The 1930 Kern County earthquake (California, USA) resulted in an increased groundwater discharge a few hours after the earthquake the latter remained high for at least 2 months (Sibson 1992). In addition, there has been a report of a dramatic increase in groundwater discharge near the fault after a major earthquake (Sibson 1981). In terms of the movement of fluids associated with fault movement, the relationship between a quartz vein and/or gold deposit in a mine and the movement of a high-angle reverse fault has been reported in detail (e.g., Sibson 19). ( 2004) showed that the aftershock activity of M5.7 and M6.0 can be explained by time-dependent changes in fluid pressure due to the diffusion of CO 2 fluid rising from depths under high pressure at depth. These seismic activities consist of relatively small earthquakes however, the relationship with fluids has also been discussed for the aftershocks of large earthquakes (Miller et al. ( 2018) to successfully explain the observed data as being due to the decrease and subsequent recovery of fault strength that accompanied the rise and diffusion of high-pressure fluid from the lower crust. Furthermore, a numerical simulation using a rate- and state-dependent friction law and considering pore-fluid pressure change allowed Yoshida et al. Yoshida and Hasegawa ( 2018) found deep to shallow hypocenter migration and temporal changes in stress drop and fault strength to be related to the swarm earthquakes that started in the Tohoku inland region seven days after the 2011 Tohoku-oki earthquake. The relationship between fluid and natural earthquake swarm activities also has been investigated (e.g., Yukutake et al.
Spatiotemporal changes in fluid pressure in the source region of the induced earthquakes were also estimated only from the focal mechanisms of the induced earthquake (Terakawa et al. In fluid-injection experiments in Basel, Switzerland, the occurrence of induced earthquakes was explained by using estimated crustal stresses together with fluid pressure (e.g., Mukuhira et al. Evaluation of fluid-injection experiments conducted in the Rangely oilfield (Colorado, USA) has empirically and quantitatively confirmed the relationship between fluid pressure and earthquake occurrence (Raleigh et al. According to this hypothesis, effective normal stress decreases as pore-fluid pressure increases, causing the fault to become slippery (Hubbert and Rubey 1959).
The Hubbert–Rubey law is a famous hypothesis concerning the mechanism by which fault strength is reduced. In extreme cases, an earthquake may occur due to a decrease in the fault strength even if the shear stress is constant. When the shear stress acting on a fault approaches the fault strength, the shear stress either increases or the fault strength decreases. Therefore, to forecast earthquakes, it is important to clarify the spatiotemporal changes in the stress field near the fault as well as its strength. These observations suggest that the aftershock activity of the Central Tottori Prefecture earthquake was controlled mainly by stress concentration rather than strength reduction due to high fluid pressure.Īn earthquake occurs when shear stress acting on a fault approaches the strength of the fault. However, the temporal change in these parameters was not apparent at depth. If fluid rises from the lower crust due to fault rupture, the locations of aftershocks and focal mechanisms may change over time, especially in the deepest part of the aftershock region. The distributions of aftershock hypocenters and T-axis azimuths are basically temporally stable, except those in limited portions in the shallow layer near the western edge of the aftershock area, where rapid decrease of aftershocks with T-axis azimuths of WSW to west was observed.
We then investigated the temporal changes in the spatial distributions of hypocenters and T-axis azimuths of focal mechanisms.
We determined the hypocenters and focal mechanisms of the aftershocks very precisely in the period from October 22 to December 15. To clarify the relationship between earthquake occurrence and fluid, we analyzed data from a dense aftershock-observation network with 69 high-gain short-period seismographs installed immediately after the mainshock occurrence (October 21) in the aftershock area of the 2016 Central Tottori Prefecture earthquake.
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