Abnormal perturbations in the F2 region ionosphere observed prior to the great San Juan earthquake of 23 November 1977

Abstract On 23 November 1977 at 0926 UT great earthquake ( M =7.4) occurred near San Juan city, Argentina; its epicenter was located at geographic latitude 31.1°S and longitude 67.8°W with depth of about 4 km. The earthquake followed by strong aftershocks at 0935 UT, November 23 ( M =6.0) and at 0158 and 0202 UT, November 24 with magnitude of 6.0 and 6.3, respectively. We have examined the ionospheric F2 region behaviour during the 5-day period before the earthquake using the data of ionospheric vertical incidence sounding at the Tucuman observatory (26.9°S, 65.4°W) which is located about 500 km north east of the earthquake epicenter. For comparison, we have also used the ionospheric data obtained at several ionospheric observatories situated much more far away from the earthquake epicenter. The results show that the F2 layer critical frequency f o F2 over Tucuman revealed statistically significant transient decreases (more than 1.5 times the standard deviation) below the f o F2 monthly median about 5 h before the earthquake onset and 15–5 h before its aftershocks of 24 November. The duration of the pre-earthquake transient negative perturbations of f o F2 was about 3 h. The f o F2 variations over the ionospheric observatories far away from the earthquake epicenter did not exhibit any statistically essential perturbations with respect to the f o F2 monthly median during the same 5-day period. Taking into account that this period was very geomagnetically quiet, we conclude that the observed negative ionospheric perturbations over Tucuman were very likely associated with the imminent earthquakes. The possible mechanism which could account for these pre-earthquake ionospheric perturbations is discussed in terms of seismogenic electric field effect on the F2 region ionosphere.