Aftershock Distribution of the 1990 Rudbar, Northwest Iran, Earthquake of M7.3 and Its Tectonic Implications

Abstract

One of the largest earthquakes in Iran occurred on June 20, 1990, at 21:00 UT at around Rudbar and Manjil Cities in Gilan Province, northwest Iran. An aftershock observation has been conducted around the source region from 30 days after the mainshock and has been continued for more than 5 months. The epicenters of the aftershocks are extending as long as 90km striking WNW-ESE in parallel with the trend of the northern and southern marginal longitudinal faults of the Qezel-Owzan basin along the Talesh-Alborz seismo-tectonic zone. A strike-slip fault, running along the aftershock zone on the uplifted side of the northern marginal fault, is proposed judging from the topographic map and the Landsat image. The part of the aftershock area southeast of the main shock epicenter is more active than the northwestern part. Particularly, microseismic activity is very high in the southeastern end close to Pakdeh, where remarkable surface fissures were observed. The Vp/Vs ratio is inferred as 1.88±0.60 for the region covered by the seismic network. The aftershock decay rate was such that p=1.5 for the modified Omori formula, although the p value for a short period of 4 days at the beginning of the observation was around 4.0.1990年6月20日21時00分13.12秒(世界時),イラン北西部ギーラーン州のルードバール市付近でM7.3の浅発地震が発生した.地震の30日後から余震観測を実施した. 60kmのスパンの3点観測網(Saravan, Abbar, Abtorsh)による観測は5ヶ月以上の期間続けられた.ほかに3ヶ所において2～5日間の短期観測(Masuleh, Bivarzin, Jirandeh)をおこなった