Sadato Ueki

Source process of very long period seismic events associated with the 1998 activity of Iwate Volcano, northeastern Japan

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 3.1×1012 J/s for the event on July 29, 1998.

Crustal deformation associated with the 1998 seismo-volcanic crisis of Iwate Volcano, Northeastern Japan, as observed by a dense GPS network

Mt. Iwate (2,038 m) is an active volcano located in northeastern Japan. Unrest of the volcano started in September, 1995 with intermediate-depth tremors. The shallow seismicity gradually became active in February, 1998, accompanying the notable crustal deformation observed by a dense GPS network. The pattern of the horizontal displacements is characterized by radially directing outward from the volcano. We estimated the source position by inversion analyses for every two-months period, assuming two models; a point pressure source (Mogi model) and a tensile fault. The comparison of AIC’s for the two models indicates that the latter is proper from February to April, while the former is preferable afterward. The tensile fault was located at about 5 km WSW of the summit and 3 km in depth, then a Mogi source was estimated at the western neighbor of the tensile fault in the successive period and moved westward as far as 10 km W of the summit with shallowing its depth. It should be noted that the seismic area also expanded westward in the same period. This synchronicity suggests that the both phenomena were caused by a movement of magma from the deeper part beneath the summit to the western shallower part.

A tensile — shear crack model for the mechanism of volcanic earthquakes

Abstract Temporary seismic observations were carried out for about 1 month at Miyakejima Island on the Izu-Bonin arc after an eruption on October 3, 1983. More than 3000 seismic events were observed and they are classified into 4 different categories: short-period earthquakes, long-period earthquakes, isolated tremors and continuous tremors. Two active source regions of the short-period earthquakes were located along the old caldera rim of the Miyakejima volcano. One region was very close to the eruptive fissures. Another was about 1–2 km northwest of the fissures. Almost all the earthquakes in the former region showed dilatant first motions of P-waves at all the stations in the island. On the other hand, almost all events in the latter region showed compressive first motions of P-waves at all the stations. The first motions of P-waves for these earthquakes cannot be explained by the quadrant-type pattern due to a double-couple force system. Nucleation of a tensile crack coupled with a shear crack (a tensile-shear crack model) is proposed as a source mechanism of the short-period earthquakes. Moment tensor elements of tensile and shear cracks have been determined by the least squares method using observed P- and S-wave data. Results suggest that tensile cracking (opening or closing) is dominant for generating those earthquakes in comparison with shear cracking. The strikes of the tensile cracks for the earthquakes are shown to be nearly parallel to the strike of the fissures. The earthquakes that occurred in the northwestern region of the fissures are considered to be generated by a sudden opening of tensile-shear cracks due to the excess pressure of intrusive magma. In contrast, the earthquakes occurring beneath the fissures are probably generated by a sudden closing of tensile-shear cracks, and this suggests that the magmatic pressure under the fissures rapidly decreased after the eruption. Moment tensor analysis of the tensile-shear crack model in this study demonstrates the non double-couple source mechanism for volcanic earthquakes related to magmatic activity.

Investigation of the Onikobe geyser, NE Japan, by observing the ground tilt and flow parameters

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 certain periods these times randomly shortened to about 60 s and 6 mins, respectively. Tilt records show a strong correlation with the short and long effusion times, reflecting water movement in at least two chambers beneath the vent. We are able to empirically predict the duration of effusion from tilt data, although flow pressure does not vary with effusion time.

Migration of seismic activity during the 1998 volcanic unrest at Iwate volcano, northeastern Japan, with reference to P and S wave velocity anomaly and crustal deformation

Abstract We describe the seismicity at Iwate volcano, northeastern Japan, during the volcanic unrest of 1998 with reference to a three-dimensional P and S wave velocity model from tomographic analysis. The abnormal seismic activity beneath Iwate volcano started under the caldera in February, 1998 and migrated westward in the period February to August, 1998. Previous geodetic modeling [Sato and Hamaguchi, Chikyu Monthly 21 (1999) 312–317] suggested the growth of a dike in the time of the seismic activity. Comparing the seismicity and dike extension with the tomographic images of the P and S wave velocity structure, we find that the trace of the growing dike coincides with the region of the high Vp and high Vp/Vs ratio beneath the volcano. The seismic and geodetic data are consistent with an intrusion of magma or other fluid under the caldera in 1998. Another pressure source causing the predominant crustal deformation at Iwate volcano was detected from geodetic data, which was located in the region with high Vp/Vs ratio under the western end of the volcano through the period from February to August. It is suggested that the activation of the point pressure source probably associated with the inflation of a hot fluid reservoir relate to a geothermal region adjacent to the western edge of the volcano.

Broadband seismic signals associated with the 2000 volcanic unrest of Mount Bandai, northeastern Japan

Abstract We have observed volcano-tectonic (VT) earthquakes, volcanic tremor and very long-period events (VLPEs) associated with the volcanic unrest of Mount Bandai since 2000 using a broadband seismic network closely deployed around the volcano. VT earthquakes are characterized by high-frequency (>10 Hz) signals with clear onsets of P and S phases, whereas volcanic tremor is characterized by a long coda comprising frequencies ranging from a few hertz to more than 10 Hz. In contrast, waveforms of VLPEs consist of very long-period (∼10 s) signals preceded by short-period (

Volcanic earthquake swarms at Mt. Erebus, Antarctica

Abstract Mount Erebus is an active volcano in Antarctica located on Ross Island. A convecting lava lake occupies the summit crater of Mt. Erebus. Since December 1980 the seismic activity of Mt. Erebus has been continuously monitored using a radio-telemetered network of six seismic stations. The seismic activity observed by the Ross Island network during the 1982–1983 field season shows that: (1)Strombolian eruptions occur frequently at the Erebus summit lava lake at rates of 2–5 per day; (2)centrally located earthquakes map out a nearly vertical, narrow conduit system beneath the lava lake; (3)there are other source regions of seismicity on Ross Island, well removed from Mt. Erebus proper. An intense earthquake swarm recorded in October 1982 near Abbott Peak, 10 km northwest of the summit of Mt. Erebus, and volcanic tremor accompanying the swarm, may have been associated with new dike emplacement at depth.

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.

Earthquake swarms on Mount Erebus, Antarctica

Abstract Mount Erebus (3794 m), located on Ross Island in McMurdo Sound, is one of the few active volcanoes in Antartica. A high-sensitivity seismic network has been operated by Japanese and US parties on and around the Volcano since December, 1980. The results of these observations show two kinds of seismic activity on Ross Island: activity concentrated near the summit of Mount Erebus associated with Strombolian eruptions, and micro-earthquake activity spread through Mount Erebus and the surrounding area. Seismicity on Mount Erebus has been quite high, usually exceeding 20 volcanic earthquakes per day. They frequently occur in swarms with daily counts exceeding 100 events. Sixteen earthquake swarms with more than 250 events per day were recorded by the seismic network during the three year period 1982–1984, and three notable earthquake swarms out of the sixteen were recognized, in October, 1982 (named 82-C), March–April, 1984 (84-B) and July, 1984 (84-F). Swarms 84-B and 84-F have a large total number of earthquakes and large Ishimoto-Iidas “m”; hence these two swarms are presumed to constitute on one of the precursor phenomena to the new eruption, which took place on 13 September, 1984, and lasted a few months.

New Seismic Evidence for the Origin of Arc and Back-Arc Magmas

We used high-resolution seismic tomography to probe into the arc and back-arc magmatism and volcanism of the Japan subduction zone. Prominent zones with low-velocity, high Poisson’s ratio, high-attenuation and strong seismic anisotropy are revealed in the crust and uppermost mantle beneath the active volcanoes, and they exist in the central portion of the mantle wedge under the volcanic front and back-arc region, roughly parallel with the subducting oceanic plate. The anomalous zones in the mantle wedge are connected with the subducting slab at a depth of 90–150 km, while they also exhibit along-arc variations. These seismological results indicate that the arc and back-arc magmas are caused by a combination of corner flow (convection) in the mantle wedge and fluids resulting from the dehydration process of the subducting oceanic plate.