  Low-frequency earthquakes are a particular class of slow earthquakes that
provide a unique source of information on the mechanical properties of a
subduction zone during the preparation of large earthquakes. Despite increasing
detection of these events in recent years, their source mechanisms are still
poorly characterised, and the relation between their magnitude and size remains
controversial. Here, we present the source characterisation of more than 10,000
low-frequency earthquakes that occurred during tremor sequences in 2012-2016
along the Nankai subduction zone in western Shikoku, Japan. We show that the
seismic moment versus corner frequency scaling for these events is compatible
with an inverse of the cube law, as widely observed for regular earthquakes.
Our result is thus consistent with shear rupture as the source mechanism for
low-frequency earthquakes, and suggests that they obey to a similar physics of
regular earthquakes, with self-similar rupture process and constant stress
drop. Furthermore, when investigating the dependence of the stress drop value
on the rupture speed, we found that low-frequency earthquakes might propagate
at lower rupture velocity than regular earthquakes, releasing smaller stress
drop.
