4.3 Upward migration of the aftershocks along several planes
The aftershock sequence of
the Kagoshima Bay earthquake sequence follows Omori’s law (Fig. 1d),
suggesting that this sequence was triggered by the M5.3 mainshock.
However, the aftershock sequence slightly deviates from the prediction
based on the ETAS model (Figs 9b–c). The transient increase in the
background seismicity rate suggests that the Kagoshima Bay earthquake
sequence may have been affected by physical processes other than
earthquake-to-earthquake interactions, especially during this period (20
to 40 days) and that these aseismic processes may have led to the
largest aftershock (ML 4.4) that occurred 44 days after
the mainshock. Based on the model simulations of fluid injection-induced
seismicity, Hainzl and Ogata (2005) pointed out that the background
seismicity rate of the ETAS model is sensitive to the amount of injected
water. In previous studies, similar observations were made for fluid
injection-induced seismicity and natural earthquake sequences (Llenos &
Michael, 2013; Yoshida & Hasegawa, 2018b; Kumazawa et al., 2019).
Our results indicate that the aftershock hypocenters migrated toward the
shallower portion on multiple planes. Such upward movements of
hypocenters have been previously reported for earthquake swarms
following nearby large earthquakes and it has been concluded that they
reflect the upward pore pressure migration associated with the
fault-valve behavior (Shelly et al., 2015; Ruhl et al., 2016; Yoshida &
Hasegawa, 2018a, b). Examples are the earthquake swarms that occurred in
northeastern Japan following the 2011 Tohoku-Oki earthquake (Yoshida et
al., 2016a; Yoshida & Hasegawa, 2018a, b). The earthquake swarms might
originate from the pore pressure increase because (1) they occurred in
the stress shadow of the 2011 Tohoku-Oki earthquake with a time delay of
a few weeks despite the reduction in the shear stress, (2) they are
located beneath the caldera structures that are believed to host shallow
igneous bodies, with hydrothermal fluids immediately below, (3) they are
located a few kilometers above S-wave reflectors and the low-velocity
zone including fluids, and (4) their hypocenters migrate upward (Yoshida
& Hasegawa, 2018a; Yoshida et al., 2019a). The Kagoshima Bay swarm was
also located beneath an ancient caldera and involved the upward
migration of aftershocks, which can be explained by an increase in the
pore pressure. Fluid paths in the crust may have expanded due to the
deformation and shaking associated with the mainshock. Pore pressure
migration may explain deviations in the seismicity rate from Omori’s
law. These observations are consistent with the prediction based on the
fault-valve model proposed in Sibson (1992), that is, upward fluid
discharge after the mainshock. In recent geodetic studies, a porosity
wave associated with the fault-valve action was detected (Rossi et al.,
2016 and 2018).
We presume that the subducting Philippine Sea Plate is the source of
fluids, similar to the model reported in Hasegawa et al. (2005), which
is based on the geophysical and geological observations in northeastern
Japan. This hypothesis is supported by seismic data obtained in Kyushu
using tomography, which indicate that the existence of an inclined
low-velocity layer continuously distributed in the mantle wedge and
reaching right below the volcanic front as northeastern Japan (Zhao et
al., 2012). The low-velocity zone is considered to represent the
ascending flow portion of the secondary convection within the mantle
wedge and therefore contains fluids from the slab and resultant melts
(Hasegawa et al., 2005). The buoyancy facilitated the upward migration
of the fluids, as shown in simulations (e.g., Iwamori, 1998; Wada et
al., 2015; Horiuchi et al., 2016), and the fluids reached the source
region of the Kagoshima Bay sequence.