Geological Characteristics and Problems in and Around Osaka Basin as a Basis for Assessment of Seismic Hazards

Nakagawa, Kōichi; Shiono, Kiyoji; Inoue, Naoto; Sano, Masato

doi:10.3208/sandf.36.special_15

Cited by 17 publications

(5 citation statements)

References 7 publications

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“…In this study, we set the mean sedimentation rate at the OD-1 core site to 0.4 m/kyr from 2.6 Myr to 3.0 Myr, even though the sedimentation rate from ~0.4 Ma to ~0.05 Ma actually decreased from 0.5 m/kyr to 0.2 m/kyr (Uchiyama et al 2001). Furthermore, in the present simulation Nishiwaki et al 36 models, the subsidence rate in west Osaka decreases with distance from the western fault, even though, based on gravity and seismic reflection data (e.g., Nakagawa et al 1996b;Iwabuchi 2000), the top of the basement deepens toward the west and the deepest part of the Osaka Bay is located ~27 km from the Uemachi fault, where the NNE-SSW-trending Osaka-wan fault is located. The Osaka-wan fault is an ESE-side-down reverse fault, having a mean vertical displacement rate of 0.5-0.6 m/kyr after 1 Ma (Yokokura et al 1998).…”

Section: Cases With Two Preexisting Fault Zonescontrasting

confidence: 45%

Evaluation of Dip Angles of Active Faults Beneath the Osaka Plain Inferred from a 2D Numerical Analysis of visco-elasto-plastic Models

Nishiwaki¹,

Okudaira²,

Ishii³

et al. 2020

Preprint

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The geometries (i.e., dip angles) of active faults from the surface to the seismogenic zone are among the most important factors used to evaluate earthquake ground motion, which is crucial to seismic hazard assessments in urban areas. In Osaka, a metropolitan city in Japan, there are several active faults (e.g., the Uemachi and Ikoma faults), which are inferred from the topography, the attitude of active faults in surface trenches, the seismic reflection profile at shallow depths (less than 2 km), and the three-dimensional distribution of the Quaternary sedimentary layers. The Uemachi and Ikoma faults are N–S-striking fault systems with total lengths of 42 km and 38 km, respectively, with the former being located ~12 km west of the latter; however, the geometries of each of the active faults within the seismogenic zone is not clear. In this study, to examine the geometries of the Uemachi and Ikoma faults from the surface to the seismogenic zone, we analyze the development of the geological structures of sedimentary layers based on numerical simulations of a two-dimensional visco-elasto-plastic body under a horizontal compressive stress field, including preexisting linear high-strained weak zones (i.e., faults) and surface sedimentation processes, and evaluate the relationship between the observed geological structures of the Quaternary sediments (i.e., the Osaka Group) in the Osaka Plain and the model results. Based on a comparison between the simulation results and the geological observations/interpretation, we propose geometries of the Uemachi and Ikoma faults from the surface to the seismogenic zone. When the friction coefficient of the faults is ~0.5, the dip angles of the Uemachi and Ikoma faults near the surface are ~30°–40° and the Uemachi fault has a downward convex curve at the bottom of the seismogenic zone but does not converge to the Ikoma fault. Based on the analysis in this study, the dip angle of the Uemachi fault zone is estimated to be approximately 30°–40°, and the downward extension of the Uemachi fault zone nearly coincides with the epicenter of the 2018 northern Osaka earthquake.

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Section: Cases With Two Preexisting Fault Zonescontrasting

confidence: 45%

Evaluation of Dip Angles of Active Faults Beneath the Osaka Plain Inferred from a 2D Numerical Analysis of visco-elasto-plastic Models

Nishiwaki¹,

Okudaira²,

Ishii³

et al. 2020

Preprint

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“…Bedrock depths from several tens to hundreds of m, especially in valley-or basin-like configurations will affect the surface ground response at longer periods (typically exceeding 1 s), which are of interest only for the taller and more flexible structures (more than, say, 10-12 stories in height), that are in fact present in significant number in some of the studied cities. As the lesson of the 1995 Kobe earthquake has shown, in the presence of basin-like configurations the deeper geological structure must be carefully considered, because it may generate complex ("basin edge") effects leading to strong ground motion amplification and catastrophic damage in limited portions of the urban area (Nagakawa et al 1996). Only deep reflection geophysical surveys, if feasible, can in such cases resolve the deeper geological structure.…”

Section: Levels Of Zonation and Key Factorsmentioning

confidence: 99%

Seismic hazard assessment for derivation of earthquake scenarios in Risk-UE

Faccioli

2006

Bull Earthquake Eng

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The main features of the Risk-UE project approach to assessing the ground-shaking (and related hazards) distribution within urban areas are described, as a basis for developing seismic damage scenarios for European cities. Emphasis was placed in the project on adoption of homogeneous criteria in the quantitative treatment of seismicity and in constructing the ground-shaking scenarios, despite wide differences in amount and quality of data available for the cities involved. The initial steps of the approach include treatment of the regional seismotectonic setting and the geotechnical zonation of the urban area, while the hazard assessment itself takes the form of both a deterministic analysis, and of a probabilistic, constant-hazard spectra analysis. Systematic 1D site response analyses were used, mostly in the softer soil zones, to modify (when needed) the obtained ground motion maps. Earthquake induced hazard effects, such as liquefaction and landsliding, are also briefly dealt with at the end.

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“…The top of the bedrock (pre-Tertiary rocks), which corresponds to the most significant unconformity and the origin of the velocity discontinuity, was modelled based on available exploration datasets described above. The S-wave velocity (V S ) was derived from the empirical relationship between V P and V S proposed by Nakagawa et al (1996) and the density was obtained using Gassmann's equation (Gassmann 1951). Thus, the sediments within the basin were expressed as a volume with spatially variable seismic velocities and densities, rather than as a stack of layers with constant material parameters.…”

Section: -D Velocity Structure Model In the Osaka Sedimentary Basinmentioning

confidence: 99%

“…Sporadic Oligocene and Miocene sediments and Late Pleistocene terrace deposits are exposed on the hills and the Plio-Pleistocene Osaka Group is present under the alluvial plains (e.g. Nakagawa et al 1996;Itihara et al 1997;Itoh et al 2000). It has an elliptical shape with a long axis of about 90 km and a short axis of about 40 km.…”

Section: Introductionmentioning

confidence: 99%

Modelling of wave propagation and attenuation in the Osaka sedimentary basin, western Japan, during the 2013 Awaji Island earthquake

et al. 2016

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an inland earthquake of M w 5.8 occurred in Awaji Island, which forms the western boundary of the Osaka sedimentary basin in western Japan. The strong ground motion data were collected from more than 100 stations within the basin and it was found that in the Osaka Plain, the pseudo velocity response spectra at a period of around 6.5 s were significantly larger than at other stations of similar epicentral distance outside the basin. The ground motion lasted longer than 3 min in the Osaka Plain where its bedrock depth spatially varies from approximately 1 to 2 km. We modelled long-period (higher than 2 s) ground motions excited by this earthquake, using the finite difference method assuming a point source, to validate the present velocity structure model and to obtain better constraint of the attenuation factor of the sedimentary part of the basin. The effect of attenuation in the simulation was included in the form of Q(f) = Q 0 (f/f 0 ), where Q 0 at a reference frequency f 0 was given by a function of the S-wave velocity, Q 0 = αV S . We searched for appropriate Q 0 values by changing α for a fixed value of f 0 = 0.2 Hz. It was found that values of α from 0.2 to 0.5 fitted the observations reasonably well, but that the value of α = 0.3 performed best. Good agreement between the observed and simulated velocity waveforms was obtained for most stations within the Osaka Basin in terms of both amplitude and ground motion duration. However, underestimation of the pseudo velocity response spectra in the period range of 5-7 s was recognized in the central part of the Osaka Plain, which was caused by the inadequate modelling of later phases or wave packets in this period range observed approximately 2 min after the direct S-wave arrival. We analysed this observed later phase and concluded that it was a Love wave originating from the direction of the east coast of Awaji Island.

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Geological Characteristics and Problems in and Around Osaka Basin as a Basis for Assessment of Seismic Hazards

Cited by 17 publications

References 7 publications

Evaluation of Dip Angles of Active Faults Beneath the Osaka Plain Inferred from a 2D Numerical Analysis of visco-elasto-plastic Models

Evaluation of Dip Angles of Active Faults Beneath the Osaka Plain Inferred from a 2D Numerical Analysis of visco-elasto-plastic Models

Seismic hazard assessment for derivation of earthquake scenarios in Risk-UE

Modelling of wave propagation and attenuation in the Osaka sedimentary basin, western Japan, during the 2013 Awaji Island earthquake

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