Sand-Venting in the M5.7 Earthquake 3 Jan 2017 and in the Much Larger Penultimate Liquefaction Event with their Sedimentary Setting in an Upstream Valley of the Ganges-Brahmaputra Delta: Implications for Earthquake Hazard
Abstract
The Ganges-Brahmaputra Delta (GBD) is near the cusp between Sunda subduction and Himalayan collision. Abundant water and fertile sediment support a huge population, but large earthquakes along these broad convergence boundaries have repeatedly caused widespread liquefaction and destruction. The 3 Jan 2017 M5.7 32 km deep (USGS) Ambassa (Tripura, India) earthquake accommodated down-dip extension of the Indian slab where it subducts eastward from the GBD below Burma. This is typical for current seismicity below and east of the GBD, although much larger and shallower thrust earthquakes are anticipated based on GPS. Generally, reported effects in the broad mesoseismal area seem consistent with hypocenter depth and the assigned max MMI V (USGS), but we found surprisingly intense damage and many liquefaction sites in the alluvial northern portion of the Dolai valley in Bangladesh, 36 km NNW of the epicenter. We trenched three liquefaction sites and completed a profile of ten 50m deep wells across the 5 km wide alluvial valley. Fluvial channel sands alternate with overbank silt/clay and organic clay layers suggesting frequent changes in river course, consistent with rapid post glacial sea-level rise, transgression, high-stand aggradation and differential tectonic uplift. The Dolai is one of several short low-relief synclinal valleys in the fold belt draining northward into the Sylhet Basin (NE part of the GBD) where they meet westward drainage richer in sediment. Rapid aggradation by this cross-drainage may have a damming effect and account for the current lacustrine/marshy conditions characteristic of the northern end of these synclinal valleys. Organic rich beds derived from such conditions could encourage overpressure and raise liquefaction potential. The 1.5 m deep trenches revealed fractures and clastic dykes <15 mm wide that fed the 2017 sand vents. Their orientations were N-S, subparallel to the valley and nearby river-banks and at high angle to the fold axes. Buried 1.1m below the surface we found similarly oriented but much wider paleo-sand dykes and a lenticular apron of vented sand marking the paleosurface. Similar sand dikes and burial depths in another trench 230 m away suggest these paleo-liquefaction features record the same event. Ongoing OSL and radiocarbon ages will verify timing and implications.
- Publication:
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AGU Fall Meeting Abstracts
- Pub Date:
- December 2017
- Bibcode:
- 2017AGUFM.T43C0710M
- Keywords:
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- 9320 Asia;
- GEOGRAPHIC LOCATION;
- 7221 Paleoseismology;
- SEISMOLOGY;
- 8107 Continental neotectonics;
- TECTONOPHYSICS;
- 8175 Tectonics and landscape evolution;
- TECTONOPHYSICS