A massive debris flow induced by Typhoon Morakot buried the southern Taiwan village of Siaolin in Jiaxian township and caused the deaths of an estimated 474 people. To reconstruct the mechanisms triggering the tragic debris flow, researchers must identify the subsurface structures of the debris-flow sediments. For this purpose, we conducted 2-D, electrical resistivity imaging (ERI) surveys along networked lines where the village once stood. With the imaging results, we identified three layers, including the basement of Yenshuikeng Shale, the newly accumulated debris-flow deposits; and the old fluvial deposits amid the basement and the debris-flow sediments. According to the resistivity results, the bottom of the debris-flow deposits is under the old ground surface in three eroded areas, C1, C2 and C3. Resistivity anomalies in the debris-flow sediment layer are well correlated with the locations of houses and the major roads in the piling area (P1) and the eroded area (C2). Hence these findings indicate that the basal erosion of the debris flow may have occurred in areas C1, C2 and C3 since a specific mass movement may undercut into the basal sediments or rocks and forms a filled trench in its basement. These eroded areas may be related to different events of mass movements due to their different orientations of basal erosion. From the resistivity image we estimated the volume of debris flow is underestimated for about 24.5 per cent to the estimated volume of the debris flow from digital terrain models (DTMs) in the study area. We conclude that the volume of a debris flow may be underestimated because of the basal erosion if only data from DTMs are used for its calculations and present new means for its correction by combining DTM and ERI results.