Low Seismicity, Low Coda-Q and Discontinuities of the Upper Crust in the Vicinity of the Iznik-Mekece Fault, the North Anatolian Fault Zone, Turkey
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概要
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An attempt to detect microearthquakes was made in the Iznik-Mekece Fault region, a proposed seismic gap in the western part of the North Anatolian Fault Zone, during the period from July 6 to August 13, 1986. We set up six temporary stations installing very small vertical-component seismometers and portable analog event recorders equipped with either paper tapes or magnetic cassette-tapes. A large number of earthquake events, 135 at most for one station, were registered, the majority of which were either distant earthquakes with S-P times over 15 sec or artificial explosions. Eight hypocenters were determined by using data from the three stations, and three more epicenters were estimated from data from two of the stations. All the epicenters are located outside of the central part of the seismic gap region. Only four microearthquakes, which cannot be located, were identified in the vicinity of the Iznik-Mekece Fault. The microseismic activity rate around our network is statistically estimated to be 23.0 events/year with a significance level of 10%. Coda Q values are found to be very low, between 30 and 200, in the fault region at frequencies from 2 to 10 Hz compared with the values, between 70 and 300, of the surrounding region. Two possible explanations are proposed for this: scattering due to concentrated heterogeneous medium and temporal increase of intrinsic absorption related to preparation of a large earthquake.北アナトリア断層帯西部の地震空白域,イズニック・メケジェ断層付近において,1986年7月6日から8月13日の間,はじめての高感度地震観測を実施した.地震計は固有周波数2Hzの上下動成分のみを使用し,6ケ所に観測点を設けた.3点には可視記録方式,あとの3点にはアナグロのカセットテープの記録計を用いた.11個の電源が決められたが,これらはいずれも当該断層から数10km離れている.震源が決まらなかった断層付近の微小地震は4回であった.統計的には10%の危険率で23回/年の微小地震が発生すると言える.カセットテープに記録された数少ない地震の波形を解析し,断層付近でコーダQが低いことや,その直下,深さ8～9km付近に地震速度の不連続面が存在することが反射波の解釈から推定された.これらの性質はその地域の地震予知やテクトニクスの問題にも関わっている.
- 1989-03-31
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