Temporal changes of event size distribution during episodes of shallow
tectonic tremor, Nankai trough
Slow earthquakes follow a power-law size distribution with an
exponential taper for the largest events. We investigated changes in the
size distribution of shallow tectonic tremor events during two prolonged
tremor episodes (>1 month) along the Nankai trough and
found that the slope of the size distributions increased while the
cut-off magnitudes decreased late during each episode, as tremor
activity waned. Interpreting these changes with the two-dimensional
probabilistic cell automaton model of slow earthquakes, we found that a
decrease in event ignition probability or an increase in energy
dissipation during slip can qualitatively explain the observed changes.
These changes imply that a decrease in accumulated stress or pore-fluid
pressure on the fault interface occurred during each tremor episode.
Because the tremor source migrates during an episode, the changes in the
size distribution parameters can be attributed to spatial variations or
temporal changes in the source characteristics.