Assessing Multiple Liquefaction Events in the Dolai Syncline Valley: IndoBurma Ranges, NE Bangladesh
Abstract
Sediment liquefaction depends on geologic-geomorphic-hydrologic site conditions, and the strength and duration of earthquake shaking. Alluvial areas of the Ganges-Brahmaputra Delta (GBD) have experienced widespread liquefaction. Excavations reveal sand dikes linked to both large and smaller earthquakes. The seismicity is associated with active faults that accommodate convergence between India, Eurasia and IndoBurma plates. On January 3, 2017 a Mw5.7 earthquake occurred 32km beneath Manu in Tripura, India. In NE India and Bangladesh, the earthquake damaged buildings and caused liquefaction, which we document in the Dolai valley 40km NNW from the epicenter. Dolai is a syncline valley in the IndoBurma subduction zone that is accreting the GBD. For the Tripura earthquake, ground motion was amplified 8-25x as documented at a seismograph on alluvial sediment, relative to another on an anticline, 53 and 55km from the epicentre.
To explore liquefaction potential, on March 2017 we excavated 3 trenches and drilled 10 45m deep wells across the 5km wide Dolai valley. In the trenches we identified a much larger paleoliquefaction event dated by optical stimulated luminescence. This older event was crosscut in 2017 by narrow 1cm dikes, which were partially filled with vf sand despite much vented sand. Well logs and C14 ages shows a characteristic stacking from the base to the top: 5-10m of Late Pleist. mud interbedded with sand; 5-10cm of earliest Holocene red, oxidized, indurated clay capped by gravel; early Holocene sand (10-15m) capped by mid-late Holocene sand interbedded with mud (5-10m). We interpret the stratigraphy as due to river entrenchment during the late Pleist. sealevel lowstand, and fluvial aggradation during the Holocene transgression and highstand. Sand encased in mud is liquefaction prone: when subjected to shaking, the saturated sand increases pore pressure, and fluidizes before breaching the mud cap. The isolated channel sand encased in overbank mud of the uppermost stratigraphy is liquefaction prone. Future will focus on drilling wells in other syncline valleys of the IndoBurma ranges to evaluate our hypothesis. Bangladesh is densely populated and rapidly urbanizing. A greater understanding of liquefaction potential will help estimating earthquake hazard and designing measures for risk reduction.- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2018
- Bibcode:
- 2018AGUFM.T23C0381M
- Keywords:
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- 8104 Continental margins: convergent;
- TECTONOPHYSICSDE: 8169 Sedimentary basin processes;
- TECTONOPHYSICSDE: 8175 Tectonics and landscape evolution;
- TECTONOPHYSICSDE: 8177 Tectonics and climatic interactions;
- TECTONOPHYSICS