  A comparative study is performed on volcanic seismicities at Icelandic
volcano, Eyjafjallaj\"okull, and Mt. Etna in Sicily from the viewpoint of
complex systems science, and the discovery of remarkable similarities between
them is reported. In these seismicities as point processes, the jump
probability distributions of earthquakes (i.e., distributions of the distance
between the hypocenters of two successive events) are found to obey the
exponential law, whereas the waiting-time distributions (i.e., distributions of
inter-occurrence time of two successive events) follow the power law. A careful
analysis is made about the finite size effects on the waiting-time
distributions, and the previously reported results for Mt. Etna (Abe and Suzuki
2015) are reinterpreted accordingly. It is shown that the growth of the seismic
region in time is subdiffusive at both volcanoes. The aging phenomenon is
commonly observed in the "event-time-averaged" mean-squared displacements of
the hypocenters. A comment is also made on (non-)Markovianity of the processes.
