Electrical conductivity of a locked fault: investigation of the Ganos segment of the North Anatolian Fault using three-dimensional magnetotellurics

Abstract

This study attempts to reveal the fault zone characteristics of the locked Ganos Fault based on electrical resistivity studies including audio-frequency (AMT: 10,400-1 Hz) and wide-band (MT: 360-0.000538 Hz) magnetotellurics near the epicenter of the last major event, that is, the 1912 Mürefte Earthquake (M w 7.4). The AMT data were collected at twelve stations, closely spaced from north to south, to resolve the shallow resistivity structure to 1 km depth. Subsequently, 13 wide-band MT stations were arranged to form a grid enclosing the AMT profile to decipher the deeper structure. Three-dimensional inverse modeling indicates highly conductive anomalies representing fault zone conductors along the Ganos Fault. Subsidiary faults around the Ganos Fault, which are conductive structures with individual mechanically weak features, merge into a greater damage zone, creating a wide fluid-bearing environment. This damage zone is located on the southern side of the fault and defines an asymmetry around the main fault strand, which demonstrates distributed conduit behavior of fluid flow. Ophiolitic basement occurs as low-conductivity block beneath younger formations at a depth of 2 km, where the mechanically weak to strong transition occurs. Resistive structures on both sides of the fault beneath this transition suggest that the lack of seismicity might be related to the absence of fluid pathways in the seismogenic zone.

Electrical resistivity structure at the North-Central Turkey inferred from three-dimensional magnetotellurics

“…Analogously, lack of fluids in the vicinity of faults can infer to near hydrostatic pore pressures that may lead to longer times of stress accumulation and larger rupture events. Previously, MT measurements made on locked strikeslip faults demonstrated a characteristic portrayed by the presence of resistive bodies at both sides of the fault (e.g., Unsworth et al 1999;Goto et al 2005;Tank et al 2003;Tank 2012;Karaş et al 2017). Both the metamorphic units of CPS (R 2 ) and mafic units south of the NAF (R 1 ) can be interpreted as reasonable geological bodies to resist such inclusion of fluids due to their low porosity values or poor interconnection between their pores.…”

“…Many segments can be interpreted as seismic gaps (Ergintav et al 2014), while the surface creep (Çakır et al 2005) or microseismic activity (Örgülü 2011) is also valid explanations for release of stress. Karaş et al (2017) showed that even in small distances within the same segment of the NAF, structures of faults might show drastically different characteristics, thus leading to heterogeneous rheological environments.…”

“…Kaya et al (2009) were able to image the resistive asperity zone and claimed that this zone was responsible for super-shear earthquake occurred along the Düzce Fault in 1999. Likewise, Ganos Fault, which appears as an anomalously quiescent seismic gap, depicted with resistive structures appearing at both sides of the fault, representing mechanically strong units (Karaş et al 2017). Avşar et al (2013) carried out a survey in Erzincan Basin and found a shallow conductor related to NAF.…”

New deep electrical resistivity tomography in the High Agri Valley basin (Basilicata, Southern Italy)

“…Several recent works focus on geophysical survey for deep geological and tectonic quaternary basin characterization (Boncio et al 2016;T€ un et al 2016;Civico et al 2017;Mekkawi et al 2017;Nocentini et al 2017;Onnis et al 2017). Furthermore, most recently reflection seismic and magnetotelluric surveys provide high-resolution images of fault systems (Improta et al 2017(Improta et al 2010Balasco et al 2015;Beckers et al 2015;Beilecke et al 2016;Beka et al 2017;Karaş et al 2017;Buttinelli et al 2016) giving a relevant support to seismologists and structural geologists.…”

Magnetotelluric, Basin Structure and Hydrodynamics; South West of Western Australia

“…Several studies indicate a separation between TE and TM mode in both apparent resistivity and phase at low frequencies in proximity to the coast line (Pous et al, 2002;Lezaeta & Haak, 2003). Large scale faults also impact magnetotelluric data and inversion, and similarly many MT studies have investigated the influence of these structures (Unsworth & Bedrosian, 2004;Bedrosian et al, 2002;Wannamaker et al, 2004;Karaş et al, 2017). In most of these studies, MT data was collected across the fault zone.…”