Misure di rumore sismico (L’Aquila)

INGV-UniRoma3Corso di Sismologia e Rischio Sismico

8 maggio 2013

Rapporto sugli effetti del terremoto aquilanodel 6 aprile 2009

Coordinamento del rilievo macrosismico a cura di P. Galli e R. Camassi

Siti di misura

Analisi H/V

AMBIENT NOISE

SESAME: Site EffectS assessment using AMbient Excitations

http://sesame-fp5.obs.ujf-grenoble.fr/

“Understanding the physical nature and composition of the ambient seismic noise wavefield, especially in urban areas, requires answering two sets of questions, which are not independent of each other:

• What is the origin of the ambient vibrations (where and what are the sources)?• What is the nature of the corresponding waves, i.e., body or surface waves? What is the ratio of body and surface waves in the seismic noise wavefield? Within surface waves, what is the ratio of Rayleigh and Love waves? Again within surface waves, what is the ratio of fundamental mode and higher modes?”

Nature of ambient vibration wavefield

From SESAME……

The wavefield being a mixture of Love, Rayleigh and body waves, the origin of the H/V peaks (and troughs) is multiple: Rayleigh wave ellipticity, Airy phase of Love wave modes, resonance of S and/or P body waves.

Nature of ambient vibration wavefield

Most of the H/V peak frequencies agree within a deviation range of 10-20% with the 1D frequencies, the most extreme deviation are found for receivers that are located at the border of the model or above local topographical troughs.

Used for the identification of the resonance frequency (amplitude values are not always reliable)

theoretical 1D resonance frequencies (1D transfer function for vertically incident Swaves)

H/V peak frequencies computed

Ambient noise: HVNSR, or “Nakamura Technique”

FOUNDAMENTAL FREQUENCYthere are a good agreement between fundamental frequency obtained from earthquake and ambient noise non-agreements specially for low requencies (noise data can not shown the fundamental frequency predicted spectral ratio method).

fo(SSR) earthquake

fo(H

VN

SR

) n

ois

e

Ao(SSR) earthquake

Ao

(HV

NS

R)

no

ise

HVNSR versus SSR

AMPLITUDEfor many of the sites the amplification factors obtained from noise data are smaller than those obtained by earthquakes.

AMBIENT NOISE ANALYSIS

GEOPSY free software

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Analisi H/V

Analisi H/V