The largest magnitudes of earthquakes associated with some historical volcanic eruptions and their volcanological significance

We know several reports of earthquake swarms associated with volcanic eruptions in the 19th century or older periods when seismographs were not yet available. Even if we have no seismographs, the largest magnitudes of earthquakes can be estimated by the maximum distances of perceptibility which are determined by reports of felt shocks or by records of earthquake movements at various distances from the origins. For example, the largest magnitudes of the earthquake swarms associated with the 1815 eruption of Tambora is estimated at 7 by the reports of felt shocks from three sites and that of the 1883 eruption of Krakatau is estimated at 5 by examination of magnetograms in place of seismograms. The magnitude of the 7 class is exceptionally large as volcanic origin, but we know a few examples besides Tambora. The magnitude 5 of the Krakatau eruption is rather small contrary to our expectation, and implies that crustal stresses had not accumulated much at the Krakatau area to cause larger earthquakes. The earthquake magnitudes associated with volcanic eruptions are not necessarily proportional to explosion magnitudes or volume of volcanic ejecta, and may have a volcanological significance. Such volcanic earthquakes may be closely related to readjustments of tectonic stresses caused by magma movements or phase transitions of magmatic material beneath and around volcanoes, and may be called magmatotectonic earthquakes.