11:00 AM - 11:15 AM
[SGL28-08] Fault activity during Quaternary Period off the Shimane Peninsula based on the profiles acquired by the high-resolution seismic survey
Keywords:offshore area of the Shimane Peninsula, seismic profile, bottom sampling, tectonic evolution, Quaternary period, fault
A number of faults are shown off the Shimane Peninsula by Tamaki et al. (1982), Japan Coast Guard (1991, 1992), Tokuyama et al. (2001) and others. The Chugoku Electric Power Co., Inc. (Energia) carried out seismic reflection survey by using mainly Sparker in 1981 in the area including some faults described by the above-mentioned authors. The investigated area was about 60 kilometers long along the shoreline and about 30 kilometers long perpendicular to the shoreline. Seismic survey lines were deployed at the interval of about 1 to 4 kilometers in it. Since then, Energia has continually been investigating to grasp more detailed locations and characteristics of the faults in the area by using the latest technology such as multi-channel seismic system and so on. Furthermore, Energia has referred to the concepts of sequence stratigraphy in its interpretation to improve the reliability of the evaluation for fault activity. Government organization MLIT, Cabinet Office and MEXT referred to the Energia's fault evaluation in their document published in 2014.
Structures found off the Shimane Peninsula mostly extend ENE-WSW, forming reverse faults and flexures. Ito and Arato (1999) and others indicate that the stress field off the San-in district was north-south compression during Late Miocene epoch and has been east-west compression since Pliocene epoch, and many folds and reverse faults extending ENE-WSW in this region were developed during Miocene epoch. Therefore, most faults off the Shimane Peninsula are considered to develop around Late Miocene epoch under the north-south compression stress field and end their activity by Latest Miocene epoch. However, Middle to Upper Pliocene (Unit-B) off the Shimane Peninsula is distributed in a narrow area, and it is also extremely thin. These factors have made the evaluation for fault activity during Quaternary period difficult.
On the basis of the above information, we conducted seismic survey in 2014 by using various sound sources off the Shimane Peninsula to grasp the detailed geological structure from shallow to deep. We also conducted bottom sampling for dating on the seismic line at about 7 kilometers off the coast of Kashima town in Matsue city. The samples collected in Unit-A were analyzed by tephrochronology and radiocarbon dating. As a result, we found the Sanbe-Unnan (SUn : about 50ka. Machida and Arai, 2011) tephra and a few samples dated as Late Pleistocene. This shows that most Unit-A is possibly Upper Pleistocene except its surface portion. We confirm that the conventional sparker profiles cannot distinguish Unit-A in more detail, but the high-resolution profiles acquired by multi-channel boomer system can distinguish it into more units : Middle to Upper Pleistocene and Holocene.
From the above, we confirm that faults existing within the area where Middle to Upper Pleistocene, have been inactive at least since Late Pleistocene epoch. Pleistocene possibly exist much more widely compared to the previous interpretation off the Shimane Peninsula. Additional multi-channel high-resolution boomer seismic survey may contribute to reveal the detailed fault activity under the east-west compressive stress field in Quaternary period.
Structures found off the Shimane Peninsula mostly extend ENE-WSW, forming reverse faults and flexures. Ito and Arato (1999) and others indicate that the stress field off the San-in district was north-south compression during Late Miocene epoch and has been east-west compression since Pliocene epoch, and many folds and reverse faults extending ENE-WSW in this region were developed during Miocene epoch. Therefore, most faults off the Shimane Peninsula are considered to develop around Late Miocene epoch under the north-south compression stress field and end their activity by Latest Miocene epoch. However, Middle to Upper Pliocene (Unit-B) off the Shimane Peninsula is distributed in a narrow area, and it is also extremely thin. These factors have made the evaluation for fault activity during Quaternary period difficult.
On the basis of the above information, we conducted seismic survey in 2014 by using various sound sources off the Shimane Peninsula to grasp the detailed geological structure from shallow to deep. We also conducted bottom sampling for dating on the seismic line at about 7 kilometers off the coast of Kashima town in Matsue city. The samples collected in Unit-A were analyzed by tephrochronology and radiocarbon dating. As a result, we found the Sanbe-Unnan (SUn : about 50ka. Machida and Arai, 2011) tephra and a few samples dated as Late Pleistocene. This shows that most Unit-A is possibly Upper Pleistocene except its surface portion. We confirm that the conventional sparker profiles cannot distinguish Unit-A in more detail, but the high-resolution profiles acquired by multi-channel boomer system can distinguish it into more units : Middle to Upper Pleistocene and Holocene.
From the above, we confirm that faults existing within the area where Middle to Upper Pleistocene, have been inactive at least since Late Pleistocene epoch. Pleistocene possibly exist much more widely compared to the previous interpretation off the Shimane Peninsula. Additional multi-channel high-resolution boomer seismic survey may contribute to reveal the detailed fault activity under the east-west compressive stress field in Quaternary period.