Akashi Strait Research Articles

兵庫県南部地震の3次元差分シミュレーション-西宮市域および明石海峡周辺の強震動

兵庫県南部地震の3次元差分シミュレーション-西宮市域および明石海峡周辺の強震動

A fault model of the 1995 Kobe earthquake derived from the GPS data on the Akashi Kaikyo Bridge and other datasets

Co-seismic horizontal displacements, which have been obtained from recently released GPS observations on the Akashi Kaikyo Bridge, are examined for their consistency with displacements observed in the vicinity of the bridge. An E18.7°S displacement of 25.0 cm should be removed from them. The adjusted data indicate an additional fault segment beneath Akashi Strait. We construct a new fault model by adding this segment to a model assumed previously. We then recover the slip distribution over the new model by inverting these data together with other geodetic observations. The displacements calculated from the recovered distribution fit the observations well, and the distribution of slip indicates that the additional segment is closely related to the southern main segment of the previous model. Joint inversion of the geodetic and waveform datasets suggests large slips with longer duration in the shallow parts of these segments.

Source model of the 1995 Hyogo-Ken Nanbu earthquake and simulation of near-source ground motion

AbstractThe 1995 Hyogo-Ken Nanbu earthquake struck the heavily populated Kobe and adjacent cities in western Japan. More than 6400 people were killed, and more than 150,000 buildings were destroyed. The characteristics of mainshock ground motions in the heavily damaged area are needed to understand how buildings and bridges performed and why they reached failure. Unfortunately, very few strong ground motions were recorded in the heavily damaged area during the mainshock. In this study, we attempt to estimate mainshock ground motions by using the empirical Green's function method (EGF method). First, we assume an initial source model with the asperities based on the rupture process obtained by inversion of strong-ground-motion records. For simplicity, we consider each asperity as a subevent with uniform stress drop in a finite extent. Then, the initial model was improved by matching the synthetic and observed ground motions using a trial-and-error procedure. The final model consists of three subevents: subevent 1 with stress drop of 163 bars, under the Akashi Strait around the rupture starting point; subevent 2 with stress drop of 86 bars, under the Nojima Fault in Awaji Island; and subevent 3 with stress drop of 86 bars, under Kobe. Finally, we estimate strong ground motions using aftershock records at sites where the mainshock was not recorded. The near-source motions in Kobe synthesized with the best-fit model are characterized by two large pulses with a duration of 1 to 3 sec. The pulses are caused by forward rupture directivity effects from subevents 1 and 3. Peak horizontal acceleration and velocity of the synthesized motions at the heavily damaged sites are about 1000 cm/sec2 and 130 cm/sec, respectively, while those at a rock site in the near-source region are about 300 cm/sec2 and 60 cm/sec.

SLICKS APPEARED IN AKASI-STRAIT BY SUCCESSIVE PICTURES OBSERVATION

The Akashi Strait connects Osaka Bay to Sea of Harima-nada, where maximum tidal velocity is beyond 7 knots with strong vertical mixing. It is important to improve the understanding of the flowstructure in Osaka Bay and Harima-nada for evaluating water exchange and material transfer through the Akashi Strait. The surface flow pattern in Akasi Strait was observed from the top of AKASHI KAIKYOU OHASHI bridgetower whose height is 300m, From this platform, the slicks in Harima-nada and off the coast of Iwaya in Awajisima-Island was observed at eastward current. The slicks were found to be discontinuous lines of thevelocity by the wave line caused by ships The slicks off the coast of Iwaya moved slowly and amalgamatedwith the other slicks. The slicks coincided with the edge of separated vortices at Matsuhozaki in Awaji Island.

明石海峡のテクトニクスと兵庫県南部地震

We propose a tectonic model associated with formation of the Akashi strait. The epicentral distribution of aftershocks and geodetic data show that the earthquake rupturing occurred not only along the Nojima Earthquake Fault and the Rokko Fault Zone striking NE-SW, but also occurred along the faults striking NW-SE in the Akashi strait. The earthquake surface ruptures striking NW-SE and NE-SW were also found in the northwestern end area of Awaji Island. The vertical displacements along these surface ruptures and the GPS data measured on the piers of Akashi strait bridge indicate that the central part of the Akashi strait subsided a few tens of cm among the 1995 Southern Hyogo Prefecture Earthquake. Based on these data, we show that the Akashi strait is a pulling-apart area between the right-lateral strike-slip faults of the Rokko Fault Zone and the Nojima Earthquake Fault, and that the Rokko-Awaji mountains are the areas showing the character of pressure ridge formed by pushing-up.

Aftershocks and Faults of the Hyogo-ken Nanbu Earthquake beneath Akashi Strait.

We carried out a seismic marine survey around Akashi Strait employing reflection and side-scan sonar imaging methods and an ocean bottom seismograph (OBS) array in order to closely examine the aftershocks in the immediate vicinity of the rupture initiation area of the 1995 Hyogo-ken Nanbu earthquake. A total of 71 aftershocks and some mechanisms were determined using OBS data for a period of 1 month with land temporary station data starting 12 days after the main shock. We identified a number of seismologically characteristic clusters to understand the heterogeneity that seem to exist around Akashi Strait. We determined that two major faults, the Nojima Fault and the Rokko fault system, are right-stepping within the strait cause en-echelon faultings in between. The Nojima Fault seems to terminate by deflecting westward in the strait. Focal mechanisms of these events are generally concordant with an E-W compression stress field, which is the tectonic trend. Events beneath the northern part of Awaji Island occur as a cluster dipping eastward, but mechanisms vary.

Co-seismic Displacements of the 1995 Hyogo-ken Nanbu Earthquake.

We present co-seismic displacements of the Hyogo-ken Nanbu earthquake of January 17, 1995, detected by continuous GPS (Global Positioning System) observation, campaign type GPS survey and leveling. Continuous GPS observation gives a consistent pattern of displacements with those expected from a right lateral slip on a NE-SW trending vertical fault in far field: stations about 50 km east and west of the epicenter moved toward the epicenter by about 4 cm, while stations north and south moved away from the epicenter. By comparing with line lengths obtained by geodolite about 10 years ago, the campaign type GPS revealed most control points on Awaji Island moved to the southwest or south, which may be attributed to the movement of the Nojima Fault which cut the surface. On the other hand, control points northwest of the Rokko fault system moved toward the northeast and those on the other side moved slightly to the west, in and around Kobe. Leveling data revealed uplift of 19 cm on the northwestern side of the Suma Fault, a member of the Rokko fault system, and subsidence of 7 cm just east of this fault. Furthermore, uplift of about 5 cm was observed in the central part of Kobe City, and subsidence of 5 cm was detected east of Kobe. There is no significant gap in horizontal and vertical displacements around the northern extension of the Nojima Fault, which implies a complicated rupture process of this event. Leveling on the east coast of Awaji Island revealed a significant uplift of about 20 cm with slight subsidence at both edges of this uplift region during the past 20 years. By fitting the above geodetic data, we searched for an optimal set of parameters of a dislocation model. We assumed six nearly vertical faults trending NE-SW from Kobe to Awaji Island on the basis of aftershock distribution and focal mechanism. About 250 cm of the right lateral slip for the fault on Awaji Island is derived from large horizontal displacements near the Nojima Fault. The fault in Kobe may be divided into two segments with 100-200 cm slip by a slip-free zone which roughly corresponds to the cluster of aftershocks. The southern part of the Nojima Fault, segments near the Akashi Strait, and north of central Kobe may have significantly large thrust components of 100 cm. We also examined the possibility of buried faults beneath the zones of severe damage. Since this model cannot explain the observed geodetic data and the estimated slips are inconsistent with focal mechanism, these possible buried faults may not play a significant role, if any.

Minute Locating of Faulting beneath Kobe and the Waveform Inversion of the Source Process during the 1995 Hyogo-ken Nanbu, Japan, Earthquake Using Strong Ground Motion Records.

We estimated the source process of the 1995 Hyogo-ken Nanbu (Kobe), Japan, earthquake based on: 1) locating buried fault planes in the Kobe area by examining particle motion at observation stations in the near-source area and 2) multi-time window linear waveform inversion of strong ground motion seismograms. S-wave particle motion diagrams created theoretically show reverse rotation in the horizontal plane at two stations located on opposite sides of the intersection of the earth's surface and the extension of the buried fault plane. We simulated ground motion at various locations surrounding the buried fault plane and compared their particle motions with observed records to obtain constraints on the location of the fault plane. At least two planes are needed for the rupture area northeast of the epicenter if the causative fault is assumed to be made up of a few perfectly planar structures, and the rupture extended at least 26 km northeast from the epicenter.Using a fault model consisting of three planes, two planes on the Kobe side and on the Awaji side, and assuming a step-over at the Akashi strait based on the aftershock distribution, we performed a waveform inversion. The main rupture extended about 45 km horizontally. Three regions had relatively large moment releases: 1) around the rupture starting point; 2) in the shallow (less than 10 km) part of the Nojima fault, which extends along the northwest shore of Awaji Island: and 3) deep (about 10 km) under Kobe City. Even though the second subevent on the Nojima fault had a large moment release, it did not generate pulsive waves, because its rise time was long. The two remarkable pulses seen in the seismograms in the Kobe area came from the first and third subevents.

兵庫県南部地震に伴って淡路島北西岸に出現した地震断層

Associated with the Hyogo-ken Nanbu earthquake of January 17, 1995 (MJMA 7.2), a 10.5km long surface fault ruptures, the Hokudan Earthquake Fault System, appeared along the Nojima Fault and the Mizukoshi Flexure on the northwest coast of Awaji Island, central Japan. The surface ruptures consist of a NE-SW trending right-lateral fault system with high-angle reverse component. The system has two fault strands, namely the Nojima Earthquake Fault of 8.8km long and the Ogura Earthquake Fault of 3.0km long. These two surface faults show the right-stepping arrangement and form an extensional jog at the southwest of the rupture system. Composite displacement of the right-lateral and vertical shift along the fault system are generally 1.4 to 1.8m (1.2 to 1.6m in right-lateral and 0.5 to 0.8m in vertical). The maximum displacement measures 2.5m (2.1m in right-lateral and 1.3m in vertical) at the central part of the surface faults. At the southwest of the surface ruptures, the fault displacement gradually decreases to non-traceable in a distance of 1.9km. On the other hand, surface ruptures reach as far NE as to the coast of Akashi Strait, about 4km west of the epicenter, with large displacement. No surface rupture appeared in the severely damaged Kobe area, northeast of the epicenter. Geodetic and seismological data suggest that the main faulting event of the earthquake occurred in the Awaji Island area, followed by a smaller event (or events) in the Kobe area. Based on the long-term slip-rate of the Nojima fault in the late Quaternary and the amount of slip of the 1995 event, the recurrence interval of large faulting events is estimated to be 1.3 to 2.5 thousand years.

A Numerical Simulation Study on the Tidal Exchange at the Osaka Bay

It has been reported that the seabed in the Akashi Strait, a crucial entrance to the Osaka Bay, has risen 5 m due to a recent major tremor, a scale believed to be certain to cause a major change in the residual circulation of the Osaka Bay. The present study investigates tidal exchange phenomenon carrying out a float dispersion study using both square-shaped model bay and Osaka Bay. The result shows that either embankment or seabed swelling does affect the major circulation and hence tidal exchange of a bay

Tidal-jet and vortex-pair driving of the residual circulation in a tidal estuary

Many tidal estuaries are composed of narrow constrictions and wide basins, forming a “strait-basin system”. A typical example of such a system is that of the Akashi Strait and Osaka Bay in the Seto Inland Sea, Japan. In this system, tidal currents in the straits are much stronger than those within the basins. Therefore the mass transport processes are advection rather than diffusion and governed by strongly non-linear effects. In order to reveal the flow and transport mechanism in the strongly non-linear effect, field observations have been conducted. The observations include flow-visualization experiments by the simultaneous use of several shipboard acoustic Doppler current profilers. In the strait-basin system, the tidal-jet from the strait generates a moving vortex-pair, which is smaller than the tidal excursion. This vortex-pair carries vorticity originally generated at the separation point in the strait far into the basin during each tidal cycle. The moving vortex replenishes the vorticity of the basin-wide residual circulation, which is larger than the tidal excursion. This residual circulation further acts on the trajectories of the moving vortices. Generation of moving vortices from a tidal-jet is not unique but common and essential phenomena in many strait-basin systems. These systems are filled with vortices.

Wind tunnel experiments using 3D models and response prediction for a long-span suspension bridge

A series of wind tunnel experiments on a long-span suspension bridge has been conducted. A proposed design for Akashi Straits Bridge having a box stiffening girder was selected as the prototype for full aeroelastic models and taut strip models to a scale of 1:350. The wind-induced response (flutter) under smooth flow and the prediction calculated from the results of sectional model tests using the strip theory are presented. It is found that agreement between experiment and prediction is very good. Response under turbulent flow and its prediction are also presented.

FATIGUE STRENGTH OF CORNER WELDS OF TRUSS CHORDS CONTAINING BLOWHOLES

One box section member whose dimensions were almost equal to the truss chord of the Akashi Strait Bridge was tested under repeated bending by using a 4MN fatigue testing machine. The four corner joints consist of partially penetrated groove welds from outsides and fillet welds from insides. These corner joints contain blowholes with various sizes at the roots. The fatigur strengths of corner joints decrease remarkably with the increasing of blowhole size at the root of corner joints.

ANTI-Ro IN SJÖGREN’S SYNDROME AND SYSTEMIC LUPUS ERYTHEMATOSUS

Anti-Ro autoantibodies are frequently found in the sera of patients with Sjögren's syndrome, systemic lupus erythematosus, and subacute cutaneous lupus erythematosus as well as in the sera of mothers of infants with the neonatal lupus syndrome. Close associations have been found between anti-Ro and a number of clinical manifestations, particularly including hematologic cytopenias, heart block, and photosensitive skin rashes. Serologic and genetic associations have been found between anti-Ro and anti-La, rheumatoid factor, hypergammaglobulinemia, the histocompatibility alleles DQ1 and DQ2, and alleles of the T-cell receptor beta chain gene. The origin of anti-Ro and other autoantigens is thought to relate to the etiology of Sjögren's syndrome and systemic lupus erythematosus and remains the most fundamental unanswered question preventing a comprehensive understanding of these diseases.

Wind tunnel experiment of long-span suspension bridge using 3D models

A series of wind tunnel experiments on a long-span suspension bridge was conducted. A proposed design for the Akashi Straits Bridge having a box stiffening girder was selected as the prototype for full aeroelastic models and taut strip models to scale 1: 350.In this paper, an outline of the experiment, the wind-induced response (flutter) under smooth flow, and its prediction computed from the results of sectional model tests using “strip theory” are presented. It is found that agreement between experiment and prediction is good.

Wind tunnel study of long-span suspension bridge under smooth and turbulent flow

Wind tunnel experiments of long-span suspension bridge are described. The proposed design of the Akashi Straits Bridge having box stiffening girder (Ref. 1) is selected as the prototype for the aeroelastic full models with scale 1:350. Tests are conducted under smooth and turbulent flow. Preliminary results are presented. Wind-induced oscillations of the towers on completion are also mentioned.

Biological and physical parameters of the population dynamics of sand ee1 larvae in the eastern Seto Inland Sea.

An analytical method for estimating the biological and physical parameters of larvae population from field-observed body length distribution is developed. The hatching intensity function and the survival rate function used in this analysis are given deductively from field data without any assumption on the form of these functions. We applied this method to the population analysis of sand eel larvae in the eastern Seto Inland Sea. The survival rate function is an exponential function with an apparent diminishing coefficient of 0.071 day-1. The hatching intensity function is a normal function whose central date, standard deviation, and maximal value are December 29, 16 days, and 6.86×1010 individuals/day, respectively. The total number of hatched larve is 2.76×1012, which includes the number of larvae hatched in the Bisan Strait (1.79×1012) and near the Akashi Strait (0.97×1012). The 86% of the larvae hatched in the Bisan Strait are transported into the Harima-Osaka area within one-month after hatching. The eastward transportation of the larvae from the Bisan Strait is well explained by the current driven by westerly monsoon wind in winter.

Tidal Vortices relating to the Conditions of Fishing Grounds around the Akashi Strait

Tidal Vortices relating to the Conditions of Fishing Grounds around the Akashi Strait

Formation of a Fishing Ground around Akashi Strait

Formation of a Fishing Ground around Akashi Strait

A Numerical Experiment on Water and Salt Exchange through the Akashi and the Naruto Straits

Abstract To study the relation between the water-volume exchange rates Ev and the salt-exchange rates Ez of the Akashi and the Naruto Straits in the Seto Inland Sea of Japan, we use an observed salt distribution near the straits, the calculated Eulerian velocity of the M2 current and the calculated location of about 20 000 labeled particles tracked during an M2 period. The salt-exchange rate Ez was about one-third and one-twelfth that of the water-volume exchange rate Ev for the Akashi Strait and the Naruto Strait, respectively. In general, the exchange rates of Ez and Ev do not equal each other, because Ez largely depends on the spatial distribution of salt near a strait. Therefore, Ez is not an adequate measure for the tidal exchange. We found that the salt distribution near the straits and the salt-exchange rates Ez were dependent on the deformation of a water column and that this deformation is produced not only by the velocity shear of the nonlinear tidal current but also by tide-induced transient ed...