1

DETECTION OF DIRECTIVITY IN SEISMIC SITE RESPONSE

FROM MICROTREMOR SPECTRAL ANALYSIS

1.Dipartimento di Geologia e Geofisica, Universit`a degli Studi di Bari, Italy

2.Istituto di Ricerca per la Protezione Idrogeologica, Consiglio Nazionale delle Ricerche, Bari, Italy

3.Istituto di Metodologie per le Analisi Ambientale, Consiglio Nazionale delle Ricerche, Tito Scalo, Potenza, Italy

4.Dipartimento di Strutture, Geotecnica, Geologia Applicata all’Ingegneria, Universit`a degli Studi della Basilicata,Potenza, Italy

V. Del Gaudio1, S. Coccia1, J. Wasowski2, M. R. Gallipoli3, and M. Mucciarelli4

報告者：林子翔指導教授：李錫堤報告日期：06/09

Recent observations have shown that slope response to seismic shaking can be characterized by directional variations of a factor of 2–3 or larger, with maxima oriented along local topography features (e.g. maximum slope direction).

This phenomenon appears infl uenced by slope material properties and has occasionally been detected on landslide-prone slopes, where a down-slope directed amplifi cation could enhance susceptibility to seismically-induced landsliding.

We tested the applicability of a method commonly used to evaluate site resonance properties (Horizontal to Vertical Noise Ratio – HVNR or Nakamura’s method) as reconnaissance technique for the identifi cation of site response directivity.

INTRODUCTION

Research Motivation

The analysis presented by Del Gaudio and Wasowski (2007) suggests that these phenomena are not caused by a purely topographic eff ect.Indeed, a fundamental role seems to be played by anisotropies in slope material properties, which, in diff erent settings, can be either induced by factors controlling also the topographic relief (e.g. tectonics) or directly controlled by the topography (e.g. as eff ect of gravity-driven mass movements).

However, until now, the limited amount of observational data has not allowed a thorough understanding of factors controlling directional amplification and the definition of practical recognition criteria.

PREVIOUS WORK

Research Motivation

site eff ect evaluations by reference site method and non-reference sitemethod

SITE EFFECT EVALUATIONS

Reference site method

HSS=Horizontal Spectrum of Sedimentary site

HSR=Horizontal Spectrumof Reference site

HNS=Horizontal Noise of Sedimentary site

HNR=Horizontal Noise of Reference site

Borcherdt(1970)

Non-reference sitemethod

HNS=Horizontal Noise of Sedimentary site

VNS=Vertical Noise of Sedimentary site

HSS=Horizontal Spectrum of Sedimentary site

VSS=Vertical Spectrum of Sedimentary site

Lermo and Chávez-García (1993)Nakamura(1989)

Research Method

The HVNR technique, despite its somewhat unclear theoretical bases (see Bonnefoy-Claudet et al., 2006a), has been diff usely applied in microzonation studies, mainly because peaks in the H/V spectral ratios have proved to be an eff ective indicator of the resonance frequency of low-impedance surface layers. The technique remains popular even though the reliability of the associated estimate of the amplification factor is deemed questionable (Bonnefoy-Claudet et al., 2006b).

ABOUT HVNR

Research Method

ORFENTO AND ORTA RIVER VALLEYS

high reliefactive river erosionstrong permeability

contrasts between diff erent lithologies

abundant rainfallclose to active seismogenic

structuresa case triggered by an event

that was quite far away (with epicentral distance more than 100 km)

ROMA

Study Area

7

ACCELEROMETRIC NETWORK

地質年代 (百萬年 ) 岩性代號 英文 岩性第四紀 (1.8-today) Bq Quaternary limestone megabreccias 石灰質火山角礫岩

第四紀全新世 (0.01-today) Sqh Quaternary and Holocene soils (colluvium and artificial ground) 土壤 ( 崩積層與人工地面 )

第三紀上新世 (5.3-1.8) Mp Pliocene mudstones 泥岩第三紀中新世晚期 (7.246–

5.332)Me Messinian sandy-silty deposits with carbonate

breccia 粉砂沉積物混和碳酸鹽質角礫岩

第三紀中新世 (23-5.3) Lm Miocene 石灰岩未確認年代 L(m) limestones of uncertain Miocene age 石灰岩

Study Area

= accelerometric stations.

= microseismic noise measurements.

Etna

Tromino

GEOLOGIC PROFILES

Car1

Car2

Car3Car4

Car5

Study Area

5

HVNR MEASUREMENTS

Study Area

10

WEAK MOTION DATA2002-2008

11

CAR1 VS CAR2 & CAR4 VS CAR3

2002.11.12 09:27:49 East

-6

-4

-2

0

2

4

6

0 10 20 30 40 50

Time (s)

Acc

eler

atio

n (

gal

)

CAR2CAR1

Data Analysis

2006.06.05 00:07:50 East

-15

-10

-5

0

5

10

15

0 10 20 30 40 50

Time (s)

Acc

eler

atio

n (

gal

)

CAR3CAR4

12

POLAR DIAGRAMS OF NORMALIZED ARIAS INTENSITY

Max,0.99,260°

Min,0.55,170°

Max,0.90 ；min,0.83

Max,0.89 ；min,0.59

Max,0.96,290°

min,0.33,200°

Data Analysis

dttag

Ia )(2

2

13

Horizontal - to -Vertical Spectral Ratio (HVSR) from seismic “weak motion” data until 2008

POLAR DIAGRAMS OF HVSR

Data Analysis

14

HVNR RESULTS

15

Horizontal - to -Vertical Noise Ratio (HVNR) from microtremor recordings

POLAR DIAGRAMS OF HVNR

Data Analysis

Tromino

16

HVSR VS HVNRData

Analysis 𝐌𝐦𝐐=𝐇𝐕𝐌𝐇𝐕𝐦

HVM: at the frequency the maximum spectral ratio of H/V

HVm: at the frequency the minimum spectral ratio of H/V

↓

以 Mm

Q >

1.5

作為門檻值了解有無方向性

17

HVSR VS

HVNR

Data Analysis

𝐌𝐦𝐐=𝐇𝐕𝐌𝐇𝐕𝐦

18

Horizontal - to -Vertical Noise Ratio (HVNR) from microtremor recordings

POLAR DIAGRAMS OF HVNR

Data Analysis

Etna

13.114.7

19

INFLUENCE OF INSTRUMENTAL PROPERTIES

Data Analysis

13

Etna/Tromino

𝐻𝑉𝑁𝑅=𝐻𝑉

20

POLAR DIAGRAMS OF HVNRData

Analysis

21

HVSR VS HVNRData

Analysis 𝐌𝐦𝐐=𝐇𝐕𝐌𝐇𝐕𝐦

HVM: at the frequency the maximum spectral ratio of H/V

HVm: at the frequency the minimum spectral ratio of H/V

2.4 90 5.3 1.7

1. Peaks with significant H/V spectral ratio values (not less than 2) having a significant directivity (maximum/minimum quotient larger than 1.5) should be identified in the azimuthal distribution of HVNR values

2. If only one peak with a significant directivity is found, repeated measurements have to be performed at diff erent hours and week days, to rule out the possibility that the single directional maximum is due to a source of polarised noise and thus may not reflect site properties

3. Major directional peaks should have similar directions (within an azimuth interval of 20–30); otherwise diff erently oriented pronounced peaks could reflect the coexistence of diff erent sources of polarised noise at diff erent azimuths.

PROVISIONAL CRITERIA FOR SITE RESPONSE DIRECTIVITY IDENTIFICATION FROM HVNR

Discussion and Conclusions

1. several directional spectral peaks can be found at diff erent frequencies along similar directions.

2. The calculation of horizontal to vertical spectral ratios derived from seismic events (HVSR) or microtremor measurements (HVNR) can provide quite variable peak frequencies in relation to the signal-to-noise ratio characterizing recordings in diff erent frequency bands.

3. under the conditions of site response complexity typical of slopes with both topographic and lithostratigraphic amplification, HVNR and HVSR measurements alone cannot provide a complete and reliable identification of resonance frequencies and amplification factors.

Discussion and Conclusions

24

The End

波的傳導，根據 Snell’s Law ，在不同介質中情況

此為討論建立於平行層狀構造上，但事實上地震波所經過的地球內部應是呈現球狀層狀構造。

V

V’

i

I’

V’>V

場址效應： 方向性 (directivity) ：提到方向性與地形無關：Vidale et al.(1991) at Los AngelesBonamassa and Vidale (1991) at California ( 認為是非均質地質條件造成 )提到方向性與地形有關：Squdich et al.(1996) at California ( 沿著最大坡面有方向性 )與山崩構造有關：Xu et al.(1996) ( 透過模型發現沿著山崩滑脫面 S 波產生偏振現象 )其他：Rial(1996)( 波被滯留在山波內低速物質而產生放大 )Vahdani and Wikstrom(2002)( 地震波通過基盤時產生傾斜 )Gallipoli and Mucciarelli(2007)(HVSR 峰值方向與滑移方向相同 )

回顧研究方向性 文獻