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
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.
Jahan Ramezani is currently a research scientist in the
Department of Earth, Atmospheric and Planetary Sciences of Massachusetts
Institute of Technology.
Figure 1. The Alpine-Himalayan Belt. Image from Cornell
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.
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.