Low-frequency earthquakes are peculiar energy-release events mostly occurring at the transition between the seismogenic and the freely creeping zones of a subducting slab. The source characterization of these events is of fundamental importance to understand physical processes that govern the slow out of equilibrium evolution of the subduction interface that may lead to the generation of large, destructive earthquakes. Nevertheless, their source mechanisms still remain unclear. Here, we estimate the source parameters of ~23,000 low-frequency earthquakes continuously detected from 2013 to 2015 in Shikoku, Japan. We show that a cubic moment-duration scaling characterizes these events, suggesting a self-similar process as for regular earthquakes. However, their high-frequency fall-off suggests an omega-cube decay in contrast to the omega-squared model of earthquakes. Source characteristics do not change when low-frequency earthquake bursts occur during the analyzed three years. On the other hand, we observe a coherent along-strike variation of the product of stress drop and the cube of rupture velocity, possibly related to a weaker behavior of tremor patches in central Shikoku. Secondary microseismic noise and network-dependent completeness magnitude lead to missing event detections that do not allow discriminating between Gutenberg-Richter event size distribution and any deviation from it. Our findings suggest that the same observational limits might affect worldwide detection of low-frequency earthquakes.