2:15 PM - 2:45 PM
[MIS15-02] How much we had understood a history of giant earthquakes along the Japan trench -progress and remaining problems-
★Invited Papers
Keywords:subduction zone great earthquakes, tsunami deposit, Japan trench
The 2011 Tohoku earthquake disaster was broadcasted as “unexpected disaster” by non-geologists but also surprised them because it had been not well known that geologists had previously studied geological evidence for 869 Jogan earthquake and tsunami: a potential predecessor of the 2011 event.
The Jogan tsunami deposit was first reported in Sendai by Abe et al. (1990 Zishin) and Minoura and Nakaya (1991 J. Geol.). Sugawara et al. (2001 Research Report of Tsunami Engineering) later found a similar sand deposit in Soma, Fukushima prefecture. Building on their research, research groups of Geological Survey of Japan, AIST traced the Jogan sandy deposits using several hundred cores along leveled transects in Sendai, Ishinomaki, and Minami-Soma regions. On the basis of such geological records, the source of the Jogan tsunami was examined by comparison between simulated inundation areas and the observed extent of Jogan tsunami deposits (Namegaya et al., 2010 AFRC reports; Satake et al., 2008b AFRC reports). These results had been submitted to the Japanese government, but they were not employed by governmental assessments of subduction zone earthquakes until the 2011 Tohoku earthquake.
After the 2011 disaster, many researchers began to study tsunami geology along the Tohoku coast to extend tsunami history in millennium time scale. In Iwate and Aomori prefectures, they identified Jogan tsunami deposits as well as historical (Goto et al. 2015 Mar. Geol.) and prehistoric (Tanigawa et al. 2014 JQS) tsunami deposits. Also, a tsunami of 1454 in the Sendai plain was reexamined on the basis of historical documents (Namegaya and Yata, 2014 Zishin) and stratigraphic and geomorphological evidence (Sawai et al., 2015 GRL).
In the last two decades, tsunami geology has developed along the Pacific coast of Tohoku region, but there still remain issues that should be tackled. For example, the moment magnitude of the Jogan earthquake may have been greater than the present model proposed by Namegaya and Satake (2014 GRL). They constrained their fault parameters using distributions of tsunami deposits at multiple sites along the trench, but the modeled fault may be extended farther north or south if the recent geological data were involved. Also, a parent earthquake is still unknown for a tsunami deposit in 17th-century along the Pacific coast of Aomori prefecture (Tanigawa et al. 2014 JQS). These problems may be solved by future comprehensive research.
This presentation is based on description in Sawai (2020 ESR).
The Jogan tsunami deposit was first reported in Sendai by Abe et al. (1990 Zishin) and Minoura and Nakaya (1991 J. Geol.). Sugawara et al. (2001 Research Report of Tsunami Engineering) later found a similar sand deposit in Soma, Fukushima prefecture. Building on their research, research groups of Geological Survey of Japan, AIST traced the Jogan sandy deposits using several hundred cores along leveled transects in Sendai, Ishinomaki, and Minami-Soma regions. On the basis of such geological records, the source of the Jogan tsunami was examined by comparison between simulated inundation areas and the observed extent of Jogan tsunami deposits (Namegaya et al., 2010 AFRC reports; Satake et al., 2008b AFRC reports). These results had been submitted to the Japanese government, but they were not employed by governmental assessments of subduction zone earthquakes until the 2011 Tohoku earthquake.
After the 2011 disaster, many researchers began to study tsunami geology along the Tohoku coast to extend tsunami history in millennium time scale. In Iwate and Aomori prefectures, they identified Jogan tsunami deposits as well as historical (Goto et al. 2015 Mar. Geol.) and prehistoric (Tanigawa et al. 2014 JQS) tsunami deposits. Also, a tsunami of 1454 in the Sendai plain was reexamined on the basis of historical documents (Namegaya and Yata, 2014 Zishin) and stratigraphic and geomorphological evidence (Sawai et al., 2015 GRL).
In the last two decades, tsunami geology has developed along the Pacific coast of Tohoku region, but there still remain issues that should be tackled. For example, the moment magnitude of the Jogan earthquake may have been greater than the present model proposed by Namegaya and Satake (2014 GRL). They constrained their fault parameters using distributions of tsunami deposits at multiple sites along the trench, but the modeled fault may be extended farther north or south if the recent geological data were involved. Also, a parent earthquake is still unknown for a tsunami deposit in 17th-century along the Pacific coast of Aomori prefecture (Tanigawa et al. 2014 JQS). These problems may be solved by future comprehensive research.
This presentation is based on description in Sawai (2020 ESR).