EVALUATION OF ACTIVE TECTONICS OF THE SOHREIN FAULT ZONE USING MORPHOMETRY AND GEOMAGNETIC SURVEY

In this study, mechanism and geometry of the Sohrein Fault zone, located in the North West of Zanjan, is investigated. Before this study, the ~75-km-long Sohrein Fault was supposed as a main seismogenic source of the region. Nearly eighty percent of the fault length was mapped based on subtle geomorphic features in late Quaternary deposits. Discovering true surficial extend, geometry and mechanism of this fault is thus a major interest for seismic hazard assessment in the Zanjan region and surrounding areas. These objectives were achieved by a combination of neotectonic and geophysical studies. The interplay of tectonic activity and erosion was investigated through field investigation and morphometric analyses. Two geomorphic indices of stream gradient index (SL) and valley width-to-height ratio (Vf) were evaluated using satellite imageries and digital elevation models (DEMs). The geomorphic response of streams in related to tectonic activity do not changes along the while length of the fault. Shallow geometry of the fault was investigated using magnetic survey, induced polarization and resistivity methods. The evidence of Quaternary faulting and even the shallow expression of the fault zone is not distinguished by these geophysical studies. The only effect of faulting is seen in induced polarization method along a small part (<4 km) of the active sector of the Sohrein Fault. These two lines of evidence suggest that the linear Quaternary risers previously interpreted as the surface trace of the Sohrein Fault are likely the landforms produced by retrogressive erosion in fluvial terraces around the Zanjan Rud River. This can be easily formed after a regional change in the base level of the river.

کلیدواژه ها:

Zanjan ، Sohrein Fault ، Seismic hazard Morphometery ، Magnetometery