Tectonic tremors occurring on subducting plate boundaries are known to
migrate at various timescales and migration speeds. Spatiotemporal
patterns of tremor migration are key to investigating the rupture growth
of slow earthquakes. However, spatiotemporal patterns are not
sufficiently simple to visually define tremor migrations. This study
developed a space-time Hough transform to objectively extract tremor
migrations. The space-time Hough transform enables the extraction of
multiple tremor migrations with various durations, migration directions,
and migration speeds. We applied this method to a catalog of tremors for
the period from 2012 to 2014, which was determined from the data
analysis of a dense seismic array deployed in the Kii Peninsula,
Southwest Japan. We successfully extracted 1,010 tremor migrations with
durations ranging from 10 min to 24 h. Along-strike migrations
propagating southwestward were predominant in the northeastern part of
the Kii Peninsula, whereas those propagating northeastward were
principal in the southwestern part. Regarding the along-dip direction,
tremor migrations propagating in the up-dip directions were predominant
in the deep part, and those propagating in the down-dip directions were
principal in the shallow part. The patterns of along-strike migrations
were related to the distribution of tremor energies, suggesting that
tremor migrations may be controlled by heterogeneous structures of
frictional properties on the plate interface. We further found that the
migration speed is proportional to the inverse of the square root of the
duration. This relation implies that a diffusion process controls the
growth of fault ruptures behind tremor migrations.