抄録
We present a detailed seismicity in and around the Tjörnes Fracture Zone (TFZ) on the northern insular shelf of Iceland, based on the experiment conducted there in August 1991 using 18 ocean bottom seismographs (OBSs) and 24 land seismographs. The TFZ is a broad zone (75km×100km) that connects the Northern Volcanic Zone (NVZ), the northern part of the neovolcanic zone that is thought to be a portion of the Mid-Atlantic Ridge (MAR) within Iceland, with the Kolbeinsey Ridge, a segment of the MAR 100km to the west. The aim of this experiment, which consisted of a seismic refraction survey and a microearthquake observation, was to understand the detailed structure of TFZ. Though seismicity in the TFZ was extremely low during the observation period judging from the permanent land station data (Icelandic Seismological Station Network, ISSN), the OBS array could record sufficient number of events and the crustal velocity structure deduced from the refraction study allowed us to get a clear image of hypocentral distribution to discuss detailed microseismicity in relation to the regional tectonics. The results may be summarized as (1) the Grimsey Zone appears to consist of at least three subzones rather than a single linear zone, (2) the high seismicity along the Eyjafjardaráll trough, whose depth distributions suggest listric faultings, related to rifting along the Kolbeinsey Ridge. We suggest that the NVZ and the Kolbeinsey Ridge, the left stepping MAR segments, are being connected by the two major presently active components, that is, the Grimsey Zone and the Húsavík-Flatey fault.
