Hitoshi Mori

Global positioning system measurements of ground deformation caused by magma intrusion and lava discharge: the 1990–1995 eruption at Unzendake volcano, Kyushu, Japan

Abstract Global positioning system (GPS) measurements made around Unzendake volcano, Kyushu, Japan, since January 1991 have detected ground deformation caused by magma intrusion and lava discharge. In the intermittent phreatic and phreatomagmatic eruption stage, the ground was inflating. After growth of the lava dome and following frequent pyroclastic flows at Unzendake volcano, the ground began deflating. These ground deformations are explained by the inflation and deflation of a Mogis source model (a point source model) located about 6 km west of the active crater at a depth of 11 km, at an aseismic region. The observed horizontal displacement vectors pointed radially away from the estimated pressure source during inflation and pointed to the pressure source during deflation. The horizontal displacements at the reference GPS station calculated from contraction of the estimated pressure source coincide well with the actual horizontal displacements observed from other GPS baseline systems. These observations validate our estimates for the pressure source. Based on the relation between the deformation volume of the ground surface and the discharged volume of the lava, it is estimated that during the eruption there was magma supply from the deeper portion as well as magma discharge at the crater. Magma is estimated to be supplied to the reservoir at an average rate of 1.1×105 m3/day; magma intrusion began in December 1989 at the latest and continued for 1.9×103 days.

Elastic models for the magma intrusion associated with the 2000 eruption of Usu Volcano, Hokkaido, Japan

Abstract After 23 years of dormancy, Usu Volcano (Hokkaido, Japan) erupted on March 31, 2000. Many observations (seismicity, deformation rates, gravity data, groundwater level monitoring) show that the period of intense activity was short, starting abruptly, and continuing for ca. 5 months with a decreasing rate. Uplift was observed at two successive and separate locations at the time of the eruption. We obtained GPS and microgravity data at Usu Volcano for two intervals, the first from August 1996 to July 1998, once every 2–4 months, and the second in November 2000, 2 months after the end of the eruption. Between July 1998 and November 2000, the displacements and gravity variations are among the largest ever recorded on an active volcano in association with an eruption. We review three different elastic models commonly used in volcano-geodesy (sphere, fault system, fissure zone) and invert the high-quality data using each of these models. The combined inversion of GPS and microgravity data leads to the best solution in the least-squares sense. It is compatible with the intrusion of approximately 5×10 11 kg of new magma into the western part of Usu Volcano. This appears to have occurred in a subvertical fracture zone (about 2.4 km length, 0.1 km width) aligned in the east–west direction. The fracture zone is between 0.4 and 3.3 km depth with an extension of about 30 m. The fractures are likely to be filled with material having a density slightly higher than the density of old products of Mount Usu, i.e. about 2400 kg m −3 . This model is consistent with the locations and magnitudes of the earthquakes recorded during the period of intense seismic activity in April and May 2000. These earthquakes correspond to the boundaries of the intruded magma body. The model suggests that the two locations of uplift are not independent.

Preparatory process preceding the 2014 eruption of Mount Ontake volcano, Japan: Insights from precise leveling measurements 5.Volcanology

Preparatory activity preceding the 2014 eruption of Mount Ontake volcano was estimated from vertical deformation detected using a precise leveling survey. Notable uplift (2006–2009) and subsidence (2009–2014) were detected on the eastern flank of the volcano. We estimated pressure source models based on the vertical deformation and used these to infer preparatory process preceding the 2014 eruption. Our results suggest that the subsidence experienced between 2009 and 2014 (including the period of the 2014 eruption) occurred as a result of a sill-like tensile crack with a depth of 2.5xa0km. This tensile crack might inflate prior to the eruption and deflate during the 2014 activity. A two-tensile-crack model was used to explain uplift from 2006 to 2009. The geometry of the shallow crack was assumed to be the same as the sill-like tensile crack. The deep crack was estimated to be 2xa0km in length, 4.5xa0km in width, and 3xa0km in depth. Distinct uplifts began on the volcano flanks in 2006 and were followed by seismic activities and a small phreatic eruption in 2007. From the partially surveyed leveling data in August 2013, uplift might continue until August 2013 without seismic activity in the summit area. Based on the uplift from 2006 to 2013, magma ascended rapidly beneath the summit area in December 2006, and deep and shallow tensile cracks were expanded between 2006 and 2013. The presence of expanded cracks between 2007 and 2013 has not been inferred by previous studies. A phreatic eruption occurred on 27 September 2014, and, following this activity, the shallow crack may have deflated.