Application of polarization ellipse technique for analysis of ULF magnetic fields from two distant stations in Koyna-Warna seismoactive region, West India

Abstract

Abstract.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 (M L >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 wellCorrespondence to : Gautam Rawat (rawatg@wihg.res.in) 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.

Analysis of the local lithospheric magnetic activity before and after Panzhihua <i>M</i><sub>w</sub> = 6.0 earthquake (30 August 2008, China)

“…It is clear that for far sources (both ionospheric and magnetospheric) the PE major axes ratio is close to unity. For lithospheric source, if its distance to the measuring points is less than 50-100 km, this ratio usually differs from 1 (in detail about selection criteria see Dudkin et al, 2010). The calculation of M-lines' intersection with chosen crustal block for each elementary time-frequency window of dynamical Fourier spectra and the use of selection criteria allow the discrimination of lithospheric sources from ionospheric/magnetospheric ones.…”

“…(Non-lithospheric signals are considered as ionospheric|magnetospheric). This method was introduced by Dudkin et al (2010) and applied to the Koyna-Warna (India) region. In this article, we demonstrate an application of the proposed method for the study of crustal magnetic activity before and after Panzhihua M w = 6.0 EQ which occurred on 30 August 2008 at 8:30:52 UT in the southern part of Sichuan province, China.…”

“…In order to avoid these obstacles a new method of pre-EQ lithospheric magnetic field signal detection has been proposed (Dudkin et al, 2010, see also a recent review of related problems therein). This methodology is based on space selection of these signals with the use of two or more 3-component magnetometers placed at distance 50-100 km from EQ hypocentre and on the calculation of ULF magnetic field polarization ellipse (PE) parameters.…”

“…However the intensity of the ULF lithospheric magnetic field is very weak and often masked by much stronger ionospheric and magnetospheric signals. The study of pre-earthquake magnetic activity before the occurrence of a strong earthquake is a very hard problem which consists of the identification and localization of the weak signal sources in earthquake hazardous areas of the Earth's crust.For the separation and localization of such sources, we used a new polarization ellipse technique (Dudkin et al, 2010) to process data acquired from fluxgate magnetometers installed in the Sichuan province, China. Sichuan is the region of the strongest seismic activity on the territory of China.…”

“…For the separation and localization of such sources, we used a new polarization ellipse technique (Dudkin et al, 2010) to process data acquired from fluxgate magnetometers installed in the Sichuan province, China. Sichuan is the region of the strongest seismic activity on the territory of China.…”