Hydrological and Co-seismic Perturbations in Gravity Field: Example from Central
Himalaya using Superconducting Gravimeter
Baldev R. Arora1, Naresh Kumar1 and Gautam Rawat1
1
Wadia Institute of Himalayan Geology,
33 GMS Road, Dehradun 248 001, India
arorabr@wihg.res.in; Fax +91 135 2525200
Abstract
In 2007, India’s first Superconducting Gravimeter (SG) was installed at Ghuttu,
Central Himalaya. High resolution and high sensitivity gravity measurements are part of
the Multi-Parameteric Geophysical Observatory established to study earthquake
precursors in integrated manner. Time-varying gravity measurements at sub-μGal level
are influenced by tidal forces, atmospheric pressure, changes in water table, moisture etc.
The quantification and reductions of such effects is a challenging task prior to the
residual fields can be searched to isolate the earthquake precursors. The estimation and
elimination of most dominating tidal and atmosphere pressure effects show gravity
residuals ranging up to 30-35 µGal, especially during the rainy periods, The correlation
with precipitation as well as water level changes recording in a 68 meter deep borehole at
about 200m distance from the SG station show that in almost all cases the steep changes
are associated with intense rainfall events while the slow gravity variations are related to
water balance changes as revealed by ground water level fluctuations. While the sharp
gravity changes are concurrent to strong precipitation event, the slow gravity respond to
water fluctuations with time delays from a few days to tens of days. The presentation
shall focus on the success and limitation in quantifying these influences in estimating the
secular gravity changes resulting from active geodynamics, including effect of
accumulating stresses.
However, a careful scrutiny of residual gravity data revealed unambiguous coseismic jump of 2 μGal in relation the Kharshali Earthquake (M=4.9) of July 22, 2007,
strongest earthquake recoded since the establishment of the station. The proximity of
epicenter, only about 50 km, as well as location of SG and epicenter on other side of the
causative fault, were conducive to produce large co-seismic gravity change by
dislocation. Examination of local and distant earthquakes have shown that such gravity
changes are a common feature in many parts of the world but cause of these co-seismic
changes is not clear. High time resolution of data permits to infer that such co-seismic
gravity changes are not coincident with the origin time of earthquakes but are co-seismic
with arrival of surface waves at SG station. This suggests that in addition to dislocation of
fault, possibly propagating shock front induced by high pore-pressure fluids in the hypocentral zone may be alternative source. Demonstrating relation of such co-seismic gravity
change with magnitude, distance, the role of additional data from strain meter, GPS etc in
constraining the alternative source mechanisms for co-seismic gravity changes will be
outlined.
ORAL Presentation