Evidence for surface sediment remobilization by earthquakes in the Nankai forearc region from sedimentary records

Abstract

Submarine landslides triggered by earthquakes can generate turbidity currents. Recently, several studies have reported that the remobilization of surface sediment triggered by earthquakes can also generate turbidity currents. Such sedimentary processes may be influenced by sediment characteristics, seafloor morphology and seismic motions. Here, we verify surface sediment remobilization using sedimentary records from the Nankai forearc region, SW Japan. We collected multi-core and piston core samples from a small confined basin, mainly composed of silty clay or very fine sand. Radiocaesium measurements of the multi-core show consistently high values in the upper 17 cm and low values below this depth. Rapid sediment deposition after 1950 is assumed, and the most likely cause is the 2004 off the Kii Peninsula earthquake. Based on calculations using bathymetric maps and palaeocurrent data, settlement of the upper 17 cm can be explained by redeposition of the surface (c. 1 cm) slope sediment around the basin. Muddy turbidites are also identified in the piston core. The gap in radiocarbon age observed around 2.0 m bsf (metres below seafloor) implies similar sedimentary processes. Our study represents the first examination of surficial remobilization from sedimentary cores in the Nankai forearc region.The recurrence intervals of great subduction earthquakes have been estimated using historical documents and ruins, terrestrial archives (e.g. Cisternas et al. 2005;Nelson et al. 2006) and marine archives (e.g. Adams 1990; Patton et al. 2015). However, palaeoseismic records obtained from historical documents and ruins are limited in number and can only be found up to a certain time after earthquakes. Moreover, terrestrial archives may incompletely record past seismic events due to erosion over time

The Finicky Nature of Earthquake Shaking‐Triggered Submarine Sediment Slope Failures and Sediment Gravity Flows

“…Flows require available unstable sediment slopes, and thus susceptibility to sediment failure and remobilization should correlate with sedimentation rates (Sawyer et al, 2017). Although sedimentation rates in Nankai have been variable over time, the most recent rate estimates are exceptionally high for deep-sea environments (e.g., 2,000 m/Ma; Taira & Niitsuma, 1986) and the maximum sediment thickness in the region reaches 3,000 m. We suggest that this abundance of sediments makes it unlikely that the 2004 Kii earthquake shaking (Okutsu et al, 2019) would have depleted the supply of surficial sediment available for further remobilization or for destabilization in slope failures in 2016.…”

“…Multiple turbidites in two cores from sites near the edge of the prism show that pathways exist through the DONET (Taira & Niitsuma, 1986). The most recent example of shaking‐triggered flows followed the 2004 Kii earthquakes, which led to the deposition of muddy turbidites observed in cores located between sites KMD13 and KMD16 (Okutsu et al., 2019) and suspended sediments sampled in the water column between sites KMC12 and KMC09 (Ashi et al., 2014; Figure 1). These cores also revealed 23 earlier‐formed muddy turbidites and deposits indicative of slumping that were inferred to be earthquake‐generated because the sites are isolated from coastal/river systems (Okutsu et al., 2019).…”

“…The aforementioned studies show that plausible sediment transport pathways may exist from sources near the coast to sites within the Kumano Basin and from its seaward edge to the deepest sites. There the bathymetry indicates pathways near KMD15 or between KMB05 and KMB06, but other routes appear blocked by ridges and elongated depressions formed by the strike‐slip component of oblique subduction (Okutsu et al., 2019).…”

Unravelling a 2300 year long sedimentary record of megathrust and intraslab earthquakes in proglacial Skilak Lake, south‐central Alaska

“…However, the process of surficial sediment remobilization has up to now only been demonstrated in a few studies, i.e. in Chilean lakes (Moernaut et al ., 2017; Molenaar et al ., 2021) and offshore Japan (McHugh et al ., 2016; Molenaar et al ., 2019; Okutsu et al ., 2019; Ikehara et al ., 2020). In both cases, the remobilized sediments consist predominantly of diatomaceous mud and ooze.…”