Fig. 8

Redefinition of the time delay of the arrivals of seismo-acoustic signals. a Seismic and infrasound waveforms and their arrivals. \(\Delta t\) is the time delay between the seismic and high-frequency (\(\sim\)15 Hz) infrasound signals. \(\Delta t'\) is that between the seismic and low-frequency (\(\sim\)2 Hz) infrasound signals. b Range of the time delay explained by the horizontal change in the source position when the seismic propagation velocity \(v_\text {p0}\) is 23 m/s (same as Fig. 7c). c PDF distribution of the observed \(\Delta t\) and d that of the observed \(\Delta t'\) when \(f_\text {0} = 0.5\) Hz. A case with \(v_\text {p0}\) = 100 m/s is shown in e, f, and g. When \(v_\text {p0}\) = 23 m/s, the range of time delays explained by the horizontal change in the source position is wide, as shown in b. Although the upper half of the distribution of \(\Delta t\) is not included within the range of b (c), most of the distribution of \(\Delta t'\) is included in this range (d). On the other hand, if \(v_\text {p0}\) = 100 m/s, both distributions of \(\Delta t\) and \(\Delta t'\) cannot be completely included in the range of the time delays explained by the horizontal source change (f, g). h The fraction of the PDF of time delay explained by the horizontal change in source position against the PDF of time delay in the whole range (which means dark-colored area/whole area of the distribution in c, d, f, g). This fraction increases as the seismic propagation velocity \(v_\text {p0}\) decreases. If \(v_\text {p0} < 23\) m/s, the fraction becomes \(\sim\)0.9 for all four cases of the lava pond’s radii