Subionospheric VLF perturbations observed at low latitude associated with earthquake from Indonesia region

Subionospheric propagation from a Very Low Frequency (VLF) transmitter (VTX, 18.2 kHz) received at a low latitude station Suva, Fiji over a Transmitter-Receiver Great Circle Path (TRGCP) length of 11,400 km has been utilized to identify any possible ionospheric perturbations associated with the earthquakes that occurred in the Indonesia region during the period December 2006-October 2010. Out of five earthquakes that occurred with their epicenter in the fifth Fresnel zone, only an earthquake on 18 December 2006, in the North Sumatra region, has shown convincing evidence of lower ionospheric perturbations on the VTX transmission. The magnitude of this earthquake was 5.8 measured on the Richter scale and occurred at a depth of 53 km with its epicenter located 45 km off the TRGCP. The VLF amplitude data for this earthquake was analyzed using (1) terminator time (TT), (2) average nighttime and daytime amplitude variation, and (3) nighttime fluctuation (NF) methods. The results show that the sunrise TTs deviated considerably in the period 14-22 December 2006 measuring up to ~20 min on the day of the earthquake. The results also show that the average nighttime as well as the average daytime signal amplitudes decreased by about 5 dB and 3 dB, respectively, during the period of the earthquake. The NF method revealed a decline in the trend at least 2 days before the earthquake though not exceeding the 2σ criteria and enhancements in the NF exceeding 2σ mark, however, the normalized values of the trend, NF and dispersion did not reveal an increase above the 2σ marks as reported by previous researchers. This could be due to the very long path length and the prevalence of lightning activity along the TRGCP in the Asia-Oceania Region.