3:30 PM - 3:45 PM
[HCG23-06] The modern and historical depositional process of the Notsukesaki barrier spit system controlled by seismotectonics around the Kuril subduction zone
Keywords:depositional process , Notsukesaki barrier spit system, seismotectonics, Kuril subduction zone, eastern Hokkaido, northern Japan
The Notsukesaki Barrier Spit system (NBS) is the largest barrier spit system in Japan distributed around Notsuke Bay facing the Nemuro Strait (the Sea of Okhotsk) on the eastern coast of Hokkaido, northern Japan (Nanayama, 2021). The NBS is known the typical compound recurved spit landform with a total length of 28.9 km extending southeast from the mouth of the Shibetsu River. Since 2015, we have been conducting research on the modern and historical depositional process and geomorphological development of the NBS. As a result, three major points are obtained as bellows.
(1) We estimated the NBS has been established since more than 4000 years ago, because of the presence of Ma-d tephra (ca. 4.0 cal ka) in the basal horizon of the peat bed of Chashikotsu Marsh. The earliest sand spit was already eroded, and it is no longer preserved as a local landform. From the remaining NBS landform, we recognized four branching barrier spits (NBS1 to NBS4) based on interpretation of aerial photographs and geological field survey.
(2) The NBS1 as the youngest barrier spit without tephra. The height of the upper limit of the backshore (BH (NBS1)) located at +0.60-1.00 m. The NBS1 appeared after the 17th century and continues to grow the modern deposition around Cape Arahama. The NBS2 originates from Cape Kiraku, branches off to the NBS1 near Nakashibetsu, and continues to Ryujinzaki. The ruins of Tsukoya (late Edo period) are located on this barrier spit. The BH (NBS2) is located at +1.47m. This barrier spit is estimated to have emerged in the 17th century because of covered Ta-a (AD1739) and Ko-c2 (AD1694) tephra. The emerged age of the NBS3 is predicted to be the 12th/13th century because of relationship of tephra horizon, which extends from Cape Ipponmatsu to Notsukesaki Lighthouse. The BH (NBS3) is located at +2.28 m. The NBS4 is the oldest barrier spit distributed only in Onnenikle. The pit dwelling ruins (Satsumon period) is located on top of this barrier spit. We obtained the AMS ages of 2.7-2.3 cal ka and Ta-c tephra (ca. 2.5 cal ka) from the top horizon of the past beach gravel bed. The BH (NBS4) is located at +2.66 m. We infer from the direction of the tip of the barrier spit that the main body of NBS4 distributed far to the east of the modern coast.
(3) The mega-earthquakes (Mw 8.8~) have occurred every 500 years in the southern Kuril subduction zone, and the elevation in this area was uplifted 1 to 2 m during (or after) the 17th century mega-earthquake (Nanayama, 2021). However, the modern subsidence rate in the Notsukesaki area is known to reach 15 mm/year and occurring rapid transgression. We think the seismic crustal deformation of the Kuril subduction zone has been controlled the appearance of the branched barrier spits of the NBS.
Acknowledgment: This research was supported by JSPS Grant-in-Aid for Scientific Research (C) 22K03744.
References:
Nanayama, F., 2021, Evidence of giant earthquakes and tsunamis of the 17th-century type along the southern Kuril subduction zone, eastern Hokkaido, northern Japan: A review. GSL Special Publications, 501, 131-157. https://doi.org/10.1144/SP501-2019-99
(1) We estimated the NBS has been established since more than 4000 years ago, because of the presence of Ma-d tephra (ca. 4.0 cal ka) in the basal horizon of the peat bed of Chashikotsu Marsh. The earliest sand spit was already eroded, and it is no longer preserved as a local landform. From the remaining NBS landform, we recognized four branching barrier spits (NBS1 to NBS4) based on interpretation of aerial photographs and geological field survey.
(2) The NBS1 as the youngest barrier spit without tephra. The height of the upper limit of the backshore (BH (NBS1)) located at +0.60-1.00 m. The NBS1 appeared after the 17th century and continues to grow the modern deposition around Cape Arahama. The NBS2 originates from Cape Kiraku, branches off to the NBS1 near Nakashibetsu, and continues to Ryujinzaki. The ruins of Tsukoya (late Edo period) are located on this barrier spit. The BH (NBS2) is located at +1.47m. This barrier spit is estimated to have emerged in the 17th century because of covered Ta-a (AD1739) and Ko-c2 (AD1694) tephra. The emerged age of the NBS3 is predicted to be the 12th/13th century because of relationship of tephra horizon, which extends from Cape Ipponmatsu to Notsukesaki Lighthouse. The BH (NBS3) is located at +2.28 m. The NBS4 is the oldest barrier spit distributed only in Onnenikle. The pit dwelling ruins (Satsumon period) is located on top of this barrier spit. We obtained the AMS ages of 2.7-2.3 cal ka and Ta-c tephra (ca. 2.5 cal ka) from the top horizon of the past beach gravel bed. The BH (NBS4) is located at +2.66 m. We infer from the direction of the tip of the barrier spit that the main body of NBS4 distributed far to the east of the modern coast.
(3) The mega-earthquakes (Mw 8.8~) have occurred every 500 years in the southern Kuril subduction zone, and the elevation in this area was uplifted 1 to 2 m during (or after) the 17th century mega-earthquake (Nanayama, 2021). However, the modern subsidence rate in the Notsukesaki area is known to reach 15 mm/year and occurring rapid transgression. We think the seismic crustal deformation of the Kuril subduction zone has been controlled the appearance of the branched barrier spits of the NBS.
Acknowledgment: This research was supported by JSPS Grant-in-Aid for Scientific Research (C) 22K03744.
References:
Nanayama, F., 2021, Evidence of giant earthquakes and tsunamis of the 17th-century type along the southern Kuril subduction zone, eastern Hokkaido, northern Japan: A review. GSL Special Publications, 501, 131-157. https://doi.org/10.1144/SP501-2019-99