The northward motion of Arabia relative to Eurasia causes the Iranian plateau to be deformed. Most of this deformation occur within the political boundaries of Iran, mainly in the Zagros, Alborz and Kopeh-Dagh mountains. The Zagros mountains trends NW-SE and consist of at least two zones: The High Zagros (HZ) in the NE and Simply Folded Belt in the SW, and separated from the Central Iran by the Main Recent Fault (MRF) and High Zagros Fault (HZF). There are two zones of mainly metamorphic rocks (Sanandaj-Sirjan) and volcanic (Urumieh-Dokhtar) in the SW margin of the central Iran with trends parallel to the Zagros. A series of N-S trending strike-slip faults in eastern Iran, taking up the right-lateral shear resulted due to northward movement of central Iran relative to stable plate of Afghanistan.
The main aim of seismotectonic group at GSI is to improve our understanding of present-day continental deformation within Iran. This provide the basis for our better understanding of earthquake hazard and for a better readiness and reduction of the community's exposure to risk.
Major structural units of Iran: Solid lines are major faults from Berberian & Yeats (2001). White arrows are the directions and raters (mm/year) of the overall Arabia-Eurasia motion from DeMets et al.(1994, big arrow) and Sella et al. (2002, small arrow). The inset shows earthquakes of greater than 5 from the catalogue of Engdahl et al. 1998.
Main Research Themes: Research projects within the sesimotectonic group contribute to our better understanding of active faulting and improving disaster management in Iranian communities. A brief description of what this group does is summarized as follows:
Geodetic survey and monitoring of active faults: Since 1985 seismotectonic group at GSI has been monitoring active faults near major cities including the capital Tehran, Mashad in the NE, and Tabriz in the NW. These measurements were initially started by using ground based optical surveying instruments ( theodolite). Recently GPS measurements have been conducted for the same network, and additional benchmarks have been added. We have in mind to develop our measurements to other regions to cover more active faults around other major cities.
Case study of earthquakes : Each earthquake gives a new lesson and contains valuable information about seismotectonic of the region. In order to make maximum use from this unique information, it is necessary that these data, especially co-seismic surface ruptures, to be documented immediately after earthquake occur and before natural erosion and human destruction remove them for ever. Field geologists in seismotectonic group at GSI are always ready for such a quick response and have successfully recorded destructive events in the last few years. Field observation were later combined with seismological and geomorphic data and published in international journals.
Coseismic surface rupture of 1994 Sefidabeh earthquake in eastern Iran (after Berberian et al 2000).
Aftershock study and local seismic network : The sismotectonic group at GSI runs local seismic network to monitor aftershocks immediately after the main shock. We also run local seismic network to study seismic activity of some important faults.
Geomorphologic study of active faults : Traditionally, geologists have looked at the final result of continental deformation by studding old geological structures in places that are now inactive. However, such regions have deformation histories spanning many million of years, the structures developed in them can be very complicated, and their interpretation can be difficult or ambiguous. Looking at re gions that are actively deforming at the present-day provides the opportunity to look at the process of continental deformation while they operate. Observing active continental deformation not only aids the estimation of seismic hazard for local population, but also help us to understand how continental regions respond to tectonic stress, and how deformation of the continental lithosphere as a whole is reflected in the distribution and style of discontinuous brittle faulting in the upper crust. As some earthquakes in Iranian plateau occur on blind faults with no clear co-seismic surface rupture, the ability to interpret the geomorphology of regions in which concealed thrust faults occur is important for estimating seismic hazard in Iran, as these faults may be capable of generating destructive earthquakes in the future.
A classic triangular facets ('flat-iron') with deeply incised canyons along the range front caused by vertical motion of the fault (afterTalebian & Jackson 2002).
Study of historical earthquakes : The about 35 mm/yr convergence between the Eurasian and Arabian plates accommodates by reverse and strike-slip faults within Iranian plateau, with relatively low slip rates in a zone 1000 km across. A rich historical and archeological record in Iran spans several thousand years, long enough to establish recurrence intervals of 1000 to 5000 yr on individual fault segments. Study of earthquakes in historical literature, together with information obtained from other sources can increase our understanding of seismic hazard in the country. "Destruction occurred within a radius of 12 farsangs [~72 km] from Tabriz city; …very long fissures developed in most mountains, the fissures extended for approximately 7 farsangs [~42 km] along the road to Shebli [located along the North Tabriz active fault line]…" (Zonuzi, 1801 describing the 1780.01.08 Tabriz earthquake of Io~ IX +, Ms ~7.4 +, after Berberian M. & Yeats R. S. 1999).
A 12th century monument near Ghazvin damaged by Avaj earthquake in 2002 (photographed by F.Ansari 2003)
Paleoseismology: In order to improve our understanding of the size and frequency of earthquakes along the active fault, we conduct trenching along the faults and in places where seismic activity of the fault is recognizable in geomorphology as well as in historical and recent seismic activities. We give priority to major active faults near highly populated cities.
A trench across the North Tehran fault i excavated during a joint paleoseimology project between GSI and university of Montpellier II. The North Tehran fault is known to be the most active major fault near Tehran, a city with more than 10 million population).
Seismology : Relation between earthquakes and faults on the surface is not always clear. It is extremely rar e for co-seismic surface faulting to
beassociated with earthquakes in Zagros. Several other earthquakes in central Iran and Alborz occur on blind faults with no clear surface rupture. So, the most accessible information relating to active faulting comes from earthquake seismology. As geologist need to construct structural cross-section at depth, an important constraint on their interpretation is the known distribution and orientation of the faulting revealed by earthquakes. Seismic hazard assessments require better understanding of location and characteristic of these active faults. We determine source parameters of some major earthquakes through the inversion of teleseismic long-period P and SH body waves. The focal mechanisms and centroid depths of the earthquake determined in this way are the most reliable information about the source parameters of earthquakes.
Waveform modeling of 1997 earthquake in the Zagros ( after Talebian & Jackson 2003).
Microtremore study : It is well known that, degree of damage during earthquakes strongly depends on dynamic characteristics of buildings as well as amplification of the site. Majorities of the cities in the Iranian plateau are located on thick alluvial sediments on foothill of mountains, or intra-mountain playas. Thickness and characteristic of the alluviums vary from place to place and therefore have different effect on the waves passing through. Microtremore records can be used to estimate the response of these sediments to seismic waves. These data along with other subsurface geotechnical information will be used for urban planning, especially on selection of appropriate site for multi-story buildings and process of seismic hazard assessments.
Predominant period of the ground in Kerman based on microtremore study (after M. Roustaei 2001).
Earthquake hazard assessment : The aim of various geological hazard programs is to reduce the economic and social cost of such hazards to Iran. This includes researches aimed at understanding the processes that cause earthquakes. It is through such improved understanding that we are better able to estimate the future likelihood and effects of geological hazard events. The uncertainties are unavoidable and still very large, however, so further development is required to produce models that are sufficiently robust for loss estimation and risk-based decision making. There are different methods for evaluating seismic hazards. But more than formulation, it is the quality of the input data, and our understanding about active faults, which affect the degree of uncertainty in the seismic hazard estimation. Seismotectonic department at GSI is premier provider of such information and by running research projects has access to the most updated information about active tectonic of Iran.