Abstract
The 1965–1967 Matsushiro earthquake swarm in central Japan exhibited two unique characteristics. The first was a hydro-mechanical crust rupture resulting from degassing, volume expansion of CO2/water, and a crack opening within the critically stressed crust under a strike-slip stress. The other was, despite the lower total seismic energy, the occurrence of complexed seismo-electromagnetic (seismo-EM) phenomena of the geomagnetic intensity increase, unusual earthquake lights (EQLs) and atmospheric electric field (AEF) variations. Although the basic rupture process of this swarm of earthquakes is reasonably understood in terms of hydro-mechanical crust rupture, the associated seismo-EM processes remain largely unexplained. Here, we describe a series of seismo-EM mechanisms involved in the hydro-mechanical rupture process, as observed by coupling the electric interaction of rock rupture with CO2 gas and the dielectric-barrier discharge of the modelled fields in laboratory experiments. We found that CO2 gases passing through the newly created fracture surface of the rock were electrified to generate pressure-impressed current/electric dipoles, which could induce a magnetic field following Biot-Savart’s law, decrease the atmospheric electric field and generate dielectric-barrier discharge lightning affected by the coupling effect between the seismic and meteorological activities.
Highlights
Kuribayashi, Collections of Matsushiro Earthquake Center14. (A) The earthquake lights (EQLs) observation site: (B) the atmospheric electric field (AEF) observation site (Matsushiro Seismological Observatory), JMA16: (C) the geomagnetic observation station by ERI, University of Tokyo[3,4,5,6,7]: (D) the levelling resurvey point where the maximum upheaval movement was observed by ERI6: (E) the water outflow observation sites at the Kasuga hot spring[17]
Yellow circle: the EQL area estimated from Photo 3 with a fisheye lens
The present results suggest that as many water/CO2 gas-bearing pores are distributed in the shallow seismic zone, the dipole generation gave rise to short-term transient and temporally decaying electrification at the ground level
Summary
Since the seismic activities of the Matsushiro earthquake swarm occurred over a long period, i.e., 1965–1967, a considerable amount of reliable data has been reported on the seismic activities[1,2,3,4,5,6] and the complexed seismo-EM activities[7,8,9,10,11,12,13,14,15,16] as given in Figs 1,2 and 3, where the source of the map in Fig. 1 is Geospatial Information Authority of Japan website (http://maps.gsi.go.jp/#12/36.553224/138.221397/&base=std&ls=std%7C_ort&disp=1 1&lcd=_ort&vs=c1j0l0u0f0) created from the map data source: Landsat[8] mosaic image (GSI, TSIC, GEO Grid/AIST), Landsat 8 image (courtesy of the U.S Geological Survey), Geological Information Authority of Japan. Insert in the upper right: I (field) vs S(field) of 0.45 and 0.9 km[2] in the fault zone as estimated using Eq (1) and (c) the fault model showing a dipole current generated by the coupling interaction between CO2 gases and fracutring rocks.
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