I. Yokoyama's research while affiliated with Hokkaido University and other places

A compact data taking electronics was developed for high-speed multi-layer muon radiography in order to minimize the operation failure rate. By requesting a linear trajectory within the number of redundant position sensitive detectors (PSDs), the background (BG) events produced by vertical electromagnetic (EM) showers are effectively reduced. In order to confirm the feasibility of this method, the system comprising 4 PSD layers were tested by imaging the internal structure of a parasitic cone and the adjacent craterlets formed in the 1910 eruption at the base of Usu volcano, Hokkaido with a conventional (MURG08) readout system (Kusagaya et al., 2012; Tanaka et al., 2012). The new mountain has been believed to be a cryptodome since its formation. According As knowledge on lava domes is are accumulated at various volcanoes, the definition of "cryptodome" is now doubted in its validity. The results of the preliminary 290-h muon radiographic survey revealed that the "cryptodome" is not underlain by any lava mass and that a main craterlet is accompanied by magma intrusions at shallow depths. The former verifies that the new mountain is not a cryptodome but a volcanogenetic mound, and the latter interprets the phreatic explosions forming the craterlets as intrusions of magma into the aquifer. However, a higher data taking failure rate was observed with a software-based MURG08 system when the size of the active area of the detection system was enlarged to improve the detection ability of the system. The newly developed MURG12 is a complete hardware-based electronics system that can simultaneously process signals from 192 scintillation counters of data size of 600 kbps ch-1 without operation failure. We anticipate that the observation speed would be further improved by employing MURG12. At the base of Usu volcano, in 20th century, four eruptions occurred. Some of them demonstrated three characteristic magma intrusions. First, a magma branch remained at a depth leaving an upheaval of the ground, second, it rose and reached aquifers causing phreatic explosions but not extruded, and third, it reacted with aquifers causing phreatic explosions and further extruded over the ground forming a lava dome. In order to clarify the eruption mechanism of Usu, it is necessary for us to image many parasitic cones. Based on the result of the test measurement, we anticipate that MURG12 would be a strong tool for high-speed muon radiography.

Compact data-taking electronics were developed for high-speed multilayer muon radiography in order to minimize operation failure rates. By requiring a linear trajectory within the position sensitive detectors (PSDs), the background (BG) events produced by vertical electromagnetic (EM) showers are effectively reduced. In order to confirm the feasibility of this method, the system comprising four PSD layers was tested by imaging the internal structure of a parasitic cone and the adjacent craterlets formed in the 1910 eruption at the base of the Usu volcano, Hokkaido with a conventional (MURG08) readout system (Kusagaya et al., 2012; Uchida et al., 2009). The new mountain is believed to be a cryptodome since its formation. As knowledge on lava domes is accumulated at various volcanoes, the definition of "cryptodome" is now doubted in its validity. The results of the preliminary 290 h muon radiographic survey revealed that the "cryptodome" is not underlain by any lava mass and that a main craterlet is accompanied by magma intrusions at shallow depths. The former verifies that the new mountain is not a cryptodome but a volcanogenetic mound, and the latter interprets the phreatic explosions forming the craterlets as intrusions of magma into the aquifer. However, a higher data taking failure rate was observed with a software-based MURG08 system when the size of the active area of the detection system was enlarged to improve the detection ability of the system. The newly developed MURG12 is a complete electronics system that can simultaneously process signals from 192 scintillation counters with a data size of 600 kbps ch−1 without operation failure. We anticipate that the observation speed would be further improved by employing MURG12. At the base of the Usu volcano, in the 20th century, four eruptions occurred. Some of them demonstrated three characteristic stages of magma intrusions. First, a magma branch remained at a depth leaving an upheaval of the ground; second, it rose and reached aquifers causing phreatic explosions without extrusions; and third, it reacted with aquifers causing phreatic explosions and further extruded over the ground forming a lava dome. In order to clarify the eruption mechanism of Usu, it is necessary for us to image many parasitic cones. Based on the result of the test measurement, we anticipate that MURG12 would be a strong tool for high-speed muon radiography.

The MU-RAY project has the challenging aim of performing muon radiography of the summit cone of Mt. Vesuvius. The muon telescopes developed for this purpose will be available for the radiography of other volcanoes, in particular Stromboli. The scientific goals, the strategy for their implementation and the baseline detector design are discussed in detail. A tentative time schedule for the project is drawn.

The activity of Usu volcano situated at southern Hokkaido is characterized by the persistent occurrence of earthquake swarms and remarkable ground deformations including the formation of lava domes, both of which are due to the high viscosity of its magma.After the pumice eruptions of August 1977, the central part of the summit crater bounded by a U-shaped fault upheaved day after day at the maximum rate of 1 m/day. Although the rate has decreased, the total upheaval reached to about 180 m as of September 1981 resulting in formation of a new cryptodome. The doming at the summit has caused the northeastern rim to thrust towards the northeast about 180 m, and consequently caused corrugation of the ground at the northeastern foot of the volcano.The two kinds of the deformations, upheavals and thrusts, are closely related with each other. To examine the relationship between the earthquake occurrences and the deformations, a quasi-continuous observation of the distance from the northeastern rim of the summit crater to the foot of the volcano, was carried out by an optical distancemeter for 44 hours. It was found that the thrusts were episodic and accompanied by relatively large earthquakes. This supports that the earthquake swarms are caused by stick-slip motions of frictional sliding at the planes of the doming faults.

An earthquake swarm, and the major pumice eruptions in August 1977 which followed, marked the start of the dacitic doming activity of Usu volcano in southwestern Hokkaido, Japan. The sequence of magma intrusion processes was investigated in detail by means of seismological and other geophysical data. The distribution of the abundant hypocenters shows clearly an earthquake-free zone beneath the summit crater. The hypocenters migrated in a manner consistent with the development of the observed asymmetrical surface deformations, considered due to magma intrusion into this earthquake-free zone. The earthquake mechanism solutions are mostly of dip-slip type and are interpreted in terms of the doming deformations. The existence of earthquake families (earthquakes with similar waveforms) is the main cause of the peculiar occurrence of earthquakes in space, time and magnitude. The concept of scattered barriers of different sizes and strengths can explain well the distinct characteristics of the occurrence of the swarm, and the observed episodic deformations.

The 1977–1978 eruption of Usu volcano is discussed from the geophysical standpoint as a classic example of dacite volcanism. The activities of dacitic volcanoes are characterized by persistent earthquake swarms and remarkable crustal deformations due to the high viscosity of the magmas; the former include shocks felt near the volcanoes and the latter accompany formation of lava domes or cryptodomes.The hypocenters of the earthquakes occurring beneath Usu volcano have been located precisely. Their distribution defines an earthquake-free zone which underlies the area of doming within the summit crater. This zone is regarded as occupied by viscous magma. The domings within the summit crater forming the cryptodomes have amounted to about 160 m. In addition to uplift they showed thrusting towards the northeast. As a result, the northeastern foot of the volcano has contracted by about 150 m. The relation between crustal deformation and earthquake occurrence is examined, and it is found that the abrupt domings are accompanied by the larger earthquakes (M = 3–4.3). Both the seismic activity and the ground deformation are shown to have a unique and common energy source.The energy of activities of Usu volcano consists of the explosive type, the deformation type and the seismic type; the second and the third are in parallel with each other in discharges, and both energies are complementary to the explosive energy. The explosive energy and the seismic energy have been calculated for an explosion sequence, and it is concluded that the deformation energy is about 10 times greater than the seismic energy. The discharge rate of the seismic energy and the upheaval rates of the cryptodomes have continued to decrease since the outburst of the eruption, except for a small increase at the end of January 1978. Eruptions are governed not only by the supply of the energies but also by the depth of the magma, which has gradually approached the surface. The last eruption occurred in October 1978; however, the crustal deformations and the earthquake swarms are still proceeding as of January 1980, albeit at a lower rate of activity.

Muon radiography was first applied to the pyramid Giza in 1968 while volcanology has long history since the 0079 eruption of Vesuvius. Some traditional conception of volcanology for many centuries sometimes disturbed its progress. It has been difficult for volcanologists to correctly explore subsurface structure of volcanoes by conventional means even though some of them were formed under their eyes. Volcanologists would complain that they could not see through the earth beneath their feet. Very recently muon radiography gives us a clue for visualization of the interior of volcanoes though its detection ranges are limited at present. In 2007, Tanaka et al. (1), (2) succeeded in visualization of the explosion deposits inside the crater of Asama, and of the subsurface structure of the 1944 lava dome of Usu. Tanaka et al. (1) carried out muon radiography of the summit part of Asama after its 2004 eruption, and found the deposits of the 2004 eruption inside the summit crater. By such radiography, we can visualize the internal structure inside the erupting crater and vent. Then we can judge whether the vent is plugged with lava or is drained of magma to the deeper parts. This is useful for prediction of the coming consequences in volcanic eruption. Lava domes are one of the conspicuous topographic features in volcanic fields and afford us important data to discuss magma risings through conduits and resultant deformations of the ground. The knowledge of subsurface structure of lava domes is indispensable to discuss their formation mechanisms. The 1944 eruption of Usu produced a lava dome at its eastern foot. The activity began first with earthquakes and the ground began to uplift forming a mound. After 6 months, explosion took place and another 4 months later, a lava spine extruded at the top of the mound. Finally a lava dome was completed in 22 months after the first earthquake. The height of the mound is roughly 200 m and the relative height of the lava dome reaches approx. 100 m above the mound. In 1952-1955, Hayakawa et al. (3) applied various methods to explore the underground structure of the lava dome and succeeded in finding the distribution of seismic velocities inside the dome, but could not detect the shape of the deeper part. Later Yokoyama (4) analyzed the results of precise levels carried out during the 1944 eruption by Minakami et al. (5) and obtained "pseudo growth curves"