Abstract
The paper provides a comparative analysis of precursory phenomena in the ionosphere and atmosphere for two strong earthquakes of the same magnitude M7.1 that happened in the same region (North-East from Los Angeles) within a time span of 20 years, the Hector Mine and Ridgecrest earthquakes. Regardless of the similarity of their location (South-Eastern California, near 160 km one from another), there was one essential difference: the Hector Mine earthquake happened during geomagnetically disturbed conditions (essential in the sense of ionospheric precursors identification). In contrast, the quiet geomagnetic conditions characterized the period around the time of the Ridgecrest earthquake. The Hector mine earthquake happened in the middle of the rising phase of the 23-rd solar cycle characterized by high solar activity, while the Ridgecrest earthquake happened by the very end of the 24th cycle under very low solar activity conditions. We provide a comprehensive multi-factor analysis, determine the precursory period for both earthquakes and demonstrate the close similarity of ionospheric precursors. Unlike the majority of papers dealing with earthquake precursor identification based on the “abnormality” of observed time-series mainly determined by amplitude difference between “normal” (usually climatic) behavior and “abnormal” behavior with amplitudes exceeding some pre-established threshold, we used the technique of cognitive recognition of the precursors based on the physical mechanisms of their generation and the morphology of their behavior during the precursory period. These permits to uniquely identify precursors even in conditions of disturbed environment as it was around the time of the Hector Mine earthquake. We demonstrate the close similarity of precursors’ development for both events. The leading time of precursor appearance for the same region and similar magnitude was identical. For the Hector Mine it was 11 October 1999—5 days in advance—and for 2019 Ridgecrest it was 28 June—7 days before the mainshock and five days before the strongest foreshock.
Highlights
The M7.1 Hector Mine earthquake on October 16, 1999, was preceded by a very strong foreshock, which the whole seismological world experienced after a heated discussion in the pages of the Nature journal, started on 25 February 1999, by Robert Geller of the University of Tokyo [1]
We present results from analyzing the ionosphere and atmosphere for two strong earthquakes of the same magnitude, M7.1 in 1999 at Hector Mine and the Ridgecrest earthquakes of 4 July 2019, which occurred in the same region
Our main emphasis will be put on ionospheric precursors, which were registered by the local network of GPS receivers in California, Global Ionospheric Maps provided by NASA in IONEX format, and ground-based vertical sounding ionosonde at Point Arguello
Summary
The M7.1 Hector Mine earthquake on October 16, 1999, was preceded by a very strong foreshock, which the whole seismological world experienced after a heated discussion in the pages of the Nature journal, started on 25 February 1999, by Robert Geller of the University of Tokyo [1]. At almost the same time, a paper was published that showed that electromagnetic coupling is possible between processes in the earth’s crust and in the ionosphere, where the Global Electric Circuit’s atmospheric electric field (GEC) plays the main role [3] Following this idea, studies were carried out during the past 20 years, which demonstrated that the earth’s crust affects the physical processes in the upper geospheric shells in seismically active regions. We carefully analyzed the arguments of critics of ionospheric precursors, describing the Hector Mine case In that case, they are all based on playing with the amplitude variations of the Total Electron Content (GPS TEC) and attempt to explain all the observed anomalies by high geomagnetic activity in October 1999. Atmosphere 2021, 12, 262 analysis and Ionosphere Mapping, Section 6 focuses on achieving results, and Section 7 reports the final remarks of the work
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