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We have investigated the spatial variation of the stress field along the fault rupture zone of the 1999 Izmit earthquake (Mw7.4) using first-motion polarity data at seven distinct aftershock clusters. In our approach, the first-motion polarities of all the aftershocks in a cluster are simultaneously inverted to determine the stress tensor parameters and fault plane solutions of individual events, in accordance with the method of Horiuchi et al.(1995). Where post-seismic slip was significant (e.g., Sapanca, Sakarya-Akyazı, and Karadere segments), we obtained stress tensors with the fault parallel or fault normal maximum (σ1) and minimum (σ3) principal compressive stress axes, which may imply either low frictional coefficients or fault weakness. A stress tensor with similar features was derived from the Çınarcık cluster, where the aftershocks lie in a low-velocity zone beneath the geothermal area. The maximum principal stress axis tends to remain parallel to the trend of the pre-mainshock σ1around the Yalova segment; this segment experienced little to no co-seismic displacements. The stress tensor around the Gölcük segment, where the largest surface displacement of 5.5 m was observed, was determined to be 20–25° counterclockwise rotated, but the aftershock alignment remained fault parallel. We interpret these results in terms of the strong crust. On the other hand, both the aftershock alignment and the stress tensor were found to be rotated in the Izmit earthquake epicentral region despite the lower co-seismic displacements. We attribute this feature to the weak crust.

Magnetotelluric (MT) surveys were carried out along some profiles crossing the fault rupture zone associated with the İzmit earthquake which took place on 17 August 1999 in the western part of the North Anatolian Fault Zone (NAFZ). In this paper, we focus on the western part of the fault rupture zone where two different groups of seismicity followed the İzmit earthquake. One group was seen along a narrow belt and corresponds to aftershocks occurring along the fault rupture zone. The other was seen in a circular region and represents a swarm activity, presumably triggered by the occurrence of the İzmit earthquake. Two-dimensional inversion was performed for the MT data acquired along two profiles; one crosses the western end of the fault rupture zone and the other is located in the west of the swarm activity area. In the former case, aftershocks tended to occur in a resistive zone underlain by a moderately conductive zone, as was the case for the hypocenter area. In the latter case, the swarm activity tends to be confined in a conductive zone below a highly resistive zone. This activity is likely to be triggered through pore-pressure changes associated with the İzmit earthquake.

Electric and magnetic data were obtained above the focal area in association with the 1999 Izmit, Turkey earthquake. The acquired data are extremely important for studies of electromagnetic phenomena associated with earthquakes, which have attracted much attention even without clear physical understanding of their characteristics. We have already reported that large electric and magnetic variations observed during the earthquake were simply due to seismic waves through the mechanism of seismic dynamo effect, because they appeared neither before nor simultaneously with the origin time of the earthquake but a few seconds later, with the arrival of seismic wave. In this letter we show the result of our further analyses. Our detailed examination of the electric and magnetic data disclosed small signals appearing less than one second before the large signals associated with the seismic waves. It is not yet solved whether this observational fact is simply one aspect of the seismic dynamo effect or requires a new mechanism.

On August 17, 1999, a destructive earthquake occurred in the western part of the North Anatolian Fault Zone, Turkey. The earthquake source region has been designated as a seismic gap and an M7-class earthquake has been supposed to occur someday in the future so as to fill this seismic gap. So far we have undertaken various kinds of observations in this area and we could obtain some valuable data before, during and after the mainshock. Here we report some of the preliminary results of our recent studies, which include field work started in late July this year and continued during and after the earthquake occurrence just in the earthquake source region and its vicinity, in addition to seismic observations carried out for several years before the mainshock. Much emphasis is put on magnetotelluric field data acquired during the mainshock; in fact, large variations caused by seismic waves were recorded. Such variations could be interpreted in terms of electromagnetic induction in the conducting crust caused by the velocity field interacting with the static magnetic field of the Earth. In particular, the first motion of seismic wave could be identified in the records and used for precise determination of the hypocenter of the mainshock.

A project for detecting time-dependent electrical resistivity changes in crustal rocks by measuring short-period variations of the magnetic and telluric fields has been in operation since October, 1974 in a seismically active region northeast of Quebec City, Canada. Significant changes in amplitude and direction of telluric fields have been observed for variations ranging from 5 to 120 min. A change with a duration of a year or so seems to be a predominant feature. Superposed on it, a more rapid change with a duration of a few months is seen in late spring and early summer. Although the observed changes show a history similar to a premonitory change as expected from a model of dilatancy with fluid diffusion, they are unlikely to have any bearing on a dilatancy process in the crust since no earthquakes of magnitude greater than 3 associated with the changes have taken place in the survey area. It is pointed out, however, that the magnetotelluric method provides a useful means of detecting changes in electrical resistivity in the shallow crust. Thus the possibility exists of detecting resistivity changes associated with shallow earthquakes.

In order to investigate changes in the geomagnetic field associated with seismic activity in the western part of the North Anatolian Fault Zone (NAFZ), seven continuous measurement stations and eighteen repeated-survey sites were set up in the İnik-Geyve region, taking into account the fault trace and its features. Simple differences in the total intensity between these continuous stations have been derived for detecting possible anomalous phenomena associated with earthquake occurrences. Theoretical studies imply that changes in the total intensity associated with fault activity are larger in the case of non-uniform distribution of rock magnetization than those in the uniform case. Meanwhile, inversion analyses of magnetic anomalies at the North Anatolian Fault Zone (NAFZ) suggest a dyke-like structure extending parallel to the fault strike. This structure enables us to test our strategy, based on non-uniform magnetization, for detecting pre- and/or co-seismic changes in the geomagnetic field. We made a theoretical estimation of geomagnetic changes for the case of a magnetic dyke structure intruding a non-magnetic body parallel to the strike of a fault. It turns out that, unlike the uniform case, the magnetic field arising from stress-induced magnetization is enhanced. A tectonomagnetic observation system talcing into account such a non-uniform magnetization structure in the İnik-Geyve region of the NAFZ is also described.

Westward migration of large earthquakes along the North Anatolian Fault Zone seems to have been suspended after the 1967 Mudurnu Valley earthquake, which may imply the formation of a seismic gap. However, the westward extension of the North Anatolian Fault Zone has not been clearly defined; two branches have been proposed but which one of them is the true extension is a matter of controversy. In this paper we review some of the results which we have obtained so far from field work in order to derive some basic information as to the characteristic features of the respective branches.

A temporary microseismic network was installed with nine stations around the western extension of the 1967 Mudurnu Earthquake fault. The area studied is a part of one of the seismic gaps of the first kind along the North Anatolian Fault Zone. The fault zone branches into two fault zone in the area. It is found that the hypocentres are concentrated on the northern branch. It seems that the seismicity close to the western end of the 1967 Mudurnu Earthquake fault is dominated by normal faulting.

Laboratory experiments and theoretical considerations have suggested that anomalous dilatant regions can develop in the earth's crust during the period of strain accumulation prior to an earthquake. For moderate and major earthquakes such anomalous regions could be tens or even hundreds of kilometers in extent and should be detectable at the surface with appropriate survey or sounding techniques. Since electrical resistivity is one of the rock properties likely to be strongly modified in a dilatant zone, magnetotelluric impedance and geomagnetic transfer functions might be expected to show time-dependent precursory effects if monitored over a period of time above the focal region of an impending earthquake. Such experiments have been conducted in Japan and in other parts of the world and several examples of resitivity changes in the crust prior to earthquake occurrence have been reported. These results and their association with local seismicity are reviewed in this paper. The available evidence indicates that transfer functions and impedance can display significant time-dependent response to changing crustal conditions in some regions. However, the correspondence between these effects and earthquake occurrence is usually not very clear.