Fig. 3
Slow earthquakes as broadband fault slip phenomena. a Moment–duration scaling relations of slow and fast earthquakes. Blue and magenta symbols represent slow earthquakes. LFE, VLFE, SVLFE, and S-SSE indicate LFEs (Ide et al. 2007b), deep VLFEs (Ito et al. 2007; Ide et al. 2008), shallow VLFEs (Ito and Obara 2006), and short-term SSEs (Hirose and Obara 2005) in the Nankai Trough, respectively. Other symbols are as follows: IA, a small short-term SSE near the Kii Peninsula, southwest Japan (Itaba and Ando 2011); BO, a short-term SSE near the Boso Peninsula, eastern Japan (Sagiya 2004); NZ, short- and long-term SSEs in Hikurangi, New Zealand (Wallace et al. 2012); CA, short-term SSEs in Cascadia (Dragert et al. 2004); BU, a long-term SSE in the Bungo Channel, southwest Japan (Hirose and Obara 2005); MX, a long-term SSE in Guerrero, Mexico (Kostoglodov et al. 2003; Yoshioka et al. 2004); TK, a long-term SSE in the Tokai region, central Japan (Miyazaki et al. 2006); AL, a long-term SSE in Alaska (Ohta et al. 2006); IB, aseismic transients in the Izu–Bonin subduction zone (Fukao et al. 2021); and SR, the 1992 Sanriku-Oki ultraslow earthquake in the northern Japan Trench (Kawasaki et al. 1995). The thick red line represents the scaling relationship of fast earthquakes. The red ellipse indicates tsunami earthquakes (Kanamori 1972; Ide et al. 1992). The gray curve represents the approximate observational limit of seismological instruments suggested by the US Geological Survey low-noise model (Peterson 1993). This panel is adapted from Ide (2014). b Average maximum amplitude of stacked LFE waveforms for each frequency band. The maximum amplitudes of seismic signals in each frequency band are averaged for each component of each station. Blue inverted triangles represent stacked LFE waveforms in the Nankai Trough (Masuda et al. 2020). Green symbols indicate stacked LFE waveforms in the Cascadia subduction zone (Bostock et al. 2015; Ide 2019b). For comparison, the average maximum amplitude of synthetic waveforms that obey the omega-square model (Aki 1967) with a 2 Hz corner frequency is shown. This panel is adapted from Masuda et al. (2020)