No abstract

An M JMA 6.5 earthquake (foreshock) and M JMA 7.3 earthquake (mainshock) struck Kumamoto Prefecture on April 14, 2016, and April 16, 2016. To evaluate the effect of crustal deformation due to the earthquake on the Aso magma system, we detected crustal deformation using InSAR and GNSS. From InSAR analysis, we detected large crustal deformations along the Hinagu Fault, the Futagawa Fault, and the northeast extension of the latter fault. It extended to more than 50 km, and the maximum slant-range change exceeded 1 m. Although the obtained crustal deformation was approximately explained by the right-lateral strike-slip on the fault, its details could not be explained by such simple faulting. Additionally, we found complex surface deformation west of the Aso caldera rim, suggesting that shallow fault slips occurred in many known and unknown faults associated with the earthquake. Most of the crustal deformation could be reasonably explained by four rectangle faults located along the Futagawa Fault, in the northeast extension of the Futagawa Fault, alongside the Hinagu Fault, and in the eastern part of the Futagawa Fault. The first three of faults have high dip angles and right-lateral slip. The other was a fault with a low dip angle that branched from the shallow depth of the fault along the Futagawa Fault. The normal-dip right-lateral slip was estimated for this segment. Based on the estimated fault model, we calculated the displacement and stress field around the Aso volcano by the finite-element method (FEM) to evaluate the effects on the Aso magma system. In this calculation, we assumed a spherical soft medium located at a 6-km depth beneath the area south of the Kusasenri region as the magma system and considered only static effects. The result shows complex distributions of displacements and stresses, but we can notice the following significant points. (1) The spherical magma system deformed to an ellipsoid, and the total volume was slightly increased, less than 1%. (2) The differential stress around the upper portion of the magma system was as large as 3.5 MPa. This is strong enough to open pre-existing cracks and can cause the migration of magma.

The National Research Institute for Earth Science and Disaster Prevention (NIED) developed volcano observation stations at the Kirishima volcanic group in 2010. The stations observed remarkable crustal deformation and seismic tremors associated with the Shinmoe-dake eruption in 2011. The major eruptive activity began with sub-Plinian eruptions (January 26) before changing to explosive eruptions and continuous lava effusion into the summit crater (from January 28). The observation data combined with GEONET data of GSI indicated a magma chamber located about 7 km to the northwest of Shinmoe-dake at about 10 km depth. The tiltmeter data also quantified detailed temporal volumetric changes of the magma chamber due to the continuous eruptions. The synchronized tilt changes with the eruptions clearly show that the erupted magma was supplied from the magma chamber; nevertheless, the stations did not detect clear precursory tilt changes and earthquakes showing ascent of magma from the magma chamber just before the major eruptions. The lack of clear precursors suggests that magma had been stored in a conduit connecting the crater and the magma chamber prior to the beginning of the sub-Plinian eruptions.

The Miyakejima observation network had been constructed by the National Research Institute for Earth Science and Disaster Prevention mainly until early 1999. This observation network has provided the crustal deformation data by tiltmeters and GPS and the seismic data by short-period and broadband seismometers in association with the 2000 Miyakejima eruption. The subsurface magma movement at the first stage of the present activity, during the period from June 26 to 27, was successfully detected mainly by the tilt measurements. The tilt change observed at five stations indicates the migration of magmas from the eastern part of Miyakejima to the western part. The most distinctive phenomenon appearing after the first stage is tilt steps, which started on July 8 with the first eruption from the summit crater. Each tilt step indicates an abrupt uplift of the summit area. These tilt steps continued until the eruption of August 18, which is the largest eruption up to early September, 2000. 45 tilt steps in total were observed in this period. The seismic data show a variety of seismograms including VT (volcano-tectonic) earthquakes, LF (low frequency) earthquakes and volcanic tremor. At the time of the tilt steps, very long period events with predominant periods of about 100 s were detected by the broadband seismometers. As the activity has still continued, this report summarizes the observation during June, July, and August, 2000.

[1] The lava flow simulation code LavaSIM has been developed to give accurate predictions of volcanic disasters and to support evacuation plans. The code uses three-dimensional analysis with free surface evaluation, including boundary transport between the melt and the crust. It is therefore applicable to various types of lava flows and flow behaviors such as flood basalts, subaqueous lava flows, and levee formation. Heat transfer between the lava and the ground, air, and water, and between the melt and crust of the flow is calculated by using appropriate relations. The code has been verified by applying it to actual lava flows, and the simulation results have been compared with observations of the Izu-Osima flows of 1986. The lava flow rate, temperature, and properties were assigned values based on data from existing literature. The comparisons demonstrated the code's capability for prediction of inundated areas and maximum flow lengths. The present code is expected to be useful for real-time prediction during eruptions, in addition to assessing volcanic hazards and designing protection structures for existing installations against possible future lava flows.