Preliminary results of paleo-tsunami study in Aceh …mizu.bosai.go.jp/wiki/wiki.cgi?action=ATTACH&page=%C4%C5...Preliminary results of paleo-tsunami study in Aceh Province Shigehiro

Preliminary results of paleo-tsunami study

in Aceh Province

Shigehiro FUJINO (Active Fault Research Center, AIST),

Yuichi NISHIMURA (Institute of Seismology and Volcanology, Hokkaido Univ.),

Purna Sulastya PUTRA and Nandang SUPRIATNA(Research center for Geotechnology, LIPI)

2004 tsunami deposit

Meulaboh

Simeulue Is.

Observation of modern tsunami deposit on Aceh mainland

TOPIC 2

Possible inundation depth

The 2004 tsunami at north Meulaboh

c.a. 1 km from shore

Mr. Hamidi Tengku witnessed that 30-40 cm

thick, black mud was left after the 2004 tsunami,

and that the mud smelled sulferous.

According to local eyewitnesses, inundation

height of the 2004 tsunami exceeded the height

of the roofs.

The 2004 tsunami at Meulaboh

In general, it is assumed that tsunami deposits are coarser

than background sediments because of their sedimentation

under higher energy condition. Hunting for paleotsunami

deposit actually tends to focus on sand layers in muddy

successions in coastal lowlands.

2004 and 2005 tsunami deposits

Paleotsunami deposit 1

Paleotsunami deposit 2

peat

Are all tsunami deposits coarser than background sediments?

Busong, Simeulue Is.

The 2004 tsunami sand

Pre-2004 peaty sand

(beach-ridge sand)

Sandy tsunami deposit

just behind sand ridges

Deposit of the 2004 tsunami at

the point 1 is composed of

coarser sediment than

underlying background

sediments as “usual” tsunami

deposits.

Well sorted sand infers that

they were entrained from

beaches, and provably shallow

sea floor, in front of the site 1.

The 2004 tsunami deposit

Peaty sand of pre-2004 sand ridge

Pe

aty

sa

nd

of

pre

-20

04

sa

nd

rid

ge

Org

an

ic-r

ich

mu

d

of

the

20

04

ts

un

am

i

Thin sand layer Inland muddy tsunami

deposit

Deeply eroded inter-ridge swamp

Possible source of the muddy tsunami deposit

c.a. 700 m from the shore

seeing southeast (parallel to the shore)

Wide swampy area behind sand ridges,

that were previously used as paddy fields, were eroded by

the 2004 tsunami to be deep ponds.

Landward

(sites of muddy tsunami deposits)

Seaward

(site of sandy tsunami deposit)

1 2 3 4erosion

mud supply

muddy tsunami deposit

sandy tsunami deopsit

sand supply

from beach

and seafloor

Paleo-tsunami sand?

fine sand

peat

peat

3

Another sand layer was

intercalated within peat

succession at the site 3.

This is dated to about

1,000 yrBP, and probably

consistent with one of the

pre-2004 tsunami deposits

reported at the sites

15 km apart from site 3.

(Monecke et al., 2008)

Monecke et al. (2008)

Nature, 455, 1232−1234

Discovery of muddy tsunami deposit signifies that

1. Facies of tsunami deposits are highly influenced

by source sediments, and are not necessarily

coarser than background sediment.

2. Past tsunami records can be overlooked

at where large amount of mud is available,

because muddy tsunami sediments

are indiscernible in muddy sediment successions.

This may be one of the reasons why we cannot

see any palotsunami evidences at some sites

despite written past tsunami record.