Does an ionospheric hole appear after an inland earthquake?

Ionospheric disturbances occurred as a result of the tsunami associated with the 2011 M9.0 off the Pacific Coast of the Tohoku earthquake (EQ). The ionospheric disturbances propagated radially from the tsunami source area, termed the traveling ionospheric disturbance. In addition to the traveling ionospheric disturbance, an ionospheric plasma depression lasting for approximately 1 h occurred above the tsunami source area, called a tsunami ionospheric hole. In this study, we compare the ionospheric disturbances caused by large inland and submarine EQs to investigate whether an ionospheric plasma depression only occurs in association with a tsunami. Note that we term an EQ with a tsunami a submarine EQ. To investigate the presence of a plasma depression, i.e., an ionospheric hole, associated with an inland EQ, data on total electron content between the global positioning system satellite and its receivers were used. Comparison of two inland and two submarine EQ events with similar magnitudes around 7 showed that ionospheric holes were observed only for the submarine EQs. This discrepancy might be attributed to the different excitation amplitudes of the atmospheric acoustic waves between the unidirectional fault displacement and the tsunami uplift/depression, corresponding to quarter and one-period variations. From this hypothesis, we predicted that an ionospheric hole could be observed after a significantly large inland EQ with a sufficiently large vertical ground displacement. In fact, we recognized the ionospheric hole generated by the large inland EQ that recently occurred in the Nepal with the magnitude of 7.8 on 25 April 2015.