Ground observation of electromagnetic emissions related to clusters of earthquakes

Ultra-low frequency (ULF) and very low frequency (VLF) data obtained from three ground-based experiments at Agra station (latitude 27.2 degrees N, longitude 78.0 degrees E) in India, namely measurement of ULF magnetic field emissions using a three-component search coil magnetometer, the vertical component of VLF electric field emissions with a borehole antenna, and the phase and amplitude of fixed frequency VLF transmitter signals using an Absolute Phase and Amplitude Logger (AbsPAL) receiver, were analysed in search of possible precursors of two major seismic activities that occurred in Sumatra (Indonesia) during the post-tsunami period between January and April 2005. These two major seismic events occurred as clusters of earthquakes during 27-29 January and 28-30 March 2005. The results show that, apart from the borehole antenna, all the experiments showed precursors due to these clusters of earthquakes. Such precursors were not seen in the case of isolated large-magnitude earthquakes. Furthermore, the precursory duration was influenced by a magnetic storm that occurred about a week before the clusters. The mechanism of ULF propagation to long distances between Sumatra and Agra and perturbations in the ionosphere before the clusters are discussed.