On the Generation of Ionospheric Atmospheric Gravity Waves (AGWs) before Major Earthquakes
Abstract:When the Earth surface heaves during major earthquakes, it acts like the membrane of a drum, sending air pressure waves upward. With increasing height and decreasing air density these waves increase in amplitude, leading to ionospheric perturbations known as Atmospheric Gravity Waves (AGWs). However, prior to many major earthquakes, without any up and down movement of the Earth surface, similar ionospheric perturbations are often recorded above the future epicenters hours to days before the seismic events. The origin of these pre-seismic AGWs is unknown.
In this study, we examine possible formation mechanisms of pre-seismic AGWs based on the findings that, when rocks are stressed, highly mobile electronic charge carriers are activated. Those charge carriers are positive holes, ie. defect electrons in the oxygen anion sublattice. They are able to flow out of the stressed rock volume and able to traverse kilometers of overlying rocks. At the Earth surface they cause microscopic but steep electric fields, capable of field-ionizing air molecules.
This process leads to the injection of massive amounts of predominantly, often exclusively, positive airborne ions at the ground-to-air interface, often over a wide area around the future epicenter. The air ionization is not a steady process but tends to occur in bursts, lasting from seconds in laboratory experiments to minutes to tens of minutes in the field. The introduction of positive ions at the Earth surface is bound to cause a variety of effects, including electric phenomena and – importantly – air pressure pulses, which will propagate upward. We describe early theoretical work on the injection of positive airborne ions, on their upward movement through the atmospheric column, and on how this process may cause pre-seismic AGWs in the ionosphere.