Rayleigh wave signature in ionograms induced by strong earthquakes

After the magnitude 9.0 earthquake off the Pacific coast of Tohoku (near the east coast of Honshu, Japan), which occurred on 11 March 2011, an unusual multiple-cusp signature (MCS) was observed in ionograms at three ionosonde stations across Japan. To investigate the general characteristics of MCSs, we examined ionograms obtained during the period 1957 to 2011 from five ionosonde stations in Japan after earthquakes with a seismic magnitude of 8.0 or greater. A total of 43 such earthquakes occurred after 1957, as recorded in the database of the United States Geological Survey. MCSs were observed in 8 of 43 earthquakes, allowing us to investigate the specific conditions of MCS appearance. The appearance of MCSs at different epicentral distances exhibits traveling characteristics at a velocity of similar to 4.0 km/s, which is in the range of Rayleigh waves. There is a similar to 7 min offset in delay time after the earthquake at each epicentral distance in the travel-time diagram. This offset is consistent with the propagation time of acoustic waves from the ground to the ionosphere launched by Rayleigh waves. MCSs in ionograms provide a snapshot of vertical disturbances induced by a wave because the sweep time for the whole ionospheric echo traces is much shorter than the propagation time of acoustic waves. The vertical scale of the disturbances is 10 similar to 30 km, which corresponds to an acoustic wave period of 20 similar to 50 s at ionospheric heights.