Abstract
Abstract. Indonesia is one of the most seismically active regions in the world and mitigation of seismic hazard is important. It is reported that Ultra low frequency (ULF) geomagnetic anomalies are one of the most convincing phenomena preceding large earthquakes (EQs). In this paper we have analyzed geomagnetic data at Pelabuhan Ratu (PLR) (7.01° S, 106.56° E), Sukabumi, West Java, Indonesia, from 1 September 2008 to 31 October 2010. There are twelve moderate–large EQs (M ≥ 5) within 160 km from the station during the analyzed period. The largest one is the M =7.5 EQ (depth = 57 km, epicentral distance = 135 km, 2 September 2009) based on EQ catalog of Indonesian Meteorological, Climatological and Geophysical Agency (BMKG). To investigate the ULF geomagnetic anomalous variations preceding all the EQs, spectral density ratio at the frequency range of 0.01 ± 0.003 Hz based on wavelet transform (WT) and detrended fluctuation analysis (DFA) have been carried out. The spectral density ratio results show the enhancements a few weeks before the largest EQ. The enhancement persists about one week and reaches a maximum on 16 August 2009. At the same time, the result of the DFA presents the decrease of α value. For other EQs, there are no clear increases of the spectral density ratio with simultaneous decrease of α value. When these phenomena occur, the value of Dst index shows that there are no peculiar global geomagnetic activities at the low latitude region. The above results are suggestive of the relation between the detected anomalies and the largest EQ.
Highlights
Electromagnetic phenomena have been considered as a promising candidate for the short-term prediction of large earthquakes (EQs)
The purpose of this paper is to evaluate whether there are Ultra low frequency (ULF) electromagnetic anomalous variations associated with large EQs occurred in Indonesia, especially in Java Island region or not
The analysis of spectral density ratio based on wavelet transform (WT) and detrended fluctuation analysis (DFA) have been carried out for a single station in this research to detect anomalous variations
Summary
Electromagnetic phenomena have been considered as a promising candidate for the short-term prediction of large earthquakes (EQs). There are three types of measurement for electromagnetic phenomena. They are (1) passive ground-based observations for lithospheric emissions in a wide frequency range (from DC to microwave frequency); (2) ground-based observations with the use of transmitter signals as active monitoring of seismo-atmospheric and seismoionospheric perturbations; and (3) satellite observations of plasma perturbations, thermal anomalies and radio emissions associated with EQs in the upper atmosphere (e.g., Molchanov and Hayakawa, 2008; Hayakawa, 2009, 2012). The usage of the ULF range gives an advantage of detecting precursory EQ signatures due to deep skin depth
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