EERI Special Earthquake Report — April 2004 Learning from Earthquakes Preliminary Observations on the Bam, Iran, Earthquake of December 26, 2003 Following the earthquake, EERI Introduction sent a reconnaissance team to the Prior to the earthquake, Bam was A magnitude 6.6 (Ms) earthquake affected area in cooperation with the one of the richest cities in Iran. Sur- struck the city of Bam in southeast International Institute of Earthquake rounded by deserts, Bam had a tra- Iran at 5:26:52 AM (local time) on Engineering and Seismology dition of very successful agriculture, Friday, December 26, 2003. The (IIEES) of Iran. The EERI team was thanks to man-made irrigation sys- city’s population was about 90,000, composed of structural engineers tems (qanats) built and maintained with 200,000 total residents in the Farzad Naeim (team leader), Mike by locals over many centuries. It greater Bam area. The U.N. Office Mehrain, and Mohsen Rahnama; produced more than 100,000 metric for the Coordination of Humanitarian strong-motion specialist Yousef tons of the finest quality dates per Affairs (OCHA) indicates that the Bozorgnia; environmental engineer year and a large amount of premium Bam earthquake caused the deaths Elahe Enssani; geotechnical engi- citrus fruits. East of the city, a mod- of approximately 43,200 residents neers Ali Bastani and Farhang ern industrial complex was built that, and injured approximately 20,000. Ostadan; medical doctor Haasan among other things, assembled Some 75,600 people (14,730 house- Movahedi; and remote-sensing about 15% of automobiles produced holds) were displaced, and 25,000 specialist Babak Mansouri. in Iran each year. dwellings were razed. An additional 24,000 dwellings were destroyed in Ron Eguchi, Beverley Adams, and Seismicity and Strong the rural areas. The vast majority of Charles Huyck, remote sensing Ground Motions buildings in the city collapsed, and specialists with ImageCat, Inc., most of the remaining buildings According to the USGS (2003), the made significant contributions to the were severely damaged. In terms earthquake epicenter was located at reconnaissance effort by preparing of human cost, the Bam earthquake 29.004 N, 58.337 E, on a predomi- detailed satellite-based damage ranks as the worst disaster in Iran- nantly right-lateral strike-slip fault. maps and a GPS-based software ian history. In addition, Bam’s an- The focal depth was estimated at system that was used by the team cient citadel (Arge-Bam), probably 7 km (BHRC 2004). Currently, sci- at the earthquake site. Satellite the oldest and largest adobe com- entists from Iran, the United States, imagery was purchased by EERI plex in the world, with 2,000 years of and the United Kingdom are carry- and the University of California at history, was substantially lost. ing out comprehensive investiga- Irvine. The IIEES team was composed of Mohsen Ghafory Ashtiany (team leader), Sassan Eshghi, Mehdi Zare, Kambod Amini, Mahmood Hussaini, Kazem Jafari, Behrokh Hashemi, A. S. Moghadam, Farhang Pas, Farokh Parsizadeh, Khaked Hessami-Azar, Kiarash Asadi, Mehran Razaghi, Massoud Ahaari, Mehrtaash Mota- medi, Mohammad Reza Mahdavifar, Mohammad Bakhshaiesh, and Massoume Rakhshandeh. All photos in this report were taken by team members except where noted. The publication of this report Figure 1 Satellite image of Bam area. The locations of few strong-motion recording is supported by EERI’s Learning stations are marked. For each station, the larger horizontal acceleration as well as from Earthquakes Program under peak vertical acceleration are given. For the Bam station, three peak accelerations 2 National Science Foundation grant are provided. All values are in cm/sec (imagery courtesy of DigitalGlobe, www. # CMS-0131895. digitalglobe.com). 1 EERI Special Earthquake Report — April 2004 EERI Special Earthquake Report — April 2004 2003 Bam (Iran) Earthquake, Recorded at Bam Figure 3 Response spectra of the recorded ground motions at Bam for 5% damping. Figure 2 Ground accelerations recorded at the Bam station. station are plotted in Figure 3. An tions to identify the location and ex- important severe near-fault ground Evolution of Arias Intensity (AI) for tent of the causative fault of the motions. The instrument was on the the N278E component of the Bam Bam earthquake. ground floor of the two-story County station is plotted in Figure 4. With a Building. The building suffered se- 5-95% evolution of AI, the ground Southeast Iran has had previous vere damage and partial collapse at motion at Bam had a strong-motion major earthquakes. In 1981, two two locations, but the instrument duration of approximately eight earthquakes struck the area: in the room had only minor cracks in its seconds. Golbaf earthquake (Mw 6.6) 3,000 wall. Following the earthquake, the people lost their lives; in the Sirch staff of BHRC inspected the instru- In Figure 5, distributions of peak earthquake (Mw 7.1), 1,500 people ment room and the SSA-2 instru- horizontal and vertical ground ac- were killed. The other significant ment and documented their obser- celerations are plotted against the earthquake in the area was the vations (BHRC 2004). They con- seismogenic distance to the Bam 1998 Fandoqa earthquake, with a cluded that the instrument was not fault. The results of Campbell and magnitude of 6.6 (Berberian et al. damaged and did not malfunction Bozorgnia (2003) attenuation rela- 2001; Walker and Jackson 2002). during the earthquake. tions for the stiff soil sites are also plotted for comparison. It is evident Iran has 983 digital strong-motion The recorded ground accelerations that the recorded peak accelerations instruments and 71 SMA-1 analog at the Bam station are plotted in Fig- at Bam exceed the median value of strong-motion instruments. They are ure 2, which shows that the Bam attenuation models for both hori- maintained by the Building and station recorded very severe ground zontal and vertical components. Housing Research Center (BHRC). motions. The N278E component, In the Bam earthquake, 24 Kinemet- which is roughly in the fault-normal Geotechnical Aspects rics SSA-2 digital strong-motion in- direction, recorded a severe long- Geology and local soil condi- struments recorded the strong period pulse. The pulse corresponds tions: Surface deposits in the gen- ground motions in the area (BHRC to a high peak ground velocity of eral area of Bam consist of alluvial 2004). Figure 1 shows the Bam about 123 cm/sec (BHRC 2004). deposits from major seasonal flood- area, with a few locations of the The instrument also recorded very ing over time. The thickness of the strong-motion stations. strong vertical acceleration with a alluvium ranges from less than a peak value of about 1g. The re- few meters to about 50 m (IIEES A tri-axial strong-motion instrument sponse spectra of the uncorrected 2003), depending on the location. In located in downtown Bam recorded ground accelerations at the Bam the northeastern part of the city and 2 3 EERI Special Earthquake Report — April 2004 EERI Special Earthquake Report — April 2004 2003 Bam, Iran, Bam Station (N278E) soil effects on the high-intensity ground motion recorded in the city. Landslides: The area is generally flat, and there are no major man- made earth structures, but there were landslides along riverbanks and man-made channels. A typical riverbank landslide is shown in Fig- ure 6. Large blocks of soils moved and tumbled. Most soil blocks al- ready had tension cracks due to the dry environment. There was no di- Figure 4 Evolution of Arias Intensity (normalized) for the N278E component of the rect damage caused by the land- strong motion recorded at Bam. slides, but the soil blocks need to be removed to clear the riverbed for the next seasonal flooding. Distribu- Figure 5 There was no surface tion of the recorded Liquefaction: peak ground accel- evidence of liquefaction due to the erations versus seis- depth of the ground water table. mogenic distance to There were no reports of any dam- the Bam fault, for the age or failure caused by liquefac- geometric mean of tion. the horizontal com- ponents (top) and Qanats: A qanat is a horizontal aque- vertical acceleration duct system that conveys water from (bottom). The actual an aquifer in mountainous alluvial site-to-source distances may be fans to lower-elevation irrigated fields, less than those as shown in Figure 7. It consists of shown here, if the a series of vertical shafts in sloping causative fault is ground spaced between 30-50 m, somewhat west of interconnected with an underground the Bam fault. tunnel. The soil removed is placed around the shafts forming a circular Arge-Bam, a rock outcrop is visible. The underlying layers consist of re- cent quaternary alluvium, late quat- ernary sandstone and siltstone, sed- imentary and volcanic rocks. The seasonal riverbed (Posht-Rood), mostly dry, is in the northern part of the city. Due to heavy use of deep water wells, the groundwater table is low — at depths in excess of 30 m. At the time of the reconnaissance effort (January 2003), drilling was underway near the building where the only strong-motion instrument in the city of Bam was located. The soil layering and properties will be available in near future, and will be very useful for evaluating the local Figure 6 A typical landslide along a riverbank. 2 3 EERI Special Earthquake Report — April 2004 EERI Special Earthquake Report — April 2004 Bam. Satellite photos of a neighbor- hood in Bam with 80%-100% dam- age are illustrative. Figure 9 was ta- ken on 9/30/2003 before the earth- quake, and Figure 10 was taken on 1/3/2004 after the earthquake. An isoseismal map developed by IIEES is overlaid on a population map in Figure 11.