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On the signifcance of requisite criteria in detecting pre earthquake ionospheric precursors: a case study of the Tohoku earthquake of March 11, 2011

Chukwuma Victor U., Adekoya Bolarinwa J., Thomas Jewel E., Emah Joseph B., Oloruntola Racheal F.

Acta Geophysica

2021

Vol. 69, no. 4

1545--1566

In the advent of the Tohoku earthquake (Mw 9.1 occurred at 14:46 LT on March 11, 2011), the Japan Meteorological Agency issued earthquake early warning announcements to the general public; however, there were problems with the precision of the early warning systems. This early warning systems issue, and the scale of consequential loss of human, material and technological resources of the Tohoku earthquake and those of about 57 earthquakes with magnitude M ≥ 7.5 from August 2008 to August 2018 around the world compel a need for investigation for detection of some signals before the earthquake that should be examined and interpreted as a precursor by examining the changes in the ionospheric phenomena. In this light, discrimination of pre-earthquake ionospheric anomalies of seismic origin from those of other drivers is essential for understanding, justifcation and physical interpretation of the earthquake precursors phenomena. Presently using a set of criteria that adequately diferentiates pre-earthquake anomaly of seismic origin from those of other sources, this study has shown concordant results with the existing studies namely that foF2 data obtained from 8 Chinese and Japanese stations revealed signifcant enhancements 2–6 days before the Tohoku earthquake and that the anomalies extended to 2774 km from the epicentre, thereby underscoring the necessity of requisite criteria in detecting pre-earthquake ionospheric precursors. Furthermore, the study suggests that the anomalous variations observed during periods of earthquake preparation are of seismo-magnetosphere origin.

Automatic scaling of critical frequency foF2 from ionograms recorded at São José dos Campos, Brazil: a comparison between Autoscala and UDIDA tools

Pezzopane M., Pillat V. G., Fagundes P. G.

Acta Geophysica

2017

Vol. 65, no. 1

173--187

This paper considers a dataset of ionograms recorded by the CADI ionosonde installed at São José dos Campos (SJC; 23.2°S, 45.9°W, magnetic latitude 14.1°S), Brazil, to compare two autoscaling systems: Autoscala, developed by the Istituto Nazionale di Geofisica e Vulcanologia, and the UDIDA-scaling, developed by the Universidade do Vale do Paraíba. The analysis, focused on the critical frequency of the F2 layer, foF2, shows that the two systems work differently. The UDIDA-scaling gives always a value of foF2 as output, regardless of the presence of the ionogram trace and its definition, while Autoscala tends to reject ionograms for which the digital information is considered insufficient. As a consequence, the UDIDA-scaling can autoscale more foF2 values than Autoscala, but Autoscala can discard a larger number of ionograms for which the trace is unidentifiable. Discussions are made on the accuracy of the foF2 values given as output, as well as on the main shortcomings characterizing the two systems.