報告書番号

N09008

タイトル（和文）

岩盤上観測記録に基づく2009年駿河湾の地震の強震動特性と震源過程

タイトル（英文）

Strong-Motion Characteristics and Source Process during the Suruga Bay Earthquake in 2009 through Observed Records on Rock Sites

概要 （図表や脚注は「報告書全文」に掲載しております）

2009年8月11日に発生した駿河湾の地震(M6.5)は，震央が東海地震の想定震源域内に位置するものの，震源深さやメカニズム解から，陸域プレートの下に沈み込むフィリピン海プレートの内部で発生した地震であると推定された．電力中央研究所が設置・展開している露頭岩盤上強震観測網RK-netでは，伊豆半島を中心とした多数の地点で本震および主要な余震の震源近傍における強震波形記録が得られた．本報告ではこれらの観測記録から岩盤上地震動特性を明らかにするとともに，表層地盤の影響を受けない特長を生かして震源スペクトルの特性を求めた．さらに，中部電力浜岡原子力発電所内の鉛直アレイ観測点や防災科学技術研究所KiK-netの地中記録を追加したデータセットを用いて本震の震源インバージョン解析を実施した．その結果，断層の主要なすべり領域を示すアスペリティは，破壊開始点周辺と，主断層から副断層に破壊が伝播する境界領域に存在することを明らかにした．また高周波地震動の生成に影響する実効応力分布は主断層の東側で大きく，これは主として伊豆半島地域で観測される大振幅のパルス波を説明しているものと考えられる．

概要 （英文）

On 11 August 2009, a moderate earthquake of M6.5 occurred in the Suruga Bay region, south off Shizuoka prefecture. During this event, JMA Seismic Intensity reached 6 lower in several cities around the hypocenter, and at Hamaoka nuclear power plant of Chubu Electric Power reactors were automatically shutdown due to large ground motions. Though the epicenter is located at the eastern edge of source area for the assumed great Tokai earthquake of M8, this event is classified into the intra-plate (intra-slab) earthquake, due to its focal depth lower than that of the plate boundary and fault geometry supposed from the moment tensor solution. Dense strong-motion observation network has been deployed mainly on the rock outcrops by our institute around the source area, and the waveform data of the main shock and several aftershocks were obtained at 13 stations within 100 km from the hypocenter. The observed peak ground motions and velocity response spectral amplitudes are both obviously larger than the empirical attenuation relations derived from the inland and plate-boundary earthquake data, which displays the characteristics of the intra-slab earthquake faulting. Estimated acceleration source spectra of the main shock also exhibit the short period level about 1.7 times larger than the average of those for past events, and it corresponds with the additional term in the attenuation curve of the peak ground acceleration for the intra-plate earthquake. Detailed source process of the main shock is inferred using the inversion technique. The initial source model is assumed to be composed of two distinct fault planes according to the minute aftershock distribution. Estimated source model shows that large slip occurred near the hypocenter and at the boundary region between two fault planes where the rupture transfers from primary to secondary fault. Furthermore the broadband source inversion using velocity motions in the frequency up to 5 Hz demonstrates the high effective stress area on the east of hypocenter generates a high-frequency velocity pulse observed at the stations on the Izu Peninsula region.

報告者

キーワード