Vibrational Characteristics of Microseisms and Their Applicability to Microzoning in a Sedimentary Basin.

Akamatsu, Junpei; Fujita, Masayuki; Nishimura, Keiichi

doi:10.4294/jpe1952.40.137

Cited by 11 publications

(4 citation statements)

References 14 publications

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“…Cette méthode a été validée expéri-mentalement par plusieurs auteurs (Duval et al 1996;Riepl et al 1998). Le pic de la courbe, en fréquence, représentant ce rapport coïncide avec la fréquence fondamentale de résonance du site constatée par mouvement forts; ceci a été particulièrement mis en évidence dans les zones de la baie de San Francisco les plus atteintes par les séismes (Ohmachi et al 1991), à Nagano lors du séisme de 1984 (Akamatsu et al 1992) et dans plusieurs villes des Philippines touchées par le séisme de 1990 (Ohmachi et Nakamura 1992). Lermo et Chavez-Garcia (1993) montrent que la valeur du pic peut donner une estimation de l'amplification observée lors de séismes réels en trois sites au Mexique.…”

Section: Méthodes ''H/v Bruit De Fond'' Et ''H/v Séisme''unclassified

Effet de site sismique, méthodes d'évaluation: application au site de la ville de Tunis

Bouden-Romdhane

Mechler

2002

Bulletin of Engineering Geology and the Environment

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Tunis n'est pas située dans une zone sismique très active; toutefois, une grande partie de la ville est construite sur un bassin quaternaire et la qualité géotechnique faible des terrains l'expose à des effets de site sismiques. Deux campagnes de mesures comparent les réponses dynamiques de plusieurs sites obtenues à partir de l'enregistrement de la séismicité et de mesures de bruit de fond sismique. Les résultats montrent que les fréquences de réso-nance peuvent être déterminées au moyen du bruit de fond. Les mesures réalisées en 300 sites de la ville mettent en évidence de très basses fréquences (0.4 à 1.5 Hz) dans des sites à remplissages sédimentaires. Les effets de site les plus marqués sont attendus dans les sites à remplissage peu consolidé. Une modéli-sation numérique attribue les modes propres de ces sites aux couches sous-consolidées superficielles, du moins pour les sites éloignés des reliefs.Abstract The seismic site effect reflects the magnification versus frequency undergone by a seismic wave when crossing the subsurface layers. The methods most frequently used to obtain estimates are geotechnical structure modelling (assuming the information is available) or recording the actual site seismicity compared with the seismicity recorded on a reference rock. The latter method, also called the sediment to bedrock spectral ratio, provides the best experimental estimation as the required parameters are measured directly. However, it involves major practical difficulties, particularly the long duration of the measurements (and hence high cost) and the difficulty of undertaking seismic measurements in towns due to the ambient noise. The ''H/V background noise'' method (H/V, horizontal/vertical) consists of recording the site seismic background noise and computing the spectral ratio between the signals obtained on the vertical and two horizontal components. This ratio is known to give a good estimation of the fundamental soil resonance frequency.The main part of Tunis city is built in a Quaternary basin. The presence of saturated silty and plastic clayey formations up to 50 m thick, overlying a more compact clayey-sandy layer, suggests the likelihood of seismic site effects. No seismic microzonation was available for Tunis city. To prepare such a zonation, two field surveys have been carried out to evaluate the dynamic site responses. Both of the methods discussed above were used. The experimental validity of the H/V background noise was checked and very low fundamental resonance frequencies were detected in the basin and on the higher ground. These appear to be caused by a deep reflector, probably the top of the limestone and marls of Eocene age. There are strong site amplitudes in areas with a silty infill. The amplification factor increases with the thickness of the deposits.Non-linear responses were also modelled using the computer program EERA which uses the equivalent linear theory. Such modellings are based on the unit weight, shear wave velocity and thickness as well as the shear modulus of the soil la...

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Section: Méthodes ''H/v Bruit De Fond'' Et ''H/v Séisme''unclassified

Effet de site sismique, méthodes d'évaluation: application au site de la ville de Tunis

Bouden-Romdhane

Mechler

2002

Bulletin of Engineering Geology and the Environment

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“…The technique, originally proposed by Kanai & Takanaka (1961) and based on a simple spectral analysis of microtremor measurements, was mainly improved by Japanese researchers in the following 30 yr as an exploration tool (e.g. Ohta et al 1978; Kagami et al 1982; Nakamura 1989; Akamatsu et al 1992; Yamanaka et al 1994). In the last 15 yr, several authors have investigated both the nature of the environmental noise and the phenomenology behind the HVSR in order to better understand the overall physics as well as to improve the processing techniques and its interpretation in term of underground structure (e.g.…”

Section: Introductionmentioning

confidence: 99%

The buried shape of an alpine valley from gravity surveys, seismic and ambient noise analysis

Barnaba¹,

Marello²,

Vuan³

et al. 2010

Geophysical Journal International

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It has long been observed that damage due to earthquakes depends greatly on local geological conditions. Alpine valleys represent a typical populated environment where large amplifications can take place owing to the presence of surface soils with poor mechanical properties combined to complex topography of the rock basin. In the framework of the EU Interreg IIIB SISMOVALP Project ‘Seismic hazard and alpine valley response analysis’, a stretch of the Tagliamento River Valley (TRV), located in the north-western part of the Friuli Region (Italy) and close to the epicentre of the 1976 Mw = 6.4 earthquake, has been investigated with the aim to define the buried shape of the valley itself.\ud Two non-invasive, lowcost, independent geophysical methodswere used: (i) detailed gravity survey and (ii) H/V spectral ratio (HVSR) of microtremors.\ud Because of structural geological complexity and active tectonics of the Friuli region, an irregular valley shapewas expected in this area. The independent analysis performed by gravity and passive noise, and complemented with refraction seismic velocity profiles, confirms this hypothesis and leads to two models that were consistent, but for some small scale details. The maximum depth estimated is about 400–450 m in the southern part of the valley, while a mean value of 150–180 m is estimated in the northern part. The sediment thickness obtained for this stretch of the TRV is quite large if compared to eastern Alps Plio-Quaternary rates; therefore the valley shape imaged by this study better corresponds to the top of carbonate rocks.\ud Finally, on the basis of the obtained morphology and some direct measurements,we conclude that the TRV features an overall 1-D seismic response (i.e. the resonance is related only to the\ud sediment thickness rather than to the cross-section shape), but in its deepest part some limited 2-D effects could take place

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“…Microseism, or long-period microtremor (T\ 1 second), has been studied extensively from the viewpoint of seismic microzonation of urban regions (e.g., KAGAMI et al, 1982KAGAMI et al, , 1986AKAMATSU et al, 1991AKAMATSU et al, , 1992a. It is expected to be a useful indicator in evaluating soil amplification of earthquake ground motions.…”

Section: Introductionmentioning

confidence: 99%

Stochastic Simulation of Microseisms Using Theory of Conditional Random Fields

Morikawa

Akamatsu

Nishimura

et al. 1998

Pure and Applied Geophysics

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We examine the applicability of conditional stochastic simulation to interpretation of microseisms observed on soft soil sediments at Kushiro, Hokkaido, Japan. The theory of conditional random fields developed by KAMEDA and MORIKAWA (1994) is used, which allows one to perform interpolation of a Gaussian stochastic time-space field that is conditioned by realized values of time functions specified at some discrete locations. The applicability is examined by a blind test, that is, by comparing a set of simulated seismograms and recorded ones obtained from three-point array observations. A test of fitness was performed by means of the sign test. It is concluded that the method is applicable to interpretation of microseisms, and that the wave field of microseisms can be treated as Gaussian random fields both in time and space.

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Vibrational Characteristics of Microseisms and Their Applicability to Microzoning in a Sedimentary Basin.

Cited by 11 publications

References 14 publications

Effet de site sismique, méthodes d'évaluation: application au site de la ville de Tunis

Effet de site sismique, méthodes d'évaluation: application au site de la ville de Tunis

The buried shape of an alpine valley from gravity surveys, seismic and ambient noise analysis

Stochastic Simulation of Microseisms Using Theory of Conditional Random Fields

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