Line12: Single Channel Seismic Reflection Result (H. Yokose)

Introduction

In order to identify the boundary between Nuuanu and Wailau landslide deposit near Oahu and Molokai, single channel seismic reflection surveys were carried out. Because this study area has a bench like topography in northeast flank of Oahu at about 4000 mbsl, which is very similar to Hilina landslide area in their topographic characteristics, it is very interesting to compare and contrast with the existing detailed Hilina and Nuuanu results. The boundary is assumed based on the bathymetric map（Moore, 1964） and rock chemistry (Shinozaki et al., 2002) and is shown with a broken  line in figure 1.
　Identification of landslide region corresponding to each event is very important to asses the scale and to understand the detailed process. During 1998 and 1999 Hawaii cruise sponsored by JAMSET, precise bathymetric maps for Nuuanu and Wailau landslides were obtained（Smith et al., 2002）.

Purpose

3D projection of topography based on a precise digital elevation model is very useful for diversifying the imagination of underwater world . Topographical characteristics of the study area can be divided into 4 categories：bench like landslide block, flat plane between megablocks, hummocky terrain, flat basin in the landslide block. The submarine flat plane between megablocks continu submarine channel which is start from Oahu or Molokai. Main part of Nuuanu landslide deposit is topographically  higher than that of Wailau landslide（see Figs. 1 and 2）. The boundary line (dot line) assumed previously is not consistent with topographical nature, such as alignment of landslide blocks and development of submarine canyon. It is more reasonable that the line move to southeast as indicated in a heavy solid line in fig. 2.

Figure 1　Figure 2　　

During Leg 1B, we collected single channel seismic reflection data two lines across boundary zone between Nuuanu and Wailau landslides (line 12-1 and 12-3) and one line along Wairau landslide(Line 12-2).Initial on-board processing consisted of bandpass filter (20-100Hz) and scaling. Additional processing was applied at the University of Hawaii during Leg 2B.

result

Line 12-1

West to East

Line12-1 straddles  three different topographic categories: bench like landslide block, flat plane between megablocks, hummocky terrain. Because the power of air gun was not sufficient to penetrate deep subbottom strata, we can not investigate on internal structure of the bench like megablock and main part of both huge landslide deposits. We can investigate near ocean floor sedimentary deposits about 500m in their thickness, where is proximal boundary zone between Nuuanu and Wailau landslide deposite.
　The sedimentary deposit around  Line12-1 can be divided into four units (unit A：well bedded turbidities layer, unit B:　Hummocky and chaotic sequence, unit C：　Chaotic sequence, unit D: bedrock ). the most upper part of the flat area, which is the western half of the line  is covered with  well bedded turbidity  strata which is about 100m thick. Unit B is a hummocky and chaotic sequence with 200m thick and onlap unit C.   Unit B in hummocky terrain was folded (slumped) and tilted toward  west. This unit did not find in Wailau landslide area and overlay the landslide block of Wailau landslide unconformly. Unit C, 300-400 ｍ thick, extends from western side of bench like landslide block to Wailau landslide block.

Line 12-2

South to North
We find northern extension of unit B and unit C in southern part of landslide block on Line12-2. Two landslide blocks derived from Wailau landslide sprit unit C into two parts north and south. Unit C is inferred to be younger than the landslide blocks based on lack of continuity of  unit C. The flat basin surrounded by landslide blocks dose not have unit A which is prevailing in both LINE12-1and Line 12-3.

Line 12-3

South to North
In gently dipping ocean bottom plane  from southeastern foot of the Tuscaloosa seamount to Wailau landslide block  stratified sediment (unit A) 300m thick is imaged  and as is interpreted recent turbidities sequence. Unic B is not observed. Unit C  abuts  on Tuscaloosa sea mount. Information of under unit C can not obtained  due to insufficient air gun power.

Conclusion
Although the SCS　SYSTEM using in this cruse is not powerful enough to investigate the internal structure of huge landslide deposit included in megablocks, many important information about shallow region of the strata were obtained as follow :

（1）Flat ocean floor prevailing between landslide blocks is covered with thick (ca. 100m) well bedded turbidities sequence which is derived from Oahu and Molokai thought submarine channel.

（2）Unit B  is small scale debris avalanche deposit based on the stratigraphic feature and is younger than  Nuuanu and Wailau landslide.