Ionospheric anomalies possibly associated with M ≥ 6 earthquakes in Japan during 1998–2011: Case studies and statistical study

Abstract

In this paper, we examine pre-earthquake ionospheric anomalies in time series and perform a statistical test by using total electron content (TEC) derived from global ionosphere maps (GIM). The normalized GIM-TEC (GIM-TEC*), which is computed based on 15 days backward running mean of GIM-TEC, have been investigated for minimizing possible confounding effects of consecutive earthquakes and identify the abnormal signals. Meanwhile, to reduce the effect of strong geomagnetic activities such as geomagnetic storms, a criterion for removing the GIM-TEC data have been adapted; that is Dst index exceeds -60nT. Temporal variations of GIM-TEC* for large and destructive earthquakes in Japan have been studied; which are the 2004 mid-Niigata Prefecture Earthquake (M6.8), its aftershock (M6.1), the 2007 offshore mid-Niigata Earthquake (M6.8), and the 2008 Iwate-Miyagi Nairiku Earthquake (M7.2). Although there are some positive and negative TEC anomalies before and after the four earthquakes, there is a tendency that positive TEC anomalies appear 1-5 days before all the above earthquakes even in the quiet geomagnetic condition. Superposed epoch analysis has been performed for statistical analysis of TEC anomalies associated with M ≥ 6.0 Earthquakes during the 12-year period of May 1998-May 2010. The statistical result indicates the significance of the positive TEC anomalies 1-5 days before earthquakes within 1000 km from the epicenter around Japan.