Search

A new approach is developed to find the source azimuth of the ultra low frequency (ULF) electromagnetic (EM) signals believed to be emanating from well defined seismic zone. The method is test applied on magnetic data procured from the seismoactive region of Koyna-Warna, known for prolonged reservoir triggered seismicity. Extremely low-noise, high-sensitivity LEMI-30 search coil magnetometers were used to measure simultaneously the vector magnetic field in the frequency range 0.001-32 Hz at two stations, the one located within and another ∼100 km away from the seismic active zone. During the observation campaign extending from 15 March to 30 June 2006 two earthquakes (EQs) of magnitude (ML >4) occurred, which are searched for the presence of precursory EM signals. Comparison of polarization ellipses (PE) parameters formed by the magnetic field components at the measurement stations, in select frequency bands, allows discrimination of seismo-EM signals from the natural background ULF signals of magnetospheric/ionospheric origin. The magnetic field components corresponding to spectral bands dominated by seismo-EM fields define the PE plane which at any instant contains the source of the EM fields. Intersection lines of such defined PE planes for distant observation stations clutter in to the source region. Approximating the magnetic-dipole configuration for the source, the magnetic field components along the intersection lines suggest that azimuth of the EM source align in the NNW-SSE direction. This direction well coincides with the orientation of nodal plane of normal fault plane mechanism for the two largest EQs recorded during the campaign. More significantly the correspondence of this direction with the tectonic controlled trend in local seismicity, it has been surmised that high pressure fluid flow along the fault that facilitate EQs in the region may also be the source mechanism for EM fields by electrokinetic effect. [ABSTRACT FROM AUTHOR]

The Chagos-Laccadive Ridge (CLR) is a prominent aseismic topographic feature in the northern Indian Ocean. The evolution and origin of the CLR continues to be a subject of great debate. Geomagnetic depth sounding studies have been carried out in five different Lakshadweep Islands. Transfer functions, summering relationship between anomalous and normal components of transient geomagnetic variations, by virtue of their sensitivity to sub-surface electrical conductivity distribution facilitate to constrain the deep structure and thermal state of the CLR. The induction arrows over a wide range of periods indicates dominance of island effect. The residual induction arrows (obtained after accounting for the Island effect) suggest the concentration of anomalous induced currents in an elongated structure corresponding with track of the Reunion hotspot. Further, the thin sheet modelling of the residual induction arrows brings out a narrow belt of high-conductivity zone close to the eastern margin of the CLR. This is interpreted in terms of partial melt associated with the northward drifting of Indian plate over the Reunion mantle plume (65 Ma). The increase in the longitudinal conductance from north to south can be attributed to the degree of partial melting with younging of the age of volcanic intrusion to the south. [ABSTRACT FROM AUTHOR]