The observatory is located 4 km E of the active summit crater. It was inaugurated in 1935 and is now a branch of the Earthquake Research Institute, University of Tokyo. The observatory witnessed a period of severe eruption activity centered around 1930-1940. Two contrasting types of volcanic earthquakes, now widely known as A-type and B-type, were recognized by Minakami through the seismic observation here: A-type earthquakes have deep (1-10 km) focal depth and clear P and S phases on seismograms, while B-type earthquakes have extremely shallow (< 1 km) focal depth and vague P and S phases, characterized by relatively long period waves. At Asama, the number of B-type earthquakes sometimes increases markedly before an explosive eruption.

Although the volcano is quiet at the moment, the telemetered network constantly collects the data of earthquakes, ground tilts, temperatures, and other parameters realtime.

A cross-section outside of the observatory reveals a 2-m thick layered deposit of pumice fallout from the plinian column of 1783, the last major eruption of Asama. The lower half consists of finely stratĚds while the upper half consists of homogeneous and coarser-grained pumice. Considering old documents, the lower half is interpreted as a fallout from unstable eruption column established from the night of 2 August to 1400 hours of 4 August, and the upper half is a fallout from the succeeding climactic phase of stable high column lasted until 1000 hours of 5 August.

A number of thin layers of pink silt intercalated in the lower half are fallouts from thermal clouds developed over the Agatsuma pyroclastic flows discharged during the daytime of 4 August.

Three kinds of particles may be identified: (1) pale brown, well vesiculated pumice, which are quenching products of erupting magma, (2) dark blue, angular and non-vesiculated lithic fragments, which were torn off from the conduit wall during ascent of magma, and (3) yellow brown, well-vesiculated glassy fragments formed by heating and melting Neogene sediments at depth.

1783 pumice

The eruption deposits of 1783

Isopach map of the 1783 pumice (As-A; Machida and Arai, 1992)