An Experiment to Investigate Polarization Anomalies in North Anatolia

Abstract

A three-component seismometer network, 15 km in aperture, was set up over some swarm activity just East of the Sea of Marmara. In an eight week period several hundred small earthquakes were recorded beneath or immediately adjacent to the network. Almost all the shear wave trains of these earthquakes display polarization anomalies indicative of dilatancy anisotropy.

A three-component seismometer network, TDPNET, was set up during the summer of 1979 a few kilometres South of Izmit, near the Northern Anatolian Fault at the Eastern margin of the Marmara Sea. The network was designed to monitor the polarization anomalies suggested by theoretical and numerical investigations (Crampin 1978). A site was chosen on the basis of swarm activity recognised during routine epicentral locations at Kandilli Observatory (see Fig. 1 of Üçer, Crampin & Miller 1980, this volume). One of the MARKET stations, KLT, recorded a high level of local activity throughout the winter of 1978/79. This activity, centred a few kilometres to the South of the Northern branch of the Northern Anatolian Fault, was monitored for eight weeks by a closely-spaced radio-linked network of six three-component stations recording on analogue magnetic-tape.

Several hundred earthquakes were recorded within and near TDPNET during this period (Fig. 1). They were located with HYP071 (Lee & Lahr 1975) at depths between 8 and 15 km, by assuming an isotropic crustal-structure determined by using local quarry blasts. The magnitudes (^M _L) range from 0.8 to 2.5, and an event of 3.5 ^M _L