Investigating seismoionospheric effects on a long subionospheric path

We examine the possibility of earthquake precursors influencing the subiono spheric propagation of VLF transmissions. We consider the long (12 Mm) path from northeastern United States to Faraday, Antarctica (65 degrees S, 64 d egrees W), during 1990-1995 and investigate the subionospheric amplitude va riation of signals from the NAA communication transmitter (24.0 kHz, 1 MW) in Cutler, Maine, with particular emphasis on possible changes induced by s eismic events occurring in South America. We have analyzed the changes in t iming of modal minima generated by the passage of the sunrise terminator ov er the Andes, i.e., the "VLF terminator time" (TT) method. The anomalous va riations in timing throughout the year are of a size and occurrence frequen cy similar to those previously reported, i.e., +/-0.5-1 hour and 1-2 per mo nth. However, we find that, in these anomalous cases, the time of the sunri se modal minimum does not change significantly, but rather, the minimum bec omes insufficiently deep to be detected, and the time of the next nearest, minimum is logged. Our analysis indicates that the occurrence rate of succe ssful earthquake predictions using the TT method cannot be distinguished fr om that of chance. Additionally, the level of false earthquake prediction u sing the TT method is high.