Ionospheric GNSS Imagery of Seismic Source: Possibilities, Difficulties, and Challenges

Up to now, the possibility to obtain images of seismic source from ionospheric Global Navigation Satellite Systems (GNSS) measurements (seismo-ionospheric imagery) has only been demonstrated for giant earthquakes with moment magnitude Mw >= 9.0. In this work, we discuss difficulties and restrictions of this method, and we apply for the first time the seismo-ionospheric imagery for smaller earthquakes. The latter is done on the example of the Mw7.4 Sanriku-oki earthquake of 9 March 2011. Analysis of 1-Hz data of total electron content (TEC) shows that the first coseismic ionospheric disturbances (CID) occur similar to 470-480 s after the earthquake as TEC enhancement on the east-northeast from the epicenter. The location of these first CID arrivals corresponds to the location of the coseismic uplift that is known as the source of tsunamis. Our results confirm that despite several difficulties and limitations, high-rate ionospheric GNSS data can be used for determining the seismic source parameters for both giant and smaller/moderate earthquakes. In addition to these seismo-ionospheric applications, we raise several fundamental questions on CID nature and evolution, namely, one of the most challenging queries-can moderate earthquake generate shock-acoustic waves? Plain Language Summary Ionosphere is a layer of charged particles of the Earth's atmosphere located at altitudes similar to 60-800 km. However, despite being high above the Earth's surface, the ionosphere is sensible to numerous near-ground geophysical events (earthquakes, tsunamis, volcano eruptions, etc). Acoustic and gravity waves emitted by these events propagate upward and generate atmospheric/ionospheric perturbations. Ionospheric disturbances generated by earthquakes are known as coseismic ionospheric disturbances (CID). Recently, it has been suggested that analysis of CID and their first arrivals can provide information on the position and on the structure of seismic fault ruptured in earthquake. This method is known as ionospheric imagery of seismic source. However, so far, this method has been only applied to giant earthquakes (Mw >= 9.0). In this work, for the first time, we apply the ionospheric imagery for smaller earthquakes on the example of the M7.4 Sanriku-oki earthquake that occurred on 9 March 2011 in Japan. Our results show that this method is applicable to smaller earthquakes, and despite some difficulties, it can indicate the position of coseismic uplift similar to 8 min after the earthquake. The uplift generates CID but also tsunamis. Therefore, our method can be used as independent or complementary one for near-real-time tsunami warning systems.