Ns Component Research Articles

Local Effects on Strain Seismograms at Matsushiro Seismological Observatory - 2. Rayleigh Waves

We evaluate local effects on strain seismograms for a Rayleigh wave observed at Matsushiro Seismological Observatory, Japan Meteorological Agency, central Japan, by applying a method proposed in a previous report (Okamoto et al. 2007). The method involves examination of polarization angles, local phase velocity, and accuracy of velocity seismograms. The results are as follows: 1) Polarization angles of observed strain seismograms agree with expected ones from those of velocity seismograms also observed at Matsushiro; 2) Local phase velocity estimated by comparison between strain and velocity seismograms is 54% larger than the theoretical value calculated from the PREM velocity model; 3) Velocity spectra observed at Matsushiro have almost the same amplitude as an average of those at F-net observation stations near Matsushiro. These results indicate that both EW and NS component strain seismograms observed at Matsushiro have been reduced by 35% in amplitude for a Rayleigh wave due to local heterogeneity. The local effects on a Rayleigh wave are quite different from that on a Love wave obtained in the previous report.

447 構造物を対象とした発電式振動抑制装置の開発と特性調査

In the previous paper the authors proposed a new vibration control device (V.C.D.) using power generator. The device has an inertial force created inertia disk, and damping force derived an energy dissipation of the power generator. The trial experimental model was made, and the vibration tests were carried out in our laboratory. In this paper, for the purpose of applying the device to a real large structure, new two V.C.D.s for 15 kN range are manufactured, and shipped to NCREE, Taiwan. In order to investigate about dynamic properties of the V.C.D., performance tests are carried out, and the resisting force characteristics are measured. Next, vibration tests of 3-story structure when the V.C.D.s are attached are carried out using by shaking table. Seismic responses at each story are measured when the earthquake wave of Imperial Valley El Centro NS component is inputted. The control law based on minimizing the Lyapunov function is used, and it is controlled by Bang-Bang type. The effects of vibration suppression using the V.C.D. are confirmed.

Modern evaluation of the seismic hazard for the Romanian Plain

A new technique is developed to perform a deterministic hazard analysis in terms of physical parameters, i.e. displacements, velocities, maximum accelerations, and civil engineering specific ones, design accelerations. The method is adapted for the specific features of the Romanian Plain seismicity including the latest theoretical results in both seismotectonic and prediction fields, and data acquisitions. The deterministic analysis is performed computing free surface synthetic seismograms corresponding to different seismic sources and structural models, for crustal and intermediate-depth events separately. The DGA (design ground acceleration) greatest values and the NS and EW components resultant for the velocities and horizontal displacements are considered as a measure for seismic hazard level.

Power‐law decay characteristic of coda envelopes revealed from the analysis of regional earthquakes

Seismic envelope of coda waves is an efficient measure to estimate physical parameters of the Earth heterogeneity. For NS component seismograms of regional earthquakes recorded by 11 IRIS stations, applying the Hilbert transform to make seismic envelopes in period bands from 0.25–0.5s to 16–32s with lapse time as long as 4000s, we discovered that coda envelopes decay according to some power of lapse time measured from the origin time. It is new and different from the present knowledge about coda envelopes which have been described by several bending curves with different decay rates. The ‘power‐law’ decay characteristic of seismic envelopes indicates that the spectra amplitude of seismic coda could be simply expressed as of the form A(t,TC) ∝ where t is lapse time and TC is central period in second. Applying regression analysis, we found that the exponent α value has period dependence and ranges α1 = 1.3∼6.5 (0.25–0.5s∼16–32s), α2 = 0.6∼6.1 (0.5–1s∼16–32s) for before and after ScS arrival, respectively. Moreover, it shows no significant difference between shallow and deep focus events for periods shorter than 8s. The simple and distinct characteristics of seismic coda envelopes could provide reliable information to determine physical characteristics such as attenuation and scattering to identify the regional difference of medium heterogeneity in the Earth.

Singular spectral analysis and principal component analysis for signal discrimination of ULF geomagnetic data associated with 2000 Izu Island Earthquake Swarm

In order to extract any ULF signature associated with earthquakes, the singular spectral analysis (SSA) and the principal component analysis (PCA) have been performed to investigate a possibility of discrimination of signals from different sources (geomagnetic variation, artificial noise, and the other sources (earthquake-related ULF emissions)). We apply SSA to the time series data observed at closely spaced stations; Seikoshi (SKS), Mochikoshi (MCK), and Kamo (KAM) stations. Then, PCA is applied to the time series data sets filtered at 0.01 Hz of NS component at three stations. In order to remove the most intense signal like the first principal component, we make the differential data sets of SKS–MCK and MCK–KAM for the above data. The major findings are summarized as follows. (1) It is important to apply simultaneously SSA and PCA, because SSA gives the structure of signals and the number of sensors for PCA is estimated. This makes the results more convincing. (2) There is a significant advantage using PCA with differential data sets of filtered (0.01 Hz band) signal between SKS–KAM and MCK–KAM in NS component for removing the most intense signal like global variation (solar–terrestrial interaction). This yields that the anomalous changes in the second principal component were detected more clearly, and the contribution of the second principal component is found to be 20–40%. It is enough to prove mathematical accuracy of the signal. Further application is required to accumulate events. These facts demonstrate a possibility of monitoring the crustal activity with using the SSA and PCA.

Shaking table tests on seismic response of steel braced frames with column uplift

Previous studies have suggested that rocking vibration accompanied by uplift motion might reduce the seismic damage to buildings subjected to severe earthquake motions. This paper reports on the use of shaking table tests and numerical analyses to evaluate and compare the seismic response of base-plate-yielding rocking systems with columns allowed to uplift with that of fixed-base systems. The study is performed using half-scale three-storey, 1 × 2 bay braced steel frames with a total height of 5.3 m. Base plates that yield due to column tension were installed at the base of each column. Two types of base plates with different thicknesses are investigated. The earthquake ground motion used for the tests and analyses is the record of the 1940 El Centro NS component with the time scale shortened by a factor of 1/√2. The maximum input acceleration is scaled to examine the structural response at various earthquake intensities. The column base shears in the rocking frames with column uplift are reduced by up to 52% as compared to the fixed-base frames. Conversely, the maximum roof displacements of the fixed and rocking frames are about the same. It is also noted that the effect of the vertical impact on the column associated with touchdown of the base plate is small because the difference in tensile and compressive forces is primarily due to the self-limiting tensile force in the column caused by yielding of the base plate. Copyright © 2006 John Wiley & Sons, Ltd.

Principal component analysis and singular spectrum analysis of ULF geomagnetic data associated with earthquakes

Abstract. In order to extract any ULF signature associated with earthquakes, the principal component analysis (PCA) and singular spectral analysis (SSA) have been performed to investigate the possibility of discrimination of signals from different sources (geomagnetic variation, artificial noise, and the other sources (earthquake-related ULF emissions)). We adopt PCA to the time series data observed at closely separated stations, Seikoshi (SKS), Mochikoshi (MCK), and Kamo (KAM). In order to remove the most intense signal like the first principal component, we make the differential data sets of filtered 0.01Hz SKS-KAM and MCK-KAM in NS component and 0.01 Hz band. The major findings are as follows. (1) It is important to apply principal component analysis and singular spectral analysis simultaneously. SSA gives the structure of signals and the number of sensors for PCA is estimated. This makes the results convincing. (2) There is a significant advantage using PCA with differential data sets of filtered (0.01 Hz band) SKS-KAM and MCK-KAM in NS component for removing the most intense signal like global variation (solar-terrestrial interaction). This provides that the anomalous changes in the second principal component appeared more sharply. And the contribution of the second principal component is 20–40%. It is large enough to prove mathematical accuracy of the signal. Further application is required to accumulate events. These facts demonstrate the possibility of monitoring the crustal activity by using the PCA and SSA.

Effects of Damping Ratio of Restoring force Device on Response of a Structure Resting on Sliding Supports with Restoring Force Device

Effects of damping ratio of the restoring force device on the response of a space frame structure resting on sliding type of bearing with restoring force device is studied. The NS component of the El – Centro earthquake and harmonic ground acceleration is considered for earthquake excitation. The structure is modeled considering six-degrees of freedom (three translations and three rotations) at each node. The sliding support is modeled as a fictitious spring with two horizontal degrees of freedom. The response quantities considered for the study are the top floor acceleration, base shear, bending moment and base displacement. It is concluded from the study that the displacement of the structure reduces as the damping of the restoring force device increases. Also, the peak values of acceleration, bending moment and base shear decreases as the damping of the restoring force device increases.

Response of seismically isolated buildings with buffers subjected to near-source ground motions and possible alternative isolation systems

The response of a seismically isolated building with lead rubber bearings (LRB) to near source ground motions from large earthquakes was investigated. The building was assumed to have a buffer to limit the maximum bearing displacement in a rare event of large magnitude and the buffer gap was assumed to be only 150mm (the level of maximum isolator displacement used in the 1980s). The structure was assumed to be designed for 1.5 times the NS component of the 1940 El Centro record. The 15% damped (an amount of damping which is close to the equivalent damping ratio for an seismically isolated building at its isolator design displacement) displacement spectrum of the design motion is only 40% that of the Sylmar County Hospital Parking Lot record from the 1994 Northridge earthquake (Mw= 6.7) in the period range around the first modal periods of both isolated and un-isolated structure used in the present study. Among the near-source records that are available, the near-source Sylmar record from the 1994 Northridge earthquake was found to have a very large displacement demand in a period range of 2.0 - 3.0s and this record is thought to be a better representation of the expected near-source motions than the 1.5 times the 1940 El Centro record. Structure-buffer impact was found to impose very large inter-storey drifts and produce very large storey accelerations, when the building was subjected to the excitation of the Sylmar record. The structure-buffer impact was found to be detrimental to the structural response if the structure was not designed to provide inelastic deformation capacity, and the structural response did not improve when the gap was increased to 200-250 mm, the expected maximum displacement capacity of the LRBs used in the building. An alternative isolation system of LRBs and hysteretic dampers was investigated and found to be adequate for resisting near-source motions. A large initial damper stiffness and relatively small buffer stiffness (compared with the total initial stiffness of LRBs) were found to be effective in reducing inter-storey drifts and storey accelerations at floors except for the base and roof of the structure. A disadvantage of such a system is the relatively large base and roof accelerations. The system has relatively large inter-storey drifts and storey accelerations compared with an isolated structure using LRBs only when the structure was subjected to either the 1940 El Centro type ground motions or the Joshua Tree type ground motions with backward directivity effect. Such an isolation system would still enable the structure to respond essentially elastically under the excitation of the Sylmar record even though the isolated structure was designed for a much lower level of ground shaking. As the upper structure of a seismically isolated building is usually designed to respond essentially elastically, the detailing used in the design of a reinforced concrete structure to provide inelastic deformation capacity was generally uncommon and was not fully accounted for in the present study.

MAPAS DE VELOCIDAD MÁXIMA DEL SUELO PARA LA CIUDAD DE MÉXICO

El objetivo de este artículo es presentar los mapas de distribución de velocidad máxima del suelo de la Ciudad de México para cuatro eventos sísmicos de interés. Se describe el procedimiento de cálculo, el programa de computadora empleado y su calibración, la cual permite que los valores de velocidad calculados se aproximen a los medidos. Además, se propone un grupo de ecuaciones de ajuste, en función de la magnitud del evento, con el propósito de calcular mapas de velocidades para sismos postulados. Finalmente, se presentan mapas de velocidad máxima (componentes NS y EW) para un sismo postulado con magnitud igual a 8.4 y distancia focal de 300 km, como aplicación de las funciones de ajuste propuestas.

Strong ground motion recorded by high-rate sampling GPS at the closest site to the 2003 Tokachi-oki earthquake

The Mw 8.1 earthquake occurred on September 25, 2003, off the southeast coast of Hokkaido, Japan. Since 2000 we have conducted high-rate sampling GPS measurements and precise gravity surveys in Erimo Peninsula, the closest site to the source region of the 2003 event. Strong ground motion recorded by GPS at the point of Erimo Peninsula, located just above the second asperity of the earthquake, shows two major pulses as large as about 56 cm on the EW component. Displacements obtained from the integration of accelerograms very close to our GPS site are consistent with each other, showing the absolute displacement field generated by the magnitude 8-class earthquake. Synthetic seismogram from a similar fault model by Yamanaka and Kikuchi (2003) would predict the amplitude of the second pulse to be about one half of that observed. Synthetic NS component from the GSI fault model (2003) is not consistent with our observations both on amplitude and polarity. The amplitude of ground motions detected by our GPS observation is more than one order larger than the noise level of the GPS survey, so this discrepancy is not due to insufficient GPS observation. We rather think that this suggests that our observations closest to the earthquake would give an insight into the detail of the source processes of the earthquake, which cannot be resolved from observations away from the source region. Static deformation at the point of Erimo Peninsula is consistent with the GSI fault model but not with the Yamanaka and Kikuchi model. The static analysis of our GPS measurement evidently describes the continuous post-seismic deformation as well as the co-seismic displacement in the source region until November.

ULF geomagnetic anomaly associated with 2000 Izu Islands earthquake swarm, Japan

Electromagnetic phenomena associated with large earthquakes have been investigated to establish any method to monitor the crustal activity such as earthquakes and volcanic eruptions. The ULF geomagnetic approach is considered to be one of the most promising methods for the imminent prediction. In this paper, a new method of principal component analysis (PCA) has been applied to the ULF geomagnetic data associated with 2000 Izu Islands earthquake swarm. This swarm activity started on June 26, 2000 and terminated in September, 2000, and during this activity there were observed 5 large earthquakes whose magnitude was grater than 6 (July 1, 8, 15, 30, and August 18). During this period our ULF stations were fortunately in operation, and there are three stations closely distributed (about 5 km distances). The epicentral distances are about 80–100 km. The PCA has been applied to the ULF horizontal NS component, because we can distinguish a few noises sources based on the orthogonal decomposition in the PCA. We investigate the temporary variations of eigenvalues and eigenvectors of PCA results, and the results are summarized as follows. The first principal component is found to be the signal originated in the solar–terrestrial effects such as geomagnetic pulsation. The variation of eigenvector for the first principal component suggests that this signal is very stable over the whole analyzed period. As for the second principal component, the local artificial noise is included. As for the smallest third component in the local midnight at the Izu Peninsula, it indicated an apparent increase in the third eigenvalue a few days before the large earthquakes. Also about three months before the beginning of swarm activity, the level of the third eigenvalue was slightly enhanced. Correspondingly, the pattern of eigenvector direction in the signal subspace is changed simultaneously and it recovered to the original position after the swarm activity. These features are likely to be correlated with large earthquakes. Finally we want to emphasize that PCA approach is promising for monitoring the crustal activity.

Seismic effectiveness of tuned mass dampers considering soil–structure interaction

This paper presents how soil–structure interaction affects the seismic performance of Tuned Mass Dampers (TMD) when installed on flexibly based structures. Previous studies on this subject have led to inconsistent conclusions since the soil and structure models employed considerably differ from each other. A generic frequency-independent model is used in this paper to represent a general soil–structure system, whose parameters cover a wide spectrum of soil and structural characteristics. The model structure is subjected to a stationary random excitation and the root-mean-square responses of engineering interest are used to measure the TMD's performance. Extensive parametric studies have shown that strong soil–structure interaction significantly defeats the seismic effectiveness of TMD systems. As the soil shear wave velocity decreases, TMD systems become less effective in reducing the maximum response of structures. For a structure resting on soft soil, the TMD system can hardly reduce the structural seismic response due to the high damping characteristics of soil–structure systems. The model structure is further subjected to the NS component of the 1940 El Centro, California earthquake to confirm the TMD's performance in a more realistic environment. Copyright © 1999 John Wiley & Sons Ltd.

Seismic effectiveness of tuned mass dampers considering soil-structure interaction

This paper presents how soil–structure interaction affects the seismic performance of Tuned Mass Dampers (TMD) when installed on flexibly based structures. Previous studies on this subject have led to inconsistent conclusions since the soil and structure models employed considerably differ from each other. A generic frequency-independent model is used in this paper to represent a general soil–structure system, whose parameters cover a wide spectrum of soil and structural characteristics. The model structure is subjected to a stationary random excitation and the root-mean-square responses of engineering interest are used to measure the TMD's performance. Extensive parametric studies have shown that strong soil–structure interaction significantly defeats the seismic effectiveness of TMD systems. As the soil shear wave velocity decreases, TMD systems become less effective in reducing the maximum response of structures. For a structure resting on soft soil, the TMD system can hardly reduce the structural seismic response due to the high damping characteristics of soil–structure systems. The model structure is further subjected to the NS component of the 1940 El Centro, California earthquake to confirm the TMD's performance in a more realistic environment. Copyright © 1999 John Wiley & Sons Ltd.

Sampled-dataH2 optimal output feedback control for civil structures

An optimal control method involving sampled data is considered for use in earthquake and wind engineering applications. The structure is modelled as a continuous system attached to a discrete-time controller using zero-order sample-and-hold devices. Examples of two buildings with active base isolators and a 163 m tall planar frame with an active mass damper are considered. The buildings with the base isolators are subjected to excitation input using the 1940 El Centro earthquake (NS component) as an example, while the planar frame is subjected to assumed sinusoidal gusts with a period close to that of the frame. The controlled responses (with and without time delays) are studied. To further analyze the features of the control designs, the building examples with base isolators are subjected to five other different earthquake excitation records. Trends in control performance and effectiveness are presented and discussed. The results suggest that such systems are potentially suited for implementation in the vibration control of civil infrastructures; such potentiality becomes more realistic with the current trends in software development and the increased use of digital computers. Copyright © 1999 John Wiley & Sons, Ltd.

Deuterium isotope effect on the solvation dynamics of a dye molecule in methanol and acetonitrile

Solvation dynamics of a dye molecule in methanol, acetonitrile, and their deuterated analogs has been measured by the three-pulse stimulated photon echo peak shift. The effects of deuterium substitution on the stationary fluorescence spectra and the time domain data are found to be significant in both solvents. Moreover, the deuterium isotope effects are contrasted in methanol and acetonitrile. The ultrafast component, found to be 200–260 fs, is not slowed down by the deuterium substitution of the hydroxyl group in methanol, and it is even slightly shortened by deuteration in acetonitrile. The isotope dependence and the time scales suggest that the ultrafast component is not originated from the inertial free rotation of the solvent molecules but from collective dispersive solvent motion, at least for the present system. In addition to the ultrafast and ps solvation time scales usually observed in methanol and acetonitrile, a large amplitude ns component is observed in all solvents. The amplitudes of the ns component are substantially different in isotopomers, showing the opposite trend in methanol and acetonitrile.

Crustal deformation in the mid-May 1991 crisis preceding the extrusion of a dacite lava dome at Unzen volcano, Japan

Tilt observations at FG1 station located about 680 m west of the central crater of Unzen volcano, southwestern Japan, revealed details about the process of magma ascent during the mid-May 1991 crisis, just before the appearance of a lava dome. Crustal deformations measured during this period can be interpreted by a combination of upward growth of a buried magma column and lateral intrusion of a dike, modeled by a vertical line source and a planer tensile dislocation in an elastic half space. Marked change in tilt was observed at first in the EW direction at FG1 during May 11–14, suggesting the ascent of a magma column with a diameter of about 40 m from about 300 m to 160 m in depth beneath the central crater. After the interruption of the rise of the magma column, magma began to intrude laterally as a dike on May 15 toward approximately the N80°W direction. This resulted in a large tilt change essentially in the NS component at FG1. The horizontal extent of the dike might have exceeded 400 m in a few days with a thickness of about 6 m. On May 17, the magma column started to rise again, probably because the impediment to magma ascent was fractured by the growth of the dike. The top of the magma column finally reached to the crater bottom probably on May 19, and a new lava dome was actually observed on May 20. After which, the remarkable upward tilt to the south at FG1 due to the dike intrusion decreased rapidly. The deformation after May 20 can be interpreted by a gradual increase in the thickness of the dike up to 13 m by the end of May 1991. Although large errors are inevitable in estimated values because of a lack of sufficient data, the present results confirm the importance of making continuous observations of crustal deformation near the summit areas of active volcanoes in order to clarify the underground processes of magma movement leading to eruptions.

On the origin of the `35-μs kinetics' of P680 +⋅ reduction in photosystem II with an intact water oxidising complex

The origin of the `35-μs kinetics' of P680 +⋅ reduction in photosystem II (PS II) with an intact water oxidising complex has been analysed by comparative measurements of laser flash induced changes of the 830-nm absorption and the relative quantum yield of chlorophyll (Chl) fluorescence. The latter parameter was monitored at a time resolution of 500 ns by using newly developed home built equipment [Reifarth, F., Christen, G. and Renger, G. (1997) Photosynth. Res. 51, 231–242]. It was found that: (i) the amplitudes of the unresolved ns-kinetics of both 830-nm absorption changes and the rise of fluorescence yield exhibit virtually the same period four oscillation pattern when dark adapted samples are excited with a train of saturating laser flashes; (ii) the corresponding oscillation patterns of the normalised extent of the 35-μs kinetics under identical excitation conditions are strikingly different with maxima after the 3rd and 5th flash for the 830-nm absorption changes vs. pronounced maxima after the 4th and 8th flash for the rise of the fluorescence yield. The period four oscillations unambiguously show that the `35-μs kinetics' of P680 +⋅ reduction are characteristic for reactions in PS II entities with an intact water oxidising complex. However, the disparity of the oscillation patterns of (ii) indicates that in contrast to the ns components of P680 +⋅ reduction the 35-μs kinetics do not reflect exclusively an electron transfer from Y Z to P680 +⋅. It is inferred that a more complex reaction takes place which comprises at least two processes: (a) P680 +⋅ reduction by Y Z and (b) coupled and/or competing reaction(s) which give rise to additional changes of the chlorophyll fluorescence yield.

Meteor trail induced backscatter in MST radar echoes

The amplitude variations and Doppler spectra of the 50 MHz radar echoes from meteor trails are investigated. The radar echoes from underdense meteor trails particulary during nighttime do not always fit the conventional specular reflection model. The echo was found to spread, sometimes, over as large as 8–10 km in range. In each range bin, the spread echo from the trail is produced with an altitude dependent time delay after the passage of the meteoroid producing the ionized trail. The Doppler spectra of the spread echo in each range bin is asymmetric with a high frequency wing comprised of narrow spectral peaks and successive spectral peaks amplitude decrease with frequency. These observations are interpreted as due to multiple reflections of the radar pulse between the striations generated parallel to the geomagnetic field along the length of the meteor trail. There is evidence to show that within the dynamo region the EW and NS components of the currents at lower and higher altitudes could be in opposite directions during the early morning hours of Sq electric field reversal.

地震動のやや長周期成分からみた1923年関東地震の震源特性

Records observed at the Takada observatory by an Imamura's type strong motion seismograph are useful in Japan to investigate a source process of the 1923 Kanto earthquake (M=7.9). Ground motion in the vertical component is completely recorded to the end. Those in horizontal components are also recorded to the end, though the amplitudes of them are saturated in the midst of the records. The instrumental response of the seismograph has been already clarified by TAKEMURA and NOZAWA (1996), and many records from the recent earthquakes occurring near the focal region of the Kanto earthquake are also obtained at the same site with modern instruments. First, a crustal structure model from the source to the station is established so as to explain the dispersion characteristics of Love-waves observed for the recent earthquakes, and their records are simulated by the normal mode theory using the obtained structure to confirm its validity. Secondly, the records from the Kanto earthquake are reproduced using the same structure model. NOZAWA et al. (1995) proposed the two possible source models for the Kanto earthquake through the simulation of the records at the Gifu observatory. Two big subevents with a time interval of about 12s are assumed on the fault plane in both the models, while the focal mechanism and the focal depth of the second subevent are different between these models. It is found that these models similarly explain the records at the Takada observatory from the Kanto earthquake. The maximum displacements of the reproduced horizontal ground motions are estimated about 20cm in NS component. and about 10cm in EW component. These results are consistent with the fact that the amplitudes of the records in NS component are strongly saturated compared with those in EW component.