Japan Agency for Marine-Earth Science and Technology(JAMSTEC)
Prompt reports of
"The underwater survey of the SUMATRA earthquake source area"

(The date without the annotation in the text is local time of natsushima.)



18th Feb. (1st day of the research cruise)

State of Affiars
The Research Vessel "Natsushima" of JAMSTEC passed across the Malacca Strait at midnight of February 18th. (Fri), 2005 (local time), then arrived at the survey area off the Sumatra Island. At 06:30am, all onboard persons dedicated flowers and prayers to mourn over the victims of the disaster.
(Position)On the northern part of the Ache Basin in the northern and western part of the research area.
(Wheather and Sea Surface Condition) Cloudy, weak swelling.
(Today Research Schedule)
OBS: 17 OBSs are being deployed on the scheduled sites in the Ache Basin and the neighbouring outer high to the east.
Bathymetric Survey: Seafloor topographic survey started using SEABAT 3000 system with a backscatter imagery data acquisition.
(co-chief)


PHOTO:Group photo on the shipboard scientific party in front of ROV Hyper Dolphin.

PHOTO:Group photo on the shipboard scientific party in front of ROV "Hyper Dolphin".

Press Release
State of Marine geology and geophysical survey in immediate response to the
occurrence of the 2004 Off Western Coast of Sumatra Earthquake by The
Research Vessel "Natsushima"



19th Feb.(2nd day of the research cruise)

State of Affiars
(Position)On the outer high neighbouring the Ache Basin in the northern
part of the research area.
(Weather and Sea Surface Condition) Cloudy/weakly swelling
(Today's Research Schedule)
OBS: 17 OBSs were deployed on the scheduled sites in the Ache Basin and the neighbouring outer high to the east.
SCS: After the OBS deployment, a single channel seismic profile survey
started across the outer high to the Sunda Trench.
Bathymetric Survey: ongoing using SEABAT 3000 system with a backscatter
imagery data acquisition.
(co-chief)


20th. Feb. (3rd day of the research cruise)

1) State of Affairs
Position: Near the Sunda Trench in the northern part of the research area.
Weather and Sea Surface Condition: Slightly cloudy, calm conditon with wind weaker than yesterday.
Status: We conducted the shallow crustal structure survey using a single-channel seismic (SCS) profiler., and obtained a seismic profile across the outer high. The topography is rough and the reflection from deeper part is weak.

2) Research Report
SCS: Survey was completed in the morning.
Bathymetric Survey: SEABAT bathymetric survey was conducted around Hyper Dolphin survey area on 21st. Feb.

3) Survey plan after 21 Feb.
 Hyper Dolphin and bathymetric surveys.

(Co-chief)


21st. Feb. (4th day)

1)State of Affairs
Position:On the outer high in the northern part of the research area. Water depth: 2000-3000m.
Weather: Slightly cloudy. Calm state.
Status: Bathymetric survey continues on the outer high, along a steep slope with a 900m difference in water depth.

2) Research Report
Bathymetric and Hyper Dolphin surveyed at 3000m depth along one of the slopes of the outer high.

< Scientific Report of Hyper Dolphin Dive (Dive No. 382) >
Target for today's dive was a seaward slope of outer high, where large-scale slope failures form steep and rough cliff topography. It is possibly caused by the thrusting through repeated earthquakes. Dive started at water depth of 3000m. Up to 2730m we observed very turbid water with only 1m visibility. It may be caused by the seafloor landslides or tudbidites, which would be specified by monitoring the temperature anomalies of the bottom-water. We obtained three sediment and two water samples. We observed outcrops of muddy sediments on the rough seafloor with little biological activities. No obvious evidence for 'new' fault planes were found today. We continue survey from 2600m to 2100m tomorrow.

ハイパードルフィンによる海底観察の様子 ハイパードルフィンによる海底観察の様子


3) Survey plan after 22 Feb.
Hyper Dolphin and bathymetric surveys.
Deployment of 2 long-term OBSs is planned after 23 Feb.

(Co-chief)


22nd Feb.(5th day of the research cruise)

(Weather and Sea Surafce Condition) Fine and calm sea state.
(Summary of Survey)
Continued diving survey using ROV Hyper Dolphin. During today’s dive #383 we had a better visibility of the seafloor. Surface deformation such as steps and fissures was observed more often than yesterday. Small fissures were observed on the upper plateau of a cliff at 2184 m depth. We obtained rock samples and made heat flow measurements. We will be able to determine the age of the rock samples if they contain nanofossils. The result was released to the onboard reporters.
(co-chief)




Press Release
Finding, steep topography, collapse and slope failure caused by Sumatra Earthquake: Sea floor survey by Research Vessel “NATSUSHIMA”


23rd Feb.(6th day of the research cruise)

(Position) Northern part of the survey area on the outer high (2000m depth)
(Weather and Sea Surafce Condition) Slightly cloudy, and calm sea state with ~1m swell.
(Summary of results so far)
Two dives were conducted along the slope between two basins with 3400m and 2500m water depths. We believe we found direct lines of evidence for slope failures and landslides caused by this earthquake. They include fresh open cracks near the edge of slope-break basin, slope failures with sharp-edges, and a 300m-thick turbulent layer, with its visibility less than 1m, above the seafloor at > 3000m depth. Sediment coversexist on the steep slope, and no benthos, burrows or trajectories were found.
(Summary of Today’s Survey)
Bathymetric survey using SEABAT was conducted along topographic contours at the outer high area.
(Tomorrow’s Plan)
After the bathymetric survey we start Single-Channel Seismic (SCS) survey. We also study where to deploy long-term OBSs (Ocean Bottom Seismometers) after Feb. 24.

(co-chief)


24th Feb.(7th day of the research cruise)

(Position) Northern part of the survey area. Heading from the outer high (2000m depth) to the Sunda Trench.
(Weather and Sea Surafce Condition) Fine and calm sea state with 2-4m swell.
(Summary of results so far)
Bathymetric survey using SEABAT was conducted on the north-eastern side of the outer high.
Two dives were conducted along the slope between two basins with 3400m and 2500m water depths. We believe we found direct lines of evidence for slope failures and landslides caused by this earthquake. They include fresh open cracks near the edge of slope-break basin, slope failures with sharp-edges, and a 300m-thick turbulent layer, with its visibility less than 1m, above the seafloor at > 3000m depth. Sediment coversexist on the steep slope, and no benthos, burrows or trajectories were found.
(Summary of Today’s Survey)
Bathymetric survey using SEABAT was conducted along topographic contours at the outer high area.
(Tomorrow’s Plan)
After the bathymetric survey we start Single-Channel Seismic (SCS) survey. We also study where to deploy long-term OBSs (Ocean Bottom Seismometers) after Feb. 24.

(co-chief)




25th Feb.(8th day of the research cruise)

1. Present Status
(Position) Northern part of the survey area on the Sunda Trench.
(Weather and Sea Surface Condition) Fine and calm sea state with 2 m swell.
2. Summary of results so far
Detailed topographic relief and its internal structure was investigated on the ridges near the axis of Sunda Trench.
3. Today’s survey plan
More SCS surveys will be carried out across the Sunda Trench, including a long survey line through the trench, outer high and the Ache Basin.
4. Plan after tomorrow
Finalize current SCS line survey, then move to the next line across the trench on the SE side of the OBS network. After that another bathymetric survey and dives are planned near the SW boundary of the Ache Basin. We also deploy the second long-term seismometer.

(co-chief)


26th Feb.(9th day of the research cruise)

1. Present Status
(Position) Northern part of the survey area on the outer high.
(Weather and Sea Surafce Condition) Rain and calm sea state with 1 m swell.
2. Summary of results so far
We investigated rugged topographic features with deformation found on the axis of the Sunda Trench, based on several SCS lines across this structure. The mound with this rugged topography has a relative height of 600 m, and its internal structure is not clear because of irregular acoustic reflections near its summit area. Further analyses are in progress using PDR data as well as SCS and SEABAT data.
3. Today’s Plan
We continue SCS survey around the outer high and Sunda Trench areas until 10:00am, Feb. 26. This line goes across one OBS, the outer high and the SW edge of Ache Basin. We also carry out bathymetry survey along the SCS line.
4. Plan after tomorrow
After finalizing SCS survey, we plan to carry out ROV dives near the SW edge of Ache Basin, followed by the detail bathymetry survey around the dive site. The objective is to identify potential faults or deformation in this area. Bathymetry survey will continue with SCS lines across the outer high.

(co-chief)


27th Feb.(10th day of the research cruise)


1. Present Status
(Position) Around 15km off Sumatra Island. Near the end of SCS and bathymetry survey line.
(Weather and Sea Surafce Condition) Very fine and calm.
2. Summary of results so far
We compared our bathymetry data with that obtained by the British Bessel SCOTT. The SCS record record was obtained across the outer high, deployed OBSs, and the Ache Basin. Its quick look tells us that there is 700m-thick sediment beneath the Ache Basin, and that slightly tilted from east to west. We decided where to deploy the long-term OBS (l-OBS2).
3. Today’s Plan
SCS survey will be terminated at 10:30, then streamer and GI gun will be recovered. 25 days-long bathymetric grid survey near the western boundary of the Ache Basin.
4. Plan after tomorrow
High-density bathymetric survey near the western margin of Ache Basin is planned until March 1. This is necessary for the dive surveys planned on March 2 and 3 in the area. The objective is to know the relationship between the faults (Mentawai fault and west Andaman fault) or deformations and the Sumatra earthquake. After the dive on March 3, we plan to deploy the other long-term OBS (l-OBS2).

(co-chief)




28th Feb.(11th day of the research cruise)

1. Present Status
(Position) Western edge of the Ache Basin
(Weather and Sea Surafce Condition) Very fine and calm.
(Status) Gridded bathymetry survey in the western edge of Ache Basin
2. Summary of results so far
Interpretation of SCS data across the outer high and Ache Basin.
3. Today’s plan
Gridded bathymetry survey in the western edge of Ache Basin.


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