Shigekazu Kusumoto

Geometric difference between non-feeder and feeder dikes

Feeder dikes bring magma to the surface; non-feeder dikes become arrested and never reach the surface. The differences, if any, between these dike types remain largely unexplored because in the field it is normally unknown if a particular dike is a feeder or non-feeder. Here we present measurements of feeder and non-feeder dikes exposed from depths of >200 m to the surface in the walls of the A.D. 2000 caldera collapse of the Miyakejima Volcano, Japan. A typical feeder thickness reaches a maximum of 2–4 m at the surface, decreases rapidly to ∼1 m at a depth of 20–40 m, and then remains constant to the bottom of the exposure. By contrast, a typical non-feeder thickness reaches a maximum of 1.5–2 m at 15–45 m below the tip, and then decreases slowly with depth to 0.5–1 m at the bottom of the exposure. We propose that free-surface effects and magmatic overpressure (driving pressure) changes during the eruption cause the overall shape of a feeder to differ from that of a non-feeder.

Restoration of the depression structure at the eastern part of central Kyushu, Japan by means of dislocation modeling

Abstract We have attempted to restore a subsurface structure in the eastern part of central Kyushu, Japan, by combining fault motions which were modeled as dislocation planes embedded in an elastic isotropic half space. The simulated crustal deformation pattern was compared with the subsurface structures estimated from gravity anomalies and/or seismic prospecting. The modeling procedure successfully restored all tectonic basins in the area without any need for motive forces for uplift or subsidence. The results also suggest that two major tectonic events have occurred in this region. Those are (1) the formation of half-graben caused by north–south extension, and (2) the formation of the pull-apart basin caused by east–west compression.

Caldera geometry determined by the depth of the magma chamber

The depth of the magma chamber is shown to be an important factor governing the initial type, scale, and collapse of a caldera. The collapse of the magma chamber is approximated by the contraction of a sphere in an elastic medium, and the distribution of plastic and/or rupturing area on the surface is calculated using the Coulomb failure criterion under the assumption of an elastic-perfectly plastic material. It is found that the necessary contraction for the formation of a caldera is described by fifth-power polynomial expression of the depth of the magma chamber, and that the radius and geometry of the caldera are dependent on the depth of the magma chamber.

Mechanism of long-standing Cenozoic basin formation in central Hokkaido: an integrated basin study on an oblique convergent margin

The basin-forming process along a convergent margin off the eastern coast of Eurasia was pursued on the basis of geological, geochemical, and geophysical approaches. Central Hokkaido has been a site of vigorous tectonic events throughout the Cenozoic reflecting the long-standing subduction of oceanic plates in the region. Geochemical modeling provided an estimate of the eroded Paleogene unit in the study area. Data on the considerable thickness of the missing unit implied continued subsidence of the forearc region and its subsequent exhumation under the emergence of a compressive regime synchronous with the back-arc opening stage. Spatially large facies variety in the Paleogene system suggests that basin compartmentalization occurred as a result of the trench-parallel component of the plate convergence. Right-lateral motion seems to have been the dominant type in Hokkaido and the forearc of northeast Japan since the Late Cretaceous, except for a left-lateral episode during rapid subsidence of the Izanagi Plate around 110 Ma. Numerical modeling demonstrated that dextral slip on a bunch of longitudinal strike-slip faults restored the Neogenedepocenters in central Hokkaido, together with an east-west compressive regime related to an arc-arc collision.

Displacement and stress fields around rock fractures opened by irregular overpressure variations

Many rock fractures are entirely driven open by fluids such as ground water, geothermal water, gas, oil, and magma. These are a subset of extension fractures (mode I cracks; e.g., dikes, mineral veins and joints) referred to as hydrofractures. Field measurements show that many hydrofractures have great variations in aperture. However, most analytical solutions for fracture displacement and stress fields assume the loading to be either constant or with a linear variation. While these solutions have been widely used, it is clear that a fracture hosted by heterogeneous and anisotropic rock is normally subject to loading that is neither constant nor with a linear variation. Here we present new general solutions for the displacement and stress fields around hydrofractures, modelled as two-dimensional elastic cracks, opened by irregular overpressure variations given by the Fourier cosine series. Each solution has two terms. The first term gives the displacement and stress fields due to the average overpressure acting inside the crack; it is given by the initial term of the Fourier coefficients expressing the overpressure variation. The second term gives the displacement and stress fields caused by the overpressure variation; it is given by general terms of the Fourier coefficients and solved through numerical integration. Our numerical examples show that the crack aperture variation closely reflects the overpressure variation. Also, that the general displacement and stress fields close to the crack follow the overpressure variation but tend to be more uniform far from the crack. The present solutions can be used to estimate the displacement and stress fields around any fluid-driven crack, that is, any hydrofracture, as well as its aperture, provided the variation in overpressure can be described by Fourier series. The solutions add to our understanding of local stresses, displacements, and fluid transport associated with hydrofractures in the crust.

Numerical Modeling of Sedimentary Basin Formation at the Termination of Lateral Faults in a Tectonic Region where Fault Propagation has Occurred

地震や火山活動で生じた地殻変動の定量的解釈によく用いられるディス ロケーション・モデリング手法に,断層の活動履歴を取り込めるよう解の 重ね合わせを行い,堆積盆地形成のプロセスについての議論を試みた. 一般に,ディスロケーション・モデリングとは,半無限弾性体中に仮定 された断層面に食い違い量(ディスロケーション)を与え,地表面や地中の 変形やひずみなどを計算する手法の一つである.Steketee (1958)にはじまる ディスロケーションの研究は,半無限均質弾性体中の任意の傾斜角をもつ 断層面に対応した媒質表面と内部での変位やひずみに対する解が,解析的 に閉じた形で導かれるという格好で,Okada (1985)あるいは Okada(1992)に より一応の完成をみている.不均質構造を取り入れたモデルや,粘弾性媒 質を仮定したモデル(例えば,Matu’ura et al., 1981; Du et al., 1994)の解も与え られているが,実際の地殻変動データの解釈やモデリングでは, Okada(1985)や Okada(1992)で与えられている均質媒質中での解析解が多く 用いられ,またその解が実際の地殻変動データをよく説明している.本研 究でも,横ずれ断層端に形成されるプルアパートベイスンの形成モデリン グに,Okada(1985)の式を用いた. ディスロケーション・モデリングを用いたこれまでのプルアパートベイ スン形成シミュレーションでは,地殻中に仮定した断層に,一度だけの断 層運動を仮定している(例えば,Kusumoto et al., 1999).実際には,地質学的 時間スケールで何度も運動している.そこで,ディスロケーション理論が 線形弾性論に基礎をおいていることに着目し,同じ断層の解の重ね合わせ を行うにより,断層の活動履歴をモデルに取り込むことを試みた.例えば, 断層の活動域が変化する(進展あるいは後退する)ことが地質学的に明らか にされている場合,それらの条件に従って重ね合わせる断層の長さを変化 させる.このモデリング手法では,断層運動の様式をこまめに変化させる ことができるため,地質学的な証拠に基づいた断層運動の活動履歴をモデ ルに組み込むことができ,詳細なテクトニクスの議論を可能にする.実際, Itoh et al. (2008),Itoh et al (2009),Tamaki et al (2010)らは,このモデリング 手法により,断層の活動履歴や運動量の変化についての議論を行っている. 北海道中部に分布し,約 48Ma から 12Ma 頃にかけて形成された堆積盆 地群は,現在地表で観測されている断層の逆断層運動によるものなのか, 或いは,横ずれ運動によるものなのか未だ決定的な結論が出されていない. 本研究では,これらの堆積盆地の分布や形状を,上述のディスロケーショ ン・モデリング手法により再現することを試みた.その結果,北海道中部 地域の堆積盆地は,基本的に地表面で観測されている断層の右横ずれ運動 により形成されるプルアパートベイスン群として再現できることが明らか にされた.顕著な逆断層運動は,川端期(15Ma-12Ma)後期の日高山脈形成 時だけであった.48Ma 頃から川端期前期までは右横ずれ断層が卓越して いたが,川端期後期から突然,逆断層運動が卓越するようになる.現段階 では原因(力源)は特定できていないが,この時期に広域応力場の大きな転 換があったと考えられる.

A distinction technique between volcanic and tectonic depression structures based on the restoration modeling of gravity anomaly: a case study of the Hohi volcanic zone, central Kyushu, Japan

Abstract In this study, we propose a numerical modeling technique which restores the gravity anomaly of tectonic origin and identifies the gravity low of caldera origin. The identification is performed just by comparing the restored gravity anomalies with the observed gravity anomalies, thus we do not need detailed geophysical and geological information around the buried caldera. The technique has been successfully applied to distinguish the gravity low originated in the buried Shishimuta caldera from other gravity lows in the Hohi volcanic zone, central Kyushu in Japan.

Vertical movements of the Murono mud volcano in Japan caused by the Naganoken Kamishiro Fault Earthquake in 2014

We observed vertical movements of the Murono mud volcano, Niigata, Japan, caused by the Naganoken Kamishiro Fault Earthquake (M = 6.7) on 22 November 2014. Precise levelling around the mud volcano was carried out on 27 November 2014, and the obtained elevation was compared with the data obtained for 11 November 2014. It was found that an elliptical uplift area, about 46 mm in magnitude and an order of magnitude smaller than that of the ones formed due to the past movements caused by the earthquakes in the same area, had been formed. It was considered that the structures of the shallow subsurface as well as the distance between the mud volcano and epicentre might play an important role in understanding the magnitude of vertical movements and distribution patterns.

Examination of consistency of marine gravity with land gravity in and around the japanese islands using a helicopter-borne gravimeter

This paper reports a finding that marine gravity data around the Japanese Islands are inconsistent with nearby land gravity data. The comparison between land and marine gravity was made possible by the gravity measurements from the air using a helicopter-borne gravimeter (SEGAWA Model) developed by the present authors. The ground/sea truth gravity anomaly can be checked against the gravity from the air, though it is not free air gravity anomaly but gravity disturbance. The newly-developed airborne gravimeter first manufactured in 1998 shows a good performance with a 1–2 mgal average repeatability of measurement under a 90-knot flight speed. Thus we have found disagreements between ground truth and sea truth gravity anomalies on the basis of airborne gravity data in the areas in Japan where we have so far made measurements. Among them we will report the case in the area from Saitama and Ibaraki offshore to the Kashima-Nada Sea, p]Japan. Our conclusion about this area is that the past marine gravity data obtained by surface ship gravimeters involve systematic errors of more than 10 mgals. This kind of inconsistency between marine and land gravity is expected to be found in a number of other areas around the Japanese Islands.

Airborne gravimetry — a new gravimeter system and test results

Success of airborne gravity surveys mainly depends on determining the three-dimensional (3D) position of the moving platform. Recent advances in technology, especially the Global Positioning System (GPS), have made it possible to determine the velocity and position of the moving platform more frequently and with greater accuracy. Taking advantage of these advances in GPS technology, and using a newly developed system, helicopter-borne gravity measurements were successfully carried out over the Kanto and Tokai districts of Japan. This new gravimeter system is composed of servo accelerometer sensors, a stabilised platform, an optical-fibre gyroscope to control the stabilised platform, GPS receivers, and a data processor. The 3D position of the helicopter at every second was accurately determined by the interferometric GPS method. These GPS data were also used to compute various correction factors which are applied to the measured gravity acceleration. Real-time differential GPS positioning was also conducted using a separate receiver mounted on the helicopter. These real-time positioning data were used for controlling the optical-fibre gyroscope. The gravity acceleration data were processed and all necessary corrections were applied. Numerical filtering was carried out to remove high-frequency noise in the data. The observed free-air gravity anomalies were then compared with upward continuation of the ground gravity data to the flight altitude. We also compiled an airborne gravity anomaly map from the airborne data, which was compared with upward-continued ground gravity data.