Paleoseismlogical and active Fault Inevstigations In

(Rastogi B. K., M. S. Gadhvi, J. N. Malik, A.K. Tyagi, A.K. Singhvi, M. Morino,Institute of Seismological Research ,Raisan, Gandhinagar, Indian Institute of Technology, Kanpur,Physical Research Laboratory, Ahmedabad, Oyo International Corpn.)

Paleoseismology, Geomorphology and dating of recent tectonic features will establish return periods of earthquakes and help in forecasting of earthquakes by determining ages of pre-historic earthquakes in Kachchh and rates of movements along geological faults. For this study, after due observation of land by satellite stereo images and then ground combing in earthquake stricken area of Kachchh, samples have been collected from trenches. Analysis of these samples is done in OSL Lab for determining the ages of the sediments and how they were formed. Paleoseismology studies in more than 25 trenches along with geophysical investigations such as GPR and MASW survey, leveling measurements were carried out along Katrol Hill, Kachchh Mainland and Allah Bund Faults. The study revealed, for the first time in India clear evidences of neotectonic activity and pre-historic earthquakes and an active fault map of Kachchh has been prepared (Fig. 8). The highlight of the study was first to identify pressure ridges along faults by stereo images and then go for trenching. This tremendously increased chances of finding neotectonic features.

Kachchh Mainland: Pressure ridges were observed with 2-3 m height extending in E-W direction near Lodai village on KMF (NNE of Bhuj). In a trench the Quaternary deposits were observed to be widely deformed and three small faults were identified indicating three seismic events with a net slip of about 60-70cm. These are typical reverse faults with dip of 10-60 inclined to south.

At Jhura on KMF (NNW of Bhuj) pressure ridges were identified in the fan deposits of Kaila River from Satellite data. Trenching showed two events on a fault with 70 cm slip for each event (Fig.9). From this a slip of 6 m can be inferred for hard rock at depth. Some 12 m south an older fault indicates similar slip.

An active fault trace demarcating boundary between Katrol Hill and fluvial terrace is observed near Wandhay village. A warping scarp was observed on the terrace. A trench was excavated across this scarp. Three major fault strands were identified in the trench. Three seismic events were inferred along these. The northern fault strand displaces the terrace deposits along all units of the younger sequence except top layer.

The eastern part of KHF is suggested as inactive. The NE-SW extending Bhuj fault at Wandhay village is the active foreland migration of KHF. A trench across the Bhuj fault indicated one event. The fault displaced all sedimentary succession in the trench from Mesozoic rocks to thin channel deposits except the top soil cover

Allah-Bund: Indications of pre 1819 uplift along western segment of Allah-Bund were observed. Numerous paleo-channels recognized from satellite data were found to have uplifted on ground checking. These uplifts are 40 cm to 290 cm and might have occurred < 2ka. [Existence of paleo-channels was pointed out to us by Roger Bilham].

Around Vigukot (10 km north of the western segment of ABF three events were identified in two trenches on the basis of cross-cutting of liquefaction features. A few small or distant events are envisaged on the basis of small sand blows. Dating of charcoal and sediments is in progress

Satellite imageries for the eastern segment of Allah-Bund SSE of Karimsahi indicated younger drainage south of scarp (which is normally not expected). Leveling indicated abrupt slope change (20cm uplift) for a 70 m length. A trench exhibited > 2m upthrusting from north.

In the easternmost section of the Allah-Bund (8 km west of Dharamsala) numerous sand blows and dykes were observed for lengths of 70-80m which are seen to extend to shallow depths in trenches.

ACTIVE FAULT STUDIES IN KACHCHH

Fig. 1: First time ever produced active fault map of Kachchh region. Highlighted areas
indicate locations of trench investigations carried out on three major faults of the area.

Fig. 2: Observed slip along the fault traces in Jhura Trench. Slip of 144 cm observed on fault F1-1 is cumulative slip for the two events.