Takeshi Nishimura

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

We detected temporal changes of crustal structure by cross spectrum analyses of seismic waves excited by two artificial explosions that were carried out about one month before and two months after a M6.1 shallow earthquake. Phase spectrum analysis shows that seismic velocity of the upper crust around the focal area of the M6.1 earthquake and Mount Iwate decreased 0.3–1.0% during the three months. Since lager decreases of the seismic velocity were observed at the region close to the focal area an

We present one of the first studies on source location determination for volcanic earthquakes and characterization of volcanic subsurfaces using data from a distributed acoustic sensing (DAS) system. Using the arrival time difference estimated from well-correlated waveforms and a dense spatial distribution of seismic amplitudes recorded along the fiber-optic cable, we determine the hypocenters of volcanic earthquakes recorded at Azuma volcano, Japan. The sources are located at a shallow depth be

Abstract We have examined temporal changes in seismic velocity of the crust through repeated active seismic experiments at Iwate volcano, Japan, where a significant volcanic activity and an M6.1 earthquake were observed in 1998. We apply a cross spectrum moving window technique to seismic data recorded at eight stations for the six explosions detonated from1998 to 2003. The seismic velocity at the frequency range of 3-9 Hz decreased by about 1% during the three months including the occurrence of

Abstract When a large earthquake occurs near an active volcano, there is often concern that volcanic eruptions may be triggered by the earthquake. In this study, recently accumulated, reliable data were analyzed to quantitatively evaluate the probability of the occurrence of new eruptions of volcanoes located near the epicenters of large earthquakes. For volcanoes located within 200 km of large earthquakes of magnitude 7.5 or greater, the eruption occurrence probability increases by approximatel

Pressure recovery in magma subject to sudden depressurization is investigated in terms of the growth of bubbles containing magmatic volatiles. Assuming conservation of volatile mass and pressure equilibration between the bubbles, melt and a surrounding elastic medium, the final magma pressure is completely determined by its initial pressure and the magnitude of the pressure drop. Simulations show that the initial magma pressure is easily recovered, or even exceeded, when magma containing tiny bu

The aim of the present study is to clarify a scaling law for volcanic explosion earthquakes excited by a counter force of eruption. Based on the data of explosion earthquakes at three volcanoes, we derived the following results: The peak amplitude of the single force is proportional to the square of the pulse width of source time function. The initial pressure stored in a shallow region beneath a volcano before eruption is 1 MPa with a perturbation of one order. The kinetic energy of explosion e

Abstract In this study we conducted observations of the Onikobe geyser, NE Japan, by deploying a tiltmeter and an acoustic sensor close to the vent, a flow pressure sensor at the conduit exit, and measuring water temperature at ground level. The data from these instruments are consistent with the model of geysers which involves effusion process of boiling due to depressurization. During the observation period, the geyser generally effused water for about 90 s every 10 mins, although during certa

After the Fukushima Daiichi nuclear power plant (NPP) accident, new regulatory requirements were enforced in July 2013 and a backfit was required for all existing nuclear power plants. It is required to take measures to prevent severe accidents and mitigate their radiological consequences. The Regulatory Standard and Research Department, Secretariat of Nuclear Regulation Authority (S/NRA/R) has been conducting numerical studies and experimental studies on relevant severe accident phenomena and c

The finite difference method is used to calculate the magma dynamics, seismic radiation, and crustal deformation associated with a volcanic eruption. The model geometry consists of a cylindrical reservoir and narrow cylindrical conduit embedded in a homogeneous crust. We consider two models of eruption. In the first model, a lid caps the vent and the magma is overpressurized prior to the eruption. The eruption is triggered by the instantaneous removal of the lid, at which point the exit pressure

During the 1988–89 eruptive activity of Mt Tokachi, Hokkaido, Japan, volcanic tremor and many low-frequency earthquakes were observed by the National University Team at three stations close to the active crater. We calculated the spectral ratios of the tremor and the low-frequency events which were associated with the ash emission from the active crater, and showed that the ratios of both these events have similar features, indicating similar source mechanisms and focal depths. We further resolv