Montri Choowong

Medieval forewarning of the 2004 Indian Ocean tsunami in Thailand

Recent centuries provide no precedent for the 2004 Indian Ocean tsunami, either on the coasts it devastated or within its source area. The tsunami claimed nearly all of its victims on shores that had gone 200 years or more without a tsunami disaster. The associated earthquake of magnitude 9.2 defied a Sumatra–Andaman catalogue that contains no nineteenth-century or twentieth-century earthquake larger than magnitude 7.9 (ref. 2). The tsunami and the earthquake together resulted from a fault rupture 1,500 km long that expended centuries’ worth of plate convergence. Here, using sedimentary evidence for tsunamis, we identify probable precedents for the 2004 tsunami at a grassy beach-ridge plain 125 km north of Phuket. The 2004 tsunami, running 2 km across this plain, coated the ridges and intervening swales with a sheet of sand commonly 5–20 cm thick. The peaty soils of two marshy swales preserve the remains of several earlier sand sheets less than 2,800 years old. If responsible for the youngest of these pre-2004 sand sheets, the most recent full-size predecessor to the 2004 tsunami occurred about 550–700 years ago.

Erosion and Deposition by the 2004 Indian Ocean Tsunami in Phuket and Phang-nga Provinces, Thailand

Abstract The devastating December 26, 2004, tsunami produced abundant geologic effects along the Andaman coast of Thailand. The tsunami inundated the numerous sandy beaches and flowed over the adjacent aeolian dunes. On some of the dunes, the tsunami scoured circular holes 10–30 cm in diameter, and in its waning phases, it coated the holes with mud. The tsunami locally deposited a sand sheet that ranged from 0–30 cm in thickness, with an average thickness of approximately 10 cm. Sedimentary structures within the sand sheet include ripples from inflow and outflow, graded bedding, parallel lamination, and double-layered deposits. Erosion, locally severe, affected sand beaches and tidal inlets. We use these erosional and depositional features to infer the main processes that acted during inundation from the tsunami.

Tsunamis versus storm deposits from Thailand

Along the Andaman (west) coast of Thailand, the 2004 tsunami depositional features associated with the 2004 tsunami were used to describe the characteristics of tsunamis in a place far away from the effect of both recent and ancient storms. The current challenge is that a lack of precise sedimentological characteristics have been described that will differentiate tsunami deposits from storm deposits. Here, in sedimentological senses, we reviewed the imprints of the sedimentological characteristics of the 2004 tsunami and older deposits and then compared them with storm deposits, as analyzed from the deposits found along the eastern (Gulf of Thailand; GOT) coast of Thailand. We discuss the hydraulic conditions of the 2004 tsunami and its predecessors, on the Andaman coast, and compare them to storm flows found on the coast of the GOT. Similar to an extensive tsunami inflow deposit, a storm flow overwash has very similar sedimentary structures. Well-preserved sedimentary structures recognized in sand sheets from both tsunami and storms include single and multiple normal gradings, reverse grading, parallel, incline and foreset lamina, rip-up clasts, and mud drapes. All these sedimentary structures verify the similarity of tsunami and storm inflow behavior as both types of high-energy flow start to scour the beach zone. Antidunes are likely to be the only unique internal sedimentary structures observed in the 2004 tsunami deposit. Rip-up clasts are rare within storm deposits compared to tsunami deposits. We found that the deposition during the outflow from both tsunami and storms was rarely preserved, suggesting that it does not persist for very long in the geological record.

Earthquake frequency-magnitude distribution and fractal dimension in mainland Southeast Asia

The 2004 Sumatra and 2011 Tohoku earthquakes highlighted the need for a more accurate understanding of earthquake characteristics in both regions. In this study, both the a and b values of the frequency-magnitude distribution (FMD) and the fractal dimension (DC) were investigated simultaneously from 13 seismic source zones recognized in mainland Southeast Asia (MLSEA). By using the completeness earthquake dataset, the calculated values of b and DC were found to imply variations in seismotectonic stress. The relationships of DC-b and DC-(a/b) were investigated to categorize the level of earthquake hazards of individual seismic source zones, where the calibration curves illustrate a negative correlation between the DC and b values (Dc = 2.80 - 1.22b) and a positive correlation between the DC and a/b ratios (Dc = 0.27(a/b) - 0.01) with similar regression coefficients (R2 = 0.65 to 0.68) for both regressions. According to the obtained relationships, the Hsenwi-Nanting and Red River fault zones revealed low-stress accumulations. Conversely, the Sumatra-Andaman interplate and intraslab, the Andaman Basin, and the Sumatra fault zone were defined as high-tectonic stress regions that may pose risks of generating large earthquakes in the future.

Quaternary geology and sapphire deposits from the BO PHLOI gem field, Kanchanaburi Province, Western Thailand

Abstract One of the most famous blue sapphire deposits in Thailand and SE Asia is from the Bo Phloi District, Kanchanaburi Province, Western Thailand. This paper presents the results of our gemstone investigation as well as establishing the Bo Phloi depositional sequence as one of the Quaternary Type Sections in the region. Relationships among the sedimentology, depositional sequences and geomorphology were investigated in order to understand the gemstone depositional features. Sedimentary structures and textures of the sequences show that the deposition of gemstones is related genetically to fluvial processes. Gemstones are recognized in floodplain and low terrace deposits where gemstone paystreaks concentrate mostly inside layers of gravel beds and foreset-bedded gravels lithofacies. C-14 dating of wood and peat within gemstone-bearing layers indicated that the deposit formed during the middle to late Pleistocene. The gemstone-bearing gravel bed defines a north–south trend along the incised palaeo-channel of an ancient braided river system in the middle part of the basin.

Imaging and locating paleo-channels using geophysical data from meandering system of the Mun River, Khorat Plateau, Northeastern Thailand

Abstract The Khorat Plateau from northeast Thailand, the upstream part of the Mun River flows through clastic sedimentary rocks. A massive amount of sand was transported. We aimed to understand the evolution of fluvial system and to discuss the advantages of two shallow geophysical methods for describing subsurface morphology of modern and paleo-channels. We applied Electrical Resistivity Tomography (ERT) and Ground Penetrating Radar (GPR) to characterize the lateral, vertical morphological and sedimentary structures of paleo-channels, floodplain and recent point bars. Both methods were interpreted together with on-sites boreholes to describe the physical properties of subsurface sediments. As a result, we concluded that four radar reflection patterns including reflection free, shingled, inclined and hummocky reflections were appropriated to apply as criteria to characterize lateral accretion, the meandering rivers with channel-filled sequence and floodplain were detected from ERT profiles. The changes in resistivity correspond well with differences in particle size and show relationship with ERT lithological classes. Clay, silt, sand, loam and bedrock were classified by the resistivity data. Geometry of paleo-channel embayment and lithological differences can be detected by ERT, whereas GPR provides detail subsurface facies for describing point bar sand deposit better than ERT.

Geological and engineering analysis of residualsoil for forewarning landslide from highland areain northern Thailand

Abstract One devastating landslide event in northern Thailand occurred in 2006 at Ban Nong Pla village, Chiang Klang highland of Nan province after, a massive amount of residual soil moved from upstream to downstream, via creek tributaries, into a main stream after five days of unusual heavy rainfall. In this paper, the geological and engineering properties of residual soil derived fromsedimentary rocks were analyzed and integrated. Geological mapping, electrical resistivity survey and test pits were carried out along three transect lines together with systematic collection of undisturbed and disturbed residual soil samples. As a result, the average moisture content in soil is 24.83% with average specific gravity of 2.68,whereas the liquid limit is 44.93%, plastic limit is 29.35% and plastic index is 15.58%. The cohesion of soil ranges between 0.096– 1.196 ksc and the angle of internal friction is between 11.51 and 35.78 degrees. This suggests that the toughness properties of soil change when moisture content increases. Results from electrical resistivity survey reveal that soil thicknesses above the bedrock along three transects range from 2 to 9 m. The soil shear strength reach the rate of high decreases in the range of 72 to 95.6% for residual soil from shale, siltstone and sandstone, respectively. Strength of soil decreaseswhen the moisture content in soil increases. Shear strength also decreases when the moisture content changes. Therefore, the natural soil slope in the study area will be stable when the moisture content in soil level is equal to one, but when the moisture content between soil particle increases, strength of soil will decrease resulting in soil strength decreasing.