Medieval forewarning of the 2004 Indian Ocean tsunami in Thailand

Flashcards about the 2004 Indian Ocean Tsunami

Where did the 2004 tsunami run 2 km across?

A grassy beach-ridge plain 125 km north of Phuket, Thailand.

What was the impact of the 2004 tsunami on the ridges and swales?

The 2004 tsunami coated the ridges and intervening swales with a sheet of sand commonly 5–20 cm thick.

Where are the remains of several earlier sand sheets preserved?

Peaty soils of marshy swales.

When did the most recent full-size predecessor to the 2004 tsunami occur?

About 550–700 years ago.

Where did the 2004 Indian Ocean tsunami crest higher than anywhere else east of Sumatra?

Phra Thong Island.

What were the effects of the flooding at Phra Thong?

The tsunami reamed out several drainages previously cut across beach ridges as much as 300 m inland and also coated most of the island’s western half with a sheet of sand.

Where were pre-2004 sand sheets found at Phra Thong?

At 20 sites, pre-2004 sand was found interbedded with peaty soils of swales that hold standing water most of the year.

Across which two marshy swales were pre-2004 beds traced?

Swales X and Y.

When did swales X and Y form?

About 2,500 years ago.

What does peaty soil in swale X contain that resemble the overlying 2004 deposit?

Two sand sheets (B and C).

What forms a discontinuous basal layer in Sheet C?

Coarse to very coarse sand that fills pre-existing pockets in the underlying soil.

What age did three bark fragments yield?

530 to 570 radiocarbon years before AD 1950.

What does swale Y contain?

Three pre-2004 sand sheets alternate with peaty soil.

What size ruptures probably created the pre-2004 sand sheets?

The sheets probably required ruptures larger than magnitude 7.9

Which sand sheet may correlate with tsunami and earthquake evidence elsewhere?

Sheet B

What do the earlier sand sheets in swales X and Y lack?

Lack diatoms.

How are the pre-2004 sand sheets preserved?

The pre-2004 sheets are distinct and sharply bounded where the swale soil is peaty, blurred by gradational contacts where the soil is just slightly organic, and totally absent in the sandy soils of beach-ridge crests.

How far from the Equator, does this part of Thailand lie?

Less than 10 degrees.

How long is the span between sheets C and B?

1,500-1,850 years.

What do the sand sheets of Phra Thong Island forewarn?

The sand sheets represent infrequent catastrophe.