Abstract
This study reports on a possible very low frequency/low frequency (VLF/LF) subionospheric precursor to a recent earthquake in Japan. As the epicenter of this large Japanese earthquake on March 11, 2011, was located just on the great-circle path from one of our VLF/LF network stations (Chofu) to the NLK US transmitter, we examined the propagation characteristics mainly associated with the signals from the NLK transmitter, as observed at three of the stations in Japan (Chofu, Kasugai and Kochi). On March 5 and 6, 2011, a remarkable anomaly was found on the path from NLK to Chofu, which is highly likely to have been a precursor to this earthquake. The anomaly in the night-time average amplitude at Chofu was characterized by a serious decrease in the signal that exceeded −4v (v: standard deviations). The anomaly was found on the same days on the other propagation paths (from NLK to both Kasugai and Kochi), although it was less enhanced. Finally, this propagation anomaly is extensively discussed with respect to the geomagnetic activity, and we also compare this anomaly with the properties related to the former 2004 Sumatra earthquake that had nearly the same magnitude as this March 11, 2011, earthquake.
Highlights
Very low frequency/low frequency (VLF/LF)subionospheric propagation data have recently been used extensively to monitor lower ionospheric perturbations associated with earthquakes [e.g., Hayakawa 2007, Hayakawa 2009, Hayakawa 2010, Chakrabarti 2010]
Summary and discussion By making full use of our Japanase VLF/LF network, we have defined the following observational results for this large March 11, 2011, earthquake: 1. No definite anomaly was observed for the JJY–MSR path, as the earthquake epicenter was well away from the wave sensitive area of this propagation path
The most serious point for this problem is that the geomagnetic activity might have some influence on our summary
Summary
Very low frequency/low frequency (VLF/LF)subionospheric propagation data have recently been used extensively to monitor lower ionospheric perturbations associated with earthquakes [e.g., Hayakawa 2007, Hayakawa 2009, Hayakawa 2010, Chakrabarti 2010]. A recent study by Hayakawa et al [2010] established a statistically significant correlation of ionospheric perturbations detected as VLF/LF subionospheric propagation anomalies with earthquakes with large magnitudes (>6.0) and shallow depths.
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