Cratonic areas in continental interiors in general are characterized by low rates of stress accumulation and smaller slip rates. Recognizing active structures from such plate interiors is not easy. However, various studies in the cratonic hinterland show that the damaging earthquakes occur on pre-existing faults with a recurrence period of tens of thousands year. Such faults generally do not develop any dramatic fault scarps. In addition to this, weathering and erosion would neutralize any remnant of physiographic evidence of faulting. Even after these difficulties, various studies however established that the deformation related with active tectonism can be identified from careful geomorphological studies using topographic maps, aerial photographs, and satellite images and by field investigation and repeat leveling.
The region around Wadakkancheri is a part of intraplate region of peninsular India. It lies in the vicinity of Palghat Gap, a major physiographic break in western Ghats. The E-W trending Palghat Gap is also coincides with this Gap. Even though no major earthquakes occurred in this area in the historic past, a few earthquakes were reported in the vicinity of Palghat Gap. In addition since 1989 this area is also experiencing repeated earthquake events with smaller magnitudes. Among them the 1994 Wadakkancheri earthquake of M=4.3 was the biggest one recorded in this area which was followed by a number of aftershocks.
The E-W trending Bharathapuzha and its tributaries constitute the drainage network of the Palghat Gap. The lineaments within the gap are generally trend in E-W direction whereas in the southwestern end NW-SE trending lineaments dominate. The studies subsequent to the 1994 earthquake identified the abrupt change in Bharathapuzha river course near Desamangalam and a NW-SE trending structure that influence the course of the river down stream from that point. Further studies identified the NW-SE trending structure as a reverse south dipping fault which moved episodically in the present stress regime. Through ESR dating techniques on fault gouge the last movement along this fault was determined as occurs around 430 ka bp.
Geomorphic studies based on the topographic maps and satellite images identified anomalies related with the fault movements. South of Bharathapuzha signatures of drainage adjustments were picked up by these studies. The occurrence of paleochannels south of Bharathpuzha is another anomalous observation in this area. They are interlinked and abut against the cost parallel sand bars. Presently small drainages occupy these wide paleochannel valleys. Trench studies reveal that the deposits in the paleochannels are of fluvio- laccustrain origin. The studies further reveal that these channels were formed when sufficient water was flowing through it to make the valley wide. Due to lack of running water in the channels at present, which might have supplied by Bharathapuzha earlier, it is unable to cut the cost parallel sandbars across it.
In order to check whether these signatures can be picked up from topography, Shuttle Radar Topography Mission (SRTM) data is used, which has a spatial resolution of 90/90 m. The data has been cropped up for the study area using ERDAS. From this data contour had been generated for an interval of 20m using ARC GIS. Triangular Irregular Network (TIN) has been created from the contour using the 3D analyst tool of Arc GIS. From the TIN Distance- Elevation Profile has been drawn. For the identification of paleochannels as well as present channel from the SRTM data hydrogeological modeling (Hydrology Tool) of ARC GIS software has been used. The output shows an interlinking nature of paleochanels where remote sensing studies show that NE-SW trending channels are wide in comparison to other directions.
For evaluating the topographic features of the study area 21 N-S trending profiles were extracted across the river with an interval of 2 km. These are numbered from left to right. Two more profiles were drawn parallel to the coast and one profile has been drawn along the river. The profile no 1 has been taken nearly 11 km from the coast. Profile no 1, the lowest elevation reached up to Zero where ever it goes over the paleochannels in the southern side of the river. This situation is continued up to the profile no 7. The profiles further show that the valley through which Bharathapuzha is flowing in the vicinity of Desamangalam fault is very wide. Once it crosses the area of influence of the fault the river flows through a narrow zone or the river cuts down further, compared to the upstream side. The profiles across the fault indicate a sudden increase in elevation in the southern side compared to a flat valley in the northern side.
The present exercise shows that a fault can be identified through SRTM data if it is properly used. The change in valley width or a narrow path near the coast may indicate that the Desamangalam fault raised the river basin within its vicinity.