Ou Backbone Range Research Articles

Assessing the relative activity of faulting along both flanks of the Ou Backbone Range, Tohoku Region, Japan, from fluvial geomorphic analyses

The Ou Backbone Range in the Tohoku Region of Japan is bounded on its both sides by two major active fault systems: the Western Fault Zone of Kitakami Lowland in the east and the Eastern Fault Zone of Yokote Basin in the west. Although these two systems are primary active fault systems in the region, information on their long-term activity characteristics is still quite limited. Therefore, we analyzed the normalized channel steepness indexes of river valleys trending perpendicular to the range front along both flanks of the Ou Backbone Range. Our results show that the eastern flank has gentler river valleys, whereas rivers along the northwestern flank are steeper. Our field investigation shows that knickpoints in this area are mostly related to local lithologic boundaries or are check dams along the valleys, thus the river systems are likely under steady-state conditions. Hence, the steeper river valleys in northwestern Ou Backbone Range indicate a higher uplift rate of the area. Because both fault systems are primarily dip-slip reverse faults and do not have significant variations in their subsurface geometry, the faster uplift suggests that the northern segment of the Eastern Fault Zone of Yokote Basin has a higher slip rate. This is consistent with results of previous studies, and the fact that the rupture of the 1896 Rikuu earthquake, the only historical surface-rupturing event in this region, was only limited along the northern segment of this fault system.

Heat flow data and thermal structure in northeastern Japan

New heat flow data corrected for climate change over Northeastern Japan were obtained using the temperature profile of the borehole of the High Sensitivity Seismograph Network (Hi-net). The obtained spatial distribution of heat flow shows low heat flow on the forearc side, high heat flow along the Ou Backbone Range, and low heat flow in the plains on the back-arc side. However, the distribution is not clearly divided into high and low heat flow along the VF front; for example, the low heat flow extends from near the northern Kitakami Mountains on the forearc side to the Ou Backbone Range crossing the VF, while the high heat flow extends to the central Kitakami Mountains and Sendai plain on the forearc side. In addition, a crustal temperature structure model was developed that considers into account the presence of sedimentary layers, the temperature dependence of thermal conductivity, and differences in heat generation due to lithology. There is a good correlation between this temperature structure and the lower limit of the seismogenic layer, which is between 400 and 450 °C. Compared to previous studies, the crustal thermal structure calculation method assumed is a model whose estimated temperature distribution is sensitive to structural differences; however, a more accurate estimation of the temperature structure is possible if detailed structural information is available. On the other hand, it seems necessary to treat fluid behavior in more detail in areas of high heat flow. However, the estimation of crustal temperature structure, especially in regions with thick sedimentary layers, is considered an improvement over the previous study.Graphical

87Sr/86Sr Isotopic Ratio of Ferromanganese Crusts as a Record of Detrital Influx to the Western North Pacific Ocean

In this study, the Sr isotope ratios (IRs; 87Sr/86Sr) of ferromanganese (Fe–Mn) crusts are analyzed through laser ablation inductively coupled plasma multiple-collector mass spectrometry. A sample collected from off Minamitorishima Island showed uniform Sr IRs (0.70906–0.70927) similar to that of present-day seawater with more than 36 mm thickness. Meanwhile, a detritus-rich sample collected from off northeast (NE) Japan showed a wide variation in Sr IRs (0.707761–0.709963). The Sr IR variation in the Fe–Mn crust from off NE Japan suggests detrital influx contributions from both the NE Japan arc (<0.708) and aeolian dust from China (>0.718). Detrital flux from the NE Japan arc increases from the bottom to middle layers, possibly due to the uplift of the Ou backbone range that occurred after ~2 Ma. The increased influx of the aeolian dust in the outer layer is attributable to global cooling in the Quaternary that increased the loess dust transportation from China to the western North Pacific Ocean. Meanwhile, the influence of the detrital influx on the sample from off Minamitorishima Island appeared to be negligible. The Sr IR analysis with high spatial resolution proposed in this study possibly improves the burial history of Fe–Mn nodules.

Estimation of the Slip Rate Along the Unruptured Fault Segment of the M7.2 1896 Rikuu Earthquake, Northeast Japan

AbstractThe Ou Backbone Range (OBR) is one of the most seismically active intraplate regions in northeast Japan. Over the past 200 years, it has been the locus of five large magnitude earthquakes, including the M7.2 1896 Rikuu earthquake, which produced coseismic surface deformation along the northern segment of the Eastern Margin Fault Zone of Yokote Basin (EFZYB), while the southern segment remained unruptured. Despite the extensive paleoseismic investigations along the ruptured segment, the slip rate estimates, and the recurrence interval remained largely unresolved along the southern unruptured segment. This study reports the long‐term slip rate, and recurrence interval along the Higashi Chokai san Fault and the signatures of active faulting along the Kanazawa and Omoriyama Faults. Based on the seismic reflection and borehole survey, we have estimated a vertical displacement of 29.3–36.1 m across the Higashi Chokai san Fault, a long‐term slip rate of 1.6–1.9 mm/yr over the past 35 ka, and a recurrence interval of about 3,800–4,600 years.Combining our results with the already published data along the OBR, revealed two seismic gaps, first along the western margin of the OBR, along the partial rupture zone of the 1970 Akita‐ken earthquake, and second, along the eastern margin of the OBR, north of the 2008 Iwate‐Miyagi Nairiku earthquake (IMEQ) rupture zone. Given most of the slip is released elastically, we propose that these seismic gaps have a potential to produce earthquakes with magnitudes Mw7.1 and Mw7.3, respectively if they were to rupture in a single event.

Consistent estimation of strain-rate fields from GNSS velocity data using basis function expansion with ABIC

Present day crustal displacement rates can be accurately observed at stations of global navigation satellite system (GNSS), and crustal deformation has been investigated by estimating strain-rate fields from discrete GNSS data. For this purpose, a modified least-square inversion method was proposed by Shen et al. (J Geophys Res 101:27957–27980, 1996). This method offers a simple formulation for simultaneously estimating smooth velocity and strain-rate fields from GNSS data, and it has contributed to clarify crustal deformation fields in many regions all over the world. However, we notice three theoretical points to be examined when we apply the method: mathematical inconsistency between estimated velocity and strain-rate fields, difficulty in objectively determining the optimal value of a hyperparameter that controls smoothness, and inappropriate estimation of uncertainty. In this study, we propose a method of basis function expansion with Akaike’s Bayesian information criterion (ABIC), which overcomes the above difficulties. Application of the two methods to GNSS data in Japan reveals that the inconsistency in the method of Shen et al. is generally insignificant, but could be clear in regions with sparser observation stations such as in islet areas. The method of basis function expansion with ABIC shows a significantly better performance than the method of Shen et al. in terms of the trade-off curve between the residual of fitting and the roughness of velocity field. The estimated strain-rate field with the basis function expansion clearly exhibits a low strain-rate zone in the forearc from the southern Tohoku district to central Japan. We also find that the Ou Backbone Range has several contractive spots around active volcanoes and that these locations well correspond to the subsidence areas detected by InSAR after the 2011 Tohoku-oki earthquake. Thus, the method of basis function expansion with ABIC would serve as an effective tool for estimating strain-rate fields from GNSS data.

(U\u2013Th)/He thermochronometric mapping across the northeast Japan Arc: towards understanding mountain building in an island-arc setting

Topographic relief in arc-trench systems is thought to be formed by plate subduction; however, few quantitative investigations have so far been reported, with respect to the related mountain building process. This study applies apatite and zircon (U–Th)/He thermochronometry (AHe, ZHe, respectively) to Cretaceous granite rocks in the north part of the northeast (NE) Japan Arc to reveal its cooling/denudation history. Weighted mean AHe ages ranging from 88.6 to 0.9 Ma and ZHe ages from 83.9 to 7.4 Ma were determined for 10 rock samples. Using the AHe data, denudation rates were obtained for each sample. On the fore-arc side, denudation rates of < 0.05 mm/year were calculated, indicating a slow denudation process since the Paleogene. However, in the Ou Backbone Range and on the back-arc side, denudation rates at > 0.1–1.0 mm/year were computed, probably reflecting a recent uplift event since ~ 3–2 Ma. These data indicate a clear contrast in thermal and denudation histories between the tectonic units in this study area, similar to that previously reported from the southern part of NE Japan Arc. A comparison of the thermal/denudation histories between the N- and S- traverses, revealed the arc-parallel trend, the uplift model of the volcanic arc, and some minor variations of thermal/denudation histories in each tectonic unit. This study offers some further insights into the understanding of tectonic processes in an island-arc setting.

The age of the Mahirugawa Formation distributed in the Ou Backbone Range, the east of Kakunodate Town, Akita Prefecture, northeast Japan

The age of the Mahirugawa Formation distributed in the Ou Backbone Range, the east of Kakunodate Town, Akita Prefecture, northeast Japan

Thermal history analysis of granitic rocks in an arc-trench system based on apatite fission-track thermochronology: A case study of the Northeast Japan Arc

We applied apatite fission-track (AFT) thermochronometry to the Cretaceous-Paleogene granitic rocks across the southern part of the Northeast (NE) Japan Arc in order to elucidate the denudation history. As the results, AFT ages were estimated at 79.5–66.0 Ma on the fore-arc side, 29.8–5.5 Ma in the Ou Backbone range (OBR) and 19.1–4.6 Ma on the back-arc side. Thermal inverse modeling with HeFTy offered high-resolution thermal histories, which were converted into denudation rates under the assumption of appropriate geothermal gradients. On the fore-arc side, stable and slow denudation setting (denudation rates < 0.05 mm/yr) was deduced through the Paleogene. On the contrary, in the OBR and on back-arc side, rapid denudation rates from 1 to a few mm/yr were estimated at some localities reflecting the uplift event after 3–2 Ma. Besides, the uplifting style of OBR was constrained from the thermochronometric data; the doming uplift is more plausible than the tilted pop-up style, which was previously suggested based on geological observations. The Iide and Asahi mountains on the back-arc side probably have started to uplift at 3–1 Ma, coeval with the uplift of the OBR. This observation disagrees with the previous tectonic models in which the deformation on the back-arc side initiated at 5–3.5 Ma, prior to the deformation in the OBR. This disagreement possibly suggests a heterogenous tectonic history on the back-arc side.

O-21 奥羽山脈及び横手盆地の発達史から明らかになる後期中新世初期の圧縮テクトニクス(3. 堆積盆テクトニクス(液晶有),口頭発表,一般講演)

O-21 奥羽山脈及び横手盆地の発達史から明らかになる後期中新世初期の圧縮テクトニクス(3. 堆積盆テクトニクス(液晶有),口頭発表,一般講演)

First report of (U\u2013Th)/He thermochronometric data across Northeast Japan Arc: implications for the long-term inelastic deformation

(U–Th)/He thermochronometric analyses were performed across the southern part of the Northeast Japan Arc for reconstructing the long-term uplift and denudation history in the region. Apatite (U–Th–Sm)/He ages ranged from 64.3 to 1.5 Ma, while zircon (U–Th)/He ages ranged between 39.6 and 11.0 Ma. Apatite (U–Th–Sm)/He ages showed obvious contrast among the morphostructural provinces; older ages of 64.3–49.6 Ma were obtained in the Abukuma Mountains on the fore-arc side, whereas younger ages of 11.4–1.5 Ma were determined in the Ou Backbone Range (OBR) along the volcanic front and the Asahi Mountains on the back-arc side. The age contrasts are basically interpreted to reflect the differences in the uplift and the denudation histories of the provinces considering the thermal effects of magmatism and timing of the known uplift episodes. Denudation rates were calculated to be <0.1 mm/year in the Abukuma Mountains, ~0.1 to 1 mm/year in the Ou Backbone Range, and ~0.1 to 0.3 mm/year in the Asahi Mountains. The denudation rates tend to increase from the mountain base to the ridges in the OBR (and the Asahi Mountains). This relationship shows a contrast with the previous findings in fault-block mountains in the Southwest (SW) Japan Arc, where the highest denudation rates were estimated near fault(s) along the base(s). This observation might reflect a difference in mountain uplift mechanisms between the NE and the SW Japan Arcs and imply that thermochronometric approaches are useful for constraining uplift and denudation histories at the scale of an island arc, as well as continental orogens. However, careful discussion of magmatic thermal effects is required.Graphical abstract(U–Th)/He ages across NE Japan Arc, indicating different uplift/denudation histories among the morphostructural provinces.

岩手県湯田盆地に分布する不整合と火砕岩鍵層 : その奥羽山脈発達史上の意義について( 東北日本の島弧地殻構造と地質構造発達史)

岩手県湯田盆地に分布する不整合と火砕岩鍵層 : その奥羽山脈発達史上の意義について( 東北日本の島弧地殻構造と地質構造発達史)

Heterogeneous stress state of island arc crust in northeastern Japan affected by hot mantle fingers

AbstractBy considering a thermal structure based on dense geothermal observations, we model the stress state of the crust beneath the northeastern Japan island arc under a compressional tectonic regime using a finite element method with viscoelasticity and elastoplasticity. We consider a three‐layer structure (upper crust, lower crust, and uppermost mantle) to define flow properties. Numerical results show that the brittle‐viscous transition becomes shallower beneath the Ou Backbone Range compared with areas near the margins of the Pacific Ocean and the Japan Sea. Moreover, several elongate regions with a shallow brittle‐viscous transition are oriented transverse to the arc, and these regions correspond to hot fingers (i.e., high‐temperature regions in the mantle wedge). The stress level is low in these regions due to viscous deformation. Areas of seismicity roughly correspond to zones of stress accumulation where many intraplate earthquakes occur. Our model produces regions with high uplift rates that largely coincide with regions of high elevation (e.g., the Ou Backbone Range). The stress state, fault development, and uplift around the Ou Backbone Range can all be explained by our model. The results also suggest the existence of low‐viscosity regions corresponding to hot fingers in the island arc crust. These low‐viscosity regions have possibly affected viscous relaxation processes following the 2011 Tohoku‐oki earthquake.

Electrical image of subduction zone beneath northeastern Japan

AbstractWe conducted long‐period magnetotelluric observations in northeastern Japan from 2010 to 2013 to investigate the three‐dimensional electrical resistivity distribution of the subduction zone. Incorporating prior information of the subducting slab into the inversion scheme, we obtained a three‐dimensional resistivity model in which a vertically continuous conductive zone is imaged from the subducting slab surface to the lower crust beneath the Ou Backbone Range. The conductive body indicates a saline fluid and/or melt pathway from the subducting slab surface to the lower crust. The lower crust conductor is less than 10 Ω m, and we estimate a saline fluid and/or melt fraction of at least 0.7 vol. %. Other resistivity profiles in the across‐arc direction reveal that the conductive body segregates from the subducting slab surface at 80–100 km depth and takes an overturned form toward the back arc. The head of the conducting body reaches the lower crust just beneath Mt. Gassan, one of the prominent back‐arc volcanoes in the system.

Early to Middle Miocene rotational tectonics of the Ou Backbone Range, northeast Japan

AbstractIt is well known that a counterclockwise rotation occurred in the Miocene in northeast Japan. However, the detailed timing and mechanism of the rotation has been debated. Moreover, there has been no research about the relationship between rotational tectonics and the evolution of sedimentary basins. We carried out paleomagnetic and rock magnetic analyses in Nishiwaga Town, Iwate Prefecture, northeast Japan, where the stratigraphy and sedimentary basin formation have previously been clarified. We found that there was a counterclockwise rotational movement of about 45° at about 15 Ma. From our results and previous studies on the tectonics and sedimentary basin development, we are able to ascertain the following tectonic history and sedimentary basin evolution in this area: (i) before the rotational movement, sandstone and mudstone were deposited in a tranquil environment with no volcanic activity coupled with slow tectonic subsidence; (ii) between 16.4–15.1 Ma and 14 Ma, a counterclockwise rotation occurred with rapid tectonic subsidence and continuous explosive volcanism; (iii) at about 14 Ma, the counterclockwise rotation ended and there was a reduction in both subsidence and volcanism. This result shows the impact that rotational tectonics can have on sedimentary basin formation.

Crustal structure in and around the Onikobe geothermal area, northeastern Honshu, Japan, inferred from the spatial variation of coda decay

The Onikobe area is an active geothermal area situated in the Ou backbone range of northeastern Honshu, Japan. It is home to calderas from the Tertiary to Quaternary eras and active volcanoes. A systematic spatial variation of Qc has been found in this area: Qc values are lower at stations in and around calderas than at other stations. The amplitude of coda waves with high Qcs decreases more slowly after a lapse time of around 7–10s than that with low Qcs. In the present study, to determine causes for these coda decay variations, coda envelopes were synthesized in a structure model in which high attenuation zones existed beneath the Onikobe and Sanzugawa calderas and where scattering coefficients were higher in the lower crust than in the upper crust. Using hypocenters shallower than 10km, envelopes were calculated for 256 station-hypocenter pairs with epicentral distances of less than 10km. It was assumed that the coda waves were composed of S–S scattered waves, and that the scattering was single and isotropic. The observed features of the Qc distribution were reproduced in the synthesis, and synthesized envelopes were found to mostly coincide with observed decay curves. The top of high attenuation zones was thus estimated as being deeper than 7.5km. The structure assumed for the synthesis was consistent with that of previous studies. We consider that the structure model used was appropriate, and that high attenuation zones beneath calderas and the reflective lower crust caused the spatial variation of the Qc and decay curves in the Onikobe area. We also consider that studies using coda decay would be beneficial in detecting high attenuation zones and the reflective lower crust.

岩手県西和賀町周辺奥羽脊梁山脈における前期～中期中新世の火山活動と堆積盆発達史

岩手県西和賀町周辺奥羽脊梁山脈における前期～中期中新世の火山活動と堆積盆発達史

Tectonic loading processes affected by volcanoes for the 2008 Iwate–Miyagi intraplate earthquake in northeastern Japan

The 2008 Iwate–Miyagi intraplate earthquake (M7.2) occurred close to volcanoes such as Mt. Kurikoma along the southern foothills of the Ou Backbone Range (OBR), northeastern Japan. To understand the generation processes of this intraplate earthquake, 3D modeling of the faulting and tectonic loading processes around the OBR is performed using a finite element analysis considering viscoelasticity and plasticity in a compressional tectonic setting based on our previous 2D modeling. A small high-temperature area is set below the OBR, along with local high-temperature areas caused by volcanoes. In the non-volcanic area between the volcanic regions, two fault zones are generated some distance from the center of the OBR. However, near the volcanic regions, two fault zones are generated close to the center of the OBR. The fault zones corresponding to the Iwate–Miyagi intraplate earthquake are modeled to develop near the volcanic region, which is consistent with observations. The 3D geometry of fault zones and the stress loading processes for this intraplate earthquake were affected by the thermal structure caused by volcanic activity in the region.

Modeling of crustal deformation and fault development in the Ou Backbone range and the midNiigata region: Toward an understanding of the mechanisms of large inland earthquakes

内陸大地震の発生過程を理解するためには，外力としてのプレート間の相互作用による応力蓄積過程と，島弧内で応力を開放する内的な非弾性変形と断層成長過程を解明する必要がある．内的な非弾性変形と断層運動は，島弧地殻・最上部マントルの不均質なレオロジー構造に支配される．本稿では，内陸地震発生解明に向けた，不均質なレオロジー構造を考慮した東北日本および中越地域における変形と断層形成過程のモデル化を紹介する．脊梁山脈周辺では地温勾配が高く，その領域の地殻深部で非線形流動による短縮変形が生じ，それにより上部地殻で断層が形成される．他方，日本海拡大時に背弧で地溝帯形成に伴って生成された正断層が，短縮変形場の中で，逆断層として再活動することが考えられており，中越地域においては，埋没した地溝や基盤内の摩擦強度の小さな領域を考慮することで最近発生した大地震の断層成長過程を再現できる．

Strain anomalies induced by the 2011 Tohoku Earthquake (M w 9.0) as observed by a dense GPS network in northeastern Japan

We have evaluated an anomalous crustal strain in the Tohoku region, northeastern Japan associated with a step-like stress change induced by the 2011 off the Pacific coast of Tohoku Earthquake (Mw 9.0). Because the source area of the event was extremely large, the gradient of the observed eastward coseismic displacements that accompanied uniform stress change had a relatively uniform EW extension in northeastern Japan. Accordingly, the deformation anomaly, which is determined by subtracting the predicted displacement in a half-space elastic media from the observed displacement, should reflect the inhomogeneity of the rheology, or stiffness, of the crust. The difference of the EW extension anomaly between the forearc and backarc regions possibly indicates a dissimilarity of stiffness, depending on the crustal structure of the Tohoku region. The Ou-backbone range—a strain concentration zone in the interseismic period—shows an extension deficit compared with predictions. A low viscosity in the lower crust probably induced a relatively small extension. Meanwhile, the northern part of the Niigata-Kobe tectonic zone, another strain concentration zone, indicates an excess of extensional field. This is probably caused by a low elastic moduli of the thick sedimentation layer. The detection of strain anomalies in the coseismic period enables a new interpretation of the deformation process at strain concentration zones.

Precise aftershock distribution and crustal structure in and around the northern focal area of the 2008 Iwate-Miyagi Nairiku Earthquake

The 2008 Iwate-Miyagi Nairiku Earthquake (Mj 7.2) occurred on June 14, 2008, at the eastern flank of the Ou backbone range, in the central part of northern Honshu, Japan. In the northern part of the focal area, seismic reflection/refraction experiments conducted in 2006 and 2007 revealed a series of west-dipping faults. To investigate the relation between these faults and aftershocks, we conducted a high-density seismic array observation across the northern focal area. The arrival times of earthquakes were used in a joint inversion for earthquake locations and velocity structure. The V p structure shows tilted blocks and the block boundaries coincide with the revealed west-dipping faults. These faults were interpreted to have formed during the Miocene extension. The aftershock distribution shows a concentration on a plane dipping westward approximately 40° beneath the eastern margin of the Ou backbone range. The zone of aftershock concentration can be correlated to the known range-bounding fault inferred from seismic profiling and does not coincide with the known active reverse fault (Dedana fault). These results indicate the significance of the potential source fault located along the boundary of the Ou backbone range.