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Seismic exploration was conducted along a profile running through the Aira caldera located in southern Kyushu, Japan. The caldera was formed by an ignimbrite eruption approximately 30 ka BP, namely, the “AT eruption,” which produced the Ito ignimbrite and widespread Aira-Tanzawa ash. This analysis aimed to clarify the detailed P-wave velocity structure beneath the caldera. Accordingly, 829 inland seismic stations and 42 ocean bottom seismographs were deployed along the 195 km-long seismic profile to record seismic waves generated by numerous controlled seismic sources. A detailed velocity structure of the active Aira caldera was successfully obtained to depths of 20 km through travel-time tomography. A substantial structural difference was observed in the thicknesses of the low-velocity zones between the eastern and western sides in the shallowest region of the Aira caldera, suggesting that the Aira caldera is composed of at least two calderas: the AT caldera associated with the AT eruption, and the Wakamiko caldera associated with the post-AT eruption. Perhaps the most interesting feature of the caldera structure is the existence of a substantially high-velocity zone at depths of 6–11 km beneath the center area of the AT caldera, which can be interpreted as the cooled and solidified magma reservoir formed during or after the AT eruption. In addition, a low-velocity region with approximately 15 km depths indicated a deep magma reservoir. Based on these novel and past research results, a new magma supply model in the Aira caldera was proposed. Further, the spatial distribution of the magma reservoir associated with the AT eruption 30 ka BP was estimated, while the future possibility of larger eruptions in this caldera was discussed.

A new Phase-Shifted Optical Interferometry seismometer system was tested in terms of its feasibility for multi-channeled volcanological observations in two volcanos in Japan. The system is capable of both sensing ground motions and transferring its signals through optical means. The prototype of this system comprises three optical-wired stations and optical components, and was deployed in Sakurajima Volcano in 2016 and in Asama Volcano in 2017. The system successfully operated for 134 days in total and provided seismograms that are in good agreement with those obtained using conventional systems. Several obstacles for putting this system to practical use that need to be solved were found through tests. Their solutions will be explored in subsequent research.

Structural evolution beneath an active volcano is detected as the variation of seismic reflectivity through controlled seismic experiments, which is interpreted as being associated with discharging magma. The target of the present study is Sakurajima Volcano, which is one of the most active volcanoes in Japan. Six rounds of seismic experiments with controlled sources have been conducted annually at the volcano. Two seismic reflection profiles are obtained from the datasets for each successful round of experiments. The experiments reveal clear annual variation in seismic reflectivity at a depth of 6.2 km in the northeastern part of Sakurajima. The reflectivity is maximum in December 2009 upon the first intrusion of magma and decreases gradually until December 2013, which coincides with the inflation and deflation cycle of Sakurajima Volcano. Reflectivity variation occurred in the embedded clear reflector at depth. An evolving sandwiched structure in the intermediate layer is used as the reflector model. Lower-velocity magma embedded in the intermediate layer and its succeeding velocity increment explain the variation range of reflectivity. This is interpreted as a temperature decrease associated with discharging magma at depth. The present study describes a new approach for instantaneously sensing magma properties and for monitoring active volcanoes.

A gravity survey was conducted along the Mizuho traverse routes in austral summer of 1999-2000 by the 41st Japanese Antarctic Research Expedition (JARE-41). The main purpose of the survey was to obtain detail gravity anomaly along the traverse routes, where deep seismic exploration was simultaneously conducted in order to obtain a fine crustal structure of the Mizuho Plateau. By using SCINTREX (CG-3M) gravity meter, the survey was carried out at 160 sites with about 1 km interval in a distance of 190 km from S16 to Z20 on the traverse routes. Free-air and Bouguer anomalies were calculated using precise locations by GPS measurements, by taking into account an effect of the thick icesheet. The furrowed negative Free-air anomalies are identified around H192, where middle points of the whole traverse routes. Two bedrock elevation models derived both by seismic refraction analyses and radio-echo sounding might correspond to the bottom and the top of a mixture layer composed from ice-moraine rocks over the bedrock surface. Thickness of the crust estimated by Bouguer anomalies is 0.5 - 1 km larger toward inland at the terminal point of Z20 along the profile.

An active seismic survey with unprecedented density of seismometers found a high velocity zone to the west of the summit of Asama Volcano, Japan. The high velocity zone coincides with an area of magma-filled crack (dike) intrusion associated with the 2004 eruptions inferred from precise earthquake relocations and ground deformation modeling. It also coincides with an area of high resistivity surrounded by low resistivity, indicating that the solidification of magma due to repeating intrusions is responsible for the high velocity. This thus endorses the magma pathway previously speculated by seismic and geodetic observations. These findings demonstrate that dense seismic exploration combined with geophysical monitoring is an effective way to understand the dynamics of volcanic eruptions.

We investigate the basement structure of the Hokkaido Komagatake volcano, Japan, by an artificial seismic survey. By inverting first arrival time data, a three-dimensional P-wave velocity model down to 3 km b.s.l. is obtained. The resultant velocity model reasonably reflects regional geological features and is consistent with Bouguer anomalies. Around the Komagatake volcano, the velocity basement deepens to NNW on the whole, reflecting the deepening of the geological basement toward that direction. Higher velocities are observed in Oshima and Kameda Mountains where Neogene volcanic and sedimentary rocks are present. On the contrary, lower velocities are observed in lowlands surrounding the volcano, where Komagatake deposits and Quaternary sediments crop out. At the volcano, a prominent high velocity zone was detected from the surface down to a depth of 1.5 km b.s.l. Above the base, probably a contrast between the core part with higher proportion of massive lavas and intrusions and flank areas dominated by loose pyroclastic deposits caused the prominent high velocity. Below the base, it is thought that solidified intrusive bodies largely contribute to making the high velocity. Further, a high velocity elongates from Kameda Mountains to beneath the summit. This is due to a shallower of the volcano substratum.

Isolated-type tremors having two events with different dominant frequencies are characteristic seismological phenomena observed during the fumarolic activity stage at Aso Volcano. These isolated tremors are called hybrid tremors (HBT) and comprise two parts: an initial part named the “HF-part” with a dominant frequency in the high-frequency region (approximately 10 Hz) and the following part named the “LF-part” with a dominant frequency in the low-frequency region (approximately 2 Hz). The LF-part is observed after the HF-part, and the HBT is accompanied by a long-period tremor (LPT). Hypocenters and source parameters are estimated using seismograms recorded at 64 stations around Nakadake crater. The amplitude distributions of all HF-parts have almost similar trends. Similarly, the amplitude distributions of all LF-parts have almost similar trends. However, the amplitude distributions of HF- and LF-parts are not similar. From these results, we proposed that the hypocenters and source parameters of HF- and LF-parts are not common, but each of them have common hypocenters and source parameters. The hypocenter region of HF-parts was estimated to be just beneath the fumarole region south of the 1st crater: the volume fluctuation is the major source factor. The hypocenter region of LF-parts is estimated to be at a depth of approximately 300 m beneath the first crater: the strike–slip component is the major source parameter. The hypocentral depth of LF-parts is located at the upper end of the crack estimated to be the source of the LPTs. The LPTs and HBTs are observed almost simultaneously. We consider that volcanic fluid is involved in the source mechanisms of both HBT and LPT.

The three-dimensional P-wave velocity structure of Usu volcano, Japan, is investigated by using first arrival time data of the 2001 active seismic survey. The velocity structure down to the Pre-Neogene basement below the volcano is revealed. The most prominent feature of the velocity model is the deepening of the basement toward the south-southwest. The resultant velocity model contributes to the relocations of 2000 eruption precursory earthquakes. The precursory seismic activity, which reflects pre-eruptive magma movements, is divided into three parts: (1) a sub-vertical distribution indicating magma ascent to beneath the summit of the volcano, (2) a northward migration that indicates magma movement resulting in the subsequent eruptions at the northwestern foot, and (3) a horizontal southward migration possibly indicating intrusion of sill. The northward and southward migrating earthquakes are constrained by the dipping basement structure such that most of the events occur within the middle Miocene or Pre-Neogene layer. The geographical distribution of craters and the upheavals of past eruptions are also constrained by the subsurface basement. At the northern foot of Usu Volcano, the distribution of these craters and upheavals is such that they are enclosed by the subsurface basement, which becomes shallower toward the north. On the other hand, no traces of eruptions are found in the southern and southwestern foots where the basement deepens toward the south-southwest.

2003年に富士火山を対象とする人工地震探査が行われた。人工地震探査は富士山を中心として89kmの測線上に5点の発破点と468点の観測点が設定された。本報文では2003年人工地震実験の波形記録に対して山体内部の地震波反射構造の解析には擬似反射記録法を用い，深部地震波反射構造の解析にはシングルフォールド反射断面を用いた結果について報告する。 富士火山の山体内部の擬似反射断面では，南あるいは南西に傾斜する明瞭な境界面が比較的高い標高に検出され，その下位には上下で反射位相のならびのパターンが異なる境界面も検出された。前者は古富士火山上面に対比される可能性が指摘され，後者は富士山の基盤面に対応する可能性が指摘される。基盤面は富士山東斜面の下では丹沢山地の西の延長部が山頂直下まで起伏をともなって連続する一方，南西麓では急速に海面下まで深くなる。それにともなって富士山の南西麓には堆積層が発達しており，堆積層の南西縁は富士川断層系で区切られる。 富士火山とその周辺の深部地震波反射構造は，富士山南西側地域，富士山山体直下地域，富士山北東側地域の3つに大別される。富士山北東側地域では数十キロにわたるほぼ水平で連続性の良い深部反射面が複数認められるが，富士山南西側地域では南西落ちに急傾斜する深部反射面が複数認められる。これらの深部反射面のいくつかはフィリピン海プレート上面もしくは内部の速度／密度境界に対比される可能性があり，富士火山はフィリピン海プレートの傾斜が変化する場所に立地している。また，富士山山体直下では狭い領域にさしわたし数キロメートルの反射面が複数出現し，その周囲とは明確に異なる地震波反射構造をしている点が注目される。特に山頂の北東3～6kmの深さ13kmおよび16km付近にみとめられる明瞭な反射面はこれまでの研究で指摘された低周波地震の震源域と一致し，火山性流体と関係している可能性が高いと考えられる。さらに，これより上位の反射面群は深さ16kmの反射面と富士山頂を結ぶ線上に並んでいる。 以上のように富士火山は深部から地表にいたるまで南西側に傾斜した構造をもち，深さ16km付近まで明瞭で特徴的な地震反射面が複数存在することが明らかになった。

物理探査をより発展させるためには，新しい理論や手法の開発が必須である。しかし，それらと同等に重要なことは人材を育成することである。特に新しい発想をもった，過去にとらわれない若い人材育成が欠かせない。現在，理工系離れが重要な問題になっている。物理探査の発展のためには若い人，特に小中学生に物理探査のおもしろさを宣伝して物理探査に興味を持ってもらい，将来物理探査を支える人材として育てることが大切である。現在広く実施されている体験教室や科学教室では，原理の説明あるいはもの作りに重点があり，原理を応用して探査まで結びつける体験教室はほとんど実施されていない。原理を説明し，簡単な探査装置を作り，実際に探査を行う体験学習教室が効果的であると考えられる。秋田大学ではこの考えに基づき，若い人たちに物理探査に興味を持ってもらい，将来の探査技術者になることを期待して体験学習教室を実施してきた。実際に体験学習教室を実施して自分の装置が物体に反応することを体験した小中学生は，例外なく驚きの表情を浮かべ，自発的に探索を始める。このような積極性は普段の授業では考えられないことである。簡単な探査装置を教材にした体験学習は効果的であり，物理探査関係の人材育成に有効であることを明らかにした。

The three-dimensional P-wave velocity structure of the Bandai volcano has been revealed by tomographic inversion using approximately 2200 travel-time data collected during an active seismic survey comprising 298 temporary seismic stations and eight artificial shots. The key result of this study is the delineation of a high-velocity anomaly (Vp>4.6 km/s at sea-level) immediately below the summit peak. This feature extends to depths of 1–2 km below sea-level. The near-surface horizontal position of the high-velocity anomaly coincides well with that of a positive Bouguer gravity anomaly. Geological data demonstrate that sector collapses have occurred in all directions from the summit and that the summit crater has been repeatedly refilled with magmatic material. These observations suggest that the high-velocity region revealed in this study is a manifestation of an almost-solidified magmatic plumbing system. We have also noted that a near-surface low-velocity region (Vp

The subsurface structure of Aso Volcano, central Kyushu, Japan, has been imaged down to 10 km below sea level and notable features of subsurface structure in the central cones were revealed by a 3-D seismic reflection analysis. The 3-D reflection analysis included reflection enhancements and 3-D migration. The reflection enhancements and the NMO correction were applied on waveform data from the seismic experiment ASO98. The 3-D migration with a Kirchhoff integral was processed for 1397 NMO corrected traces to calculate the reflectivity at about 60 000 image points in an approximately 7 km by 7.5 km square region. The results of the 3-D migration show that the subsurface structure beneath the central cones can be divided into three parts, the western part, the eastern part, and the northwestern part. Bright reflectors appear in most of them and a significant discontinuity is inferred. The reflector horizon at 2 km below sea level spreads beneath the western and the eastern part and is probably the top surface of the Pre-Aso volcanic rocks or the basement rocks. Reflection voids appear in the western part and just beneath the active crater. The reflector void in the western part implies a hot region and is most probably the known seismic anomalies, such as low velocity region or attenuation region. Another reflector void beneath the active crater can also be associated with a hot region, apparently surrounding the conduit system of the current volcanic craters. The major discontinuity is located beneath the northwest flank of the central cones. This discontinuity divides the northwest region with weak reflectors from other parts. The discontinuity may be the trace of the Oita–Kumamoto Tectonic Line, which is the major tectonic line in central Kyushu, crossing the Aso caldera.

Seismic exploration was conducted on the Mizuho Plateau, East Antarctica, during the 1999-2000 austral summer season by the 41st Japanese Antarctic Research Expedition (JARE-41) Seismic shot records were obtained with clear arrivals of the later reflected phases by a total amount of 3300 kg dynamite charges in seven explosions along the Mizuho traverse route 180 km in length The purpose of this study is to investigate characteristics of the Moho discontinuity from reflective evidence by using explosive seismic waves First, we used the mirror image method for the travel time data of Moho reflected waves (PmP phases) Optimal image points were determined to minimize root-mean-square residuals between the observed and the calculated travel times by an iterative grid search at 1 km intervals The depth of the reflected Moho and the averaged incident angles were determined by taking the P wave velocity of the crust as a parameter The obtained Moho depth and the associated dipping angle were well correlated with those from the previous reflection study Amplitude spectrum analysis was also applied to obtain the difference in peak frequency of the spectrum between the PmP phases and the direct P phases The observed spectral ratios of the reflected PmP phases to the direct P phases indicate spectral peaks at 9 5 and 19 0 Hz, and a spectral trough at 13 8 Hz, respectively This pair of spectral peaks and the trough can be explained by the existence of a thin reflecting layer of 690-860 m thickness just beneath the Moho discontinuity The thin layer may have relatively lower velocities of 7 0-7 8 km/s than those of the surrounding uppermost mantle These relatively low velocity layers around the Moho are considered to be composed of clino-pyroxenite and/or pyroxenehornblende gneiss by comparison with high-pressure laboratory measurements of metamorphic rocks from the Liitzow-Holm Complex

A seismic survey with a high density network was conducted by the 41st Japanese Antarctic Research Expedition (JARE-41) in the austral summer of 1999-2000. The seismic line was 190 km long, along the Mizuho traverse route from the base camp S16. 160 temporary seismic stations using data loggers were installed. The first arrival times were analyzed by the refraction method. The structure of the upper crust and overlying ice-sheet are revealed down to 7 km from refraction analysis of the first arrival times. A three layered structure is estimated. The first layer with a velocity of 3.8 km/s appears to be ice-sheet. The second layer with a velocity of 6.2 km/s of P-wave velocity is the surface layer of the continental crust. The thickness of this second layer is estimated to be approximately 5 km. The third layer is defined by the P-wave velocity of 6.5 km/s. Its shape of the upper interface is not declared. The topography of the basement determined from seismic refraction matches well with that determined from ice-radar sounding except for a small difference in the estimated depth of the basement. Two abrupt changes of the second layer depth are found near shot S-1 and shot S-4. The former includes a velocity change in the second layer. The latter discontinuity corresponds to the echo-less points of the ice-radar sounding. Velocity changes in the first and second layers are not detected across the latter discontinuity.

A seismic survey with a high density network was performed by JARE-41 in the austral summer of 1999 to 2000. The seismic line is spread out over 190 km along the Mizuho traverse route. 160 seismic stations programmed on timer operation were installed. A clear reflection is obtained in some shot records. It was described as a PmP reflection in a previous study. This survey geometry enables us to apply reflection analysis. A single fold profile and corresponding velocity structures for the JARE-41 seismic line were obtained in this study. The dipping Moho reflector and dipping reflectivity layer are revealed beneath the full spread of the seismic line in the profile. The Moho reflector is located at approximately 42 km beneath the shot point S-6 and at approximately 29 km below sea level beneath the coastal end of the profile. There is a lateral change in reflection character in the reflective layer in the neighborhood of the shot point S-4, although the velocity of the upper crust from refraction analysis is constant in this region. The structure obtained might be one case of Seaward Dipping Reflector Sequences in the continental margin area of the past Gondwana super-continent.

A seismic refraction experiment was successfully conducted along the S17-Z20 profile on the Mizuho route, in East Antarctica, in the austral summer season of 1999-2000 (JARE-41). One hundred sixty seismic stations were temporarily installed along the profile about 180km long and five large shots with dynamite of about 600kg were fired. In addition, two shots with charge sizes of 250kg and 25kg were arranged along the profile. The obtained seismic records show the clear onsets of the first arrivals in a distance range of less than 100km from each large shot. In particular, seismic waves traveling through the ice sheet and the dispersed surface waves are distinctly observed. Some later phases are also detected. The first travel time data obtained show that a P-wave velocity in the ice sheet is 3.6-3.8km/s and an apparent velocity in the rock basement just beneath the ice sheet is almost 6.2km/s. This report describes the basic outline of the experiment and the seismic data obtained.

We deployed a network of broadband seismometers for one year around the Naka-dake first crater of Aso volcano in Kyushu, Japan, to reveal the mechanism of long period tremors (LPTs) emitted from the volcano. It is observed that LPTs with a dominant period of about 15 s are always emitted regardless of the surface activity of the volcano. A typical LPT has a short duration less than a minute and its spectrum shows mode peaks at 15, 7.5, 5, and 3 s. The particle motion in the frequency band for the lowest two modes at stations within a few kilometers from the crater is rectilinear, pointing in the direction of the crater. A waveform semblance technique to locate sources of LPTs is devised to utilize the rectilinearity of waveforms. The LPT sources are located at depths of 1–1.5 km beneath the bottom of the crater. When the volcano is explosively ejecting steam and mud, on the other hand, a very long period (∽100 s) displacement (VLPD) polarized outward from the crater often precedes an eruptive event by a few minutes. A typical VLPD is accompanied by a few long period pulses, first positively polarized and concurrent with the onset of VLPD, then negatively polarized just before the eruption. The source of VLPDs is inferred to coincide approximately with that of LPT. On the basis of these observations, a qualitative model is constructed for the hydrothermal system beneath the Naka-dake first crater. An explanation for the unusually long period nature of the LPT is discussed in terms of a class of slow waves, which exist in solid–liquid two-phase systems. A possibility of realtime monitoring at Aso volcano using the observed long period seismic signals is also discussed.

To constrain the source of long period tremors (LPTs), we deployed a very dense broadband seismic network consisting of totally twenty-four stations around the active crater of Aso volcano in Kyushu, Japan. The spatial variation of the observed signal amplitudes reveals that the source of LPTs consists of an isotropic expansion (contraction) and an inflation (deflation) of an inclined tensile crack with a strike almost parallel to the chain of craters. The detected crack has a dimension of 1 km and its center is located a few hundred meters southwest of the active crater, at a depth of about 1.8 km. The extension of the crack plane meets the crater chain including the active fumarole at the surface, suggesting that the crack has played an important role in transporting gasses and/or lava to the craters from below. This work also demonstrates a powerful usage of broadband seismometers as geodetic instruments to constrain subsurface structures at active volcanoes.

The May 1998 eruption sequence of Sakurajima Volcano was monitored by ten infrasonic stations, ten seismometers, and a video camera. During this seismo-acoustic experiment, we recorded hundreds of infrasonic tremor and long-period events associated with seismic signals, and observed a progression from relative quiescence to a Vulcanian eruption. The number of infrasonic events increased with escalating volcanic activity, and the dominant character of the infrasonic signals changed from impulsive to emergent. At 22:17 of May 19, Sakurajima released ash and gases to a height of 2 km above the vent, an event that was recorded continuously by one infrasonic and two seismic stations. We present the experimental setup as well as a procedure through which infrasonic signals may be incorporated into future eruption monitoring and forecasting algorithms for open-vent volcanic systems. In addition, our recordings suggest that infrasonic signals are more representative of processes occurring within the volcanic interior than are seismic signals, which are strongly altered by diffraction and scattering in the volcanic edifice.

The Kitakami massif, which is located in the eastern part of Northern Honshu, Japan, is composed of two geological units. The northern Kitakami terrane is characterized as a Jurassic accretionary complex, while the southern Kitakami terrane consists of pre-Silurian basement and Silurian-lower Cretaceous marine sediments. The boundary region of these two units, called the Hayachine tectonic belt (HTB), is composed of mafic to ultramafic rocks. The Kitakami massif experienced intense granitic intrusions in the Cretaceous. We present a detailed crustal structure model for the eastern part of the massif derived from an extensive seismic refraction experiment conducted on a 194-km N-S line. The uppermost crust is covered with a very thin (0.5–1 km) surface layer with a velocity of 3.1–5.4 km/s. The velocity structure below this layer shows remarkable lateral variation. In the northern Kitakami terrane the P wave velocity and Vp/Vs at the top of the basement are 5.85–5.95 km/s and 1.68–1.70, respectively. The seismic attenuation in this region is high (Qp = 150–200 and Qs = 70–100). In contrast, the uppermost crust in the southern Kitakami terrane is characterized by a high P wave velocity (6.05–6.15 km/s) and Vp/Vs (1.74–1.77). The Qp and Qs also show high values of 300–400 and 150–200, respectively. Such a structural difference persists to 14-to 16-km depth, at which the P wave velocity increases to 6.45 km/s. The low velocity and high attenuation in the northern Kitakami terrane represent a highly deformed structure of the accretionary complex. The high P wave velocity and Vp/Vs in the southern Kitakami terrane indicate the relatively mafic crustal composition, which may result from the fragment of the oceanic crust incorporated by the accretion process or the uplifting in the latest Jurassic-early Cretaceous. A midcrustal interface determined from wide-angle reflections shows an abrupt southward depth decrease from 25 to 20 km under the HTB. The P wave velocity and Vp/Vs between 14- and 16-km depth and the midcrustal interface are 6.45–6.55 km/s and 1.74–1.78, respectively. The Moho depth under the northern Kitakami terrane decreases southward from 34 to 32 km. In the southern Kitakami terrane the Moho dips slightly southward. The P wave velocity and the Vp/Vs ratio in the lower crust are 6.9–7.0 km/s and 1.75–1.76, respectively. The P wave velocity in the uppermost mantle is not well resolved but is probably less than 7.7 km/s. The S wave velocity derived from relatively clear Sn is 4.35–4.40 km/s. Our results show that the HTB is a prominent structural boundary extending to the Moho. The crust of Kitakami massif was not homogenized by the Cretaceous granitic intrusions, and the original structural difference remains in the upper crust.

微粒子とガスとの接触操作のために考案した粉粒流動層を用いて, 転炉ダスト (サブミクロン～数十μm径の酸化鉄) の直接還元を行い, 還元鉄微粒子を連続的に得た.転炉ダストの還元率に対する反応温度, 媒体粒子層高, ガス流速, 水素濃度の影響を調べた.本実験における最適運転条件では, 98%の還元率を得ることができた.また, 運転中に焼結が起こる反応条件を明らかにし, 定常な連続操作ができる範囲を確立した.得られた粉鉄は, 空気中で極めて酸化され易い.再酸化防止のための粉鉄の高温処理および低濃度酸素処理の効果を調べた.

An extensive seismic refraction experiment with the use of explosive sources was conducted in the Kitakami region, northern Honshu, Japan, on November 1, 1990. The experiment area is divided into two geological units by the Hayachine Tectonic Belt (HTB). The southern terrane consists of pre-Silurian basements and Silurian-lower Cretaceous marine sediments, while the northern one is characterized by a Jurassic accretionary complex. Both of the units were intruded by Cretaceous granitic rocks. An almost N-S seismic refraction profile of 194-km length was extended from Kuji City, Iwate Prefecture to Ishinomaki City, Miyagi Prefecture, on which 4 shots with a charge size of 450-700 kg were fired

A method for pseudoreflection profiling (PRP) is presented. By using transmission seismograms, this method produces time sections similar to those obtained using the common depth point (CDP) method. Pseudoreflection profiles are obtained using the following procedure. Equivalent transmission responses at sites on soft deposits are derived by Wiener filter estimation using seismograms acquired on the sites and on the basement rock. These responses are substituted into the Claerbout-Kunetz equation to obtain the pseudoreflection profiles. A 3D subsurface model is produced when the PRP method is applied to explosion seismograms. The time sections thus obtained are consistent with those obtained using the CDP method in a neighbouring area. The maximum station interval for obtaining coherent profiles with this method is estimated to be one quarter of the wavelength of the transmitted waves. This technique should be used in combination with another method such as the CDP method because it has two weak points: the velocity structure cannot be obtained directly and undesirable waves cannot be eliminated through processing.

Experiences of a series of seismic explorations conducted in Kyoto and its environs for investigating subsurface structures up to 1 km are summarized and discussed. Examples of explorations using power spectra or phase velocities of microtremors, the refraction method of various scales, the classical reflection method with the ray-tracing analysis, and the CDP method as well as a newly devised pseudo-reflection profiling (PRP) method are presented. The amount of labor and attainable resolution of the above methods are compared. The PRP method is efficient, since this is fairly high in resolution and can be applied for a wide area with relatively little effort.

The subsurface structure of the region has been surveyed with several seismic methods simultaneously, i.e., observations of microseisms, the Common Depth Point (CDP) profiling method and the Pseudo-Reflection Profiling (PRP) method. In the first stage of the survey, a probable discontinuity of subsurface structure is detected from observations of microseisms along two linear arrays on the flat, and the difference in the basement rock depth on the western and eastern parts of the flat is roughly estimated. In the second stage, two small portions of the flat are surveyed with the CDP method in order to determine subsurface structure in more detail. Moreover, the PRP method is invoked to supplement the coverage of CDP profiling method.

Broadband seismometers deployed at Aso volcano in Japan have detected a hydrothermal reservoir 1 to 1.5 kilometers beneath the crater that is continually resonating with periods as long as 15 seconds. When phreatic eruptions are observed, broadband seismograms elucidate a dynamic interplay between the reservoir and discharging flow along the conduit: gradual pressurization and long-period (approximately20 seconds) pulsations of the reservoir during the 100 to 200 seconds before the initiation of the discharge, followed by gradual deflation of the reservoir concurrent with the discharging flow. The hydrothermal reservoir, where water and heat from the deeper magma chamber probably interact, appears to help control the surface activity at Aso volcano.

Large volcanic eruptions result from the ejection of magma transported from depth. How the magma is transported to the surface is one of the fundamental questions in understanding how a volcano works. A way to address this question is to explore the seismic structures of volcanoes. One volcano that is well surveyed and instrumented through a variety of global positioning system and seismic networks is Japan's Mount Asama. Because of this, scientists were able to conduct an active-source seismic experiment on this volcano with the goal of mapping volcanic conduits (see Figure 1).

[1] The three-dimensional P-wave velocity structure of the Iwate volcano, northeastern Japan, is determined to depths of 2 km through an active seismic survey conducted in October 2000. Seismic tomography is applied to approximately 2700 travel-time data. The most prominent discovery is an existing of column-like high-velocity body (Vp > 5.4 km/s) that extends vertically for 2 km beneath the caldera. While the western part of the volcano extending from the caldera is characterized by a moderate-velocity region (4.8 < Vp < 5.4 km/s), the summit and eastern flank of the volcano are covered with very low-velocity material (Vp < 4 km/s) which represent relatively younger volcanic edifices. The spatial difference in the velocity structures between the western and eastern parts of the volcano is explained by the evolutionary history of the volcano. And we find that the western structure may give constraints on the volcanic activity in 1998.

Stations on the linear arrays are classified into two groups based on the difference of spectral characteristics of microseisms: one is the stations to the west of the Hino River that exhibited peaks around the period of 3 s in the spectral ratio to a fixed reference station, and the other is those to the east of the Hino River that had no peaks in that period range. The difference of the spectral ratios may be attributed to the lateral change of the depth of basement rock.In order to determine the depth of the basement rock on both sides of the river, we observed microseisms on both sides with vertical-motion seismometers arranged in two-dimensional arrays. We estimated the subsurface structure from phase velocities of observed microseisms by assuming that the main part of the observed microseisms consisted of the fundamental mode of Rayleigh waves, and compared the inferred structure with a deep-drilling log available in the neighborhood. The estimated depths of the basement rock are 0.9 km on the west side and 0.1 km on the east side of the Hino River. This is consistent with the existing result of gravitational survey.

Kuju Volcano lies near Aso Caldera in central Kyushu. After a few hundred years of dormancy, a phreatic eruption began with the ejection of about 20,000 m 3 ash on 11 October 1995. A number of new vents have opened on a series of lines striking east–west on the eastern slope of Mt. Hossho, one of the domes of the Kuju complex, a few hundred meters from a pre-existing fumarolic area. After the eruption, there has been continuous steam emission from the new vents. There was the second ash eruption in December 1995. Before these eruptions, seismic events were rarely observed, either near the site of the new vents, or elsewhere under Kuju Volcano. In the nearly 2 years since the first eruption, several thousand earthquakes have been recorded. These events have been very horizontally concentrated just to the north of the new vents vertically between 800 m above sea level and 1000 m below sea level. Very few earthquakes have been located on the southern side of the new vents. There was clearly a strong high-frequency attenuation affecting the seismic waves which passed through the region beneath the new vents to the seismometers south of Mt. Hossho. This evidence possibly indicates a thermal fluid content beneath the new vents, suggesting that there is a seismic attenuating zone in the feeding area of the new vents. Nearly all the earthquake spectra were of dominantly high-frequency, but the percentage of earthquakes with predominantly low-frequency spectra increased at times of enhanced volcanic activity. Volcanic tremors were also observed around the times of peak activity. Slope distance measurements have been made since the eruption. The main results of these measurements are a contraction of more than 200 ppm in distances between Mt. Hossho and points further north. The significant distance changes occurred during seismic swarms. This indicated that the seismic activities influenced ground deformation, even though some of these swarms were 3 or 5 km from Mt. Hossho. The slope distance changes indicate that an area near the top of Mt. Hossho has been moving to the northeast.