Stress release process along a crustal fault analogous to the plate
boundary: a case study of the 2017 M5.2 Akita-Daisen earthquake, NE
Japan
Abstract
Stress accumulation and release in the crust remains poorly understood
compared to that at the plate boundaries. Spatiotemporal variations in
foreshock and aftershock activities can provide key constraints on
time-dependent stress and deformation processes in the crust. The 2017
M5.2 Akita-Daisen intraplate earthquake in NE Japan was preceded by
intense foreshock activity and triggered a strong sequence of
aftershocks. We examine the spatiotemporal distributions of foreshocks
and aftershocks and determine the coseismic slip distribution of the
mainshock. Our results indicate that seismicity both before and after
the mainshock was concentrated on a planar structure with N-S strike
that dips steeply eastward. We observe a migration of foreshocks towards
the mainshock rupture area, suggesting that foreshocks were triggered by
aseismic phenomena preceding the mainshock. The mainshock rupture
propagated toward the north, showing less slip beneath foreshock
regions. The stress drop of the mainshock was 1.4 MPa and the radiation
efficiency was 0.72. Aftershocks were intensely triggered near the edge
of large coseismic slip regions where shear stress increased. The
aftershock region expanded along the fault strike, which is attributed
to the post-seismic aseismic slip of the mainshock. The postseismic slip
possibly triggered repeating earthquakes with M ~3. We
find that the foreshocks, mainshock, aftershocks, and post-seismic slip
released stress at different segments along the fault, which may reflect
differences in frictional properties. Obtained results were similar to
those observed for interplate earthquakes, which supports the hypothesis
that the deformation processes along plate boundaries and crustal faults
are fundamentally the same.