Coseismic ionospheric disturbance of the large strike-slip earthquakes in North Sumatra in 2012: M_w dependence of the disturbance amplitudes

We studied ionospheric responses to the 2012 April 11 M_w 8.6 North Sumatra earthquake using total electron content (TEC) measurements with the regional Global Navigation Satellite System network. This earthquake ruptured the oceanic lithosphere off the Indian Ocean coast of North Sumatra, and is known as the largest strike-slip earthquake ever recorded. Coseismic ionospheric disturbances (CIDs) with rapid TEC enhancement of a few TEC units propagated northward with a speed of acoustic waves (~1 kms^-1). Resonant atmospheric oscillation with a frequency ~4 mHz have been found as monochromatic oscillation of TEC lasting for an hour after the main shock and the largest aftershock. We compared CID amplitudes of 21 earthquakes world-wide with moment magnitudes (M_w) 6.6-9.2. They roughly obeyed a law such that CID amplitude increases by two orders of magnitude for theMw increase of three. The 2012 North Sumatra earthquakes slightly deviated negatively from the trend possibly reflecting their strike-slip mechanisms, that is small vertical crustal movements for their magnitudes.