Engineering Geology and Geotechnical Aspects of Bam Earthquake

(Preliminary Report)

Kambod Amini Hosseini
Mohammad Reza Mahdavifar
Mohammad Keshavarz Bakhshayesh
Masomeh Rakhshandeh

10/01/2004



International Institute of Earthquake Engineering and Seismology

1- Introduction

Bam earthquake occurred on 26^th of December 2003, at 07:56:56 GMT (05:26:26 local time) near the city of Bam that is located at southeast of Iran (Eshghi and Zare, 2003). The coordination of the epicenter of this earthquake have been determined by IIEES (IIEES, 2003) at 29.01N and 58.26E in 10km SW of Bam town that is close to the coordination mentioned by USGS (28.99N, 58.29 E (USGS, 2003)), but based on the surface evidences reported by Zare (Eshghi and Zare, 2003), the epicenter is located under the city of Bam.
The Moment Magnitude of 6.5 for this earthquake (Mw) have been measured based on the preliminary evaluations and the focal depth is estimated to be 8km based on S-P evaluation on the records obtained from the main shock (Eshghi and Zare, 2003). The macro-seismic intensity of the earthquake is estimated to be I₀=IX according to the EMS98 scale. The attenuation of the strong motion seems to be considerable as the surface evidences and damages decrease sharply at the direction perpendicular to the Bam fault (Figure 1).
Although the focal mechanism of Bam earthquake was reported to be strike slip having a small vertical component (USGS, 2003), but the strong motions recorded at the Bam station shows a considerable vertical component. The maximum PGA for the horizontal components are 0.7 and 0.8g, and 1.01g for the vertical component (Corrected values; BHRC, 2003). Bam earthquake has been accompanied by some geotechnical phenomena such as landslide, liquefaction and land subsidence. In this preliminary report, some of these geological and geotechnical aspects will be introduced. In addition a summery about the geological and geographical setting of the area will be described. It should be considered that further information about the geotechnical aspects of this earthquake will be presented later after accomplishment of the second phase of the study.

Figure 1. Intensity map of the area (Eshghi and Zare, 2003)

2- Geographical Situation

The city of Bam is located southeast of Kerman. The area of the city is about 5400 Hectares having a smooth topography and morphology. The altitude of the city is approximately 1050 meter above sea level. The main topographical feature of the city is the volcanic hills located at the north and south west of Bam. Climatologically the area has dry weather and the total amount of annual rainfall is not considerable specially during the recent years. There is one seasonal river that is nearly dry most of the year called Posht-e-Rood. It passes through the Bam city.
Due to small amount of rainfall and surface water, the main source to supply drinking and agricultural water for Bam and its vicinity are underground resources. Underground water extracted mainly by using deep wells and Qanats (underground irrigation tunnels). Lots of the Qanats have been excavated at the area during the past decades. Before Bam earthquake there were about 126 active Qanats at the area to supply 50% of the required water for the city. The rest of the required water supplied with deep wells.

3- Geological Setting

A simplified geological map of the area is presented at the figure 2, based on the 1:250,000 geological map prepared by GSI (Geological Survey of Iran).
Five different lithologies can be observed in the main geological formations of the area including: recent Quaternary alluvium, late Quaternary sandstones and siltstones, Paleogene sedimentary rocks, Eocene volcanic rocks, and intrusive igneous rocks (Granodiorite). Quaternary fine sands and silts form the alluvium around the Bam town and its vicinity. These sediments are yellow to brown sand and silt (Qm1), coarse grain brown gravel deposits of flooded plains (Qm2), coarse grain gravel of alluvial fans (Qf2) and coarse grain deposits of the rivers, respectively. Qm2 deposits covered nearly most of the Bam and Baravat areas. The thickness of these sediments having low to medium compaction is about 50 meters. The effects of deep erosion can be observed in these sediments (figures 3 – 5).
Bam fault is the main tectonic feature at the area that overlaid the old Quaternary sediments on younger sedimentary layers at east of Bam. As a result, the old Quaternary sediments formed a hilly morphology that has been cut by some drainage systems at the area and made several deep channels prone to landslide.

4- Geotechnical phenomena related to Bam Earthquake

4-1- Landslides

As shown in figure 2, except granodioritic layers, the rest of the lithologies are sensitive to landslide; but because of the low dip of the exist slopes, the landslide prone zone is limited to the mountains and channels banks (figure 6). Figure 6 prepared by overlaying of geology and slope map of the area.
In aerial photos that have been taken 2 days after the earthquake (NCC, 2003), extensive fall zones, earth block slides and earth slides can be identified at the natural channels of east and southeast of Bam (figures 7-12).

Figure 2. The simplified geologic map of the area

Figure 3. Sandy and silty sediments (Qm2) (west of Baravat)

Figure 4. Deep erosion in Qm2 alluvium (west of Bam)

Figure 5. Alluvial fan (Qf2) (South of Bam)

Figure 13 presents the areas which landslide (mostly falls) occurred during the earthquake. Two areas are shown in the figure:
 

• The area with small numbers of falls and slides (between 10-50/Km2)

• The area with large numbers of falls and slides (more than 150 / Km2)

This figure shows that landslide numbers is large at the channels banks in the east and south east of Bam. There is also a limited area with considerable number of falls at the northwest of Bam.
The reasons for more instability of the channels banks compared to hilly areas, could be explained as follows:

As a result of Bam fault movement, the old Quaternary sediments formed a hilly morphology that has been cut by drainage system during the time. Most of these deep channels are formed in the weak formations (sensitive and very weak sandstones and siltstones).

Figure 6. Geotechnical hazard map of Bam area

Figure 7. Some of landslides located using one of the aerial photo

Figure 8. Some earth falls due to Bam Earthquake (east of Bam)

Figure 9. Some earth falls due to Bam earthquake (East of Bam)

Figure 10. Some earth falls due to Bam earthquake (Southeast of Bam)

Figure 11. A block earth slide due to earthquake (North of Rahmani Village)

Figure 12. A block earth slide due to earthquake (North of Rahmani Village)

Figure 13. Landslide map of Bam area
 

As a result of high energy release of Bam earthquake at Bam city and its vicinity and the very high vertical component of the earthquake (based on BHRC records, 2003) the stability of the natural channels have been reduced considerably and many falls and slides occurred at the area. In addition several tensional crack and separation of the blocks with the main body can be observed at the area.
Up to now, There is no confirmed reason for explaining the large number of falls and slides at the NW of Bam (Figure 13) and the intensity map presented by Eshghi and Zare (2003) can not explain the phenomenon correctly. This problem is now under investigation and the results will be presented in the next reports.
By a statistical estimation, the number of different types of landslide triggered by Bam earthquake can be classified as follows:

4-2- Liquefaction

The liquefaction potential of Bam area can be evaluated using Geological data, ground water level and soil condition. Although the soil condition at the most parts of Bam city and its vicinity shows high percentages of fine grain sediments (sand and silt) but due to low level of ground water, the risk of liquefaction is not considerable at the most parts of the city. In addition there is no report indicating the damages due to liquefaction.
The preliminary investigations also confirmed that there is no evidence of liquefaction at south and south west of the region due to low level of ground water.
Of course at the north and north-east of Bam near the Posht-e-Rood River, Esfikan and Chehel Tokhm; some evidences of liquefaction can be observed at the aerial photos taken 2 days after the earthquake. At these parts the ground water level is higher and the sediments are prone to liquefaction. Complementary investigation at these areas is now in progress that the results will be presented in the next reports.

4-3- The effects of earthquakes on Qanats of Bam

As mentioned before one of the main sources of drinking and agricultural water at Bam area is underground irrigation systems called Qanats. Before the earthquake 50% of the required water of the area have been supplied with 126 active Qanats. Most of these Qanats can be observed in the areal photos (figures 14 –16). In addition there are several trends of old Qanats related to the past decades or centuries that their locations are unknown now. Of course most of these old Qanats are now dry and partially collapsed. Most of the Qanats of the area have been damaged due to Bam Earthquake. In some cases the collapse of some of theses Qanats caused severe damages to the building and lifelines. At this part the effects of Bam Earthquake on Qanat systems exists at the area will be presented breifly.

4-3-1- The behavior of Qanats during Iran’s historical earthquakes

Earthquake may damage or even destroy the tunnels and access wells of new and old Qanats. Due to partial or complete collapse of the Qanats tunnels, water flow can be affected in different levels and land subsidence in form of sinkholes can be occurred on the ground surface.
Damage to the Qanats have been reported in several seismic events in Iran that a summery of them is presented at table 1. It can be observed that most of the strong earthquake of Iran that occurred at the dry regions caused different levels of damages on the Qanats.

4-3-2- The behavior of Qanats during Bam Earthquake

Bam Earthquake has considerable effects on a lot of Qanats that excavated at the Bam area and its vicinity. Based on the preliminary evaluations, about 40 percents of these Qanats have been collapsed or experienced severe damages due to the earthquake. In some cases the collapse of the Qanats stopped the water flow completely.
The Qanat network of the area can be observed in aerial photos. Some of these trends have been marked at the figures 14 to 16. Figures 17 and 18 show some of the access wells of Qanats of the area. Most of these Qanats have been supported with hand made arcs called “Kaval”.
 

Table 1: The effects of historical and recent earthquakes on Iran’s Qanats (Ambraseys and Melville, 1982)

Figure 14: The Location of some Qanats on aerial photo at West Baravat

Figure 15: The Location of some Qanats on aerial photo
(North East of Bam)

Figure 16: The Location of some Qanats on aerial photo
(South part of Bam)

Figure 17: A Qanat trend at the 8th km Bam to Kerman road

Figure 18: The effects of old Qanats near Khajeh Askar village

4-3-2- The effects of Bam Earthquake on Qanats

Qanats in Bam and its vicinity experienced severe damages during the earthquake. Site investigations carried out during the first days after the earthquake showed different levels of damages to Qanats.
In addition, at some locations the collapse of the Qanats had secondary effects on the buildings and lifeline of the area and increased the damages to them. These effects will be discussed at the following parts.

4-3-2-1- Subsidence above the tunnels and access wells of Qanats:

The most important effects of Bam Earthquake on Qanat systems are damages to the access wells and tunnels. Several sinkholes induced due to the earthquake above the tunnels and wells due to the collapse of underground openings.
Based on the previous experiences underground openings should be more resistant against seismic loads, but the damages to the Qanats of Bam area were severe. Most of the damages observed at the access wells but near the Bam Fault underground tunnels also have lost their stability and collapsed. It should be considered that most of the Qanats of the area have been supported with hand made arcs, but these supporting systems could not affect considerably on the stability of Qanats when dynamic loadings applied on them.
High concentration of sinkholes observed at the vicinity of the Bam Fault. Most of the occurred collapses were also observed in a narrow band close to the Bam Fault. Of course it should be considered that other damages to the structures and lifelines were also in a limited zone around the Bam fault.
Far from the Bam Fault the effects of earthquake on Qanats are less important and only some fissures and cracks can be observed along the tunnels of access wells.
Figure 19 to 22 presents some of the effects of Bam Earthquake on Qanats systems. As it is shown at some places the induced sinkholes have very large dimensions with several meters diameters.
 

Figure 19: Sink hole due to collapse of a Qanat tunnel at west of Baravat

Figure 20: Concentration of sink holes due to collapse of Qanat tunnels and access well (South of Bam)

Figure 21: Sink hole due to collapse of a Qanat tunnel at west of Baravat
 

4-3-2-2- The effects of sinkholes on structures and lifelines
The induced sinkholes by Bam Earthquake in urban area cause some damages to the structures and lifelines. These damages are more considerable at Baravat and south of Bam. Most of these damages are due to constructing on tunnels and access wells of old Qanats as the surface evidences of old Qanats can be disappeared during the time. In such cases the underground openings of Qanats may remain without considerable changes on a critical stability. Due to dynamic loading of earthquake this stability may change and collapse will occur. Such collapse will have effects on the above structures and may increase the damages to the structure.
Figure 22 and 23 show some of the damages to the roads due to collapse of Qanats. Some sinkholes induced under the main roads and bypasses of the area and increased the traffic and affect the performances of rescue teams, considering the importance of first hours after an earthquake for helping the injured and trapped persons. Figures 24 and 25 presents the damages to some houses and building due to sinkhole cause by collapsing the underground Qanats.
 

Figure 22: Sinkhole due to collapse of a Qanat under a bypass at Baravat

Figure 23: Sinkhole observed close to main road of Baravat; same sinkhole occurred under the main road that is now filled

Figure 24: Damage to a building due to collapse of an old Qanat (Baravat)

Figure 25: Damage to a sign due to the collapse of a Qanat (Baravat)

 

References:
 

• Ambraseys, N. N. and Melville, C. P. (1982) A History of Persian Earthquakes, Cambridge University Press, Britain, translated by Radeh, A., Agah Publishers, Tehran, 1991.

• Building and Housing Research Center (BHRC). 2003. Website: http://www.bhrc.gov.ir/bhrc/reports/bam/bam_pdf.pdf ;

• Eshghi, S. and Zare, M. (2003) Bam (SE Iran) earthquake of 26 December 2003, Mw6.5: A Preliminary Reconnaissance Report , Website: http://www.iiees.ac.ir/English/bam_report_english_recc.html

• Geological Survey of Iran (GSI), Geological quadrangles of Bam, Sabzevaran, allah Abad, Jahan Abad , Scale:1:250000

• International Institute of Engineering Earthquake and Seismology (IIEES). 2003. Preliminary Report of Bam Earthquake. Website: http://www.iiees.ac.ir/English/Bam_report_english.html

• National Cartographic Center, 2003. Aerial photos of Bam earthquake affected area in the scale of 1:10000 (printed in the scale of 1:2500)

• Seismology Department (IIEES). 2003. Seismological aspects of Bam earthquake of 26th December 2003, Mw 6.5.
  Website: http://www.iiees.ac.ir/Bam_Report_2.pdf

• United State Geological Survey (USGS). 2003. Websites: http://earthquake.usgs.gov/recenteqsww/Quakes/uscvad.htm ;
  http://neic.usgs.gov/neis/eq_depot/2003/eq_031226/neic_cvad_q.html