, Keisuke Yoshida1*, Toru
Matsuzawa1, and Akira Hasegawa1
1 Research Center for Prediction of Earthquakes and
Volcanic Eruptions, Graduate School of Science, Tohoku University,
Sendai 980, Japan
† Now at Japan Meteorological Agency, Japan
*Corresponding author: Keisuke Yoshida
(keisuke.yoshida.d7@tohoku.ac.jp)
Key Points:
- Intensive foreshocks migrate via one plane.
- Aftershock hypocenters migrate toward shallower levels via several
planes.
- Upward pore pressure migration explains the occurrence of the
foreshock–mainshock–aftershock sequence.
Abstract
Determining fluid migration and
pore pressure changes within the Earth is key to understanding
earthquake occurrences. We investigated the spatiotemporal
characteristics of intense fore- and aftershocks of the 2017
ML 5.3 earthquake in Kagoshima Bay, Kyushu, southern
Japan, to examine the physical processes governing this earthquake
sequence. The results show that the foreshock hypocenters moved upward
on a sharply defined plane with steep dip. The mainshock hypocenter was
located at the edge of a seismic gap formed by foreshocks along the
plane. This spatial relationship suggests that the mainshock ruptured
this seismic gap. The corner frequency of the mainshock supports this
hypothesis. The aftershock hypocenters migrated upward along several
steeply dipped planes. The aftershock activity slightly differs from the
simple mainshock–aftershock type, suggesting that aseismic processes
controlled this earthquake sequence. We established the following
hypothesis: First, fluids originating from the subducting slab migrated
upward and intruded into the fault plane, reducing the fault strength
and causing a foreshock sequence and potentially aseismic slip. The
continuous decrease in the fault strength associated with an increase in
the pore pressure and the increase in shear stress associated with
aseismic slip and foreshocks caused the mainshock in an area with
relatively high fault strength. The change in the pore pressure
associated with post-failure fluid discharge contributed to aftershocks,
causing the upward migration of the earthquake. These observations
demonstrate the importance of considering fluid movement at depth not
only earthquake swarms but also foreshock—mainshock–aftershock
sequences.