The research has been carried out to localize the possibility of the suspected leakage area based on microtectonic study. Toba Lake is the largest lake around Indonesia Archipelago; surface water area is about 1.072,16 km2 that was occurred by volcano tectonic depression. The lake is an important asset of North Sumatera Province for tourism and drinking and irrigation water resources. During the last decade the water level of Toba Lake was decrease until 2,86 m th in 1994. There was a hypothetic that the decreasing of the water hypothetic that the is caused by water leakage within the base of the lake that probably resulted from young tectonic activities. This second research period consists of a geological mapping, tectonic element identification and element data collecting, microtectonic analysis and geological & technical synthesis. The results of this research indicates that the fault N 154 - 164 and N 99 - 111 as relatively young age faults and cut younger and older lithology, but the fault do not cut the old river deposit and the alluvial deposit. At one of the locations of the old river deposits was found soils with thickness 0,20 meters, it means that the youngest tectonic were occurred at about two hundred years ago. Some spring water which are found at Tiga Dolok - Pematang Siantar - Kisaran area probably related to the recharge area around of Toba Lake or related to the leakage of Toba Lake.
* Staf P2BGGN
** Staf ITM, Medan
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