2003年イラン･バム地震(Mw＝6.5)の強震動

The acceleration waveforms of the mainshock of the 2006 Bam Earthquake, in southeast Iran (Mw 6.5) have been analyzed to derive several characteristics of strong ground motion. The near field effect of the main shock caused a huge maximum acceleration of about 1G at the Bam station. Waveform analysis of this record shows a big effect of directivity with a strong motion in the fault normal direction. The fault normal component of the near field record shows a maximum displacement of about 30 cm. This effect is also shown when comparing the response spectrum of fault normal and fault parallel components of strong motion. The fault normal response spectrum shows a spectral displacement almost 2 times that of the parallel component, especially for periods greater than 1 s. The attenuation relations derived for both vertical and horizontal components have a very good correlation. Comparisons of the attenuation relation of the Bam earthquake with predictive attenuation relations in other regions (North of Iran, Turkey, and Japan) show a higher PGA gradient of decay in the Bam earthquake surrounding area. Calculations of duration of strong ground motion records show that up to 100km strong motion duration increases with distance, and, after that, duration decreases. This could be explained by adding upper crust reflected waves up to 100 km, a weakening of wave amplitude, and decreasing total arrived energy at each station with distance increasing beyond 100 km. From a calculation of maximum displacements at all triggered stations the propagation pattern is also derived

2003年イラン･バム地震の余震観測と初期震源決定

We investigate the hypocenter distribution of aftershocks of the 26 December, 2002 Bam earthquake Mw 0.5 using a temporal seismic network installed for 1 month from 6 February, 2004. Preliminary hypocenters were determined using automatic phase-picking and calculating software. The epicenters are distributed linearly about 20km in length parallel 3.5km west of the Bam fault, and extend from the south of Bam city to the heavily damaged area of eastern Bam city including Arg-e-Bam. The hypocenters are distributed vertically or slightly leaning toward the east in the depth range from 0km to 15 km. Using the hypocenter distribution we propose a schematic 3-D structure model of the source fault distinguished from the Bam fault. Eastern Bam city is located just above the northern part of the source fault and also on the rupture propagation direction from the asperity. It may be a major reason why eastern Bam city suffered the heaviest damage

44. Anomalous Underground Structure in the Matsushiro Earthquake Swarm Area as Derived from α Fan Shooting Technique

During explosion seismic studies in the Matsushiro Earthquake Swarm Area the seismic waves from shot B-IV were observed at the observation sites in profile A. The travel time studies as well as the frequency studies of the initial portion of P waves show clearly the coincidence between the region with a low interval velocity as well as with high attenuation, and the seismically most active region.|1967年11月～12月,松代群発地震域の地下構造の爆破地震学的調査がA,B2側線において行われた.得られた地下構造は種々の調査結果の理解に資し,松代群発地震の研究に貴重な資料を提供した.この調査の際,地下構造異常を見出す目的でA側線の観測点に対して,B-IVにおいて爆破を実施した.これは1種のPan shootingであるが,この程度の規模の,複雑な地質を有する地域で,この方法がどの程度有効かという点にも興味があった.A,B両側線において6km/s層以浅の構造がかなり詳細に得られており,この知識を用いることによって,地震活動が最もはげしかった地域を通るP波の平均速度が2%,他の地域を通るもののそれより小さいことがわかった.また,この減少の原因が最も地震活動がはげしかった地域のみに存立するとすると,5.7km/sとなり約5%小さいことになる.A,B両側線でも速度の小さい地域が地震活動の大きかった所に存在し,その値は,やはり5.65-5.75km/sとなり一致は著しい.また,千曲川の南西側の観測点では速度が系統的に距離とともにおそくなる傾向が存在する.さらに初動部分の見掛振動数,スペクトルの最大振幅を与える振動数の何れもが,異常を示した地域では周囲に比較して低い.かくして,ある程度調査地域が広くて複雑でも,主測線に適当に精密な観測資料があればFan shootingによって異常構造を推定し得ることが確かめられた.しかし,1回のみのFan shootingによっては,このように確かめられた松代地域の地下構造異常が地震の結果なのか,あるいは,群発地震が起る以前から存在していて地震の発生に関係があるかを実証するには十分でなく,次の機会を待たねばならない