Accumulation of an earthquake-induced extremely turbid layer in a terminal basin of the Nankai accretionary prism
© Ashi et al.; licensee Springer. 2014
Received: 21 February 2014
Accepted: 30 May 2014
Published: 12 June 2014
Seismic shaking is a major trigger for sediment redistribution in subduction zones, and clouds of dilute suspended sediment have been reported in association with large earthquakes. Dive observations in a basin on the slope of the central Nankai accretionary prism soon after the 2004 off-Kii Peninsula earthquakes documented a layer of suspended sediment with extremely high turbidity. We estimated the thickness of this bottom turbid layer to be more than 2.5 m by comparison of seafloor depths between surveys in 2004 and 2010 and about 2.6 m from instrumental evidence involving the submersion of a heat-flow probe. A high-resolution subbottom profiling survey across the basin revealed acoustically transparent layers thicker than 2 m. Because the slope basin is a terminal basin completely enclosed by topographic highs, we examined the possibility that the uppermost transparent layer was deposited as a consequence of the 2004 earthquakes. Considering the sediment source area and the volume of the basin fills, the mobilization and redeposition of thin surface sediments on the prism slope can account for the volume of the transparent layer. We conclude that the 2004 earthquakes caused widespread disturbance of the prism slope and concentrated surface sediments in this terminal basin. Our results emphasize the utility of a terminal basin in a subduction zone as an earthquake recorder.
Sedimentary processes in subduction zones are basically controlled by geomorphological evolution in response to short-term events related to the tectonic formation of sedimentary basins, deep-sea canyons, or steep scarps, as well as long-term sea-level changes and hinterland tectonics. Short-term sedimentation processes include various events that accompany earthquakes and volcanic activities. Subduction earthquakes, in particular, mobilize and redeposit a considerable amount of sediment as a result of slope instability and tsunamis (e.g., Seeber et al. 2007; Noguchi et al. 2012; Arai et al. 2013). Sedimentary processes associated with earthquakes are crucial to the understanding of sedimentation systems in a subduction zone. Moreover, paleoseismic histories that can be deduced from sedimentary records and the assessment of tsunami hazards associated with submarine slope failures provide important information for disaster mitigation (e.g., Goldfinger 2011; Yamada et al. 2012).
Submarine slope failures induced by seismic shaking have been investigated by seismic reflection surveys, swath bathymetry, and sediment sampling (e.g., Sultan et al. 2009; Goldfinger 2011). Autobrecciation of unconsolidated surface sediment, which is considered to represent evidence of seafloor deformation by earthquake shaking as well as sliding or slumping, was reported from the source areas of the 1993 earthquake off southwestern Hokkaido (Takeuchi et al. 1998) and the 2004 Sumatra earthquake (Seeber et al. 2007). Tsunamis were reported as an important agent of sediment redistribution from the 365 AD Cretan earthquake (Polonia et al. 2013) and the 2011 Tohoku earthquake (Noguchi et al. 2012; Arai et al. 2013; Ikehara et al. 2014). The well-known sequential breakage of undersea communication cables after the 1929 Grand Banks earthquake was the first documentation of landslide-triggered turbidity currents (Heezen et al. 1954). Suspended sediments in the water column after large earthquakes were reported by (1) in situ monitoring of light scattering and sediment traps from the Cariaco basin (Thunell et al. 1999), (2) sediment traps from the epicentral region of the 1994 Sanriku earthquake (Itou et al. 2000), (3) dive observations in the weeks after the 2004 Sumatra earthquake (Seeber et al. 2007), and (4) strong turbidity anomalies above the seafloor from turbidity sensor monitoring 1 month after the 2011 Tohoku-oki earthquake (Noguchi et al. 2012). The deep seafloor observatory in the Sagami Trough off Japan monitored mudflows caused by a moderate earthquake in real time (Kasaya et al. 2009).
Suspended sediment clouds have also been recognized after the 2004 off-Kii Peninsula earthquakes (maximum magnitude 7.4) in the Nankai subduction zone (Ashi et al. 2012). Such a layer of extremely high turbidity was observed in a basin on the prism slope. This layer is apparently similar to lutoclines observed between heavily mud-laden bottom layers and clearer overlying layers in estuaries and muddy coastal environments (e.g., Wolanski et al. 1988). This study reports details of the observation of a bottom turbid layer in the slope basin off Kii Peninsula, documents sedimentary structures from a subbottom profiling survey, and discusses the source and redeposition processes associated with the high-turbidity layer.
Geological and geophysical background
Historical and archeological records have documented recurring great earthquakes along the Nankai Trough (Ando 1975). However, few medium-sized or large earthquakes have been recorded during the period between great earthquakes. The 2004 off-Kii Peninsula earthquakes in September 2004 were a series of large events near the axis of the Nankai Trough. The epicenter of the September 5 mainshock (Japan Meteorological Agency magnitude Mj = 7.4) was northwest of Zenisu Ridge (Figure 1). About 5 h before the mainshock, there was a foreshock (Mj 7.1) about 40 km to the southwest. Small tsunamis were recorded after both of these earthquakes. Major aftershocks (Mj 6.5) occurred 20 km east of the mainshock on September 7 and 8. The aftershock distribution was mapped as extending in both the NW-SE and the ENE-WSW directions by ocean bottom seismometers (Sakai et al. 2005). The four shocks were interpreted as intraplate earthquakes in the subducting Philippine Sea Plate. Tsunami waveform inversion results suggested that the fault strike of the mainshock was perpendicular to the trough axis and that the foreshock was caused by dip-slip on an ENE-WSW-striking fault (e.g., Saito et al. 2010).
We studied this area by three cruises in 2004, 2010, and 2011. All data were obtained by the Navigable Sampling System (NSS) of the Atmosphere and Ocean Research Institute of the University of Tokyo, which is a deep-sea remotely operated vehicle (ROV) designed to collect core samples with pinpoint accuracy. The pilot vehicle has four thrusters, a payload hook, and two downward-looking video cameras: a color camera (CAM-A) hung from the pilot vehicle using an extension pipe and a black-and-white camera (CAM-B) mounted on the pilot vehicle. A pressure gauge and an acoustic altimeter provide water depth and distance above the seafloor with 0.1-m resolution. A sediment core sample was lost due to cable breakage during the 2004 survey (Ashi et al. 2012), and further core sampling was prohibited after the 2010 surveys to protect newly deployed submarine cables near the slope basin. Subbottom sedimentary structures were obtained during the 2011 cruise by a subbottom profiling (SBP) system (EdgeTech DW-106; EdgeTech, West Wareham, MA, USA) using a chirp signal generated with a frequency sweep from 1.5 to 4.5 kHz. The speed of the pilot vehicle during the SBP survey was about 0.9 knots. Profiles are shown converted from travel time to depth using a constant sonic velocity of 1,500 m/s.
Dilute suspension layer and bottom turbid layer
Ashi et al. (2012) reported seafloor disturbances due to earthquake shaking and their evolution between the cruises in 2004 and 2010. The September 2004 cruise started 2 weeks after the mainshock of the 2004 off-Kii Peninsula earthquakes. Twelve ROV dives (dives 29 to 36 and 40 to 43) were conducted near the epicenters (Figure 1). All dive observations on the prism slope noted a snow-like suspension of marine particles within 200 to 320 m of the seafloor (Ashi et al. 2012), but this dilute suspension layer (DSL) was not recognized in the forearc basin site (dives 30, 31, and 32). The DSL on the prism slope was ascribed to clouds of suspended sediment produced by mass movements at multiple locations during the earthquakes. Dives 40, 41, and 42 documented a localized layer of extremely high turbidity at the bottom of the slope basin (site NG2 hereafter) (Figure 2). The upper boundary of this high-turbidity layer was very clearly defined and exhibited a pseudo-seafloor on video images (Ashi et al. 2012). This first cruise, therefore, revealed two types of turbid layers: a widespread DSL and a localized bottom turbid layer (BTL) at site NG2. The BTL was estimated to be at least 1.5-m thick from its complete burial of a pogo-type heat-flow probe system (Figure 2).
During the 2010 cruise, two dives (dives 87 and 95) revisited the slope basin where previous dives (dives 36, 40, 41, and 42) observed both DSL and BTL in 2004 (Ashi et al. 2012). The transparency of the seawater was high without any indication of DSL and BTL.
Settlement of the BTL
Thickness of the BTL estimated by a heat-flow probe system
Ripple-like bedforms parallel to depth contours were observed on the gentle northern slope of the basin at site NG2 during dives 36 and 95 (Ashi et al. 2012). Dive 124 (in 2011) ran from the eastern slope to the northwestern slope by way of the basin center and documented well-developed ripple-like bedforms in the slope regions (Figure 2). The directions of the ripple crests were NW-SE on the eastern slope and NE-SW on the northwestern slope, roughly parallel to the depth contours. Shapes of these bedforms tended to be catenary in slope regions (Figure 2a,c) and linguoid or more complex on the flat basin floor at site NG2 (Figure 2b).
Thick transparent layers in subbottom profiles
We improved the thickness estimation of the BTL by considering regional tides. The result, which was greater than 2.5 m, was similar to the previous estimate (greater than 2.4 m) of Ashi et al. (2012) even though the regional tidal range was as large as 1.6 m (Figure 4). The two results are so consistent because three of the dives (40, 42, 87) were conducted near low tide (Figure 4). The BTL thickness was independently estimated as 2.6 m from the submersion of a heat-flow probe system into the turbid layer. The consistency between these two estimates strongly indicates that a turbid layer more than 2.5-m thick was present in the terminal basin soon after the earthquakes.
The upper boundary of the BTL was very sharp and did not measurably change its depth during the two consecutive observation days in 2004. A BTL of the type we envision would have an anomalously low settling velocity because the suspended sediment concentration would be great enough that inter-particle interference would hinder settling (e.g., Nishida and Ito 2009). Therefore, the BTL probably consisted of fluid mud and formed a kind of lutocline (e.g., Ross and Mehta 1989). Judging from the thickness of the BTL, a considerable amount of sediment may have settled out before the first ROV observations 20 days after the mainshock.
We examined the possibility that surface sediment from the prism slope flowed episodically into the terminal basin, given that the basin is isolated from terrigenous sources and has no outflow pathway. The source area capable of supplying sediment to the basin by sediment gravity flow (white dotted line in Figure 1) has an area of about 85 km2. Although this value was estimated without consideration of sediment transport beyond barriers, it is assumed that a considerable amount of sediment could pass over topographic highs. Muck and Underwood (1990) indicated that the maximum run-up elevation of a turbidity current is approximately equal to 1.53 times the flow thickness, which was determined through field observations, theory, and laboratory models. The thicknesses of dilute suspension layers during the 2004 earthquakes were more than 200 m in the study area (Ashi et al. 2012). Moreover, the thickness of the similar turbidity anomalies observed during the 2011 Tohoku-oki earthquake increased with depth up to about 1,300 m above the seafloor (Noguchi et al. 2012). Although there is no evidence about how much the height barrier turbidity currents or suspension clouds were that passed beyond, the source area capable of supplying sediment to the basin is 688 km2 for the case of a 100-m-high barrier (white dashed line in Figure 1).
The volume of the uppermost transparent layer was estimated as follows. The margin of the basin is at 3,818-m water depth, and the area of the basin is about 2.0 km2. Thus, the sediment source area without or with consideration of the over barrier is about 42.5 or 344 times larger than the basin, respectively. The transparent layer is not of constant thickness but thins toward the basin margin (Figure 7), and hence, the estimated volume of the transparent layer was about 0.0024 km3. An even layer of surface sediment from the theoretical source area just 28.2- or 3.5-mm thick could produce the transparent layer without or with consideration of the over barrier, respectively. Considering a typical porosity of 60% to 80%, the source layer would likely be twice that of the thickness. Of course, the sediment source would be heterogeneous rather than a planar layer; nevertheless, it can be said that the uppermost transparent layer could have been deposited in a short period of time. Remobilization of thin sediment veneer by earthquake shaking was also reported from turbidity sensor monitoring and surface sediment sampling 1 month after the 2011 Tohoku-oki earthquake (Noguchi et al. 2012).The ripple-like bedforms on the slope surrounding the basin are consistent with downslope gravity flows of sediment (Figure 2). Although such bedforms only indicate the paleocurrent directions during or after the fatal stage of the seismic event deposition, they can provide us with information about the paths of sediment supply to the terminal basin. The shapes of the ripple-like bedforms tend to have complex patterns on the flat basin floor, which are presumably due to the superimposition of flows of various directions and timing.
We suggest that the thick turbid layer and probably the transparent layers in the terminal basin developed by accumulation of fine-grained sediment derived from multiple directions. Widely observed clouds of suspended sediment soon after the 2004 earthquakes suggest that earthquake shaking caused sediment gravity flows to dump mud into the terminal basin and form thick acoustically transparent layers. The series of these transparent layers documented by subbottom surveys, then, is indicative of repeated events supplying muddy sediment during past seismic shaking. Stratigraphical continuity from the wedge-shaped chaotic body below the hummocky seabed relief to the lowermost transparent layer (Figure 7c) strongly suggests that a submarine landslide was a source for sediment remobilization. Many slump scars appeared in the detailed seafloor morphology of the upslope region Strasser et al. (2011); these are also candidates for the sediment sources.
The water layer of extremely high turbidity observed in a terminal basin on the prism slope soon after the 2004 off-Kii Peninsula earthquakes was studied by a high-resolution subbottom profiler. Comparing the seafloor depths from surveys in 2004 and 2010, the thickness of the BTL was estimated to be more than 2.5 m. The BTL thickness was independently estimated to be 2.6 m by submersion of a heat-flow probe system in the turbid layer. The existence of this thick BTL suggests that a large amount of sediment may have settled out of suspension before the first dive 20 days after the mainshock. The subbottom profile across the terminal basin was characterized by a sequence of homogeneous transparent layers. We conclude that the thin veneer of sediment on the prism slope could be mobilized to flow into the basin by earthquake shaking and create the observed BTL. Accordingly, it can be said that a terminal basin on a prism slope can serve as an excellent earthquake recorder.
We thank the R/V Kaiyo and the R/V Hakuho-maru captains, crewmembers, and technicians for their contributions to the success of the ROV surveys. Various support by Hidekazu Tokuyama, Katsura Kameo, and the shipboard scientists are highly appreciated. The authors are grateful to the editor, Masataka Kinoshita, for his helpful comments, and to Michael Strasser and the anonymous reviewer for providing many constructive suggestions, which have significantly improved our paper. This work was supported by a Grant-in-Aid for Scientific Research on Innovative Areas (No. 21107003) from MEXT of Japan.
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