Enomoto et al. Earth, Planets and Space (2021) 73:90 https://doi.org/10.1186/s40623-021-01416-1 EXPRESS LETTER Open Access Laboratory investigation of coupled electrical interaction of fracturing rock with gases Yuji Enomoto1* , Tsuneaki Yamabe1, Shigeki Sugiura2 and Hitoshi Kondo2 Abstract In the coupled electric interaction of rock fractures and gas invasion, that is, when gases interact with newly created crack surfaces, the unpaired electrons within the rock crystal defects are thermally stimulated, released into the crack due to the temperature rise at the crack tip via plastic work, and attached to ambient gas molecules to electrify them in a negative state. Using a working hypothesis that this mechanism is the source mechanism of seismo-electromag- netic phenomena, we conducted laboratory experiments in which rocks were fractured with pressurized N 2, CO2, CH4, and hot water vapour. Fractures were induced by a fat-ended indenter equipped with a fow channel, which was loaded against blocks of quartz diorite, gabbro, basalt, and granite. Fracture-induced negatively electrifed gas currents at ~ 25 °C and ~ 160 °C were successfully measured for ~ 100 μs after full development of the crack. The peak electric currents were as high as 0.05–3 μA, depending on the≥ rock species and interaction area of fractured rock and gas and to a lesser extent on the gas species and temperature. The peak current from fracturing granite, which showed higher γ-ray activity, was at least 10 times higher than that from fracturing gabbro, quartz diorite, and basalt. The results supported the validity of the present working hypothesis, that coupled interaction of fracturing rock with deep Earth gases during quasi-static rupture of rocks in the focal zone of a fault might play an important role in the generation of pre- and co-seismic electromagnetic phenomena. Keywords: Seismo-electromagnetics, Fault valve, Rock fracture, Deep Earth gas, Exoelectron emission Introduction an impermeable barrier to deep Earth fuids (Gold 1987; Geophysical evidence suggests that mantle-derived deep Sibson 1990). An impermeable layer that crosses a high- Earth fuids/gases along fault planes have an important angle reverse fault is called a “fault valve” (Sibson 1990), role in generating earthquakes, and this is signifcant for and deep Earth fuids can be stored in the lower por- modelling earthquake occurrence (Yoshida et al. 2002; tions of a fault valve during interseismic periods. Water Sano et al. 2014, 2016). According to the deep Earth gas has been found in deep scientifc boreholes to depths hypothesis, when water-bearing porous sediment extends up to 10 km, and it is likely that water extends as deep to a great depth, the diferent pressure gradients in water as ~ 20 km in stable crust (Smithson et al. 2000). Mean- and rock form a stepwise pressure distribution that builds while, the seismogenic zone of on-shore earthquakes up in the underground rock–water system. Ten, a pore- commonly also extends to depths of 10–20 km (Sibson collapsed domain develops at a critical depth and forms 1990). When both the tectonic shear stress and the pressure of the reservoir fuids reach a critical level, micro-cracks *Correspondence: [email protected] grow in the fault-valve zone and link with each other. 1 Shinshu University, Ueda Campus, 3-15-1 Tokida, Ueda, Nagano 386-8567, Japan Ten, when the linked cracks breach the fuid reservoir Full list of author information is available at the end of the article barrier, low-viscosity gases, such as hydrocarbons and © The Author(s) 2021, corrected publication 2021.. 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Earth, Planets and Space (2021) 73:90 Page 2 of 9 3He contained in the deep Earth fuids, and then expand- of gas species and rock species, not only at the room tem- ing gases degassed from deep fuids percolate into the perature of ~ 25 °C, but also at an elevated temperature linked cracks, widen them, and weaken the fault zone, of ~ 160 °C, a temperature comparable to that of the seismo- which results in acceleration of unstable ruptures, lead- genic zone (Sano et al. 2014). At this temperature, the rock ing to major earthquakes. In fact, recent observation of still exhibits brittle properties (Kawamoto and Shimamoto helium/argon isotope changes after the 2016 Kuma- 1997), but unpaired electrons are stably trapped in the crys- moto earthquake, which was a high-angle reverse-type tal lattice defects in the rock (Fukuchi et al. 1986). event, suggested that the helium/argon anomaly probably resulted from deep-seated fuids being squeezed through Methods the fault plane by the tectonic stress that caused the Fracture tests earthquake (Sano et al. 2016). Te beginning of a quasi- As illustrated in the schematic view of the experimen- static rupture of the seismogenic fault-valve zone when tal setup in Fig. 1a, a fat-ended V-shaped plate-type the linked cracks breach the fuid reservoir, leading to the indenter made of hardened carbon steel was attached to unstable acceleration of rupture of the whole seismogenic a universal testing machine and pressed against a rock zone, might correspond to an imminent precursor period sample. Te tested rocks were as-received quartz diorite of earthquakes. (from Kanagawa/Tanzawa, Japan), gabbro (from Zim- Te fault-valve model is also assumed to apply at sub- babwe), basalt (from Hyogo, Japan), and fne-grained duction zones, where an asperity on the subducting plate granite (from Fujian, China). Te water content of these is tightly connected to the pore-collapse domain in the rocks was in the range of 0.03–0.15 wt%. Te basic con- accretion wedge. In fact, Kumagai et al. (2012) showed fguration of the equipment is the same as that previously that a strongly localized asperity, possibly a subduct- reported (Enomoto et al. 2017), but some improvements ing seamount, may have been the origin of the mega- were made to conduct the experiment at elevated tem- asperity of the 2011 Tohoku-Oki earthquake. Terefore, perature (~ 160 °C) and under a variety of conditions. when a seamount on an oceanic plate encounters a A square rock sample was loosely clamped to pre- rigid impermeable barrier layer in a continental plate, it vent the edge of the rock from lifting during loading or might create the conditions for fault-valve behaviour to jumping sideways upon rupture. Two sheets of polyte- store deep Earth fuids during an interseismic period. trafuoroethylene (PTFE) 0.4 mm thick were greased for Tis model might also be supported by recent physico- lubrication and placed between the rock and the clamp chemical analyses of deep-sea waters performed after stop surface to control the slippage during fracture so that the 2011 Mw9 Tohoku-Oki earthquake, which showed the initial crack width at the time of fracture was about that 13C-enriched methane and 3He-bearing fuids were 1 mm (see Fig. 1b). At the same, a pressurized gas stored released from deep sub-seafoor reservoirs after the in a fow channel inside the indenter, which had an open mainshock passed through the plate interface in the sub- slit 20 mm long by 1.8 mm wide, was sealed with a PTFE duction zone (Kawagucci et al. 2012; Sano et al. 2014). sheet 0.4 mm thick for experiments at room tempera- When deep gases interact with the newly created crack ture of ~ 25 °C (see Fig. 1b) and lead alloy sheet 0.4 mm surfaces generated in the fault, the unpaired electrons con- thick for ~ 160 °C (not shown). As shown on the right side tained in the rock crystal defects are thermally stimulated of Fig. 1a, heated CO2, CH4, and N2 gases at a pressure (Fukuchi et al. 1986) and released into the open crack due 0.4–0.5 MPa were supplied to the gas fow channel inside to the rise in temperature (~ > 300 °C) at the crack tip, where the indenter from the gas cylinders and passed through heat is generated by plastic work dissipation (Li et al. 1996). a temperature-controlled heat exchanger. On the other Tese electrons then become attached to the gas molecules, hand, hot water vapour was supplied to the fow channel electrifying them to a negative state (Scudiero et al. 1998; from a pressured vessel having a saturated vapor pressure Enomoto 2012; Enomoto et al. 2017). We believe that this at 160 °C. Te indenter pressed the rock during loading at process is the elementary mechanism of the electromag- a crosshead speed of 0.5 mm/min, and immediately after netic phenomena that accompanies an earthquake. Note the rock was subjected to a guillotine fracture at the criti- that the geomagnetic variation caused by the 1965–1967 cal fracture load, the gas fowed into the crack gap and Matsushiro earthquake swarm could be explained quantita- interacted with the newly created fracture surface, result- tively by this model (Enomoto et al.