Figure 10. Temporal evolution of the distances between the
foreshocks and initial hypocenter. Blue circles represent the
hypocenters expressed by the size corresponding to the JMA magnitude.
The black curves show the fluid diffusion models with Dh= 0.01, 0.03, and 0.05 m2/s. Gray straight lines show
the linear spread model with migration speeds of d = 0.001, 0.003, and
0.005 km/h.
The pore pressure diffusion model with a hydraulic diffusion coefficient
of ~0.05 m2/s matches the observations
better than the linear spread model. In previous studies, it has been
estimated that the hydraulic diffusion coefficient in the crust ranges
from ~0.01–10 m2/s (e.g., Talwani et
al., 2007; Shelly et al., 2016; Yoshida & Hasegawa, 2018a), which is
similar to the foreshock migration speed of the M5.3 Kagoshima Bay
earthquake sequence. Based on the linear spread model, the propagation
velocity is ~0.001–0.005 km/h. Based on previous
studies, the migration speed of aseismic slip propagation ranges from
0.1–1.0 km/h (e.g., Lohman & McGuire, 2007; Kato et al., 2016), which
is significantly higher than the migration speed of the present
foreshock activity. If we advance the initiation timing of propagation,
the propagation speed decreases. Thus, according to the migration speed
and spatiotemporal pattern of the foreshocks, the pore pressure
diffusion model better explains the overall migration of the foreshock
hypocenters.
Aseismic creep related to the nucleation process of the mainshock might
be involved in the migration of the foreshocks. In fact, physical
simulations indicate interseismic creep in seismogenic patches from
external stable-slip regions before the occurrence of unstable slip (Tse
& Rice, 1986). Such an expansion of quasi-static slip prior to the
mainshock may explain the current migration of the foreshocks (e.g.,
Dodge et al., 1996; Yabe & Ide, 2018). However, the source of the
mainshock is smaller than the foreshock area (Fig. 6a), which
contradicts the hypothesis because the mainshock rupture zone should
include the nucleation area. Note that the size of the source of the
mainshock was estimated based on source models assuming a constant
rupture velocity (subshear rupture propagation). If the assumptions
differ from reality (e.g., supershear rupture propagation), the source
size may differ from our estimation, explaining this contradiction.
However, the aftershocks migrate upward on multiple planes (Fig. 8d),
which can be explained with the foreshock sequence if the pore pressure
migration model is adopted. Thus, we prefer the hypothesis that pore
pressure migration is primarily responsible for the generation of the
2017 M5.3 Kagoshima Bay earthquake sequence. The heterogeneity in the
permeability and/or pore pressure along the fault may explain the up-
and downward movement of the hypocenter along the plane (Fig. 8c).
However, recent observations of fluid injection-induced seismicity and
natural earthquake swarms suggest that an increase in the pore pressure
can cause aseismic slip (Cornet et al., 1997; Guglielmi et al., 2015;
Ruhl et al., 2016; Yoshida & Hasegawa, 2018a; De Barros et al., 2020).
In the presence of fluids, the effective normal stress decreases and the
critical nucleation size increases; thus, the occurrence of aseismic
slip is likely (e.g., Scholz, 1998). The increase in the pore pressure
also accelerates creep in the stable-slip segment of the fault. Both
aseismic slip and fluid movement may have contributed to the occurrence
of foreshocks. Furthermore, the poroelastic effects associated with pore
pressure migration (Segall, 1989; Goebel et al., 2018) and the
earthquake-to-earthquake interaction (Helmstetter, 2002) may contribute
to the occurrence of earthquakes.