IASPEI - Picture Gallery

IUGG XXIV General Assembly July 2-13, 2007 Perugia, Italy
(S) - IASPEI - International Association of Seismology and Physics of the Earth's
Interior
JSS014 Poster presentation 2293
Electrical resistivity imaging of seismically active frontal Himalaya
Mr. Gautam Rawat
Geophysics Group
Crustal seismicity in frontal Himalaya, like many seismically belts, exhibits sharp cut-off at depth of 15-
20 km. This plane interpreted as basement thrust fault, separating the top of the underthrusting Indian
plate from the over-riding sedimentary wedge of the Lesser Himalaya also defines the planar zone
where foci of number of moderate to large earthquakes are aligned. Given the sensitivity of resistivity to
rheology, magnetotelluric measurement are undertaken to study deep crustal structures and their
possible linkage to the space-depth distribution of seismicity. Eighteen MT sites were occupied along a
profile along Bijnaur - Mallari encompassing the major litho units of Indo-Gangetic Plains, Siwalik, Lesser
Higher and Tethys Himalaya, separated respectively by the major thrusts of Himalaya namely HFT,
MBT, MCT and STD. The tensor decomposition of robust impedances for the period band of 10 Hz -
1000 s suggest electrical strike coinciding with geological fabric. The most conspicuous feature of the
inverted resistivity section is the low resistivity zone at a shallow depth of 10 km beneath the Indo-
Gangetic Plains that dips down at a low-angle and extends as a continuous plane right up to the
northern limit of the profile. The geometry of this low resistivity zone correlates well with the plane
defined by the cutoff depth of seismicity in the region. The low resistivity of the layer may be attributed
to accreted sedimentary rocks overlying the underthrust Indian plate or may mark the brittle-ductile
transition beneath which the fluids generated by dehydration reactions are trapped. This visualization is
supported by the fact the upper crustal section above this low resistivity zone is marked by high
resistivity, interspersed by narrow low resistivity zone coinciding with surface exposure of the HFT, MBT
and MCT. The interpreted resistivity section helps to image these major thrust zones as steeply north
dipping interfaces near surface that flatten out at depth to merge with the mid-crustal low resistive
layer. The paper will discuss the tectonic and rheological significance of the resistivity imaging to
constrain the seismotectonic model of the frontal Himalaya.
Keywords: seismotectonic model, himalaya, resistivity