09:30 〜 09:45
[MIS32-03] CRISP：沈み込み帯巨大地震の特徴的マグニチュードは何が決めるのか
キーワード：Large subduction earthquake, seismogenic fault, accretion and erosive margin
Variations in earthquake magnitude and recurrence intervals of fault behavior are best understood in the context of regional tectonics. Convergent margins may be divided into two end-member types termed erosive and accretionary plate boundaries (e.g. von Huene and Scholl, 1991; Clift and Vannucchi, 2004). These margins may differ greatly in lithology, physical properties and hydrology. The Nankai accretionary margin has a 1300-year historical earthquake record with a recurrence interval of 100-150 years (Ando, 1975). Great earthquakes at Nankai are typically tsunamigenic and include the 1944 Tonankai (Mw=8.1) and 1946 Nankaido (Mw=8.1) earthquakes (Kanamori, 1977). In contrast, the Middle America trench offshore Costa Rica events of M=7.6 reoccur on average of every 40 years. The CRISP drilling area is offshore Costa Rica just northwest of the Osa Peninsula. Comparisons between these margins may produce insights into mechanisms that influence characteristic magnitudes and recurrence intervals of subduction earthquakes.The IODP Costa-Rica Seismogenesis Program (CRISP) has carried out the first step toward the deep riser drilling by characterizing the shallow lithologic, hydrologic, stress, and thermal state at offshore Osa Peninsula (Vannucchi et al., 2011; Harris et al., 2013). CRISP drilling reveals that the shallow basement of upper plate crust is forearc basin material consisting of lithic sedimentary units with terrigenous sediment accumulated at a high rate. A large sediment flux to the forearc may have originated from the uplifted back-arc Talamanca Cordillera due to Cocos-Ridge subduction (Lonsdale and Klitgord, 1978; van Andel et al., 1971). Both the Nankai and the CRISP drilling areas are characterized by the subduction of young oceanic crust with high heat flow and active fluid flow (Spinelli and Wang, 2008; Spinelli and Harris, 2011; Harris et al., 2010). The Nankai and Costa Rica margins are ideal ares to better understand the relation between the earthquake magnitudes and other subduction zone factors.