Ahadi, S., N. T. Puspito, G. Ibrahim, S. Saroso, K. Yumoto, and M. Muzli. “Anomalous ULF Emissions and Their Possible Association With the Strong Earthquakes in Sumatra, Indonesia, During 2007-2012”. Journal of Mathematical and Fundamental Sciences, vol. 47, no. 1, Apr. 2015, pp. 84-103, doi:10.5614/j.math.fund.sci.2015.47.1.7.