No abstract

We applied a 3-D seismic tomography inversion algorithm to arrival-time data obtained, during 18 years from 1985 to 2002, from local seismic networks and aftershock studies in the Marmara region, in order to better understand the upper crustal structure of the complex tectonic region. We integrated all the available data set into a common data set and relocated the events, using a 1-D velocity model. We then selected 3,949 earthquakes and obtained 92355 arrival times, in total, consisting of 59,313 P-wave and 33,042 S-wave arrival times. In this paper we present detailed crustal structures for V p, V s and V p/V s ratios from the surface down to 15 km depth with good resolution in terms of the hit count analysis of seismic rays, the checkerboard and restoring resolution tests for the studied region. The results obtained from the inversion suggest that the western part of the North Anatolian Fault Zone shows strong lateral heterogeneity. We concluded that no clear pattern exists between the distribution of microearthquakes and aftershocks and the velocity perturbations presented in this paper. This is probably due to complex tectonic and geological structures. Large coseismic slip associated with the two recent strong earthquakes was found to correspond to higher velocity anomalies, as was often found recently. On the other hand, low velocity values correspond to the sedimentary units or the alluvium regions, as supported by the low resistivity and gravity values. Thus the results presented in this paper are quite consistent with the other data such as gravity, resistivity and magnetic anomalies, indicating that our model is reliable and efficient and it should be useful for further interpretation of tectonic and geological problems in this region. It should be pointed out, however, that the results of S-wave perturbations and V p/V s ratio could not be discussed in detail because of insufficient quality of the S-wave data and the reliability of the results is not very high. Nonetheless, the fact that recent large Izmit earthquake and moderate aftershocks that occurred in or around the high velocity zones near the low velocity region suggests that high velocity regions found in the area close to Istanbul in the Marmara Sea and also at the Iznik-Mekece fault are potential sites for strain energy accumulation and release.

This is the introduction to a series of papers analysing records from three-component seismometers immediately above a swarm of small earthquakes near the North Anatolian Fault in Turkey. The networks were set up to investigate the possibility of estimating dilatancy-induced adisotropy by monitoring shear-wave splitting in shear waves from local earthquakes as suggested by the hypothesis of extensive-dilatancy anisotropy (EDA). These networks were among the first closely-spaced digital three-component .seismometers to be deployed in the shear-wave window immediately over a seismic source zone, where the polarizations of incident shear waves can be recognized from surface recordings. Consequently, the seismograms display several new phenomena that have not been previously observed: S. Crampin, R. Evans and S. B. UCerThese six observations strongly support the hypothesis of extensive-dilatancy anisotropy, which, if confirmed. suggests techniques for monitoring changes of stress for earthquake prediction studies, as well as having implications for the deformatory behaviour of all crustal rocks. S. Crampin, R. Evans and S . B. UGerEdinburgh. earthquakes, Geophys. J. R. astr. SOC., 69, 137-157. and fault motion patterns on local earthquakes, Open File Rep. U.S. geol. Surv., 73-311.