Estimating Tsunami Periods and its Behavior in the Indian Ocean through Tsunami Sedimentation

SurveyName of organization: National Research Institute for Earth Science and Disaster Prevention (NIED)Name of partners: Tohoku University, Osaka City University, National Institute of Advanced Industrial science and Technology (AIST), Hokkaido UniversityTarget Countries : Sri Lanka and IndonesiaCost of Action : US$246,680.36Stakeholders/beneficiaries: Local researchers, stakeholders and governments who planned for tsunami mitigation in the Indian Ocean. Public living tsunami threatening area and tourists.

Background and Object

• In order to conduct a future tsunami risk assessment in the Indian Ocean, it is important to know the tsunami recurrence interval. However, among countries surrounding the Indian Ocean, tsunami records can be only be traced back 400 years in maximum, which is very short for the purpose of estimating tsunami recurrence interval.

• This project focuses on historical and pre-historical tsunamis in the Indian Ocean. Large tsunamis remove sediments from sea floor and coast and transport them landward or seaward. Tsunami deposits become part of the geologic record. Tsunami deposit survey enables us to trace back past tsunami record of thousands of years.

• Period: January 2008 - March 2009

Implementation process: Sri Lanka

• In Sri Lanka, tsunamis rarely attacked in the past, because there is no subduction zone around the island and only transoceanic tsunamis such as the 2004 Indian Ocean tsunami would have reached there. In this context, Sri Lanka is one of the best countries to study the sedimentary record of the past tsunamis in the Indian Ocean.

• There is a mythical account in Mahavansa, Sri Lanka’s national Buddhist chronicle, which describes the sea flooding on land about 150 B.C.

• We conducted drilling at lagoon and lowland at the southern coast of Sri Lanka to find the sedimentary records of the past tsunamis.

Dikwella

Hambantota

Kirinda

Location map of our drilling site.

Implementation process: Indonesia• We conducted fieldwork at

coastal lowlands in Aceh and West Sumatra Provinces, because these regions repeatedly suffered by earthquakes and tsunamis occurred along the Sunda megathrust.

• Shallow excavation exposed paleo-tsunami deposits preserved in sedimentary successions as tsunami record of past thousand years.

Mode of hand coring at paddy field

A paleo-tsunami deposit exposed in a shallow pit

Tsunami deposit

Aceh P.

West Sumatra P.

Key Achievements: Sri Lanka

• We obtained continuous sedimentary record up to 7390 yr before present (B.P).

• The sedimentation rate was considerably low from 3500-4000 yr B.P. to present (0.1 mm/yr).

Drilling in a lagoon at Sri Lanka

Contd…

• Up to 8 sand layers (including the 2004 tsunami deposit) are observed in the 7390 year sedimentary record. Several sand layers can be correlated at three drilling sites.

• These sand layers were probably formed by the past-tsunamis, although additional sedimentological/ paleontological analyses are required.

Tsunami deposit formed at around 4000 yr BP in Sri Lanka

Key Achievements: Indonesia• We discovered a possible

paleo-tsunami deposit c.a. 1000 years old at west coast of Aceh (Meulaboh).

• The deposit is dated to c.a. 1,000 yrBP, and probably consistent with one of the pre-2004 tsunami deposits found at sites 15 km apart from our site by Monecke et al.(2008) .

• Our finding supports their result and infers that the tsunami 1000 yrBP affected wide area of western coast of Aceh.

Tsunami depositTsunami deposit

peatpeat

peatpeat

Monecke et al. (2008) Monecke et al. (2008) NatureNature, 455, , 455, 12321232−−12341234

Lessons Learned {What worked & what did not}

: Sri LankaWhat worked?• Sri Lanka is one of the best

countries to study the sedimentary record of past tsunamis in the Indian Ocean until 7500 year BP.

Contd…

What did not?• Due to the civil conflict, it was

unable to access to the east coast until this year.

• The sedimentation rate is significantly low from 4000 yr BP to present. Therefore, no definitive tsunami records (including Mahavansa event) have recovered during this period.

Lessons Learned {What worked & what did not}

: IndonesiaWhat worked?• Study site selection is important for

paleo-tsunami deposit hunting in tropical regions.

What did not?• Thickly forested areas and mangrove

areas are not suite for 　 preservation of past tsunami record because sediments are

highly disturbed by burrows and roots.

Thickly forested coastal lowland

How the outcomes will be sustained…• The analyses of sedimentary cores

are still ongoing. We will continue the analyses with our coworker in Moratuwa University (Sri Lanka) to estimate the tsunami recurrence interval.

• Tsunami deposit hunting is still continued in Aceh and West Sumatra provinces.

Future plans??• We are planning to take drilling

cores at east cost of Sri Lanka, where larger waves of the 2004 Indian Ocean tsunami were attacked in order to recover the high-resolution sedimentary record between 4000 yr BP to present.

• It is necessary to reveal magnitude and extent of past tsunamis in Aceh and West Sumatra Provinces based on the distribution of paleo-tsunami.