Saturday, July 22, 2017

Tsunami History Preserved In Indonesian Cave Deposits

How would you know if a coastline had been inundated by a tsunami say 5000 years ago? Well, a tsunami carries sediment stripped from the ocean bed and deposits this material over the flooded coastline, beyond the range of what a regular storm would. The problem is that such deposits have poor preservation potential and over time get eroded away. There are however some environments where such tsunami deposits may get preserved inland. These are estuaries, coastal marshes and lakes. Here, interlayered with normal estuarine, marsh or lacustrine sediment, one may find layers of sand of a distinctly different composition and texture and containing remains of organisms which live in an open marine setting. This implies a sudden incursion of marine waters into these inland coastal settings. The other coastal setting with a good preservation potential are caves. These too get flooded by storm surges and tsunamis and may preserve a record of such events in the form of sand deposits. The picture to the left (Source: Rubin 2017)  shows sand layers deposited by the 2004 tsunami.

In one such cave on the coast of Aceh, Indonesia a record of the 2004 tsunami along with sand layers deposited by 11 older tsunamis going back to 7400 years ago have been preserved.

Highly variable recurrence of tsunamis in the 7,400 years before the 2004 Indian Ocean tsunami-
Charles M. Rubin, Benjamin P. Horton, Kerry Sieh, Jessica E. Pilarczyk, Patrick Daly, Nazli Ismail & Andrew C. Parnell


 We identify coastal caves as a new depositional environment for reconstructing tsunami records and present a 5,000 year record of continuous tsunami deposits from a coastal cave in Sumatra, Indonesia (Fig. 1), which shows the irregular recurrence of 11 tsunamis between 7,400 and 2,900 years BP. The sedimentary record in the cave shows that ruptures of the Sunda megathrust vary between large (which generated the 2004 Indian Ocean tsunami) and smaller slip failures. The chronology of events suggests the recurrence of multiple smaller tsunamis within relatively short time periods, interrupted by long periods of strain accumulation followed by giant tsunamis. The data demonstrates that the 2004 tsunami was just the latest in a sequence of devastating tsunamis stretching back to at least the early Holocene and suggests a high likelihood for future tsunamis in the Indian Ocean. The sediments preserved in the costal cave provide a unique opportunity to refine our understanding of the behaviour of the Sunda megathrust, as well as study in detail the sedimentology and hydrological characteristics of tsunami deposits.

There is one point that cannot be over stressed. The average recurrence time for earthquakes and tsunamis has been estimated to be on the order of several hundred years. However, there is a great variation in the actual occurrence, with several smaller tsunamis occurring just decades apart. While our understanding of earthquake mechanisms and tsunami generation will go on improving, ultimately what will save lives is better preparedness. This includes adherence to structurally appropriate building codes, functioning tsunami warning systems and well drilled and practiced disaster management plans. South East Asia has long neglected these issues and there needs to be a renewed focus on them.

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