1028 / 2024-09-20 08:11:41

Sediment transport during submarine slides caused by the 2024 Noto Peninsula Earthquake: trace elements and isotopes approaches to clarify the origin and transport process

2024 Noto Peninsula Earthquake,Toyama Deep-sea Channel,Trace elements,Carbon and Nitrogen Stable Isotope ratios

The January 2024 Noto Peninsula Earthquake (Mw 7.6) struck at a depth of 15.5 km north of the peninsula. Tsunamis were observed along the coasts of the Sea of Japan, with run-up height of tsunami of ~6 m. Tsunami simulations show that these tsunamis were caused not only by surface ruptures, but also by the occurrence of submarine slides (horizontal displacement: ~1.1 km, vertical uplift: ~50 m), which led to the early arrival and amplification of the tsunami. There is a possibility of sediment movement from shallow area to deep sea due to the collapsed slope and tsunami backwash. On the other hand, although crab basket loss, poor catch of Japanese glass shrimp, and other damages have been reported in the offshore areas of Toyama Bay, there is little information available on seafloor changes caused by the earthquake.

Therefore, to investigate the effects of the earthquake on the Toyama Deep-sea Channel (TDSC), collecting surface sediments and bathymetric surveys were conducted surrounding Toyama Bay during the R/V Hakuho-Maru KH-24-E1 and the T/V Nagasaki-Maru NS24-134 cruises. Sediment cores from the TDSC in the northeastern part of the Noto Peninsula showed a silt layer without bioturbation overlying the fine-grained layer in the upper part. In the eastern part, fine-grained turbidite layers were identified, but there were no changes in sedimentary structure on the slope. By comparing the collected sediment samples with pre-earthquake samples, we concluded that the observed changes in the TDSC were directly caused by the earthquake.

Carbon and nitrogen stable isotope ratios indicated that sediments supplied from the earthquake have the ¹³C and ¹⁵N depleted values, rich in terrestrial organic matter (OM). The differences in total organic carbon content of these terrigenous OM suggest different origins: aged OM with well organic decomposition and young OM with include clayey silt. Young terrigenous OM was also presumed to have been laterally transported after redeposition of the aged terrigenous OM in the TDSC. According to trace element composition, the former sediment is interpreted to originate from fine-grained terrigenous sediments as relict in the TDSC with high Cr and Ni contributions, while the latter sediment can only be explained by transport from the Toyama Bay side with high Zn and Pb contributions.

These results imply that a tsunami caused by submarine slides can have a significant impact on benthic ecosystem over more than several tens of kilometers in contrast the limited coastal impact of tsunamis in Toyama Prefecture.