The NW-SE trending Tus fault is one of the active faults in the north-east of Iran . Geomorphologic evidences indicate that this fault is active and goes through Mashhad city from near Tus town. Since the population and number of pilgrims in this city are high, the study of the Tus fault is of great importance. Hence, we have carried out a new investigation to evaluate the hazardous potential of this fault. This study includes analysis of geomorphologic evidence (such as offset streams recognized in aerial photos and geological maps), geophysical, geodesy and geotechnical surveys.
The Copeh Dagh area includes active fold mountains in the north-east of Iran in contact with the aseismic cratons of Eurasia . The NW-SE trending Tus fault is one of the active faults in Copeh Dagh in the north-east of Iran near Mashhad , the capital of Razavi Khorasan province. Studies of aerial photos detected tracing of this fault on alluvial deposits. Hafezi Moghaddas et al. (2007) showed that the change of path streams in the valleys of Mashhad is correlated with the Tus fault trend. This fault enters into Mashhad from the north- west near the town of Tus and passes through Khajeh-rabee and then exits from Golshahr complex in the eastern part of Mashhad.
Occurrences of some earthquakes north of Mashhad may be related to this fault. Since Mashhad is a heavily populated city and there are old buildings in the city, the investigation of the characteristic of the fault is very important. From 1982 to 1983 an electrical resistivity study was done by the water organization of Khorasan province in order to determine the water level in mashhad area. These data were again reprocessed for tracing the fault and some cross sections were prepared again and interpreted. A land subsidence study was carried out in Mashhad during 1995 to 2005 (Motagh et al., 2007). In this study the three methods of InSAR, leveling and GPS measurements were used. The results indicated that the valley floor with a north-western-south-eastern trend subsided at a rate of 15 cmyr?1 during 2003 to 2005. In this paper we have considered an assessment of the geological characteristics and historical earthquake seismicity of the area. These investigations show that some of the powerful, historic earthquakes in Iran took place in Khorasan province at the north-eastern part of the Iranian plateau, the epicenters of some of which were located in Mashhad city (Ambrasys and Melville, 1982; Berberian et al., 1999).
The seismic exploration technique has been carried out for determining shear wave velocity, through six boreholes along the Tus fault. The results of this study show a composition of typical marls around the first borehole, while geological setting around other boreholes were composed of alluvial material such as sand, silt with traces of gravel and clay. Moreover, our results have shown that the least shear wave velocity is about 500 m/sec along BH#10.
Microtremore studies have been carried out for deriving the background vibration of the earth's surface from artificial sources such as traffic, industrial machinery and so on. In this method usually a three-component seismometer is installed for recording noise during a period of about 20 minutes. Then horizontal to vertical spectral ratio is calculated to extract the predominant period. This type of measurement is repeated for many points around the Tus fault and finally, the iso-predominant period map is prepared for the area of study. For completing the study and defining the fault some general electrical resistivity sounding point and three-dimensional electrical resistivity surveys have been taken and then measurement data are processed and interpreted. For defining the deep geometry of the fault an array of deep electrical soundages has been designed and tested. The results of the deep electrical resistivity test showed that the Tus fault has a reverse mechanism with dip direction to the south-west, towards the populated area of the city.
After revising and correlating these studies, a supplementary study with the electrical resistivity method has been conducted in the area along the Tus fault. Results show that this fault is a reverse fault and has a dip direction to south-west. The activity of this fault during the Quaternary Period is evident from displaced alluvial deposits. These results show it is necessary to prepare a suitable legal order for limiting construction near this fault and it is recommended that the present buildings should be reinforced as well.