Tremor distributions for the studied regions. Tremor depths are color-coded according to the diagram for Shikoku. Light blue triangles denote the seismic stations during the total observation period. Circles with crosses and roman numerals for Shikoku correspond to the locations of the tremor subsets shown in Figure 2. Each white and black dot with a number (or number only for Kyushu, Taiwan, and Manawatu) denotes the location corresponding to the cumulative events (white and black dots) and to the correlation integral analysis (white dot only) shown in Additional file 1: Figure S1. The circle with a cross in Shikoku, Kii-Tokai, Cascadia, Jalisco, and southern Chile denotes the spatial size of each bin applied to each tremor zone (see Table 1). For the tremors in Guerrero, the tremor subsets close to the stations were used for the analysis. For the tremors in Kyushu, Taiwan, and Manawatu, the tremors plotted in each map were used. The insets in the bottom right of the diagrams for Guerrero, Kyushu, Taiwan, and Manawatu show an enlarged view of each tremor region.