非アスペリティのマッピングから地殻内部の常時監視へ

Mapping non-asperities and continuous, active monitoring of the interior of the earth's crust

鶴我 佳代子; 笠原 順三; 三ケ田 均*; 山岡 耕春; 藤井 直之

Tsuruga, Kayoko; Kasahara, Junzo; Mikada, Hitoshi*; Yamaoka, Koshun; Fujii, Naoyuki

本論文は、巨大地震が発生する沈み込みプレート境界における地震発生場をより詳細に知るための考え「アスペリティのマッピングとモニタリング」(Exploration of Asperities-Reflectors System, EARS)を提案する。最近の研究成果から、プレート境界には、地震発生時に大部分の地震モーメントを開放するような固着度の大きい領域(アスペリティ、asperity)と、プレート間で数日数年以上で変位し、短周期の地震波動が放出するひずみエネルギーに相当あるいはそれ以上のひずみエネルギーを放出するような比較的固着度の低い領域(本論文では「非アスペリティ(non-asperity)」と呼ぶ)があることがわかってきた。ここでは、非アスペリティでのすべりの加速がそれと隣接するアスペリティのすべりを引き起こし、巨大地震の発生に関与するという考えに基づいて論旨を進める。すなわち地殻構造とプレート境界からの反射波の反射強度とその空間分布を調査することで、非アスペリティの物性と広がりをマッピングし、間接的にアスペリティの分布を知ることにつながると考える。さらに、弾性波アクロスなどの能動監視システムにより、ターゲット領域からの地震波の反射強度の時間変化を常時監視(モニター)することの重要性と、海域のプレート境界の監視海底ケーブル式地震計の将来性について述べる。

This paper proposes the idea (Exploration of Asperities-Reflectors System, EARS) for getting to know the occurrence of large - huge earthquakes which take place along a subduction plate boundary. A large earthquake along the subducting plate boundary is considered to occur repeatedly in the particular asperity, which is the region of strongly coupled zone of two plates. The regions other than asperities along the plate boundary are considered as stable - quasi-stable region in which slow-slip may release the strain energy caused by the oceanic plate subduction. The intensity of coupling and the heterogeneities in a particular large asperity is not possible to know at present. The strong seismic reflections from the subducting plate boundary were found in the aseismic forearc region in the Japan Trench. Similar strong PP reflections were also observed in the NE region of the Hamana Lake and the same region has also showed large aseismic slip detected by GPS. When we map the asperities-reflectors system, the continuous monitoring of seismic reflection intensity may give the change of the physical state at the subducting plate boundary. The rapid change may trigger a large earthquake at an adjacent asperity. We introduce the innovative measurement system, ACROSS, which has been developed for active monitoring by continuous sinusoidal elastic waves and also describe the recent submarine OBS system. To obtain real-time monitor of the transmission signals from ACROSS at the potential regions near the trenches can be done by use of submarine cables. A newly built system planned in the Tonankaki region should consider this requirement.