Recent Paleoseismic Investigations in Northern and Western Thailand

Recent paleoseismic investigations have identified a number of active faults in Northern and Western Thailand. Northern Thailand is an intraplate basin and range province, comprised of north-south-trending Cenozoic intermontane grabens and half grabens, bounded by north- to northwest-striking normal to normal-oblique faults and northeast-striking left-lateral strike-slip faults. The basin-bounding normal faults are marked by steep, linear range fronts with triangular facets and wineglass canyons and have slip rates of 0.1 to 0.8 mm/yr. Based on limited data, the average vertical displacement-per-event is about 1.0 to 1.5 m. These faults are characterized by recurrence intervals of thousands to tens of thousands of years and are capable of generating earthquakes up to moment magnitude (M) 7, and larger. The northeast-striking strike-slip faults are marked by shutter ridges, and deflected drainages. Slip rates are 3 mm/yr or less. Western Thailand is dissected by a number of northwest- and north-northwest-striking, right-lateral strike-slip faults related to the Sagaing Fault in Myanmar. Although showing much less activity than the faults in neighboring Myanmar, these faults display abundant evidence for late Quaternary movement, including shutter ridges, sag ponds, and laterally offset streams. The slip rate on these faults is estimated to be 0.5 to 2.0 mm/yr. These faults are considered capable of generating maximum earthquakes of up to M 71/2

New Insights Into the Paleoseismic History of the Mae Hong Son Fault, Northern Thailand

The Mae Hong Son Fault (MHSF) is a north-trending active fault in northern Thailand. The largest earthquake ever recorded in Thailand occurred in February 1975 with a magnitude of 5.6 and was associated with the southern end of the MHSF. Paleoearthquake magnitudes, recurrence intervals, and slip rates for the MHSF are evaluated using the morphological characteristics of the MHSF aided with a 12.5-m-resolution digital elevation model (DEM) and using fault trenching. Morphotectonic analysis, including studies of offset streams, linear valleys, triangular facets, and fault scarps, helps illustrate dextral fault movements within the MHSF zone. Two separated N–S trending basins, the Mae Hong Son to the north and the Mae Sariang to the south, are present along the MHSF. Between these basins, fault displacements decrease toward the Khun Yuam area. Surface rupture length investigation from fault segments in both basins indicates maximum credible earthquake magnitudes between 5.8 and 6.3. Fault trenching and road-cut studies show that nine earthquakes occurred along the MHSF over the past ∼43 ka. Optically stimulated luminescence (OSL) dating help define the timing of the earthquakes to ∼43, ∼38, ∼33, ∼28, ∼23, ∼18, ∼13, ∼8, and ∼3 ka. The recurrence interval of earthquakes on the Mae Hong Son Fault is ∼5,000 years and the fault has a slip rate of ∼0.04–0.15 mm/a

Missing ophiolitic rocks along the Mae Yuam Fault as the Gondwana-Tethys divide in north-west Thailand

Thailand comprises two continental blocks: Sibumasu and Indochina. The clastic rocks of the Triassic Mae Sariang Group are distributed in the Mae Hong Son-Mae Sariang area, north-west Thailand, which corresponds to the central part of Sibumasu. The clastic rocks yield abundant detrital chromian spinels, indicating a source of ultramafic/mafic rocks. The chemistry of the detrital chromian spinels suggests that they were derived from three different rock types: ocean-floor peridotite, chromitite and intraplate basalt, and that ophiolitic rocks were exposed in the area, where there are no outcrops of them at present. Exposition of an ophiolitic complex denotes a suture zone or other tectonic boundary. The discovery of chromian spinels suggests that the Gondwana-Tethys divide is located along the Mae Yuam Fault zone. Both paleontological and tectonic aspects support this conclusion. © 2004 Blackwell Publishing Asia Pty Ltd

Zircon U-Pb geochronology of the Lan Sang gneisses and its tectonic implications for the Mae Ping shear zone, NW Thailand

The Mae Ping shear zone (MPSZ), a major shear zone trending NW-SE in Thailand, is responsible for the left-lateral displacement of the N-S Triassic-Jurassic granitoid and gneiss belt. This displacement is postulated to have contributed to Cenozoic extrusion tectonics. Within the Lan Sang National Park, the MPSZ exposes several intensely deformed lithologies, collectively known as the Lan Sang gneisses. These gneisses have attracted considerable attention for their potential to substantiate the extrusion model. However, the timing of the emplacement of the orthogneiss protolith is still debated. Moreover, the origin and distribution of the Eocene syn-shearing granodiorite within this shear zone are not well understood. To shed light on the magmatic history of the MPSZ, this study utilized zircon U-Pb geochronology to systematically investigate the Lan Sang gneisses. Our findings demonstrate that these gneisses can be categorized into paragneiss and orthogneiss groups. Paragneiss samples feature zircons displaying rounded detrital cores ranging from 3,078 to 450 Ma, with metamorphic rim overgrowth of ca. 200 Ma (most Th/U <0.01). This indicates that their Paleozoic sedimentary protoliths experienced high-grade metamorphism during the Triassic-Jurassic Indosinian orogeny. On the other hand, zircon from orthogneiss samples shows that their magmatic protoliths were predominantly emplaced either around ∼200 Ma or within 45-32 Ma. The Eocene-Oligocene magmatism likely coincided with the proposed Eocene metamorphism. Since these samples were deformed by left-lateral shearing, the left-lateral motion of the MPSZ probably ended after 32 Ma. Eocene-Oligocene magmatic events have also been identified in granite, mylonite, and gneiss samples from other regions along the Sibumasu terrane, including the Three Pagodas, Klaeng, Ranong, Khlong Marui shear zones, and the Doi Inthanon area. The Eocene-Oligocene magmatism was likely linked with the movement of the shear zones and may be responsible for the regional cooling pattern. The spatial and temporal distribution of the Eocene-Oligocene magmatism within the Sibumasu terrane supports the hypothesis that the inward migration of magmatism in the overriding plate resulted from the shallowing of the Neo-Tethyan slab

Analyses of seismic activities and hazards in Laos: A seismicity approach

The seismic activities and hazards in People’s Democratic Republic Laos were analyzed using the most up-to-date seismicity data. Both the a- and b-values of the frequency-magnitude distribution model, including the return period of earthquake magnitude in the range of 5.0 - 6.0 Mw, were evaluated spatially in a region that ex­tends 300 km from Laos. Six seismic source zones with different seismic activities were found. Based on these seismic source zones and a suitable attenuation model, seismic hazards were then analyzed in both deterministic and probabilistic scenarios. The deterministic map showed a possible maximum ground shaking up to 0.4 g in Northern Laos, whereas the ground shaking calculated from the probabilistic ap­proach was 90, 70 - 90, and 20 - 40%, respectively, and was higher in the northern part. From these seismic activities and hazard analyses, Laos can be clearly separated into the three hazard zones of north­western, northeastern and southern Laos with a high, medium and low earthquake hazard, respectively. Therefore, effective mitigation plans to reduce the impact of seismic hazards should be formulated and in particular for a number of major prov­inces located in the northern part of Laos

Bulletin of Earth Sciences of Thailand One Dimensional Resistivity Sounding for Resolving Gem-bearing Gravel Layer: A Comparison of Schlumberger with Wenner Techniques at Bo Rai, Eastern Thailand

Abstract A direct detection of gem-bearing gravel is very difficult in areas covered by basaltic layer, especially in the eastern Thailand gem field, since the resistivity contrast between the gravel and basalt layers is usually much greater than the resistivity contrast within the bedrock itself. Electrical resistivity surveys are considered to be the most effective method for resolving gem-bearing gravel overlain by basalt environment. Schlumberger and Wenner electrode array configurations were applied to resistivity sounding in order to compare their effectiveness in the eastern Thailand gem field. When compared with a known lithological section in the Bo Rai area, Thailand, both survey arrays in this study were able to detect a high resistivity layer of gem bearing gravel layer, which is located under the basalt. However, the Schlumberger array is clearly more suitable for detecting zones of local gem-bearing gravel layers being more efficient, faster and more economical than the Wenner array. Thus the Wenner array is considered an option only in terrains where Schlumberger sounding cannot be conducted due to a lack of open space

Probabilistic Seismic Hazard Analysis in Thailand and Adjacent Areas by Using Regional Seismic Source Zones

We conducted probabilistic seismic hazard analysis for Thailand and adjacent areas using a method proposed by Cornell (1968). We produced seismic hazard maps showing peak ground acceleration (PGA). Twenty-one seismic source zones covering all of Thailand and extend into adjacent areas were employed. The seismicity data used in this study was a merged data set covering 1963 - 2007 from several international earthquake catalogues and a single Thai catalogue. We selected the strong ground-motion attenuation model for this study by applying several existing attenuation models to recorded strong ground-motion data and choosing the model that best fit our data. Seismic hazard analysis was carried out for 2521 grid points on a 0.25¢X ¡_ 0.25¢X mesh within a rectangle defined by longitudes 92 - 106¢XE and latitudes 0¢X - 21¢XN. The resulting PGA maps for a 2% probability of exceedance for a 50-year time period suggest that ground motion of 0.3 to 0.4 g may occur in northern and western Thailand and from 0 to 0.2 g in other parts of Thailand. The seismic hazard analysis presented here is an important step toward an accurate evaluation of a seismic hazard potential in Thailand and adjacent areas. Further work is needed to refine the analysis. More observations of strong ground motion in the region are needed and further seismo-tectonic research should be encouraged

MOESM1 of Seismic hazards in Thailand: a compilation and updated probabilistic analysis

Additional file 1: 1.txt-75.txt. Earthquake source files containing longitude (degree), latitude (degree), and depth (km), respectively. EQ-parameters.txt. 6 columns of earthquake source parameters used for this PSHA