Results of ultra‐low‐frequency magnetic field measurements during the Guam Earthquake of 8 August 1993

Abstract

We report the results of measurements of ultra‐low‐frequency magnetic noise during a large earthquake (Ms=7.1) at Guam of 8 August, 1993 (depth ∼60 km). The ULF observing system is located in the Guam Island, about 65 km from the epicenter. Several distinct features of this analysis are summarized. (1) We have proposed rather sophisticated statistical analyses (monthly mean, standard deviation) in order to estimate the wave intensity and polarization (i.e. ratio Z/H). (2) A comparison between the ULF wave activity and ΣKp, is useful in distinguishing between the space geomagnetic pulsations and non‐space emissions. (3) Then, the use of the ratio (Z/H) is found to be of essential importance in discrimating the emissions presumably of seismic origin from space plasma waves. (4) The statistical analysis of the temporal evolution of this ratio, has yielded that it shows a broad maximum only about one month before the earthquake, and this suggests that the emissions during this period are very likely to be magnetic precursors. (5) The temporal variation of Z component is similar to that for the Loma Prieta earthquake such that it shows a broad maximum ten days ∼ two weeks before the earthquake and another increase a few days before the earthquake. (6) The emissions presumably associated with the earthquake are of noise‐like nature, and their main frequency is 0.02 ∼ 0.05 Hz (with maximum intensity ∼0.1 nT).

Electromagnetic Precursors to the 2016 Kumamoto Earthquakes

“…In this sense, it is quite difficult to say that there appeared clear seismogenic ULF radiation. The most significant parameter of polarization (P z/h ) (as proposed by [25]) exhibited a significant peak in the end of February and also we can say that some peaks in P z/h exceeding unity tend to appear mainly before the…”

“…There is a ULF observatory at KNY belonging to JMA whose geographic coor- to both the geomagnetic activity (K p ) and EQ index with the use of K LS [24], the 2nd and 3rd panels correspond to the horizontal (H) and vertical (Z) magnetic components, 4th to the polarization (i.e., the ratio of vertical to horizontal component) P Z/H [25], while the 5th panel refers to the inverse of horizontal component (Dh) and the bottom panel refers to the relative depression, δDep. The vertical line is the time of the main shock on 15 April (UT).…”

Some aspects of man-made contamination on ULF measurements

“…These measurements, however, may be interesting also for other aspects of geophysical sciences: for example, geomagnetic field variations in the frequency range of micropulsations provide useful information on the structure of the Earth's crust (Meloni et al, 1985;Pilipenko and Fedorov, 1993;Vellante, 1997;. In addition, local ULF emissions are considered a promising candidate for short-term earthquake predictions (Fraser-Smith et al, 1990Molchanov et al, 1992;Hayakawa et al, 1996;Nagao et al, 2002;Gotoh et al, 2002). Lastly, special care has been dedicated to the possible influence of natural and artificial…”

Changes in total electron content associated with earthquakes (M > 5) observed from GPS station, Varanasi, India

“…Thus, all the presented ionospheric perturbations are associated with the earthquakes. However, it is known that magnetospheric variations may trigger earthquakes (Sobolev et al 2002, Anagnostopoulos et al 2010a and that earthquakes influence the magnetic field (Hayakawa et al 1996b). This can be seen when we make some close correspondence between the various parameters shown in figures 3-5.…”