Hiroshi Shimizu

• Kyushu University

The Aira caldera, located in southern Kyushu, Japan, originally formed 100 ka, and its current shape reflects the more recent 30 ka caldera-forming eruptions (hereafter, called the AT eruptions). This study aimed to delineate the detailed two-dimensional (2D) seismic velocity structure of the Aira caldera down to approximately 15 km, by means of th...

We infer the temporal changes in the pressure sources that induced crustal deformation during the 2018 Shinmoe-dake eruption using strain and tilt observations and discern that the deep magmatic activity associated with the early stage of this eruption began approximately 19 h earlier than the previously defined onset of magmatic activity. Distinct...

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 stru...

We infer the temporal changes in the pressure sources that induced crustal deformation during the 2018 Shinmoe-dake eruption using strain and tilt observations and discern that the deep magmatic activity associated with the early stage of this eruption began approximately 19 hours earlier than the previously defined onset of magmatic activity. Dist...

A magnitude 7.1 earthquake occurred west off Satsuma Peninsula, Japan in 2015, which was the largest earthquake ever recorded in the northern Okinawa Trough (OT). The northern OT is assumed to be in a beginning stage of a back-arc rifting that drives crustal extension, and the occurrence of the 2015 mainshock-aftershock sequence might be associated...

Crustal earthquake ruptures tend to initiate near fluid-rich zones. However, it is relatively unknown whether fluid-rich zones can further promote or arrest these ruptures. We image the electrical resistivity structure around the focal area of the 2016 Kumamoto earthquake sequence by using 200 sites broadband magnetotelluric data, and discuss its q...

Subsurface structures may control the migration of magma beneath a volcano. We used high-resolution seismic tomography to image a low- P-wave velocity (Vp) zone beneath Unzen Volcano, Japan, at depths of 3–16 km beneath sea level. The top of this low-Vp zone is located beneath Mt. Fugendake of Unzen volcano, which emitted 0.21 km3 of dacitic magma...

Shallow, implosive non-double-couple (NDC) earthquakes have been detected beneath Unzen Volcano, Japan, by a routinely operated seismometer network. Here, we analyze the waveforms from 14 temporary seismic stations around Mt. Fugen (Fugendake) and Heisei-Shinzan Lava Dome, which formed during the 1990–1995 eruptions, to further investigate this sha...

For understanding the physical condition of earthquake generation, we estimated the deviatoric stress fields and fault planes of the large earthquakes in the hypocentral area of the 2016 Kumamoto earthquake sequence, central Kyushu, Japan. We analyzed precise hypocenters and their focal mechanism tensors from 1993 to 2017 and found the complex faul...

Crustal earthquake ruptures tend to nucleate near fluidized zones. However, it is relatively unknown whether fluidized zones can further promote or arrest these ruptures. We image the electrical resistivity structure around the focal area of the 2016 Kumamoto earthquake sequence by using 200 sites broad-band magnetotelluric data, and discuss its qu...

Crustal earthquake ruptures tend to initiate near fluid-rich zones. However, it is relatively unknown whether fluid-rich zones can further promote or arrest these ruptures. We image the electrical resistivity structure around the focal area of the 2016 Kumamoto earthquake sequence by using 200 sites broad-band magnetotelluric data, and discuss its...

Fault behavior during an earthquake is controlled by the state of stress on the fault. Complex co-seismic fault slip on large earthquake faults has recently been observed by dense seismic networks, which complicates strong motion evaluations for potential faults. Here, we show the three-dimensional pre-stress field related to the 2016 Kumamoto eart...

The M JMA 7.3 Kumamoto earthquake that occurred at 1:25 JST on April 16, 2016, not only triggered aftershocks in the vicinity of the epicenter, but also triggered earthquakes that were 50-100 km away from the epicenter of the main shock. The active seismicity can be divided into three regions: (1) the vicinity of the main faults, (2) the northern r...

We investigate seismic wave velocity structure and spatial distribution of the seismicity in the source region of the 2016 Kumamoto earthquake sequence. A one-dimensional mean velocity shows that the seismogenic zone has a high velocity and low Vp/Vs ratio relative to the average velocity structure of Kyushu Island. This indicates that the crust is...

The remarkable eruption on Kuchierabujima, which occurred on May 29, 2015, had the potential to disrupt land seismic observation if the volcanic activities had expanded. Since relatively deep-volcano-tectonic (DVT) earthquakes under the volcano may have extended into the sea region around the island, we deployed four ocean bottom seismometers (OBSs...

We investigated the spatial variation in the stress fields of Kyushu Island, southwestern Japan. Kyushu Island is characterized by active volcanoes (Aso, Unzen, Kirishima, and Sakurajima) and a shear zone (western extension of the median tectonic line). Shallow earthquakes frequently occur not only along active faults but also in the central region...

Non-volcanic tremors were induced by the surface waves of the 2012 Sumatra earthquake around the Hinagu fault zone in Kyushu, Japan. We inferred from dense seismic observation data that the hypocenters of these tremors were located beneath the seismogenic zone of the Hinagu fault. Focal mechanisms of the tremors were estimated using S-wave polariza...

The rupture boundary of the Tonankai and Nankai earthquakes is located at the south of the Kii Peninsula. Previous researches revealed along-strike segmentation of hypocenters, P-wave anisotropy, low-frequency earthquake (LFE) distribution and subduction depth of the Philippine Sea (PHS) Plate beneath the Kii Peninsula. In addition, it was found th...

the Kii Peninsula, the distribution of low-frequency earthquakes (LFEs) forms three clusters. A previous study shows that one of the clusters has anomalously less amount of cumulative slip than the others. To understand the cause of this variation, we applied a tomographic analysis using arrival times of earthquakes recorded by both ocean bottom se...

We investigated the spatial variation in stress fields and pore fluid pressures on Kyushu Island, southwestern Japan. High seismic activity is found not only along active faults in Kyushu Island (southwestern Japan) but also in the central area of the island where there are active volcanoes. We consider the focal mechanisms of the shallow earthquak...

Volcanic tremors are indicators of magmatic behavior, which is strongly related to volcanic eruptions and activity. Detection of spatial and temporal variations in the source location is important for understanding the mechanism of volcanic eruptions. However, short-term temporal variations within a tremor event have not always been detected by sei...

We performed refraction analysis for the first P-wave arrival time data observed in the seismic experiment in 2008, and estimated a shallow velocity model up to 3 km depth beneath the Aira caldera and Sakurajima volcano. We found that a basement layer with a velocity of 4.6-5.0 km/s, which corresponds to geologically the Shimanto Group, inclines to...

Unzen volcano is located in the western part of Kyushu, Japan. We carried out a seismic reflection survey at Unzen volcano in order to elucidate the structure of the volcano. Although the survey was conducted in a volcanic area under difficult conditions, such as artificial noises and a complex structure, we were able to resolve the structure benea...

Small repeating earthquake (RE) analysis is a useful method for estimating interplate quasi-static slip, which is a good indicator of interplate coupling. We detected 170 continual-type interplate RE groups and then estimated the spatial variation in quasi-static slip in the Hyuga-nada over the past 17 years. The RE activity in this region has diff...

Recent studies on the stress field in a seismogenic zone revealed a heterogeneous feature around a fault. To model the stress field around a fault zone, we have developed an inversion method that can be applied to focal mechanism data on microearthquakes and slip data on faults. The modeling scheme takes an objective approach without an a priori mo...

The 1990–1995 eruption of Unzen Volcano in southwestern Japan was characterized by the extrusion of a dacite lava dome and frequent pyroclastic flows during the dome growth. Associated with the dome emergence on May 20, 1991, an intense swarm of high-frequency (HF) microearthquakes occurred just beneath the crater at very shallow depths. We used da...

The existence of a static fault segment boundary has been proposed for segmentation of the historical great earthquakes along the Nankai Trough, southwest of Japan. Due to the extremely low seismicity, the seismic characteristics around the boundary have remained too uncertain to allow detailed discussion of the cause of the fault segmentation. We...

We present high-resolution three-dimensional tomographic images of the crust beneath the entire Kyushu arc, and particularly the western portion. Our results reveal a velocity anomaly that correlates well with an upper crustal gravity anomaly. Significant low-velocity anomalies exist beneath the Miyazaki plane and along the Beppu-Shimabara Graben....

In the upper crust of Kyushu district, Japan, an area with high seismic activity is found in the middle part. This area is called Beppu-Shimabara graben because of existence many normal faults in this region. Many active volcanoes exist (i.e. Unzen, Aso, Kuju, Beppu), and historical large earthquakes occurred in this region. However, it is not alwa...

Kyushu Island, in south-western part of Japan is characterized by subduction from Philippine Sea Slab and Eurasian Plate (Amurian); volcanic front seen in islands arcs runs through the central part of Kyushu Island. In Kyushu, shallow and intermediate-depth earthquakes occur robustly through a depth of about 200 km. We estimated attenuation structu...

Large earthquakes have repeatedly occurred in the area off southeastern Hokkaido Island, Japan, as the Pacific Plate subducts beneath the island, which is on the North American Plate. The most recent large earthquake in this area, the 2003 Tokachi-oki earthquake (Mw = 8.0), occurred on September 26, 2003. In order to investigate aftershock activity...

Borehole USDP-4 was drilled into the Unzen volcanic conduit 9 years after its last eruption. The sub-surface mechanism of eruption at Unzen Volcano was investigated by taking cores and by geophysical logging. The drilling operations were carried out in 2003–04, as a joint research program sponsored by the Japanese Government and the International C...

Unzen Volcano erupted from 1990 to 1995 and the resultant volcanic deformations were measured by GPS, leveling, tilt meter, and EDM. The largest vertical displacement observed was subsidence of 8 cm in the western part of Shimabara Peninsula. Numerous magma chamber models were proposed based on the geodesic data obtained. However, these models coul...

On March 20, 2005, a large M JMA7.0 earthquake occurred in the offshore area, west of Fukuoka prefecture, northern Kyushu, Japan. A series of joint observations were carried out by teams from several universities in Japan with the aim of investigating the aftershock activity. Six online telemetered and 17 offline recording seismic stations were ins...

Crustal shear wave polarization anisotropy is caused by the alignment of vertical microcracks. Leading shear wave polarization directions (LSPDs) are presumed to be consistent with the maximum horizontal compressional axis in many cases. We analyzed shear wave polarization anisotropy in and around the focal region of the 2005 West off Fukuoka Prefe...

We investigated the spatial distribution of static stress drops of the aftershocks of the 2005 West Off Fukuoka Prefecture earthquake, with the aim of assessing the possibility that another earthquake will occur on the SE extension of the earthquake fault. The waveforms from six temporary online telemetry stations installed in and around the afters...

The 2005West Off Fukuoka Prefecture Earthquake (Mj=7.0) occurred on March 20, 2005 in the northern part of Kyushu, Japan. To study the aftershock activity, we deployed eleven pop-up type ocean bottom seismometers (OBSs), sixteen locally recorded temporary stations, and eight telemetered temporary stations in and around the epicenter region. We comb...

To understand the structure and growth history of Unzen Volcano in Japan and to clarify the eruption mechanisms of SiO 2-rich viscous magmas, the Unzen Scientific Drilling Project (USDP), a six-year program consisting of two phases, began in April 1999. In the first phase, two holes were drilled into the volcano's flank (USDP-1 and -2 wells). In th...

Directional drilling at Unzen Volcano in Japan during mid of 2004 penetrated the magma conduit and successfully recovered samples of the lava dike that is believed to have fed the 1991–1995 eruption. The dike was sampled about 1.3 km below the volcano’s summit vent and is intruded into a broader conduit zone that is 0.5 km wide. This zone consists...

On September 18, 2002, we conducted a high-resolution, low-altitude helicopter-borne aeromagnetic survey at two flight altitudes, using spiral trajectories for the first time, over Unzen Volcano in the framework of the Unzen Scientific Drilling Project (USDP). This study obtained more detailed and new information than the previous aeromagnetic stud...

The 2003 Tokachi-oki earthquake (Mj 8.0) occurred in the area off the southeastern coast of Hokkaido in northern Japan at 19:50 UTC, 25 September 2003. Five days after the mainshock, we constructed a dense seismic array using pop-up type ocean bottom seismometers (OBSs) to obtain a detailed aftershock distribution. The observation lasted one and ha...

The conduit of the 1990-1995 lava dome at Unzen was penetrated in the scientific drilling project (USDP). The drilling target was set in the hyprocenter zone of isolated tremors that occurred prior to magma extrusion in 1991. The drilling operation was sponsored by MEXT-Japan and ICDP and completed in the summer of 2004. Drilling was started vertic...

Unzen volcano is an active volcano in SW Japan, and its 1990-95 eruption caused the frequent dome-collapse-type pyroclastic flows and associated debris flows. Detailed observations have enabled the constructions of magma ascent and eruption models. Unzen was born in 0.5 Ma and has grown inside the regional tectonic graben. Most of its eruption prod...

The Tokachi-Oki earthquake occurred on September 26, 2003. Precise aftershock distribution is important to understand the mechanism of this earthquake generation. To study the aftershock activity, we deployed forty-seven ocean bottom seismometers (OBSs) and two ocean bottom pressure meters (OBPs) at thirty-eight sites in the source region. We start...

Thermal imaging from satellites is one methodology used for the monitoring and scientific investigation of volcanoes, including those characterised by active lava domes. To be most effective, the remote sensing techniques employed must allow the cause of any observed thermal anomaly to be identified, ideally using information contained within the r...

Following 198 years of dormancy, a small phreatic eruption started at the summit of Unzen Volcano (Mt. Fugen) in November 1990. A swarm of volcano-tectonic (VT) earthquakes had begun below the western flank of the volcano a year before this eruption, and isolated tremor occurred below the summit shortly before it. The focus of VT events had migrate...

In November 1989, a swarm of earthquakes deep beneath Tachibana Bay, Kyushu Island, Japan, heralded the inexorable rise of magma toward the summit of Unzen Volcano, some 15 km upward and 15 km eastward, on the Shimabara Peninsula. When the ``magma head'' emerged in Jigokuato Crater on May 20, 1991, a beautiful but tragic drama began. It started pea...

A disastrous earthquake with a magnitude of 7.2 hit the southern part of Hyogo Prefecture on January 17, 1995. The mainshock was located on an active fault of the Arima-Takatsuki-Rokko fault system. Its focal mechanism was consistent with a right-lateral strike-slip fault trending N40°E. Three days after the occurrence of the mainstock, a highly de...

Juvenile materials were found among products of phreatomagmatic eruptions that preceded dacite dome growth at Unzen Volcano in 1991. They give evidence showing that the hydrous magma started degassing with the resultant crystallization around 100 MPa, and was quenched soon thereafter. Ascending at a rate as low as 13 m/d while degassing, however, t...

The central Kyushu is a north-south spreading region characterized by east-west trending grabens and abundant active volcanoes. Available data indicate that the width of graben is 11-12 km apart from eruption centers of Unzen volcano, whereas it narrows down to about 7 km dose to the eruption centers. Focal mechanisms of earthquakes suggest that no...

On January 30, 1992, a shallow earthquake of magnitude 4.9 followed by about 300 aftershocks occurred in a northern area of the Koshiki channel, north-western Kagoshima Prefecture. We located seismic events observed at two stations of NOEV (Nansei-toko Observatory for Earthquakes and Volcanoes) and four stations of SEVO (Shimabara Earthquake and Vo...

本書は、1990年11月に噴火を開始した雲仙岳の火山活動について地球物理学的、地球科学的、地質学的観測、溶岩組成に基づく噴火モデルおよび眉山崩壊の土質工学的予測を含む総括的報告である。 1.Chronological Table 2.Photographic Records of the 1990-1992 Eruptions at Unzen Volcano[Ohta] 3.Photographic Records of Eruption Products at Unzen Volcano during May 1991-May 1992[Nakada] 4.Photographic Records of People's Life during the Eruption[The Nish...

Volatiles of the magma such as H2O, CO2, SO2, H2S and HCl, which are the main components of volcanic explosions, discharge to the surface of the earth as volcanic gases, hot springs, and river waters. The discharge occurs not only in volcanic eruptions but also in non-erupting activities. In order to clarify the mass balance of volatiles, the hydro...