Changes of Event Size Distribution During Episodes of Shallow Tectonic Tremor, Nankai Trough

Abstract

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.

Cataloging Tectonic Tremor Energy Radiation in the Cascadia Subduction Zone

“…(b) Tremor swarms, on the other hand, behave with an earthquake‐like frequency magnitude distribution with b values near 1. This is in agreement numerous studies from Japan (e.g., Nakano & Yabe, 2021), the San Andreas Fault (e.g., Staudenmaier et al., 2019), and Cascadia (e.g., Bostock et al., 2015; Wech et al., 2010) that have shown various slow earthquake phenomena to follow a power‐law size distribution. An exponential taper does fit the swarm data slightly better as well, but only at the very upper end of the distribution.…”