Fig. 3

Correlation coefficients (CCs) and the array analysis results. a The array CC and b SI-array CC at 5–80 Hz. Enlarged view of b around the minor peak (b1) and the prominent peak (b2). The vertical lines with the corresponding colors mark the calculated infrasound propagation time differences between the microphone pairs, assuming that the sources are located at the IWC fumarole (dotted lines) and the ISC fumarole (dashed lines). The lag time fitting results for the peaks in b1 and b2 are shown in c1, d1 and c2, d2, respectively. c The fitting residual using all pairs of SI-array CC is shown as functions of horizontal components of the slowness. The white circles indicate acoustic slowness. d The blue curves indicate the source hyperbola obtained from the SI–A1 lag time fitting assuming the propagation velocities as 335, 340, 345, and 350 m/s. The green angle range from the VSA array shows the estimated back azimuth (2 STD) obtained from c. The white and red points mark the microphones’ and fumaroles’ locations, respectively. e The power spectra densities (PSDs) normalized by the power in the main bands ranging from 5 to 50 Hz, which were calculated using the three CCs in b1 and b2 are shown in e1 by the red line and in e2 by the blue line, respectively. They are compared with the PSD for the array microphones (green) and the PSD for the SI microphone (purple). For the former (the green line), the normalized PSDs for the individual array microphones are averaged