Title	Magnetotelluric investigations in Sikkim
Participants	Prasanta K. Patro, K. Chinna Reddy, Narendra Babu and K. Shankaraiah
Sponsoring Agency	MoES

Work Description:

Sikkim region falls between Main Boundary Thrust (MBT) and Main Central Thrust (MCT). In 1988, Sikkim witnessed magnitude of 6.6 earthquake. It is believed that the seismic activity is confined to the MBT and MCT in the Sikkim Himalaya. With an aim to understand the electrical signatures of the thrust zones such as Main Frontal Thrust, MBT and MCT, NGRI acquired 18 broad-band (0.001 to 1000s) magnetotelluric stations in 2005, (Patro and Harinarayana, 2009) from Siliguri (in the south) to Yumthang (in the north).

In 2006, Sikkim region is struck with another moderate earthquake of magnitude 5.3.In order to understand the conductivity changes associated with the earthquake, MT data was acquired at 20 MT (0.001 to 1000 s) and 6 LMT (100 to 12000 s) locations during Sep, 2011 to Dec, 2011 field season (Fig 10.1). At each station, broad band data was acquired for 3 days. LMT data was acquired for 8 to 11 days. Remote reference recording was carried during the data acquisition. One of the main objectives of the study was to re-occupy old stations using same kind of instruments for same frequency range. We could successfully reoccupy some of the old stations while we could not re-occupy a few stations due to growing civilization (see Fig 10.2) and/or damage/closure of roads due to 18th September 9 Mw earthquake.The preliminary data analysis results are presented in the fig 10.3 at a site ‘ran’. In this figure, for a comparison the MT transfer function computed during 2005 is also presented.

Fig.10.1:
Location map of MT and LMT stations in Sikkim Himalayan region, India.

Fig.10.2:
Pictures to the left shows the old site now turned to a hydro power project. The picture to the right is a site in the road side jungle.

Fig.10.3:
Pictures to the left shows the old site now turned to a hydro power project. The picture to the right is a site in the road side jungle.