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Abstract

Subsidence hazards are common in Vietnam, occurring not only in mountainous regions but also in densely populated delta areas. There have been numerous subsidence incidents recorded in Thanhhoa province in the past. The results of geophysical measurements using the multi-electrode resistivity imaging method show: 1) The study area is located in an area of karst topography which includes a complex network of large and dense underground caves. 2) The geological structure of the study area can be divided into four layers with different resistivity anomalies, including a sedimentary cover ( = 13  39 m), strongly weathered layer ( = 40  150 m), less weathered layer ( = 151  214 m), and bedrock layer ( >214 m); 3) Caves in the research area have 3 characteristics including cleft-shaped caves filled with sediment originating from the soil surface and containing a lot of water with an average depth of 7  15m; sealed caves surrounded by heavily weathered bedrock and no water inside; sealed caves containing water or hydrated material have a high risk of causing subsidence.