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. Plain Language Summary The size distribution of slow earthquakes mostly follows a power law like that of ordinary earthquakes, in which the logarithm of event numbers is negatively proportional to the logarithm of event sizes, with an exponential taper for events larger than a cut‐off magnitude. We investigated the changes in this size distribution during two prolonged episodes of shallow tectonic tremor that occurred on the plate interface along the Nankai trough, southwestern Japan. We found that the ratio of smaller events increased and the cut‐off magnitude decreased as tremor activity decreased late in each episode. We interpreted this observation by using a model of slow earthquakes that divides a source fault into small cells and updates slip on each cell probabilistically. The model can explain the changes in the tremor size distribution by a decrease in the probability of event ignition or an increase in the energy dissipation during fault slip. This result implies that the accumulated stress or the pore‐fluid pressure on the source fault decreased when the tremor was less active. Because the tremor source migrates during the course of each episode, these changes indicate that the source characteristics of tremor vary at different times or locations. Key Points The slope and cut‐off magnitude of event size distribution changed during episodes of shallow tectonic tremor along the Nankai trough The 2D probabilistic cell automaton slow earthquake model can explain