  We report precursory seismic patterns prior to the 2016 Kumamoto earthquakes,
as measured by four different methods based on changes in seismicity that can
be used for earthquake forecasting: the b-value method, two methods of seismic
quiescence evaluation, and an analysis of seismicity density in space and time.
The spatial extent of precursory patterns differs from one method to the other
and ranges from local scales (typically, asperity size) to regional scales
(e.g., 2 degree x 3 degree around the source zone). The earthquakes were
preceded by periods of pronounced anomalies, which lasted in yearly scales (1.5
years), or longer (> 3 years). We demonstrate that a combination of multiple
methods detected different signals prior to the Kumamoto earthquakes. This
indicates great potential to reduce the hazard at possible future sites of
earthquakes relative to long-term seismic hazard assessment. We also found that
the seismic quiescence in a regional scale area, detected by using the two
methods of seismic quiescence evaluation, was a common precursor to the 2016
Kumamoto earthquakes and 2015 Off Satsuma Peninsula earthquake. The result
allows us to interpret both events as the onset that occurred at a section
along the tectonic line from the Okinawa Trough through the Beppu-Shimabara
graben.
