Umesh Gautam

Electrical structure of the Himalaya of central Nepal: High conductivity around the mid-crustal ramp along the MHT

Twelve broadband magnetotelluric (MT) soundings were performed across the Himalaya of Central Nepal in 1996 in order to determine the electrical structure of the crust and its relation to geological structures and active tectonics. The MT impedance tensors were obtained for frequencies between 0.001 and 500 Hz. The 2‐D section, derived from joint inversion of TE‐ and TM mode after RRI and Groom/Bailey decomposition, shows high conductivity in the foreland basin (∼30 Ω.m) that contrasts with the resistive Indian basement (>300 Ω.m) and Lesser Himalaya (>1000 Ω.m). In addition, our MT sounding reveals a major conductive feature beneath the front of the Higher Himalaya, also characterized by intense microseismic activity, and the position of a mid‐crustal ramp along the major active thrust fault (MHT). This high conductivity zone probably reflects metamorphic fluids, released during underthrusting of the Indian basement and pervading well connected microcracks induced by interseismic stress build‐up, or distributed brittle deformation around the ramp.

Seasonal modulation of seismicity in the Himalaya of Nepal

For the period 1995–2000, the Nepal seismic network recorded 37 ± 8% fewer earthquakes in the summer than in the winter; for local magnitudes ML > 2 to ML > 4 the percentage increases from 31% to 63% respectively. We show the probability of observing this by chance is less than 1%. We find that most surface loading phenomena are either too small, or have the wrong polarity to enhance winter seismicity. We consider enhanced Coulomb failure caused by a pore-pressure increase at seismogenic depths as a possible mechanism. For this to enhance winter seismicity, however, we find that fluid diffusion following surface hydraulic loading would need to be associated with a six-month phase lag, which we consider to be possible, though unlikely. We favor instead the suppression of summer seismicity caused by stress-loading accompanying monsoon rains in the Ganges and northern India, a mechanism that is discussed in a companion article.

Dipolar self‐potential anomaly associated with carbon dioxide and radon flux at Syabru‐Bensi hot springs in central Nepal

The Syabru-Bensi hot springs are located at the Main Central Thrust (MCT) zone in central Nepal. High carbon dioxide and radon exhalation fluxes (reaching 19 kg m A2 d A1 and 5 Bq m A2 s A1 , respectively) are associated with these hot springs, making this site a promising case to study the relationship between self-potential and fluids (gas and water) exhalation along a fault zone. A high-resolution self-potential map, covering an area of 100 m by 150 m that surrounds the main gas and water discharge spots, exhibits a dipolar self-potential anomaly with a negative peak reaching A180 mV at the main gas discharge spot. The positive lobe of the anomaly reaching 120 mV is located along the terraces above the main gas and water discharge spots. Several electrical resistivity tomograms were performed in this area. The resistivity tomogram crossing the degassing area shows a dipping resistive channel interpreted as a fracture zone channeling the gas and the hot water. We propose a numerical finite difference model to simulate the flow pattern in this area with the constraints imposed by the electrical resistivity tomograms, the self-potential data, the position of the gas vents, and hot water discharge area. This study provides insights on the generation of electrical currents associated with geothermal circulation in a geodynamically active area, a necessary prerequisite to study, using self-potentials, a possible modulation of the geothermal circulation by tectonic activity. Citation: Byrdina, S., et al. (2009), Dipolar self-potential anomaly associated with carbon dioxide and radon flux at Syabru-Bensi hot springs in central Nepal,

Electric potential variations associated with periodic spring discharge in western Nepal

Electric potential variations were recorded in the vicinity of the Dhor Barahi periodic spring located southeast of Pokhara, western Nepal. This spring flows for a few minutes, with a repetition rate of about 30 min. During water flow, positive electric potential pulses with an amplitude varying from 0.17 to 0.36 mV are observed on the ground surface, and negative pulses with an amplitude of 8.4 mV in the water channel. These observations provide a striking illustration of the electrokinetic effect. Such electric potential measurements may be used in the future to monitor hydraulic systems and may also improve our understanding of electrical effects associated with natural systems.