Bam (SE Iran) earthquake of 26 December 2003, Mw6.5:
A Preliminary Seismological Overview
Dr. Mehdi Zaré International Institute of Earthquake Engineering and Seismology, Tehran, P.O.Box: 19395/3913, IRAN
Introduction - Seismotectonics - Strong Motion - Macroseismic Intensity Conclusions - References
Introduction
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.
Figure 2. The historic site of Arg-e-Bam before and after the earthquake. (Click for larger picture)
The seismotectonic of the Bam earthquake prone area
General features
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 Golbaf region. 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 ISCTable 1. Known recent earthquakes in the region as reported by the NEIC and the ISC.
Focal mechanism
The focal mechanism of the Bam earthquake shows a strike slip fault with a small reverse component (Harvard 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 (source: Harvard CMT catalog) are plotted as well.
Source parameters
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 8). 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.
Figure 6. 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.
(Click for larger 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 Gap
The 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 Ground Motion
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.
Figure 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)
Figure 14. Response spectra of acceleration for 5% of damping for the vertical and horizontal-fault normal and horizontal-fault parallel components.
The macroseismic intensity and the isoseismal map and attenuation of the strong motions
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.
Figure 17. 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).
Conclusions: lessons learned in view point of seismology
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.References
- 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