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

Abstract:

A dense network of ground Global Navigation Satellite System (GNSS) receivers detected ionospheric total electron content (TEC) increase immediately before the 2011 Tohoku-oki (M_w 9.0) earthquake around the ruptured fault. It is also known that coseismic vertical crustal movements trigger acoustic wave propagating upward, reaching the F-layer of the ionosphere after 8-10 minutes. The wave front makes an N-shaped impulsive signature in TEC time series, often followed by electron depletion lasting for tens of minutes. This postseismic TEC drop has been sometimes thought to have caused artificial TEC increase before the earthquake. Our study robustly estimates three-dimensional distribution of both pre- and post-seismic ionospheric anomalies of the 2011 event by performing tomography of electron density anomalies. We set up > 6,000 blocks over the region covering the Japanese Islands, the Japan Sea, and the Korean Peninsula, as large as 1.0^o (east-west) x 0.9^o (north-south) x 60 km (vertical) up to 870 km altitude. By using TEC residuals of pairs of ~1,200 stations and 8 satellites from reference curves, we estimated electron density anomalies within individual blocks. To stabilize and regularize the solution, we applied two types of constraints, i.e. continuity constraint and altitude-dependent absolute value constraint assuming the Chapman Distribution. We also performed various tests to confirm the reliability of the solution including the checkerboard test and the test assuming realistic pre-seismic anomaly patterns. Our results suggest that the estimated pre- and post-seismic structures do not overlap each other in space (Figure 1). The estimated pre-seismic anomalies are composed of increase (~300 km height) and decrease (~500 km height) of electron density anomalies sitting along the geomagnetic field. They occurred above the land area of Northeast Japan without extending offshore, suggesting that the electric field due to surface electric charges are responsible for the preseismic anomaly. On the other hand, the postseismic ionospheric electron depletion overlapped with the off-shore region where coseismic vertical crustal movements were the largest. This supports its origin by vertical movements of the sea surface above the submarine fault. These results demonstrate that the pre- and postseismic ionospheric anomalies are independent not only temporarily but also spatially and certainly in underlying physical mechanisms.