Monitoring of buried faults and their role on the groundwater flow in the Urmia plain

The Urmia plain at west of the Lake Urmia, is situated northwest of Iran. The effect of fault activity can be distinguished in the surrounding rock units but, evidence of faulting not recorded in the Quaternary unit due to low competency of them. The displacement of the rock units proposed a normal mechanism to the faults in the studied region. The modeling of faults and groundwater flow conducted using 160 borehole data with a total 1900m excavation in the 61 points of the study area, 12 trench study and finally 4 cross-sections. The results show sandstone at a depth of 3 to 6m, marl at 5 to 12m, and gravel at 8 to 16m. The groundwater flow direction is from south to north that at the hanging wall of fault the water table depth decreased near the surface. On one cross section (DD'), gravel is at depth 30m in the hanging wall of fault (BH23) but in the footwall at depth 14m (BH40) indicating a hidden fault in this part of the plain. The water table was encountered at 6m in log 18 and at 19m in log 23 also suggesting displacement by a hidden NW-SE fault with southwest dip. In general, in four cross sections and with correlation of the stratigraphic units and tracking of water table changes, 19 hidden faults were identified mostly having NW-SE trends and dips to SW.