The Bam earthquake of December 26, 2003 with a magnitude of 6.6 Richter is one of several deadly tremors that have repeatedly struck Iran's towns and villages throughout its modern and ancient history. Understanding the geologic forces behind this tremendous natural hazard is of fundamental importance to any risk assessment or loss prevention efforts in the region.

Geological Setting of Iran

Earthquakes in Iran and neighbouring regions (e.g., Turkey and Afghanistan) are closely connected to their position within the geologically active Alpine-Himalayan belt (Fig. 1). But, what causes all that geologic activity? In general, the rigid outer shell of the solid Earth is a mosaic of interlocked slabs-known as tectonic plates&mdash that are constantly moving against each in response to Earth's powerful internal forces. The great majority of earthquakes in the Earth's crust occur along faults (ruptures in rocks) at or near the plate boundaries. In the Middle East region, modern tectonic activity is forced by the convergent movements between two plates: the Arabian plate, including Saudi Arabia, Persian Gulf and the Zagros Ranges of Iran, and the Eurasian plate that incorporates Europe, central and East Asia, as well as the interior Iran (Fig. 1 and 2). The Zagros Thrust Zone (red line in Fig. 2) constitutes the boundary between the two colliding plates.

Much of the mechanical deformation resulting from Arabia-Eurasia collision is accommodated by the Zagros Ranges in the form of folding of rocks and the rise of mountains in conjunction with fault movements at depth of the Earth. In fact, the highest frequency of earthquakes in Iran occurs in the Zagros region (Fig. 3). However, because of the diffuse nature of this deformation (i.e. simultaneous movements along a number of sub-parallel faults over a wide area) the intensities of these tremors are generally low and are recordable only by sensitive seismic devices. The interior parts of Iran, however, respond to the plate collisional forces in a different manner. In the area known as Central-East Iran deformation takes place largely in the form of strike-slip (sideway) movements focused along a complex array of intersecting faults (Fig. 2). In sharp contrast to that in Zagros, seismic activity associated with central Iranian faults is sporadic but much more localized and occurs with significantly higher magnitudes. Many of Iran's powerful tremors, such as the recent Bam earthquake, have occurred in this area. By and large similar mechanisms are responsible for large magnitude earthquakes in other parts of the country, such as Zanjan and Azerbaijan, not shown here. Figure 2 illustrates major fault structures of the Central-East Iranian region along with large centers of population in their vicinity. The locations of three largest earthquakes of the modern times are also indicated by red circular symbols. These are the Ferdows earthquake of August 31, 1968 (Magnitude = 7.3, 12,000 - 20,000 deaths), Tabas earthquake of September 16, 1978 (M = 7.8, more than 1,500 deaths), and the recent Bam earthquake (M = 6.6, more than 30,000 deaths).

Scientific Research and Earthquake Risk Assessment

Earthquake damage prevention requires organized nationwide planning and that in turn relies on successful identification of high-risk areas. Most faults, as zones of mechanical weakness prone to seismic rupture, can be located on the ground by means of geological field investigations aided by aerial or satellite imagery. However, not all faults are seismically active and not all active faults produce large magnitude earthquakes. Data collected by networks of seismic stations, both locally and globally, provide valuable information regarding the overall geologic structure of areas of high seismic activity. In addition, the geophysical analysis of seismic waves received at such stations reveals not only the timing and magnitude of earthquakes, but the sense of movement on the corresponding fault structures, as well. Through compilations of time-integrated geological and seismological information, areas of high earthquake risk can be identified.

Important advancements in the study of earthquakes have been brought forward by the more recent development of space-based Global Positioning System (GPS) technology. By simultaneous analysis of radio signals received from multiple satellites orbiting the Earth, GPS stations (Fig. 4) can pinpoint the precise geographic coordinates and elevation of any spot on the ground within fractions of a centimeter. Monitoring small Earth movements (indicated by shifts in the location of GPS stations) over extended periods of time reveals areas of strain build up along faults and helps to predict the location, style and size of potential earthquakes. A remarkable advantage of the GPS method is its ability in constraining deformation during seismically quiet motions before and after earthquakes. In places such as central Iran where ground deformation occurs through complex interactions among several fault systems, sophisticated computer models need to be employed in conjunction with both ground-based and GPS data for earthquake prediction and seismic risk assessment.

Figures

Figure 1. The Alpine-Himalayan Belt. Image from Cornell University/INSTOC.

Figure 2. The simplified structural map of Central-East Iran showing the location of major faults. Red line represents the boundary between Arabian and Eurasian plates. Large arrows indicate the direction of plate motion. Compiled from Berberian, 1981; Jackson and McKenzie, 1984; Haghipour and Aghanabati, 1989; Alavi, 1991. AZF = Abiz Fault, DRF = Doruneh Fault, GWF = Gowk Fault, KBF = Kuhbanan Fault, KMF = Kalmard Fault, MAF = Mehdiabad Fault, NAF = Nostratabad Fault, NHF = Nehbandan Fault, NNF = Na'in Fault, RJF = Rafsanjan Fault, SBF = Shahre-Babak Fault, ZRF = Zarand Fault, ZTZ = Zagros Thrust Zone.

Figure 3. Record of seismicity in southeastern Iran from 1990 to 2003. Location of the December 26, 2003 Bam earthquake is marked by a star (from USGS National Earthquake Information Center).

Figure 4. A Global Positioning System (GPS) station equipped with solar batteries in Nevada. Courtesy of Brian Wernicke, California Institute of Technology.

References

Alavi, M., 1991, Tectonic map of the Middle East: Tehran, Geological Survey of Iran, scale 1:5,000,000.

Berberian, M., 1981, Active faulting and tectonics of Iran, in Gupta, H.K., and Delany, F.M., editors, Zagros-Hindu Kush-Himalaya Geodynamic Evolution: American Geophysical Union Geodynamic Series, v. 3, p. 33-69.

Haghipour, A. and Aghanabati, A., 1989, Geological Map of Iran (2nd edition): Tehran, Geological Survey of Iran, scale 1:2,500,000.

Jackson, J. and McKenzie, D., 1984, Active tectonics of the Alpine-Himalayan Belt between western Turkey and Pakistan: Geophysical Journal of the Royal Astronomical Society, v. 77, p. 185-264.