Fluid migration before and during slow earthquakes in the shallow Nankai
subduction zone
Abstract
Fluid migration in subduction zones is a key controlling factor of slow
and megathrust earthquakes at plate boundaries. During the migration,
seismic velocity and heterogeneous structures in its pathways may be
temporarily varied, preferably triggering slow earthquakes. Here, we
show that transient changes of seismic heterogeneity occurred 0-9 months
before shallow slow earthquakes in the Nankai subduction zone, Japan,
using very long-term (6–10 y) records of ambient seafloor noise. The
heterogeneity changes preceding to shallow slow earthquakes were
observed near the margin of the source region, while concurrent changes
primarily occurred in the source region. We propose that the
heterogeneity changes are attributed to dynamic fluid migration, and the
difference in timings reflects the pore pressure level in the
corresponding source region. When fluids are supplied to a source region
under relatively low pressure, fluids are leaked out from its downdip or
updip side, and slow earthquakes occur not immediately but with a time
delay of at most 9 months. In the high pore pressure case, slow
earthquakes occur immediately with fluid migration from the source
region. This study suggests that the heterogeneous seismic structure is
possibly changed by fluid migration before slow earthquakes in the
Nankai subduction zone.