Numerical modeling of possible lower ionospheric anomalies associated with the May 12, 2015 Nepal earthquake
Abstract
We present the results obtained from numerical simulation of the anomalies in the sub-ionospheric Very Low Frequency (VLF) signal amplitude received at Ionospheric and Earthquake Research Centre and Optical Observatory (IERCOO under Indian Centre for Space Physics), India associated with a major earthquake on May 12, 2015. The signal is received from the JJI transmitter operating at 22.2 kHz before, during and after an earthquake of Richter scale magnitude M = 7.3 occurring at a depth of 18 km at southeast of Kodari, Nepal on May 12, 2015 at 12:50 PM local time. This earthquake was followed by numerous aftershocks, a major aftershock being on May 16 with magnitude M = 6.7. From the received signal, it is observed that the terminator times, both sunrise and sunset (SRT and SST) shifted towards nighttime, thereby increasing the overall day length. The maximum increase in day length is found to be 32 minutes. These shifts in terminator times is numerically reproduced using the Long Wavelength Propagation Capability (LWPC) code. The path under study (JJI-IERCOO) bears a special feature that it has a high longitudinal variation and thus the solar illumination is different at different points on the path. This is taken into account by varying the ionospheric parameters, namely, the effective reflection height (h') and the steepness parameter (β) according to the true movement of the terminators over the path. In this way, the signal amplitudes on a normal non-seismic day is reproduced. Next, to generate the additional effects due to the seismo-ionospheric coupling processes, additional modifications are made for that portion of the propagation path that lie inside the seismically affected area. By doing this, we have been able to regenerate the observed increase in day length. The maximum shift obtained from simulation is 25 minutes. We also found that increase in the ionospheric parameters resulted in the observed night ward shifts of the terminator times. We further calculated the electron density profiles using the Waits formula and found the electron density to decrease significantly before the earthquake.
- Publication:
-
42nd COSPAR Scientific Assembly
- Pub Date:
- July 2018
- Bibcode:
- 2018cosp...42E.561C