Mw Dependence of Ionospheric Electron Enhancement Immediately Before Large Earthquakes

Abstract:

Ionospheric electrons were reported to have increased ~40 minutes before the 2011 Tohoku-oki (M_w9.0) earthquake, Japan, by observing total electron content (TEC) with GNSS receivers [e.g. Heki and Enomoto, 2013]. They further demonstrated that similar TEC enhancements preceded all the recent earthquakes with M_w of 8.5 or more. Their reality has been repeatedly questioned due mainly to the ambiguity in the derivation of the reference TEC curves from which anomalies are defined [e.g. Masci et al., 2015]. Here we propose a numerical approach, based on Akaike’s Information Criterion, to detect positive breaks (sudden increase of TEC rate) in the vertical TEC time series without using reference curves. We demonstrate that such breaks are detected 20-80 minutes before the ten recent large earthquakes with M_w7.8-9.2. The amounts of breaks were found to depend on the background absolute VTEC and M_w, i.e. Break (TECU/h)=4.74M_w+0.13VTEC–39.86, with the standard deviation of ~1.2 TECU/h. We can convert this equation to M_w = (Break-0.13VTEC+39.86)/4.74, which can tell us the M_w of impending earthquakes with uncertainty of ~0.25. The precursor times were longer for larger earthquakes, ranging from ~80 minutes for the largest (2004 Sumatra-Andaman) to ~21 minutes for the smallest (2015 Nepal). The precursors of intraplate earthquakes (e.g. 2012 Indian Ocean) started significantly earlier than interplate ones. We performed the same analyses during periods without earthquakes, and found that positive breaks comparable to that before the 2011 Tohoku-oki earthquake occur once in 20 hours. They originate from small amplitude Large-scale Travelling Ionospheric Disturbances (LSTID), which are excited in the auroral oval and move southward with the velocity of internal gravity waves. This probability is small enough to rule out the fortuity of these breaks, but large enough to make it a challenge to apply preseismic TEC enhancements for short-term earthquake prediction.