Ground deformation often precedes volcanic eruptions, and results from complex interactions between source processes and the thermomechanical behaviour of surrounding rocks. Previous models aiming to constrain source processes were unable to include realistic mechanical and thermal rock properties, and the role of thermomechanical heterogeneity in magma accumulation was unclear. Here we show how spatio-temporal deformation and magma reservoir evolution are fundamentally controlled by three-dimensional thermomechanical heterogeneity. Using the example of continued inflation at Aira caldera, Japan, we demonstrate that magma is accumulating faster than it can be erupted, and the current uplift is approaching the level inferred prior to the violent 1914 Plinian eruption. Magma storage conditions coincide with estimates for the caldera-forming reservoir ~29,000 years ago, and the inferred magma supply rate indicates a ~130-year timeframe to amass enough magma to feed a future 1914-sized eruption. These new inferences are important for eruption forecasting and risk mitigation, and have significant implications for the interpretations of volcanic deformation worldwide.

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After the occurrence of the 2011 magnitude 9 Tohoku earthquake, the seismicity in the overriding plate changed. The seismicity appears to form distinct belts. From the spatiotemporal distribution of hypocenters, we can quantify the evolution of seismicity after the 2011 Tohoku earthquake. In some earthquake swarms near Sendai (Nagamachi-Rifu fault), Moriyoshi-zan volcano, Senya fault, and the Yamagata-Fukushima border (Aizu-Kitakata area, west of Azuma volcano), we can observe temporal expansion of the focal area. This temporal expansion is attributed to fluid diffusion. Observed diffusivity would correspond to the permeability of about 10 À15 (m 2 ). We can detect the area from which fluid migrates as a seismic low-velocity area. In the lower crust, we found seismic low-velocity areas, which appear to be elongated along N-S or NE-SW, the strike of the island arc. These seismic low-velocity areas are located not only beneath the volcanic front but also beneath the fore-arc region. Seismic activity in the upper crust tends to be high above these low-velocity areas in the lower crust. Most of the shallow earthquakes after the 2011 Tohoku earthquake are located above the seismic low-velocity areas. We thus suggest fluid pressure changes are responsible for the belts of seismicity.

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Abstract. We observed very long period seismic events that are associated with the 1998 activity of Iwate Volcano, northeast Japan. The events show a dominant period of 10 s and duration of 30-60 s, often with accompanying short-period waves at the beginning and at the end of the long-period signals. By analyzing the broadband seismograms we find that the source elongates in the east-west direction for-4 km at a depth of 2 km beneath the western part of Iwate Volcano. Results of moment tensor inversions show a source mechanism of mutual deflation and inflation of two chambers located at the western and eastern edges of the source region. The source region coincides with the low seismic velocity zone detected by seismic tomography and is very close to the locations of pressure sources estimated from crustal deformation data. On the basis of these results we infer that the very long period seismic events are generated by transportation and movement of magmatic fluid (hot water and/or magma) in a shallow part of the volcano. We further present a simple source model of very long period seismic events based on one-dimensional flow dynamics and propose a new parameter to characterize the size of very long period event: the energy flow rate, which is obtained by dividing the seismic moment by the dominant period. The energy flow rate was estimated as 3.1 x 1012 J/s for the event on July 29, 1998.

SUMMARY The 1998–1999 volcanic unrest of Iwate volcano, northeastern Japan, was marked by 350 deep low‐frequency earthquakes (DLFs) and 120 intermediate‐depth low‐frequency earthquakes (ILFs), as well as an intense swarm of shallow volcanic earthquakes that began during 1998 April. The rate of occurrence of the DLFs increased approximately 5 d before that of the shallow volcanic earthquakes increased, and the number of ILFs gradually increased from the middle of 1998. To investigate the relationship between the shallow volcanic activity and the activities of the DLFs and ILFs, we determined their precise hypocentres and source mechanisms by analysing waveform data recorded by a dense seismic network of 47 three‐component seismometers located on and around the volcano. The hypocentres of DLFs are concentrated within three regions: the first region is at depths from 31 to 34 km, approximately 10 km south of the summit, the second is at depths from 32 to 36 km approximately 10 km northeast of the summit and the third is at a 37 km depth, approximately 7 km northeast of the summit. In contrast, ILFs are located within a vertical pipe‐like region just beneath the summit and sometimes show a vertical migration of the focal depth. Our moment tensor inversion using spectral ratios of body waves indicates that the source mechanisms of the DLFs and ILFs have a significant double couple and a compensated linear‐vector dipole component. It is also found that a significant volumetric change is included in the source mechanisms for some DLFs. Such source mechanisms of DLFs and ILFs can be explained by the motion of a tensile crack coupled either with a shear crack or with an oblate spheroid magma chamber. However, the orientations and polarities of the crack motions are not the same in each region of DLFs and ILFs. These results suggest that a complex magma system is present at the source regions of the DLFs and ILFs.

[1] We present the three-dimensional structures of the P wave velocity (V P ), S wave velocity (V S ) as well as the P wave to S wave velocity ratio (V P /V S ) beneath Mount Fuji and the South Fossa Magna, Japan, using arrival time data collected from 2002 to 2005 by a dense seismograph array. The high-resolution data set and improved methodology reveal not only several velocity features that are consistent with previous studies but also important new details that clarify the velocity structures associated with volcanic processes beneath Mount Fuji and the collision tectonics of the South Fossa Magna. One such particular feature is a low-V P , low-V S and low-V P /V S anomaly at depths of 7-17 km beneath Mount Fuji that corresponds with the locations of deep low-frequency (DLF) earthquakes. The coincidence of the velocity anomaly and the DLF locations suggests that supercritical volatile fluid, such as H 2 O and CO 2 , may be abundant in the low-V P /V S region and may play an important role in generating DLF earthquakes. This anomaly overlies a deeper low-V P , low-V S and high-V P /V S anomaly at depths of 15-25 km that may represent a zone of basaltic partial melt. A low-V P , low-V S and low-V P /V S anomaly is seen at depths of 6-14 km beneath Mount Hakone. Isovelocity surfaces (V P = 6.0 km/s and V S = 3.5 km/s) corresponding to the upper limit of hypocenter distribution below Mount Fuji may define the upper surface of the Philippine Sea plate whose existence in a seismic gap beneath Mount Fuji has been controversial.

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