3-D Tomography of the Ionospheric Anomalies Immediately Before and After the 2011 Tohoku-Oki (Mw9.0) Earthquake

摘要

A dense network of ground global navigation satellite system receivers detected ionospheric total electron content (TEC) changes starting ~40 min before the 2011 Tohoku-oki (Mw9.0) earthquake around the ruptured fault, together with the long-lasting postseismic TEC drop. In this paper, we robustly estimate three-dimensional (3-D) distribution of both preseismic and postseismic ionospheric anomalies of the 2011 Tohoku-oki earthquake by tomographic inversions of electron density anomalies. We set up >6,000 blocks, as large as 1.0° (east-west) × 0.9° (north-south) × 60 km (vertical), over the Japanese Islands, the Sea of Japan, and the Korean Peninsula, up to 870 km altitude. By using TEC anomalies of pairs exceeding ~1,300 stations and eight satellites obtained using reference curves, we estimated electron density anomalies within individual blocks. The results showed that the preseismic and postseismic anomalies do not overlap in space. The preseismic anomalies are composed of low (~300 km height) positive and high (~600 km height) negative anomalies. They occurred above the land of NE Japan without extending offshore, suggesting its origin related to surface electric charges. On the other hand, the postseismic electron depletion occurred offshore above the region where large coseismic uplift took place. These results demonstrate that the preseismic and postseismic ionospheric anomalies are independent not only temporarily but also spatially and certainly in underlying physical mechanisms. We propose a simple model to explain how surface charges redistribute ionospheric electrons to make the observed preseismic electron density anomalies.