Abstract
The purpose of this paper is to discuss the lithosphere–atmosphere–ionosphere coupling (LAIC) effects with the use of multiparameter precursor observations for two successive Japanese earthquakes (EQs) (with a magnitude of around 7) in February and March 2021, respectively, considering a seemingly significant difference in seismological and geological hypocenter conditions for those EQs. The second March EQ is very similar to the famous 2011 Tohoku EQ in the sense that those EQs took place at the seabed of the subducting plate, while the first February EQ happened within the subducting plate, not at the seabed. Multiparameter observation is a powerful tool for the study of the LAIC process, and we studied the following observables over a 3-month period (January to March): (i) ULF data (lithospheric radiation and ULF depression phenomenon); (ii) ULF/ELF atmospheric electromagnetic radiation; (iii) atmospheric gravity wave (AGW) activity in the stratosphere, extracted from satellite temperature data; (iv) subionospheric VLF/LF propagation data; and (v) GPS TECs (total electron contents). In contrast to our initial expectation of different responses of anomalies to the two EQs, we found no such conspicuous differences of electromagnetic anomalies between the two EQs, but showed quite similar anomaly responses for the two EQs. It is definite that atmospheric ULF/ELF radiation and ULF depression as lower ionospheric perturbation are most likely signatures of precursors to both EQs, and most importantly, all electromagnetic anomalies are concentrated in the period of about 1 week–9 days before the EQ to the EQ day. There seems to exist a chain of LAIC process (cause-and-effect relationship) for the first EQ, while all of the observed anomalies seem to occur nearly synchronously in time for the send EQ. Even though we tried to discuss possible LAIC channels, we cannot come to any definite conclusion about which coupling channel is plausible for each EQ.
Highlights
Short-term earthquake (EQ) prediction is of vital importance for human beings to mitigate natural disasters [1]
Former works based on many case studies indicate that epicentral distances of 250–300 km are too far to observe such lithospheric radiation [5,72]
This figure indicates that atmospheric gravity wave (AGW) activity was very active in the stratosphere in January and early February before the first EQ
Summary
Short-term earthquake (EQ) prediction is of vital importance for human beings to mitigate natural disasters [1]. 1999 Chi-Chi EQ, the 2004 Sumatra EQ, the 2008 Wenchuan EQ, and the 2011 Japan EQ, led us to believe that there appear definitely electromagnetic precursors as possible candidates for short-term EQ prediction (e.g., [1,7,8,9,10,11]) This conclusion has been recently well evidenced by further statistical studies for various precursors in different regions of the earth, for example, DC and ULF lithospheric radiation (e.g., [12,13]), different atmospheric parameters as mainly observed from space (e.g., [14,15]), and ionospheric perturbations both in the lower part [16,17,18] and in the upper F region (e.g., [19,20]). In addition to those short-term precursors, there have been observed a few imminent (lead time of less than 1 day) precursors [8], including the recent GPS TEC ionospheric perturbation [38]
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