Investigation of electrical conductivity characteristics at the vicinty of Genos fault, northwest Turkey by magnetotellurics

Abstract

As constituting the westernmost end of North Anatolian Fault, 45-km-long Ganos Fault acts as an active segment which represents a seismic gap. In this thesis, the first systematic magnetotellurics observations are made to reveal the electrical conductivity characteristics of locked Ganos Fault. Near the epicenter of last major event, 1912 Mu¨refte Earthquake; audiomagnetotelluric (AMT) data were collected through north south aligned almost-continuous profile including twelve stations to decipher the shal low conductivity structure. Then, thirteen wide-band magnetotellurics (MT) stations surrounding the AMT profile as a grid were installed to investigate deeper structure. The dimensionality analyses performed on both AMT and wide-band MT datasets indi cate a considerable agreement with previous geological and seismological observations. While the geo-electric strike angles were determined with Swift’s method following Groom and Bailey decomposition, they were also calculated with phase tensor analy ses independently. Both methods confirm each other and verified the strike directions as∼N70oE for the AMT and∼N60oE for wide-band MT data. Two- (Rodi and Mackie, 2001) and three-dimensional (Siripunvaraporn et al., 2005; Egbert and Kelbert, 2012) numerical modeling routines were used for inverse modeling. All modeling attempts illustrate highly conductive anomalies representing so-called “fault zone conductors” along the Ganos Fault. Three-dimensional models indicate that fluctuations in spatial extents of the FZC are observed along the fault. Subsidiary oblique faults around Ganos Fault which are represented as conductive structures with individual mechani cally weak features merge in a greater damage zone and they constitute a wide fluid bearing environment. By concentrating on the southern side of the fault, the damage zone shows an asymmetry around main fault strand, which exhibits a distributed con duit behavior for fluid flow. A highly resistive block represents the ophiolitic basement beneath younger formations at a depth of 2 km, where the mechanically weak to strong transition occurs. Below this transition, the overt resistive structures at both side of the fault imply that the absence of fluid pathways through the seismogenic zone might be related to lack of seismicity in the region.