Fig. 2

Recording rate of different VEI categories: a VEI 7. b VEI 6. c VEI 5. d VEI 4. e M ≥ 7.0. f M ≥ 6.5. g M ≥ 6.0. h M ≥ 5.5. i M ≥ 5.0. j M ≥ 4.5. k M ≥ 4.0. Blue, green and red dots show the recording rate calculated with equation 1 with the window size of 8, 6 and 4, respectively. Solid lines are fitted exponential (a, b, e and f) and double exponential (c, d, g, h, i, j and k) functions (Table 3). The red solid line of (f) (M ≥ 6.5, MA = 4) and the blue solid line of (g) (M ≥ 6.0, MA = 8) are calculated as the average of both exponential and double-exponential functions weighted with their relative likelihoods (Table 3) because their AICc or AIC difference of the two models is less than 2. The recording rate of the datasets of M ≥ 4.0 (MA = 4, 6), M ≥ 4.5 (MA = 4) and M ≥ 5.0 (MA = 4) were not calculated because some of their consecutive events had the same ages and the denominator of equation 1 becomes zero