Prompt reports of "The underwater survey of the SUMATRA earthquake source area"

1st Mar.（12th day of the research cruise）

1	Present Status (Position) Western edge of the Ache Basin (Weather and Sea Surafce Condition) Very fine but some swell after a bad weather happened this morning. (Status) Gridded bathymetry survey in the western edge of Ache Basin with a steaming speed reduced from 8 knots to 6 knots.
2.	Summary of results so far We reached to a temporary conclusion of SCS data interpretation across the outer high and the Ache Basin. We also keep analyzing bathymetry in this area: In general the Ache basin is slightly tilting toward the western edge (uncorrected depth: 2700 m). On the other hand the western edge itself has steep cliff and terrace structure developed to each other. SCS record suggests that the western edge has a normal-fault character.
3.	Today’s plan Gridded bathymetry survey in the western edge of Ache Basin.

2nd Mar.（13th day of the research cruise）

1.	Present Status (Position) Western edge of the Ache Basin (Weather and Sea Surafce Condition) Very fine. A strong shower at noon.
2.	Summary of results so far We identified a fault on the western edge of Ache Basin as a result of high-resolution bathymetry and SCS surveys. This is not a main fault directly related to the M9 earthquake.
3.	Today’s plan ROV dive survey in the western edge of Ache Basin. Visual investigation of a fault on the 2200m-terrace. Bathymetry survey continues during midnight.

Dive No.384

3rd Mar.（14th day of the research cruise）

1.	Present Status (Position) Western edge of the Ache Basin (Weather and Sea Surface Condition) Very fine. (Status) In this morning, we confirmed a fishing net deployment by an Indonesian fishery boat at a scheduled ROV dive point. For safety dive, we carefully measured location of the fisher net by marking buys attached with the net and made prediction for a possible movement of it based on observed current vector (to 25°N at 0.9 knots). From the prediction, we decided that dive survey starts at 10am.
2.	Summary of results so far We dived in seafloor faults developed along the west boundary of the Ache Basin yesterday. The fault runs parallel to a small trough (2170 mbsl), which is characterized by thick clay cover and its slightly uplifted seafloor in the middle. We found cracks on the area, which are located right above the faults. We are now processing geophysical data, such as heat flow and so on, measured during the ROV expedition on the seafloor. So far we have not yet identified any abnormal signature on the geophysical data. After retrieving ROV on the deck, we re-started gridded bathymetric survey in this area.
3.	Today’s plan (ROV dive survey) Target area is the southwestward continuation of the lineament, bordered by the faults that was surveyed with ROV yesterday, and is consisting of a small tectonic basin (west part) and ridge(east part). (Bathymetry Survey) Re-start of grided bathymetry survey. (L-OBS) Deployment of long-term OBS #2 in the northernmost place of the whole survey area during the cruise.

Dive No.385

4th Mar.（15th day of the research cruise）

1.	Present Status (Position) Northernmost part of the whole survey area. (Weather and Sea Surface Condition) Very fine. (Status) We completed deployment of L-OBS#2.
2.	Summary of results so far Yesterday’s ROV dive was done along a course spanning a small trough (1270 mbsl) and neighboring ridge. The purpose of ROV dive survey was to investigate fault structure along with the small trough. We recognized that slightly tilted, step-like terraces developed in southwestern slope of the ridge. We then found a lot of rocks and breccia, an indication of slope failures. After yesterday’s ROV dive, we deployed Long-term OBS #2 and confirmed the exact location by using GPS and acoustic ranging.
3.	Today’s plan Deployment of L-OBS#2 was the final menu of Leg.1. Research Vessel “Natsushima” started to move toward a port of Malaysia for fuel supply and exchange of scientists. Leg.2 is scheduled to start off Sumatra at 10th March.

5th Mar.（16th day of the research cruise）

The Research Vessel “NATSUSHIMA” finished the first leg of the survey and left from the survey area on 4th Mar at 11:00am.
She is passing across the Malacca Strait today.

6th Mar.（17th day of the research cruise）

The Research Vessel “NATSUSHIMA” sailed into the Butterworth Port of Penang in Malaysia.

7th Mar.（18th day of the research cruise）

Embarkation/disembarkation of researchers and Preparing for the next survey

8th Mar.（19th day of the research cruise）

The Research Vessel “NATSUSHIMA” set off from the Butterworth Port and sailed into the survey area of leg2.

Press Release
Preliminary Report of the Urgent Survey Cruise off Sumatra Island by the Research Vessel NATSUSHIMA: Leg 1 (Feb.18-Mar. 4,2005)

9th Mar.（20th day of the research cruise）

The Research Vessel “NATSUSHIMA” is passing across the Malacca Strait.

10th Mar.（21th day of the research cruise: 1st day of Leg.2）

1.	Present Status (Position) Outer arc high off Sumatra, southeast part of short-period OBS network deployed during Leg.1 (Weather and Sea Surface Condition) Fine and calm. Current velocity 1 knot. (Status) Research vessel "Natsushima" arrived in the survey area mid-night. We started to G.I. gun shooting over short-term OBSs to obtain seismic velocity structure in the crust off Sumatra since 0am of today. (Scientist embarkation) New members in Leg.2 on-board scientific party are; Dr.Suehiro(JAMSTEC), Arai(AIST,Japan), Seebar(LDEO,USA), Yusuf(BPPT,Indonesia), Yudi(BPPT,Indonesia), Hanant(Marine Geological Institution, Indonesia), Celman(Metro TV, Indonesia).
2.	Summary of results so far None.
3.	Today's plan (OBS/SCS seismic structure experiment) OBS refraction experiment and SCS reflection imaging will be continued at 5am in 11th.

11th Mar.（22th day of the research cruise: 2nd day of Leg.2）

1.	Present Status (Position) West boundary of the Ache Basin, at location above short-term OBS (S-OBS) #2. (Weather and Sea Surface Condition) Fine and calm. (Status) The G.I. gun shooting, started yesterday, was completed at 7am. S-OBS#1 was recovered at 10am. We are now recovering S-OBS#2.
2.	Summary of results so far OBS refraction experiments and SCS reflection imaging were finished.
3.	Today's plan (OBS recovery) We are going to recover at maximum five S-OBSs until sunset. (Bathymetry Survey) After the sunset, we are going to do seafloor bathymetric mapping in the eastern part of the survey area.

12th Mar.（23th day of the research cruise: 3rd day of Leg.2）

1.	Present Status (Position) Outer arc high, at location above short-term OBS (S-OBS) #8. (Weather and Sea Surface Condition) Fine and calm. (Status) We re-started recovery of S-OBSs from 6am and already recovered S-OBSs #7 and 8 so far.
2.	Summary of results so far Five S-OBSs were recovered up to yesterday and then re-started bathymetry survey in northeastern part of the survey area.
3.	Today’s plan (OBS recovery) We are going to recover at maximum seven S-OBSs until sunset, 7pm. (Bathymetry Survey) After the sunset, we are going to map seafloor bathymetry in the eastern part of the survey area.

13th Mar.（24th day of the research cruise: 4th day of Leg.2）

1.	Present Status (Position) Outer arc high, at location above short-term OBS (S-OBS) #17. (Weather and Sea Surface Condition) Fine and calm. (Status) We re-started recovery of S-OBSs from 6am and just finished recovery of the last S-OBSs #17.
2.	Summary of results so far Twelve S-OBSs were recovered up to yesterday, seven of which were retrieved until 12th March.From mid-night of yesterday, we restarted to map bathymetry in northeast part of the scheduled survey area.
3.	Today’s plan (OBS recovery) We were recovering five S-OBSs from 6am, and confirming digital seismograms recorded with S-OBSs. (Bathymetry Survey) After the sunset, we are going to map seafloor bathymetry in trechward of the outer arc high.

14th Mar.（25th day of the research cruise: 5th day of Leg.2）

1.	Present Status (Position) Trenchward slope of outer arc high. (Weather and Sea Surface Condition) Fine and calm. (Status) We are surveying seafloor by using ROV from 8:30am.
2.	Summary of results so far All of seventeen S-OBSs were recovered.From mid-night of yesterday, we restarted to map bathymetry in un-covered area of outer arc high.
3.	Today’s plan (ROV dive) The dive target point is in a region that includes large-scale slope failure on a trenchward cliff (2,400-2,900 mbsl) in outer arc high and adjoining valley. We encountered highly turbidity(muddy) water layer between 2,600 and 2,900 mbsl so that ROV camera could not monitor farther than 2-3 m. Above 2,600 mbsl, small-scale slope failures and open cracks could be identified. These features are similar to ROV surveys conducted in 21 and 22 th of Feburary. (SCS) After the sunset, we will perform single-channel seismic profiling from the trench axis toward trenchward slope of outer arc high.

15th Mar.（26th day of the research cruise: 6th day of Leg.2）

1. Present Status
(Position) Trenchward slope of outer arc high.
(Weather and Sea Surface Condition) Fine and calm.
(Status) We were surveying seafloor by using ROV from 8:30am‚ but soon recovered ROV because of apparatus troubles at 12am. After retrieving ROV‚ we re-started bathymetry survey.

2. Summary of results so far
Yesterday’s ROV dive found a large-scale slope failure (see figure 1 about the location). The slope was considerably collapsed as shown in photo 1.0bserved collapse features might suggest that the slope failure is responsible for a possible strong ground(seafloor) motion rather than a simple land slide.
Results of today’s ROV dive are similar to those in 22 th‚ Feburary: turbidity water layer
Is found on seafloor deeper than 2‚650 mbsl. The presumable source of the turbidity water layer is “strongly break band” that is located shallower than 2‚600 mbsl in outer arc high. The “strongly break band” runs parallel to the trench.
Single channel seismic imaging was started at mid-night and covered from the trench axis to outer arc high. We are processing this SCS profile.

(ROV dive) Dive points are shown in figure 1.
(Bathymetry survey) After the sunset‚ we are going to map bathymetry from the trench axis toward a trenchward slope of outer arc high.

Figure 1. Survey area. Green circles‚ triangles‚ red lines‚ and purple lines show dive points‚ ocean-bottom seismographs‚ bathymetry survey lines‚ and single channel seismic lines‚ respectively. The dive points (green circles) in March 14 and 15 are middle and south‚ respectively‚ of the southwestern ends.

Photo 1. Narrow ridge with a width of 50-80 meters extends several hundred meters in “strongly break band” within slope area of outer arc high. This narrow ridge looks like “SUIDO-BASHI (bridge)”‚ an old-fashioned architecture in Tokyo. Bedrock is cracked and its crack surfaces are still sharp‚ implying that “strongly break band” might be just created by the 2004 Sumatra earthquake. The narrow ridge could be collapsed in near future (photo: height of cliff‚ which falls into right in the photo 1‚ reaches more than 100 meters).

Dive No.386

16th Mar.（27th day of the research cruise: 7th day of Leg.2）

1.	Present Status (Position) Trenchward slope of outer arc high. (Weather and Sea Surface Condition) Fine and calm. (Status) We were surveying seafloor by using ROV from 8:30am.
2.	Summary of results so far We mapped bathymetry in uncovered area.
3.	Today’s plan (ROV dive) Dive points are small flat plain (3‚000 mbsl) and adjoining cliff. (Bathymetry survey) From sunset‚ we are going to map bathymetry in uncovered area.

17th Mar.（28th day of the research cruise: 8th day of Leg.2）

1.	Present Status(Position) Trenchward slope of outer arc high. (Weather and Sea Surface Condition) Cloud with intermittent squalls. Sea condition is calm. (Status) We are surveying seafloor by using ROV from 8:30am.
2.	Summary of results so far Yesterday’s dive point was on a terrace (approximately 3,000 mbsl) and adjoining slope. The terrace is located on middle of a cliff within inner trench slope southwest of the scheduled survey area. From bathymetry survey, similar terraces are identified to scatter over along a line directing north-to-south. The terraces are considered to be gravitationally instable. The terraces are distributed trenchward of “strongly break band” that was found a day before yesterday. By dive survey, we identified that thick unconsolidated sediment with biological traces and marks covered on the instable terraces. The observations suggest that these terraces were not formed by the 2004 Sumatra earthquake. From sunset, we are mapping bathymetry.
3.	Today’s plan (ROV dive) Dive point is a flat plain behind (landward) inner trench slope. We use subbottom profiler attached on ROV to obtain a high resolution image of shallow sediment structure in the flat plain. (Bathymetry survey) From sunset, we are going to map bathymetry in uncovered area.

18th Mar.（29th day of the research cruise: 9th day of Leg.2）

Present Status
(Position) Submarine canyon through a cliff trenchward of outer arc high.
(Weather and Sea Surface Condition) Fine and calm.
(Status) We are surveying seafloor by using ROV from 8:30am.

Summary of results so far
The purpose of yesterday’s dive was to observe seafloor deformation of the region just northeast of “strong shear zone”(or “strongly break band” ) as well as subsurface imaging with subbottom profiler of ROV. We are processing data of subbottom profiler.

In-site observation with ROV, however, could not found any signature showing seafloor deformation because thick sediment covered the flat plain with well-developed ripple marks. Therefore, ROV dive survey results of these two days suggest that “strong shear zone” is strongly confined within a narrow segment (or limited region).

We processed S-OBS records and obtained preliminary results. We could identified micro-earthquakes with magnitudes down to 1. Total number of aftershocks within observation period of several weeks reaches approximately 3,000. Most of them likely occurred below the Ache Basin. We are now processing SCS, bathymetry, and aftershock data to make clear signatures of seafloor deformation due to the 2004 Sumatra earthquake.

Today’s plan
(ROV dive) Dive point target is a submarine canyon through a cliff trenchward of outer arc high. We are going to explore seafloor from halfway of the canyon to top of adjoining cliff. The purpose of this dive is to observe bedrock and layering structure below/within/above “strong shear zone”.
(Bathymetry survey) From sunset, we are going to map bathymetry in uncovered area.

Dive No. 390

19th Mar.（30th day of the research cruise: 10th day of Leg.2）

Present Status
(Position) Submarine canyon cutting deep into a cliff trenchward of outer arc high(the same location of 18 th Mar.).
(Weather and Sea Surface Condition) Fine and calm, sometimes squall.
(Status) We are surveying seafloor by using ROV from 6:30am.

Summary of results so far
Yesterday’s dive was continuation of ROV dive 18 th Mar. The purpose was to observe inside of “strong shear zone”(or “strongly break band” ). As expected, we observed lots of cracks and sheared uppermost sediment along successive ridges directing EW. At top of a ridge, we found a large, NS-striking crack or fracture zone. These cracks or fractures look fresh. ROV in-site observation on this region more or less suggests that these cracks or fractures is probably created by strong shaking of the 2004 Sumatra earthquake.

Today’s plan
(ROV dive) Scheduled dive course is along NS-striking fractures found in March 18 th. The course is also along a ridge located southwestern end of outer arc high. Today’s dive so far found that numerous fresh cracks or fractures aligned in NS-direction in this region. We confirmed that the alignment extended at least 1 kilometer.

*** Today is the last day of this cruise so that this daily report is final.


Dive No. 391	Dive No. 391

20th Mar.

The Research Vessel “NATSUSHIMA” finished the second leg of survey on Mar 19 at 2:40pm and set off to the port of Benoa, Bali in Indonesia.

21th Mar. - 25th Mar.

The Research Vessel “NATSUSHIMA” is sailing for the port of Benoa, Bali in Indonesia.

26th Mar.

The Research Vessel “NATSUSHIMA” arrived at the port of Benoa, Bali and all of the researchers got off the vessel.

28th Mar.

JAMSTEC announced a press release regarding the emergency survey, which was held around the epicenter, off Sumatra in collaboration with BPPT.
Press Release : Preliminary results from R/V NATSUSHIMA survey around the epicenter, off Sumatra