Solar and lunar geomagnetic variations in the northwestern part of Turkey

Hourly mean values of X, Y and Z components of the geomagnetic field from two observatories and the total intensity, F, of the geomagnetic field from eight monitoring stations were analysed to study the behaviour of solar and lunar geomagnetic variations in the northwestern part of Turkey. The ChapmanMiller method was applied to the data first as a whole and then by grouping it to Lloyd's seasons to calculate seasonal dependence of the harmonics. Amplitudes of solar and lunar harmonics increase from winter months to summer months. The first two harmonics of both solar and lunar origins were calculated annually to compare with sunspot relative numbers to see annual and semi-annual variation. The solar harmonics show very good sunspot dependence, but lunar harmonics do not. It means that both ionospheric and ring current systems have an important role in the formation of solar harmonics, but lunar harmonics are affected by ionospheric current system only in the northwestern part of Turkey. We would not expect and we do not find much ocean effect at these observatories since they are all remote from oceans with significant tides

Upper crustal electrical resistivity structures in the vicinity of the Catalca Fault, Istanbul, Turkey by magnetotelluric data

A magnetotelluric survey was performed at the Catalca Region, west of Istanbul, Turkey with the aim of investigating geoelectrical properties of the upper crust near the Catalca Fault and its vicinity. Broadband magnetotelluric data were collected at nine sites along a single southwest-northeast profile to image the electrical resistivity structure from surface to the 5 km depth. The dimensionality of the data was examined through tensor decompositions and highly two-dimensional behavior of the data is shown. Following the tensor decompositions, two-dimensional inversions were carried out where E-polarization, B-polarization and tipper data were utilized to construct electrical resistivity models. The results of the inversions suggest: a) the Catalca Fault extends from surface to 5 km depth as a conductive zone dipping to southwest; b) the thickness of the sedimentary cover is increasing from SW to NE to 700 m with low resistivity values between 1-100 Omega m; c) the crystalline basement below the sedimentary unit is very resistive and varies between 2000-100000 Omega m; d) a SW-dipping resistivity boundary in the northeastern part of our profile may represent the West Black Sea Fault

Rapid changes in the electrical state of the 1999 Izmit earthquake rupture zone

Crustal fluids exist near fault zones, but their relation to the processes that generate earthquakes, including slow-slip events, is unclear. Fault-zone fluids are characterized by low electrical resistivity. Here we investigate the time-dependent crustal resistivity in the rupture area of the 1999 M-w 7.6 Izmit earthquake using electromagnetic data acquired at four sites before and after the earthquake. Most estimates of apparent resistivity in the frequency range of 0.05 to 2.0 Hz show abrupt co-seismic decreases on the order of tens of per cent. Data acquired at two sites 1 month after the Izmit earthquake indicate that the resistivity had already returned to pre-seismic levels. We interpret such changes as the pressure-induced transition between isolated and interconnected fluids. Some data show pre-seismic changes and this suggests that the transition is associated with foreshocks and slow-slip events before large earthquakes