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Introduction

Iran, being located along the active Alpain-Himalian belt, has had a high potential of seismicity since the collision of the Arabic and Eurasian plates at the last mountain building process. The knowledge of the mechanisms of the earthquakes occurring in this region requires the study of the earth interior structures.
Releasing the energies stored in the earth crust causes the earthquakes, and these energies in turn, are produced due to the movements of the lithospheric plates. So, to assess the seismicity of a region, we must investigate the situation of the lithospheric plates relative to each other. Since the relative movements of the lithospheric plates take place on the upper part of the earth mantle, the investigation of these movements should accompany the study of this part of the mantle.
The knowledge of the crustal structure, in addition to help in recognition of the earthquake mechanisms, provides the needs of fundamental seismological studies. This knowledge in detail and with high precision throughout the country has always been one of the most serious needs of the seismologists.

Objectives

IIEES has conducted many projects in different region of the country in order to study the crust structure and the upper mantle. Among the project, recently IIEES in cooperation with the “Laboratoire de Géophysique Interne et Tectonophysique” of University of Joseph Fourier has carried out the project of  “ZAGROSS Transects 2000-2001”. The purpose of the Zagros transect is to investigate the lithospheric structure of a continent-continent collisional mountain range at an early stage of its evolution.
The main aim of this project is imaging the lateral variations of the velocity structure of the crust and upper mantle from the coast of the Persian Gulf in the Zagros accretionary prism (Bushehr) to the stable block of Central Iran (north of Yazd). It is expected to better understand the geometry of the collision between the Arabian and Eurasian plates, including the present geometry of the subduction.
The data from this transect will be used to investigate the following objectives: 1) is the oceanic plate still attached to the Arabian continental plate? 2) Has continental subduction started? 3) If yes, is the entire lithosphere subducting? 4) Where is the location of the suture between the two plates and its geometry at depth? And, 5) what is the geometry of the contacts between the different lithospheric blocks crossed by the transect (Zagros accretionary prism on the Arabian plate, Sanandaj-Sirjan metamorphic belt, Central Iran block on the Eurasian plate). 

Method of study

This study relies on the recording and processing of local, regional and teleseismic earthquakes by a profile of densely spaced (approximately 10 km) seismic stations from Busher to Posht-e-Badam (north of Yazd). Figure 1 shows the profile and the location of different seismological stations. Following methods will be used in this investigation:

1. To investigate tomography of the lithosphere from teleseismic arrival times, a profile of short period seismic stations with the average spacing of 10 km was installed. The stations recorded the teleseismic events through 5 month. The arrival times of these events will be used to invert for the velocity structure of the lithosphere.

2. To apply receiver functions method to study the S-wave structure of the earth crust beneath the stations, we installed broadband to medium-band (100s to 5s) seismological stations in the different tectonic region along the proposed profile. The records of teleseismic events will be used to invert, by using the receiver function method, for the S wave structure of the crust beneath the stations.
   

3. Anisotropy of the upper mantle: the records of teleseismic events (magnitude>5.5) at distance between 95°-105° containing the SKS-waves, will be used. The amount of the SKS-wave splitting will provide the evaluation of the amount and direction of anisotropy in the upper mantle.

4. Dispersion of surface waves; we deployed 3 mini-networks of broadband seismological stations, with the average spacing of 50 km in different tectonic settings. By measuring the dispersion of surface Raleigh and Love waves from teleseismic and regional events, we may determine the S-wave velocity structure of the lithosphere.

Field work and the instrumentation

As this project aimed at many different methods of investigation, different instruments and station configuration were required. In the following, a short summary of different equipment and field procedure will be described. For short period stations we used 3 types of sensors: Le-3D/5s, L22, L4C. Figure 2 shows the photos of these sensors and a brief discussion about their characteristics. 
The network included 66 stations (at the end of installation phase). 11 of them were broadband instruments with STS-2 or CMG-3 sensors (frequency bandwidth: from 100 s to 50 Hz), 8 were medium-band stations with CMG-40 sensors (bandwidth: 60 s to 50 Hz), and 27 stations had Le3D-5s sensors (natural period: 5s). The remaining 20 stations had 2 Hz 3-component sensors. Figure 3 shows one of the stations. All stations along the profile used almost the same instrumentation. The list of the stations with the locations and the type of the sensor in each station is shown in appendix A.
The broadband (BB) stations were not installed along the profile but in 3 mini-networks of about 50-km aperture. Two mini-networks of three BB stations were located around Kazerun and north of Shiraz; 5 BB stations made up a third mini-network around Yazd. This setting of BB stations was necessary to provide accurate measurements of surface-wave phase velocities. All instruments have been recording continuously for about five months.
Four teams of three persons each (1 driver, 1 Iranian scientist, 1 French scientist) worked simultaneously in the field to install 66 stations. Two teams installed 33 stations in the 
Southern part of the transect from Busher to Abadeh; their base was in Shiraz. The two other teams installed 33 stations in the northern part of the profile from Abadeh to Posht-e-Badam; their base was in Yazd. 
The logistics of 4 cars and 12 persons simultaneously in the field was heavy but proved to be very efficient, since the installation of the 66 stations was completed within 10 days. We spent the remaining days visiting the stations and trying to sort out as many technical problems as possible. This way, many problems were solved very early after installation. We encountered a reasonable (and usual) number of technical problems with the stations, as well as for finding adequate station locations.
After installation accomplished, a group of seismologists from both IIEES and LGIT visited all the stations monthly and made backup of data during the period of recording. The stations were removed from April 7 to April 17 2001. Now we have five months of continuously recorded data and an Iranian student will work on these data as his PHD at the University of Joseph Fourier. This student will use the methods mentioned above to study the velocity structure of the earth crust and upper mantle. This investigation will provide a wide variety of fundamental and applied scientific results. The following table shows the calendar of the work done by now and the coming visits.