic intensity and the isoseismal map of the 26/12/2003 Bam
earthquake, according to the reconnaissance visit by the IIEES
reconnaissance team (by S. Eshghi and M. Zaré, IIEES, December 2003).
The Bam earthquake of 26/12/2003 (Mw=6.5) has
demolished the city of Bam, having a population of about 120,000 at the
time of the earthquake. The Bam fault - which was mapped before the event
on the geological maps - has been reactivated during the 26/12/2003
earthquake. It seems that a length of about 10 km (at the surface) of this
fault has been reactivated, as it passes exactly the east of the
city of Bam. The fault has a dip towards the west and the focus of the
event was located closed to the residential area (almost beneath the city
of Bam). This caused a great damage in the macroseismic epicentral zone,
however the strong motions have been attenuated very rapidly, specially
towards the east-and west (fault normal) direction. The vertical
directivity effects caused the amplification of low frequency motions
in the fault-normal direction as well as large amplitudes of the vertical
motions. These source effects may be significant factors in the large life
and property losses in the Bam earthquake.
- Ambraseys N.N., and C.P. Melville, 1982. A History of Persian
Earthquakes, Cambridge Earth Sci. Ser.
- United States Geological Survey, USGS, Digital Data Series DDS-62-C, 2001
- Zaré M., P-Y. Bard, M.Ghafory-Ashtiany, 1999. Site Characterizations
for the Iranian Strong Motion network, Journal of Soil Dynamics and
Earthquake Engineering, 18, no.2, pp.101-123