報告書番号

N14020

タイトル（和文）

常時微動を用いた地下構造探査における地震波干渉法の適用 -若狭湾地域での深部地盤構造の推定-

タイトル（英文）

Application of seismic interferometry to an exploration of subsurface structure by using microtremors -Estimation of deep ground structures in the Wakasa bay region-

概要 （図表や脚注は「報告書全文」に掲載しております）

音海半島と大島半島に各5地点の地震計を設置し，常時微動の連続記録から地震波干渉法によるグリーン関数の合成を行った。約80日間分のデータの相互相関から分散性を示す波形が抽出され，西側を振源，東側を受振点とするグリーン関数が得られた。共通の振源に対し，複数の地点で合成されたグリーン関数から位相速度を推定する方法を提案し，若狭湾地域に適用した結果，微動アレイ観測と一致した位相速度が得られるだけでなく，より長周期の位相速度が得られることを示した。位相速度の逆解析により推定された深部地盤構造から，地震発生層上端の一つの目安となるS波速度3.5 km/s程度の層は，大島半島で深さ3.8-4.0 km，音海半島で深さ4.4-4.6 kmであり，音海半島側が若干深いが，緩やかな変化を持つ構造が示唆された。大島半島と音海半島に挟まれた浅海域を伝播するグリーン関数から推定された表面波の群速度は，推定した速度構造と同様に，浅海域の構造が両半島に連続することを示唆する理論群速度の範囲内に対応することを明らかにした。また，常時微動を用いた地下構造探査において，地震波干渉法が異なる指標に基づくモデルの検証として有効であることを示した。

概要 （英文）

We carried out continuous measurements of microtremors to synthesize Green's function based on seismic interferometry in order to estimate deep subsurface structures of the Ohshima peninsula (OSM) and the Otomi peninsula (OTM) in the Wakasa bay region. Using more than 80 days of data, dispersive waveforms in the cross correlations were identified as a Green's function from OTM(source) to OSM(receiver) based on seismic interferometry. Rayleigh-wave phase velocities at OSM and OTM were estimated by two different method using microtremors: first, by analyzing microtremor array data, and second, by applying the f-k spectral analysis to synthesized Green's functions derived from cross-correlation with a common observation station. Relatively longer period of phase velocities were estimated by the f-k spectral analysis using the synthesized Green's functions with a common observation station. This suggests that the synthesized Green's functions from seismic interferometry can provide a valuable data for phase velocity inversion to estimate a deep subsurface structure. By identifying deep subsurface structures at OSM and OTM based on an inversion of phase velocity from both methods, the depth of S wave velocity of about 3.5 km/s, considered as a top of seismogenic layer, were determined to be 3.8 - 4.0 km at OSM and 4.4 - 4.6 km at OTM, respectively. Love- and Rayleigh-wave group velocities were estimated from the multiple filtering analysis of the synthesized Green's functions. From the comparison of observed surface wave group velocities and theoretical group velocities of OSM and OTM, we demonstrated that the observed group velocities were in good agreement with the average of theoretical group velocities calculated by identified deep subsurface structures at OSM and OTM. It is suggested that the deep subsurface structure of the shallow sea region between two peninsulas is continuous structure from OSM to OTM and that Love- and Rayleigh-wave group velocities using synthesized Green's functions from seismic interferometry can be used to validate identified deep subsurface structures.

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