Anomalous ULF Emissions and Their Possible Association with the Strong Earthquakes in Sumatra, Indonesia, during 2007-2012

Abstract

Abstract. Eleven strong Sumatran earthquakes, with their epicenter less than 550 km away from the Kototabang (KTB) geomagnetic station (2007)(2008)(2009)(2010)(2011)(2012), were studied to examine the occurrence of anomalous ultra-low frequency emissions (ULF-EM). Anomalous ULF signals, possibly associated with the earthquake's precursors, were determined by the Welch ratio S Z /S H at 0.06 Hz at the KTB station. These ULF anomalies were then compared with geomagnetic data observed from two reference stations in Darwin and Davao, to prevent misinterpretation of global geomagnetic disturbances as precursors. This study aims to analyze the relationship between earthquake magnitude and hypocenter radius, and seismic index against lead time during ULF-EM anomalies. We used the polarization ratio Welch method in terms of power spectrum density to evaluate the geomagnetic data by overlapping windows and applying fast Fourier transform (FFT). The results showed anomalous variations in onset and lead time, determined using the standard deviation controlling the S Z /S H power pattern. Our positive correlation between lead time of ULF emission and earthquake magnitude as well as between lead time and seismic index. It shows a negative correlation between hypocenter distances to KTB station against lead time.

Correlations between Earthquake Properties and Characteristics of Possible ULF Geomagnetic Precursor over Multiple Earthquakes

“…Hypocentral depth, h exhibits two correlations, namely with τ and ∆ϑ % , with r = − 0.7949 (strong) and +0.8378 (very strong) respectively. The τ-h pair (Figure 8g) exhibits a negative correlation which agrees with Ahadi et al [22], i.e., for earthquakes with hypocenter located deeper underground, the precursor appears later than shallower ones. Additionally, it was shown from ∆ϑ % -h correlation (Figure 8h) that the estimated azimuthal direction tends to have a larger error when the hypocentral depth is greater.…”

“…The value of r = +0.9562 (very strong) was observed for the τ-M pair (Figure 8c). The positive correlation value suggests that, for greater magnitude earthquakes, the lead time is longer (precursors appear earlier), which corresponds to the study by Ahadi et al [22]. Similarly, K LS also showed a positive correlation with τ and had r = +0.9709 (very strong), which is the highest, where τ was shown to be longer for higher K LS (Figure 9g).…”

“…Meanwhile, the determination of the upcoming earthquake properties can be attained by studying the correlations between the precursor characteristics and the earthquake properties. Prior studies have found several possible correlations between amplitude-epicentral distance [21], lead timemagnitude and lead time-hypocentral depth [22]. Schekotov et al [3] recently showed a linear dependence of maximum ULF depression on the local seismicity index (K LS , see Equation (1)).…”

Assessing the Potential Earthquake Precursory Information in ULF Magnetic Data Recorded in Kanto, Japan during 2000–2010: Distance and Magnitude Dependences

“…The measurement of ultra-low-frequency (ULF: less than one Hertz) electromagnetic phenomena may be one of the most promising candidates, because of their deeper skin depths [ 1 , 2 , 8 , 17 ]. To date, a large number of ULF electromagnetic phenomena associated with earthquakes has been reported from different parts of the world, e.g., Greece [ 18 , 19 , 20 , 21 , 22 , 23 ], Italy [ 24 , 25 ], Russia [ 26 , 27 ]; America [ 28 , 29 , 30 ], Mexico [ 31 , 32 , 33 , 34 ], Taiwan [ 35 , 36 , 37 , 38 ], Indonesia [ 39 , 40 ], China [ 41 , 42 , 43 , 44 , 45 , 46 , 47 ] and Japan [ 48 , 49 , 50 , 51 , 52 , 53 , 54 , 55 , 56 , 57 , 58 , 59 , 60 ]. Furthermore, experiments and simulations have confirmed the existence of ULF seismo-electromagnetic phenomena [ 61 , 62 , 63 , 64 , ...…”

Production of Definitive Data from Indonesian Geomagnetic Observatories

“…one second) as BMKG wishes to detect magnetic anomalies perhaps associated with events e.g. earthquake precursors (Ahadi et al, 2015). BMKG also required all observatories to use the LEMI format (Lviv, 2008) as standard so that the old format (INDIGO format) was replaced.…”