Geoelectric characterisation of the junction of seismically active Delhi Hardwar Ridge and Delhi Sargodha Ridge.

A magnetotelluric (MT) geophysical survey for the first time has been conducted for the geoelectric characterization of the junction of the contact zone of NNE-SSW striking Delhi Hardwar Ridge (DHR) and NW-SE trending Delhi Sargodha Ridge (DSR) in the Rohtak area, Haryana which has experienced 15 earthquakes of M2.0-M4.4 from April to August 2020. A total of 08 MT sites are acquired along a NW-SE profile of length 50 km. From the 2D MT data inversion, the DHR and DSR are for the first time characterized by equal values of moderate resistivity of 100 Ohm m at two depths. The resistivity variation for DHR corresponds to 100 Ohm m from the surface to the depth of 20 km, whilst DSR is found associated with the same value of resistivity extending in the NW direction. The DHR has been found striking NE-SW with a very shallow central axis (less than 400 m) having a width of 12-15 km forming half grabens on both limbs supported by shallow faults. The DSR has been found bifurcated from DHR at a depth of 12-13 km and extended in the NW direction. The DSR has been generated due to flexure bulging caused by collision and anticlockwise rotation of the Indian plate in the Eocene period. A NE striking steep dipping reverse fault (F1) has also been identified about 15 km west of the DHR. It is inferred that the DSR got upthrusted along this fault and became shallower in the NW region. The seismicity in the Rohtak and surroundings is located at the bifurcation points of DHR and DSR and the contact zone of DSR and reverse fault F1. The reverse fault F1 is also active and has generated microseismicity in the past.

Radon variability in borehole from Multi-Parametric Geophysical Observatory of NW Himalaya in relation to meteorological parameters.

Radon has been measured continuously at an interval of 15 min at 10 m depth in a 68 m deep borehole. Five years of high resolution data of radon sampled at 15 min shows a complex trend with strong seasonal and diurnal trends. This temporal variation of radon has high emanation during summer and low concentration in winter. Diurnal radon variability was observed mainly during March-June and September-November each year. The existence of these diurnal periodicities in radon is related to the borehole temperature gradient i.e. difference between external atmospheric and internal borehole temperature at 10 m depth in the air column. The measured radon values are characterized by a diurnal cycle with a maximum in the afternoon and a minimum in the morning in all the seasons except in winter and during rainy periods. Minimum radon variations are recorded in the winter months primarily in December and January. Sudden unsystematic jumps in radon counts are observed each year in the rainy period (July and August). The atmospheric temperature was found to positively correlate with radon emanation. The data set of the borehole indicates a good correlation between atmospheric temperature and radon concentration that is observed throughout the year except in the rainy season. The spectra of radon and atmospheric temperature time series of 5 years data clearly show prominent and clear peaks at 1 and 2 cycles per day.