Tokumitsu Maekawa

Radiographic imaging below a volcanic crater floor with cosmic-ray muons

We present a novel application of cosmic-ray muon radiography to image the shallow density structure beneath Asama Volcano, Japan. We use a single detector (emulsion cloud chamber) set up in an underground vault at an elevation of 2250 m on the eastern flank of Asama, 310 m below the summit of the edifice and 1 km away from the crater. The results point to two high-density anomalies located between the original pre-2004 eruption crater floor and post-2004 eruption crater profile. A third low-density anomaly is imaged immediately below the pre-2004 eruption crater floor. The spatial extent of each density anomaly is about 100 to 200 m. To know if this method, applied to other volcanoes, would produce contrasting results, we performed the measurement in 1944 Usu lava dome. We confirmed a bulbous shape measuring approximately 300 m in diameter and narrowing downwards. The diameter of the uppermost part of the conduit is estimated at 100 ± 15 m at an elevation of 260 m a.s.l. and 50 ± 15 m at an elevation of 217 m a.s.l., demonstrating a resolution that is significantly better than that typically achieved with seismic tomography based on picks of first arrival times from earthquakes or artificial sources.

Excitation process of infrasonic waves associated with Merapi‐type pyroclastic flow as revealed by a new recording system

We investigate excitation process of the infrasonic waves associated with Merapi-type pyroclastic flows at Unzen volcano, Japan, using data set obtained by a new system that permits recording of geophysical data with accompanying visual events on a single videotape. The accurate correlation of infrasonic waves with video images reveals that: (1) successive impulsive waves and the low-frequency wave (<∼0.8Hz) are excited during the collapse of lava blocks falling from the dome, which generates a pyroclastic flow; (2) an impulsive wave is excited by an explosive blowout of the pore-gases which are released by the breakdown of the falling blocks; (3) the low-frequency wave is excited by the volumetric change which results from expansion of the pore-gases released by pulverization of the falling blocks. The comparison between the observed and the synthetic low-frequency waves shows that the amount of volume change is about 2 times that of the initial falling blocks.

Drain‐back process of basaltic magma in the summit conduit detected by microgravity observation at Izu‐Oshima Volcano, Japan

Basalt magma sometimes moves through volcanic conduits causing only minor deformation. In that case, we may detect magma movements directly by microgravity observations. After the 1986 eruption of Izu-Oshima volcano, we observed anomalous gravity variations localized at the summit. Based on a vertical cylindrical conduit model, we estimate the time variations of the head of magma in the summit conduit and clarify the magma drain-back process after the 1986 eruption.

Integrated processing of muon radiography and gravity anomaly data toward the realization of high‐resolution 3‐D density structural analysis of volcanoes: Case study of Showa‐Shinzan lava dome, Usu, Japan

We have developed an integrated processing method for muon radiography and gravity anomaly data for determining the 3-D density structures of volcanoes with a higher spatial resolution than is possible by conventional gravity inversion. In the present paper, we demonstrate the performance of the proposed method by performing numerical tests using synthesized data, and we present the results obtained by applying the proposed method to a volcano, Showa-Shinzan lava dome, Hokkaido, Japan. We obtained the detailed shape of a vent beneath the dome and detected the presence of solidified dense lava near the top of the dome. The results demonstrate the advantage of a hybrid measurement based on both gravity and muon radiography for imaging small structures with sizes of a few hundreds of meters near the surface of a volcano.

Gravity variation around Shinmoe-dake volcano from February 2011 through March 2012—Results of continuous absolute gravity observation and repeated hybrid gravity measurements

We report here on continuous absolute gravity measurements made between February 2011 and March 2012 and repeated relative gravity measurements in the vicinity of Shinmoe-dake volcano, which commenced erupting in late January 2011. We find that 20 of 24 eruptive events are associated with precursory short-term gravity decreases occurring over 5–6 hours followed by quick recoveries lasting 1–2 hours. Also evident are significant long-term gravity changes arising principally from hydrological processes around the volcano, where annual precipitation exceeds 5,000 mm. To isolate the gravity signal associated with volcanic processes, we compared gravity measurements made at 15 sites in March 2011 and again in March 2012. The gravity changes and crustal deformation observed during the one year period are well explained by 6×106 m3 inflation of a magma reservoir at a depth of 9 km and intrusion at shallower depths of a dike with dimensions of 10 km × 0.5 km × 0.5 m.

Ground deformations associated with the 1977–1978 eruption of USU volcano, Japan

Abstract 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.