Abstract
• The branches of the Thakhek Fault zone were first studied in Northeast Thailand. • A single paleoearthquake in the trench (M 7.1 moment magnitude) was examined. • This fault is active based on the inferred age of the displacement events. We conducted a preliminary paleoseismological study at the branches of the Thakhek fault zone in Northeast Thailand. Locating the fault in Thailand is challenging as there are no evident morphotectonic landforms indicative of an active fault. We utilized remote sensing to delineate fault traces, supplemented by geological and geophysical ground-based surveys. We identified a suitable site for trenching through a 2D electrical resistivity tomography (ERT) survey. Trenches were excavated to determine the succession of sedimentary units. The trench-log stratigraphy was compiled and sedimentary samples were dated by optically-stimulated luminescence (OSL) for determining the age sedimentary layers, thereby inferring the putative seismic events. Analysis of the digital elevation model revealed three minor faults extending from Laos into Northeast Thailand with lengths of 138, 159, and 35 km. The ERT survey indicated a clear resistivity contrast, which was interpreted as mudstone with low resistivity juxtaposing sediment layers with higher resistivity. Subsequently, two trenches were excavated. Displacement resulting from reverse faulting appeared to have moved older mudstone over the Quaternary sediment layers with vertical offsets of >3.5 m in the first trench; thrust faulting resulted in a transition fold to fault-bend fold with mudstone ramping over the sediment layers in the second trench. The sediment units cut by the fault plane were dated by the OSL method between 18,120 ± 1920 and 36,960 ± 2780 years in the Pleistocene. Results indicated a single paleoearthquake in the trench with greater than M 7.1 moment magnitude and 0.009 mm/a average slip rate. This fault is potentially active based on the inferred age of the displacement events.
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