Iwate-Miyagi Nairiku Earthquake Research Articles

Time-lapse imaging of seismic scattering property and velocity in the northeastern Japan

Investigating spatio-temporal changes in crustal structure is a key to understanding the dynamic processes in the earth's crust associated with large earthquakes and volcanic activities. The dense seismic network in the northeast part of Japan (Tohoku region) allows us to apply seismic interferometry to continuous records of ambient noise data for studying crustal changes related to the 2008 Mw 6.9 Iwate-Miyagi Nairiku earthquake and the 2011 Mw 9.0 Tohoku-oki earthquake. We track changes in both seismic scattering properties and velocities over five years and spatially localize them based on the coda wave decorrelation and travel time sensitivity kernels calculated from the solution of the radiative transfer equation. The 2008 Iwate-Miyagi Nairiku earthquake caused significant changes in both seismic scattering properties and seismic velocities in the vicinity of the epicenter subjected to dynamic or/and static stress perturbations. Changes in scattering properties associated with the 2011 Tohoku-Oki earthquake around some active volcanoes in the Tohoku region are likely related to geofluid migrations triggered by strong vibrations during the large earthquake. The decrease in seismic velocity exists mainly along the eastern coastal region and extends to the central Quaternary volcanic chains under the combined dynamic and static stress changes by the earthquake. The waveform correlation is possibly sensitive to the fluid content and might be a useful indicator of fluid migration in the crust. This is the first comprehensive demonstration and comparison of the spatio-temporal changes of seismic scattering properties and velocities at long-term and regional scales under the influence of two large earthquakes.

Soil Structure in Volcanic Pumice Soil of Dozou-sawa River Evaluated from In Situ and Laboratory Tests

Abstract Volcanic pumice soils are widely distributed in many countries and sometimes cause severe geo-disasters including large-scale slope failures and long-distance debris flows triggered by seismic ground motion. However, the understanding of the mechanical properties of volcanic pumice soils is still limited because volcanic pumice soils generally have a sensitive structure and are typically distributed in mountain areas that are sometimes difficult to approach. In this study, a portable in situ direct shear test apparatus was developed to evaluate shear strengths of a volcanic pumice soil without disturbing the natural soil structure. A series of in situ direct shear tests was conducted for the volcanic pumice soil in Dozou-sawa river, where a large-scale slope failure followed by debris flow occurred in the 2008 Iwate-Miyagi Nairiku Earthquake. A series of laboratory direct shear tests and triaxial tests were also carried out with intact and reconstituted specimens. The in situ direct shear tests and laboratory direct shear tests using intact specimens showed consistent shear strength. Intact specimens had an extremely loose and sensitive structure because of highly crushable porous particles and a very loose soil skeleton sustained by weak cementation between particles. Such a fragile soil structure could never be reproduced in reconstituted specimens. It was confirmed that the soil structure significantly affected peak shear strengths. For the evaluation of mechanical behavior of such sensitive soils, it is important to use as few disturbed specimens as possible. The newly developed in situ direct shear test apparatus was confirmed to be an effective tool to simply evaluate the shear strength of sensitive volcanic pumice soils on site under low disturbed conditions.

Evaluation of Near-Source Ground Displacements During the 2008 Iwate-Miyagi Nairiku Earthquake Simulated Using Stochastic Slip Distributions and the Reciprocity Theorem

Evaluation of Near-Source Ground Displacements During the 2008 Iwate-Miyagi Nairiku Earthquake Simulated Using Stochastic Slip Distributions and the Reciprocity Theorem

Critical states observed in triaxial compression tests on volcanic pumice soil related to debris flow in the 2008 Iwate-Miyagi Nairiku Earthquake

The Iwate-Miyagi Nairiku Earthquake (M7.2) that occurred on June 14, 2008, caused a slope failure in Dozo-zawa River near the summit of Mt. Higashi-Kurikoma. The debris flow caused by the slope failure flowed for about 10 km and took the lives of 7 people. The mechanisms of the slope failure and long-distance debris flow have not yet been well understood. In this study, a series of triaxial compression tests was conducted using a highly crushable pumice soil collected from the collapsed area, and shear behavior and particle breakage were discussed. Both reconstituted specimens and intact specimens showed contractancy and reached critical states that implies high flow potential. However, intact specimens preserved looser structure during consolidation and showed stronger contractancy during shearing than the reconstituted specimens, due to their natural soil structure. Significant particle breakage was observed in the tests with reconstituted specimens.

Enhancement of direct waves based on the probability density function of seismic wave amplitudes

SUMMARY The enhancement of direct waves in seismic records is desirable in various seismological situations, where the accurate picking of seismic phases is necessary for hypocentre location, spectral inversion, waveform inversion and seismic tomography. In this study, we developed a new method to enhance direct waves in seismic waveforms based on the statistical distribution (probability density function, PDF) of the amplitudes of waveforms. The seismic waveform changes from a direct wave to a coda wave as time proceeds, which also causes the PDF of its amplitude to vary with time. The PDF becomes similar to a Gaussian or Rayleigh distribution with time, and hence, direct waves can be enhanced using the dissimilarity between the PDF of the amplitude of the seismic waveform and the Gaussian or Rayleigh distribution. In the proposed method, the Kullback–Leibler divergence is used to measure dissimilarity and it is applied as a weight. We confirmed that the method can effectively enhance direct P and S waves by applying it to seismic records of the 2016 Kumamoto earthquake, synthetic waveforms with noise and volcanic events. The method was also applied to waveforms of the 2008 Iwate–Miyagi Nairiku earthquake, which revealed that direct waves were enhanced, and an overview of the wave field and rupture process was obtained. The proposed method can detect direct waves even from subevents, thereby producing detailed information on the earthquake rupture process. We detected two significant pulses and determined their source location and rupture times. The estimated pulse sources were consistent with parts of the rupture scenario proposed by the previous studies. In addition, for local and teleseismic event records, the proposed method was compared with other methods to study its merits and demerits. The present method can enhance seismic phases on records of local and teleseismic events. Our method detects or enhances direct waves based neither on their amplitudes nor on their temporal variations but on the statistical distribution of their amplitudes, which has both advantages and disadvantages. Although polarization filtering is a powerful method that can enhance seismic phases, it requires three-component waveforms. The proposed method can be applied even to a single component record, which is a significant advantage.

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.

Evaluation of Active Tectonics in the Source Area of the 2008 Iwate–Miyagi Nairiku Earthquake

Evaluation of Active Tectonics in the Source Area of the 2008 Iwate–Miyagi Nairiku Earthquake

Estimation of activity of a blind fault using the spatial variation of the relative heights of fluvial terraces : An example of the Iwai river above the focal area of the 2008 Iwate-Miyagi Nairiku earthquake

Estimation of activity of a blind fault using the spatial variation of the relative heights of fluvial terraces : An example of the Iwai river above the focal area of the 2008 Iwate-Miyagi Nairiku earthquake

2008年岩手・宮城内陸地震で発生した地すべりのすべり面の特徴と膨潤圧測定の有効性

2008年岩手・宮城内陸地震で発生した地すべりのすべり面の特徴と膨潤圧測定の有効性

Study on the Mechanism of the Peculiar Behaviors of the Aratozawa Dam During the 2008 Earthquake

During the Iwate-Miyagi Nairiku Earthquake (M7.2) of June 14, 2008, seismic motions with a maximum acceleration of 1,024 cm/s2in the stream direction were recorded at the foundation bedrock of Aratozawa Dam, a rockfill dam located approximately 16 km from the epicenter. However, the maximum response acceleration in the same direction near the center of the dam crest was 525 cm/s2, and the acceleration amplification ratio of the dam body was far lower than that normally considered for a rockfill dam. Furthermore, it was measured that the crest settled down 19.8 cm after the earthquake. In this study, the dynamic properties of the embankment materials were identified using reproduction analysis of past earthquakes, and the recorded behaviors of the dam body during the mentioned strong earthquake were simulated. The generating mechanism of the peculiar earthquake behavior was investigated based on the results of the earthquake response analysis. Furthermore, in order to understand the deformation mechanism, sliding stability analysis and cumulative damage analysis were performed. According to the results, the residual deformation of the dam body after the strong earthquake is inferred to be caused by the shaking settlement of the embankment materials.

SEISMIC RISK EVALUATION OF IRRIGATION TANKS: A CASE STUDY IN IBIGAWA-CHO, GIFU PREFECTURE, JAPAN

Landslides are one of earth’s most serious types of natural disasters and they are often inducedby earthquakes. Predicting the approximate landslide displacements caused by earthquakes is helpful forassessing earthquake hazards and for designing slopes to withstand future seismic shaking. In this research, theauthors firstly focus on obtaining the residual strength parameters and the peak strength parameters bymultistage procedure with the Bromhead ring shear apparatus, which are used in the slope stability analysis.The maximum vertical settlement (MVS) of a slope is determined by the Newmark method using representativedata from three earthquakes, i.e., Great Hanshin Earthquake, Chuetsu Earthquake and Iwate-Miyagi NairikuEarthquake. It is experimentally demonstrated that the MVS is very large in both Tanigumi Pond and SugoPond. Once a great earthquake occurs, the progressive failure phenomenon appears and people living in thearea will be in danger. The results presented in this paper can be applied to future landslide hazard assessmentsand used to promote reconstruction efforts by the government in the affected areas and for future disasterprevention and mitigation.

荒砥沢ダムにおける2008年岩手・宮城内陸地震(M6.9)の地表地震断層 : 後期中新世カルデラ縁正断層が再動した逆断層

荒砥沢ダムにおける2008年岩手・宮城内陸地震(M6.9)の地表地震断層 : 後期中新世カルデラ縁正断層が再動した逆断層

2008年岩手・宮城内陸地震による宮城県北部および山形盆地の地震地質災害(フォト)

2008年岩手・宮城内陸地震による宮城県北部および山形盆地の地震地質災害(フォト)

Late Quaternary crustal shortening rates across thrust systems beneath the Ou Ranges in the NE Japan arc inferred from fluvial terrace deformation

We documented the existence of a blind thrust along the volcanic front in northeast Japan and calculated the crustal shortening rate across the Ou Ranges. We detected broad anticlinal deformation of the longitudinal profiles of fluvial terraces dated by using tephro- and cryptotephrostratigraphy. We inferred that the Daizaki flexure, located along the southern extension of the 2008 Iwate-Miyagi Nairiku Earthquake (IMEQ) fault, indicated the presence of an underlying thrust (Daizaki fault, newly named) there. Therefore, we inferred the existence of a west-dipping thrust system along the volcanic front at the eastern edge of the Ou Ranges consisting from north to south of the Kitakami Teichi Seien Fault Zone, the 2008 IMEQ fault and the Daizaki fault. To estimate rates of crustal deformation due to this thrust, we determined the uplift rate distribution from a pair of accumulation terrace surfaces, one formed during the marine isotope stage (MIS) 6–5 glacial-to-interglacial transition and the other during the MIS 2–1 transition. Because the overall uplift-rate distribution includes crustal deformation at both regional and local scales, we calculated the fault-related deformation area by subtracting the regional uplift rate (0.15–0.18mky−1, obtained from a pair of fluvial terraces on the stable footwall) from the total uplift-rate distribution across the fault. A mass balance calculation of the fault-related deformation area showed the crustal shortening rate across the 2008 IMEQ fault to be 0.50±0.19–0.59±0.22mky−1. An east-dipping thrust system along the western edge of the Ou Ranges (eastern part of the Shinjo Basin Fault Zone) has also contributed to crustal shortening during the late Quaternary at a previously estimated rate of 0.65–1.42mky−1. Therefore, the shortening rates of the eastern and western thrust systems individually account for about 0.6–0.7% and 0.7–1.6%, respectively (total, 1.3–2.3%), of the convergence between the Pacific and Eurasian plates. This westward increase in the shortening rate across the Ou Ranges is in accordance with the crustal structure, which reflects the tectonic history since the Miocene backarc spreading of the Japan Sea.

Occurrence Mechanism of Large Acceleration in KiK-net Seismic Records during Iwate–Miyagi Nairiku Earthquake in 2008

The KiK-net is a vertical array system and seismic motions are recorded at the ground surface and in the borehole. With the peak acceleration of 24.5m/s2 at AKTH04 (EW) and 38.7m/s2 at IWTH25 (UD) during the Iwate-Miyagi Nairiku earthquake in 2008, the occurrence mechanism of large accelerations is investigated by the non-stationary Fourier spectra and the double integrated displacement profile. From the difference in the occurrence time, the frequency and the amplitude of dominant components between the surface and the borehole, it is clear that the amplification in the surface soil caused the large acceleration at the surface. In the deformation process of the surface soil, the whipping mode is picked up at AKTH04. The relative displacement profile between the surface and the borehole is analyzed by the phase curves. The orbit at AKTH04 is identified by the sinusoidal waves of the sway, up-down and rocking modes. It can be concluded that the large accelerations at AKTH04 may be due to an irregular local mode caused by the collision between the base-mat and surrounding soil. From the orbit at IWTH25, the vertical pulse can be regarded as “an induced vertical motion” due to the collision between the base-mat of an observation house and the soil beneath it.

2008年岩手・宮城内陸地震後の降雨により崩壊が発生した山地斜面の地形的特徴

2008年6月14日に発生した岩手・宮城内陸地震の直後, および3ヶ月後の9月に計測された航空レーザー測量データより作成した陰影図, 等高線図, 断面図を比較し, 地震後の降雨により崩壊が発生した斜面の崩壊前の地形的な特徴を調査するとともに, 調査結果をもとに同様な地形的特徴を持つ斜面を抽出し, 現地で確認した。地震後の崩壊は通常より少ない降雨で発生していた。崩壊が発生した斜面では, 崩壊発生前は楕円あるいは馬蹄形の緩斜面が存在し, そこに地震によって亀裂が発生していたと判断された。この地形的な特徴は, 近傍の崩壊が発生していない斜面において同様な地形を判読し, 現地調査から複数の斜面で亀裂の発生を確認したことで確かめられた。亀裂が発生している斜面では, 貫入試験と土層断面調査の結果からNc値が10～30の風化層が比較的厚く, 風化層内あるいは下層のNc値が40以上の硬い層との境界付近にNc値が5以下の軟弱な層が形成されており, 地震後に崩壊が発生した斜面ではそれらをすべり面としてその後の降雨によって崩壊が発生したことが推測された。

2008年岩手・宮城内陸地震の震源域を対象とした計測震度トモグラフィ解析による減衰構造不均質分布の推定

In the 2008 Iwate-Miyagi Nairiku Earthquake occurred at volcanic area, large amplitude strong ground motions were recorded at observatories near fault region. Because subsurface structure is very complicated in the volcanic area, it is pointed out that considering heterogeneous attenuation structure is important to investigate characteristics of earthquake ground motions. In this study, we conducted tomographic analysis using seismic intensity data of aftershock records for the purpose of estimating heterogeneous attenuation structure. We also tried to reproduce seismic intensity of aftershocks which occurred near and on the asperity of the main shock. By comparing estimated and observed seismic intensity, we indicated how the heterogeneous attenuation structure influenced amplitudes of earthquake ground motions. The tomographic analysis using seismic intensity was conducted in three phases of procedures. First, we obtained an attenuation relationship of seismic intensity using strong motion records of inter-plate and intra-plate earthquakes, and evaluated a site factor of each station. Next, we obtained the attenuation relationship for crustal earthquakes using records of aftershocks assuming the same site factor. We then calculated residuals of each observed and estimated seismic intensity data. Finally, supposing residuals to be attributed to the heterogeneous attenuation structure, a block inversion was conducted dividing the focal area of the main shock to strip form with horizontal sizes of approximately 3km. An attenuation level of each block was expressed as increase/decrease ratio of seismic intensity (we called “G” value). In this block inversion weighting by the ray length which passes through the block, 326 records of Aratozawa Dam, Kurikoma Dam and KiK-net stations IWTH25, IWTH26, MYGH02, IWTH28 and AKTH04 which located near and on the fault plane of main shock were used. To confirm a resolution of G value estimated at each block, we conducted a numerical experiment using checkerboard. Although large errors were found where the number of ray was sparse, errors were less than 10% in most domains on the fault plane of the main shock. We conducted the same inversion using observed records, and found that estimated heterogeneous attenuation structure was very complicated. As indicated in the previous studies, high attenuation zone was distributed around Mt. Kurikoma. Around IWTH25 which located just above side of the main shock hypocenter, low attenuation zone was distributed. Around Aratozawa Dam and Kurikoma Dam, attenuation structure was very complicated too. We tried to reproduce seismic intensity at IWTH25, Aratozawa Dam and Kurikoma Dam for 3 aftershocks which occurred near and on the asperity of the main shock. When uniform attenuation structure was used, the observed seismic intensity of IWTH25 and Aratozawa Dam were larger than estimated. Observed seismic intensities at Kurikoma Dam were slightly smaller than estimated. In the case of considering heterogeneous attenuation, reproducibility of observed records was greatly improved. In case that azimuths of hypocenters are different, amplitudes of the earthquake ground motions are different even if magnitude and hypocentral distance are the same. We concluded that the propagation path greatly influences amplitudes of earthquake ground motions. It is thought that this complicated attenuation structure was one of factors that large amplitude strong ground motions were recorded at particularly IWTH25 and Aratozawa Dam in the main shock.

Ambient noise tomography in the Naruko/Onikobe volcanic area, NE Japan: implications for geofluids and seismic activity

To understand the earthquake generation in volcanic areas, it is important to investigate the presence of geofluids in the uppermost crust. We applied ambient noise tomography to the Naruko/Onikobe volcanic area and constructed a detailed 3-D S-wave velocity (V s) model using continuous records from a dense seismic network and surrounding stations. The low-velocity zones were found beneath Naruko Volcano, Onikobe Caldera, and Mt. Kurikoma. The low-velocity zone beneath Onikobe Caldera may correspond to a magma reservoir, which is also characterized by surrounding S-wave reflectors. The molten magma originates from the upwelling flows in the mantle wedge. We also conducted the relocation of aftershocks of the 2008 Iwate–Miyagi Nairiku earthquake by double-difference tomography based on the obtained velocity model. Beneath Mt. Kurikoma, aftershock distribution delineates one of the unfavorably oriented fault planes of the main shock, which implies that the low-velocity zone around the fault plane is related to the presence of overpressurized fluid.

2008年岩手・宮城内陸地震を引き起こした 活断層の累積変位量

2008年岩手・宮城内陸地震を引き起こした 活断層の累積変位量

A Hybrid Approach for Broadband Simulations of Strong Ground Motion: The Case of the 2008 Iwate–Miyagi Nairiku Earthquake

Abstract We developed a hybridization code that merges the full‐wave low‐frequency (LF) signals with stochastic high‐frequency (HF) synthetics to simulate broadband strong ground motion for engineering applications. The approach is applied to reproduce, within the known source and structure constraints, the Kiban–Kyoshin network (KiK‐net) and Kyoshin net (K‐NET) accelerometric data recorded during the 2008 Iwate–Miyagi Nairiku earthquake ( M w 7.0). The results show that our approach is able to simulate satisfactorily the observed waveforms and the related response spectra over the broadband frequency range. We estimated the effects of the hybridization in the peak and spectral parameters through the goodness‐of‐fit criteria: the overall fit improves (∼15%–20%) when, instead of full‐wave calculated seismograms, we use the low‐pass filtered recordings, showing that a good match at LFs is fundamental to obtain reliable and consistent results even in the HF band. Peak ground velocities are less affected than accelerations by the inclusion of the HF contribution and are well reproduced by our method; peak ground accelerations show larger discrepancies due to the intrinsic characteristics of the stochastic model and the small‐scale heterogeneity that affect the seismic radiation and propagation at HFs. Nevertheless, simulated signals reproduce the observed ones fairly well, and an acceptable match is also found in the comparison of the spectral responses. Online Material: Figures showing the COMPEX merging procedure, comparison of recorded and simulated response spectra, and comparison of recorded and synthetic broadband signals; and tables of source parameters, 1D velocity model, and matching frequencies.