KSP Newsletter 158 – November 2017

Five years ago I wrote about the risk of a Tsunami hitting Samui and recently for some reason that topic has come up for discussion again, so I have been doing some research to see if anything has changed or there is new evidence or studies to suggest anything different.

There has still been nothing specific to Koh Samui and no new studies relating to the possibility of a tsunami in the Gulf of Thailand. An article in The Nation in December last year stated that “THAILAND is unlikely to be affected by another deadly tsunami any time soon according to an earthquake expert and that included the west coast Andaman Sea area. TRF geology researcher Passakorn Pananont said “Even though scientists still cannot preciously predict an earthquake [happening], which can cause a tsunami, it is very unlikely that the next tsunami will hit our shores soon.”

Paiboon Nuannin, a lecturer at the Department of Physics at Prince of Songkla University and a prominent earthquake expert, said that based on the earthquake record in the Andaman Sea between 2007 to this year, there were 10 earthquakes big enough to create a tsunami. He said a tsunami could only be triggered by a specific type of tectonic plate movement and not all big earthquakes can generate a seismic sea wave. “A big tsunami can only be generated by an earthquake larger than eight magnitude, and in the Andaman Sea, the recurrence interval for such a gigantic earthquake was once every 15 years. And the most recent earthquake on this scale already happened in 2012, so it will take around 10 years to gather the energy for the next big earthquake”

There are three principal causes cited for the promulgation of a tsunami, sub-sea earthquakes, landslides and volcanic eruptions – the former being the most common. Earthquakes are the consequence of the movement of the tectonic plates moving against each other and similar movements in the fault lines which cross them. Major earthquakes commonly occur along the plate boundaries and in SE Asia the ones we are concerned with are the Sunda Trench to the west of Thailand and the Manila/Luzon Trench immediately to the west of the Philippines. Any tsunami generated by an earthquake in the Sunda Trench will not affect Koh Samui due to the land mass in between. All studies I have found re the Gulf of Thailand concern themselves with the effects of earthquakes in the Manila/Luzon Trench.

“PROBABILISTIC TSUNAMI HAZARD ANALYSIS AND RISK TO COASTAL POPULATIONS IN THAILAND”
Follow the link for the full report but to summarize a seismic induced tsunami in
the southern part of the Manila Trench in excess of 8.5Mw would likely lead to a
tsunami entering the Gulf of Thailand once in 650 years to a height of 2-3 metres. The
worst affected areas would be the southern provinces.

This is demonstrated in the “2010 Tsunami Simulations for Regional Sources in the South China and Adjoining Seas” study which proposed a catastrophic earthquake at the Luzon Trench, off the western shore of Luzon producing a tsunami affecting the Narathiwat and Pattani provinces in Thailand. It also addresses the effects of a sub-sea landslide off Borneo but  generally the shallow nature (average 100m) would mitigate the magnitude of any tsunami entering the Gulf of Thailand and hence the affect on Koh Samui.
The study “Effect of Tsunamis generated in the Manila Trench on the Gulf of Thailand 2008” states “The Gulf of Thailand is affected by the diffraction of tsunamis around the southern part of Vietnam and Cambodia. The tsunami amplitude at the southernmost coastline is about 0.65 m for the Mw 9.0 earthquake. The current velocity in the Gulf of Thailand due to the Mw 9.0 earthquake is generally less than 0.2 m/s. “

Tsunami Risk Reduction Measures Phase 2 – November 2009” stated “The simulations reveal further that tsunami threat due to seismic origin to the Gulf of Thailand is almost nonexistent.” The conclusion is that , yes, Koh Samui could be affected by a tsunami, but that it would be small and not very powerful, probably less than one metre.