Abstract
We analyzed the ground geomagnetic data obtained from a 3-component fluxgate magnetometer at the Eyrewell Geomagnetic Observatory (New Zealand) (43.474 °S, 172.393 °E) from October 1 to December 31, 2016. The study aimed to investigate electromagnetic precursors associated with the M 7.8 New Zealand earthquake of November 13, 2016. This earthquake occurred 54 km northeast of Amberley (New Zealand). Its epicenter was located 158 km from the Eyrewell Observatory. We used three methods focused on the polarization ratio, fractal dimension and principal component analysis to identify anomalies in the geomagnetic data. The time series showed an enhanced polarization ratio at two times, October 20 and October 30, 2016, i.e. before the occurrence of the New Zealand earthquake, and a value ~1 or more during these instances. Since the global geomagnetic indices Kp and Dst were normal in these cases, the enhanced polarization ratio may be related to the preparation phase of the New Zealand earthquake. To further classify them, we applied the principal component analysis to the magnetic data on component H. The first three principal components showed more than 90 % of the variance of the original ultra-low frequency (ULF) magnetic field time series. The first principal component was found to be well correlated with the storm index (Dst) recorded during this period. Again, the second principal component was dominated by daily variations, which were the periodic component of the recorded ULF magnetic field. The temporal variation of the third principal component was analyzed to verify a possible correlation between the ULF emissions and the occurrence of the earthquake. The fractal dimension of components D and Z of the magnetic data decreased initially and sharply increased three days before the New Zealand earthquake.
Highlights
The proxy related to ultra-low frequency (ULF) mag netic field emissions associated with the earthquakes is be coming more significant among all proxies of earthquake precursors
Even though the radio emissions are generated as a pulse in the earthquake hypocenter, higher frequency components can not propagate over long distances in the lithosphere due to severe attenuation, but ULF waves can propagate up to an observation point near the Earth’s surface with small at tenuation
The plots showing the temporal evolution of the Z/G ratio for the five frequency bands are based on the records taken at the Eyerell Observatory at midnight (23:00–02:00 LT); the time of the New Zealand earthquake is marked by the dashed line
Summary
The proxy related to ultra-low frequency (ULF) mag netic field emissions associated with the earthquakes is be coming more significant among all proxies of earthquake precursors. Many publications have reported a significant increase in ULF emissions before the occurrence of earth quakes [Hayakawa et al, 2007; Hattori, 2004; Rawat, 2014; Chauhan et al, 2012]. The polarization ratio analysis and the fractal dimension analysis were first carried out by Hayakawa et al [Hayakawa et al, 1996] He discovered changes in the ULF magnetic field a few days before the earthquake. [Ida, Hayakawa, 2006] observed anoma lous fractal dimensions of the magnetic field 9–14 days be fore the 1993 Guam earthquake. We used the polarization ratio, fractal dimension and principal component analysis to identify anomalous magnetic data
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