1 Introduction
An earthquake is a natural
phenomenon during which a high-speed rupture propagates along a fault.
Two factors control the occurrence of an earthquake: an increase in the
shear stress acting on the fault and a decrease in the fault strength.
The results of previous studies suggested that the increase in the pore
pressure plays an important role in the earthquake occurrence (e.g.,
Hasegawa, 2017; Hubbert & Rubey, 1959; Nur & Booker, 1972; Sibson,
1992; Rice, 1992) because it reduces the fault strength.
A well-known example of fluid-driven seismicity is the seismicity
induced by fluid injection for engineering purposes (e.g., Ellsworth,
2013). There is also growing evidence that natural earthquake swarms are
closely related to fluid movement at depth. In fact, the characteristics
of many natural seismic swarms are similar to those of fluid
injection-induced seismicity including the migration behavior of the
earthquake hypocenter (e.g., Fischer and Horálek, 2003; Parotidis et
al., 2003; Bianco et al., 2004; Yukutake et al., 2011; Shelly et al.,
2016; Yoshida et al., 2016a; Ruhl et al., 2016; De Barros et al., 2019).
Based on the determination of the hypocenters and focal mechanisms of
earthquake swarm at the 2009 Hakone volcano, the diffusion of
high-pressure fluid triggered the swarm (Yukutake et al., 2011). The
spatiotemporal evolution of seismic activity in the Long Valley Caldera,
California, indicates that a pore pressure transient with a
low-viscosity fluid initiated and sustained the swarm in 2014 (Shelly et
al., 2016). It has been hypothesized that several earthquake swarms that
occurred after the 2011 Tohoku-Oki earthquake were triggered by a
decrease in the fault strength due to upward pore pressure migration
(Terakawa et al., 2013; Okada et al., 2016; Yoshida et al., 2016a,
2019a).
Not only earthquake swarms but also foreshock–mainshock–aftershock
sequences may be closely related to the fluid behavior in the Earth
interior. Sibson (1992) established the fault-valve model in which the
pore pressure cycle controls the earthquake cycle due to overpressurized
fluids that rise from the deeper portion of the fault. In this model,
fault ruptures create a transient fracture permeability within the fault
zone, which acts as a valve, promoting the upward discharge of fluids
from deeper portions of the crust. This model is supported by various
geological and geophysical observations (Sibson, 2020). Hasegawa et al.
(2005) proposed a model for the deformation process in a subduction zone
based on various geophysical observations including seismic tomography
data obtained for northeastern Japan. In this model, fluids expelled
from the subducting slab migrate upward, reach the crust, and cause
anelastic crustal deformation including earthquakes.
The migration characteristics of earthquake hypocenters can be used to
infer the origin of the seismicity (e.g., Yukutake et al., 2011; Ruhl et
al., 2016; Yoshida & Hasegawa, 2018a,b; De Barros et al., 2019).
Pore pressure migration and
aseismic slip propagation are typical mechanisms attributed to the
migration of earthquakes. In the former mechanism, the hypocenter
migration is presumed to reflect the migration of fluids (e.g., Shapiro
et al., 1997; Talwani et al., 2007). In the latter mechanism, the
hypocenter migration is presumed to be a result of aseismic slip
propagation (e.g., Lohman & McGuire, 2007; Roland & McGuire, 2009).
The spatiotemporal distribution of earthquake hypocenters can be more
precisely estimated than other seismological characteristics such as the
three-dimensional seismic velocity structure. By examining relocated
hypocenters, we may extract information on aseismic physical processes
controlling earthquakes, which is crucial to understanding the
earthquake generation. The results of previous studies showed that the
seismic activity caused by aseismic processes differs from that of the
mainshock–aftershock sequence (e.g., Hainzl & Ogata, 2005; Roland &
McGuire, 2009; Kumazawa & Ogata, 2013; Yoshida & Hasegawa, 2018b).
This suggests that investigations of the seismicity may provide clues
about aseismic processes governing earthquakes.
The volcanic front on Kyushu Island in southern Japan formed due to the
subduction of the Philippine Sea Plate. Several of the most active
volcanoes in Japan are distributed along this volcanic front (e.g.,
Sakurajima and Aso). Kagoshima Bay is located at this volcanic front
(Fig. S1), which is characterized by a low-gravity anomaly that extends
from north to south. On July 11, 2017, an ML 5.3
strike-slip earthquake occurred at a depth of ~10 km in
Kagoshima Bay (Fig. 1). Seismicity activity had been recorded near the
mainshock hypocenter since December 2016 (Fig. 1c). In total, 1.843
foreshock events were recorded and listed in the Japan Meteorological
Agency (JMA) unified seismic catalogue. The seismicity increased after
the mainshock; 12.595 events are listed in the JMA catalogue. Based on
the focal mechanisms of earthquakes in this region, these events were of
strike-slip type with a NW–SE P-axis (Fig. 1b). Only a small coseismic
step was detected by the national GNSS (Global Navigation Satellite
System) network (Fig. S2). Based on the spatiotemporal variation in the
b-value and the migration of the hypocenters, Nanjo et al. (2018)
suggested that fluid movement caused the earthquake sequence in
Kagoshima Bay, but the detailed physical process controlling this
foreshock–mainshock–aftershock sequence remains unclear.