A VLF survey using current gathering phenomena for tracing buried faults of Fethiye–Burdur Fault Zone, Turkey

Abstract

Fethiye–Burdur Fault Zone (FBFZ) produces moderate to large earthquakes which can severely affect the surrounding cities spreading towards the basins of the region. In this study, with the aim of feature city planning, very low frequency electromagnetic (VLF) survey was carried out across (FBFZ) to locate subsurface faults covered by basin fills. VLF parameters such as the apparent resistivity, phase (VLF-R response) real and imaginary parts of tipper (VLF-EM response) were obtained using GBZ, a well known radio station in England at Oxford, which give a strong signal in our survey area with a suitable frequency (19.6 kHz) for our purposes. VLF method is called as VLF-EM, when it is only relying on the measurement of horizontal and vertical magnetic fields. However, when it includes measurement of horizontal electric field, providing resistivity values, it is referred as VLF-R method. In this study, the direction of horizontal electric field is perpendicular to fault strike describing the H-polarization mode. Although this mode produces strong VLF-R (resistivity) response, we also obtained quite clear and characteristic VLF-EM anomalies along some of the survey profiles. Both VLF-R and VLF-EM responses with the contribution of the vortex and gathered (direct) currents, in E and H-polarization modes, are reviewed and VLF anomalies observed over FBFZ are discussed. In FBFZ, current gathering anomalies become stronger in the presence of high conductivity contrast and in the presence of parallel fault branches separating the medium into resistors in series. In these conditions, changing electrical charge at the contacts of faults cause successive variations in E-field. As a result VLF-R response is strengthened and also secondary magnetic field which causes VLF-EM anomalies may be enhanced, in H-polarization mode. Interpretation of resistivity, phase and tipper data with current density pseudosections, over our survey lines, showed the location of buried faults of FBFZ beneath the sedimentary covers of the basins.