Abstract
AbstractSubmarine 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.
Highlights
The recurrence intervals of great subduction earthquakes have been estimated using historical documents and ruins, terrestrial archives (e.g. Cisternas et al 2005; Nelson et al 2006) and marine archives (e.g. Adams 1990; Patton et al 2015)
Undisturbed surface sediment collected by the multiple corer is mainly composed of olive-black clay and characterized by a very thin, 6 mm thick fine sand layer at 17 cm below the seafloor (Fig. 2)
The parameters of magnetic hysteresis, which are plotted on a Day plot (Day et al 1977), are grouped within the pseudo-single domain (PSD) region
Summary
The recurrence intervals of great subduction earthquakes have been estimated using historical documents and ruins, terrestrial archives (e.g. Cisternas et al 2005; Nelson et al 2006) and marine archives (e.g. Adams 1990; Patton et al 2015). Studies that propose an earthquake trigger for the dislodging of surficial sediments are limited around the Japan Trench (e.g. Oguri et al 2013; Ikehara et al 2016) and Chilean lakes (e.g. Moernaut et al 2017) This type of sedimentary process would depend on factors such as surface sediment characteristics, seismic motion and seafloor morphology. Based on sedimentary archives and historical documents, plate boundary megathrust earthquakes have repeatedly occurred in the Nankai Trough with a recurrence time of 100–200 years (Ando 1975) Between these great earthquakes, few earthquakes have been confirmed since the onset of seismic monitoring. Offshore of Kii peninsula, there are continuous outer ridges and elongated depressions with a NE– SW-aligned trend (Fig. 1a) These depressions separate the prism slope from the Kumano forearc basin, and are interpreted to have been formed by the strikeslip component of oblique subduction (Ashi et al 2007; Martin et al 2010). The basin floor in our study site is characterized by bright colours (Fig. 1b), corresponding to low backscattering intensity
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