advances and problems in understanding the seismic response of potentially unstable slopes

ADVANCES AND PROBLEMS IN UNDERSTANDING THE

SEISMIC RESPONSE OF POTENTIALLY UNSTABLE

SLOPES

1. Dipartimento di Geologia e Geofisica, Universita` di Bari, Bari, Italy.2. Istituto di Ricerca per la Protezione Idrogeologica, ConsiglioNazionale delle Ricerche, Bari, Italy.

Vincenzo Del Gaudio1 and Janusz Wasowski2

報告者：林子翔指導教授：李錫堤報告日期：01/06

National Central UniversityGraduate Institute of Applied Geology

SEISMIC RESPONSE

Seismograph

Source effect

Site effect

Path effect

Picture from ：大地地理雜誌


September 19, 1985

SITE EFFECTS

October 17, 1989May 20 & November 15,1986


1. Why are we studying the site eff ects on unstable slopes?

2. What conditions will the site eff ects happen?

3. What will happen cause by site eff ects?

4. Our comprehension from previously studies in this problem

5. The expect of improvement and breakthrough through this study

Research motive


The influence of site eff ects on landslide triggering during earthquakes has been inferred in several studies, but its evaluation is made diffi cult by the complexity of factors controlling the dynamic response of potentially unstable slopes and also by the lack of local ground motion instrumental observations.

Considering the above, a local permanent network of accelerometric stations was sited in2002 on unstable slopes in a mountainous area of central Italy, around the town of Caramanico.

ACCELEROMETRIC MONITORING OF CARAMANICO LANDSLIDE-PRONE

SLOPES.

Research motive


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


Orfento river

Orta

river

Maiella Mts.

Alto hill

Caramanico Terme

Orfento River

CARAMANICO

0 100 m

M p

Lm

Lm

Lm

Bq

b

M p

sh

M p

1

2

3

4

5

N

LegendLm = limestones - Miocene; Mp = marly mudstones, Early Pliocene;Bq= carbonate megabreccias – Quaternary (?); Sh = soils (colluvial materials, landslide deposits, water-laid and eluvial sediments, artificial ground – Holocene); b = carbonate brecciaz (Quaternary); 1 = overthrust front of the Morrone Mt; 2 = faults; 3 = steep scarp of the megabreccia caprock; 4 = lithological limit; 5 = spring.

Caramanico Terme

Mt. Morrone

Study Area

1 km

CAR2

CAR1

CAR3

CAR4CAR

5

2002.10.10

2002.11.06

2004.12.03 2005.11.

11

2006.03.27

Accelerometric monitoring of a landslide-prone slopes at Caramanico Terme (Central Italy)

Study Area

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.

CAR4

CAR1

CAR2

CAR5

CAR3


Geologic profiles

Car1

Car2

Car3Car4

Car5

Study Area

WEAK GROUND MOTION DATA

NO. Launched

Car1 2002

Car2 2002

Car3 2004

Car4 2005

Car5 2006

Molise 2002 mainshock

2002-2008 82 152 2 5.7 161 km Period N. ev. N. record M>4 & N. rec >1 Max Mag Max Dist.

Data Acquisition


L’AQUILA EARTHQUAKE OF 6 APRIL 2009 (MW = 6.3)

Data Acquisition


Molise 2002 mainshock

L’Aquila 2009 mainshock

Period N. ev. N. record M>4 & N. rec >1 Max Mag Max Dist.

Up

North

East

Up

North

East

CAR1 recordings

2002-2008 82 152 2 5.7 161 km2009-2010 119 332 14 6.3 149 km

L’Aquila earthquake of 6 April 2009 (MW = 6.3)

Data Acquisition

WEAK MOTION DATA2002-2008

CAR1 VS CAR2

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

Amplification (16 events)

PHA(gal) Ia(m/s)

Average 1.36 1.88Min 0.83 1.06Max 1.83 3.62

CAR2 (landslide site)

CAR1 (substratum outcrop)

Data Analysis

CAR3 VS CAR4

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

CAR4 (reference)

CAR3 (on breccias)

Amplification (18 events)

PHA(gal) Ia(m/s)

Average 1.02 0.65Min 0.30 0.20Max 4.75 1.68

Data Analysis


Arias(Arias, 1970), 所定義 AI 之公式如下：

其中 g 為重力加速度（ m/sec2 ）； Td 為時間（ recording duration ）（ sec ）； a(t) 為測站接收到地震所產生的加速度值（ m/sec2 ）， AI 單位為（ m/s ）。

(corrected for the instrument response with 20 Hz cut-off frequency)

Calculate the Ia

Research Method


ellipticity ratio ：the ratio between maximum and minimum of Ia values measured along horizontal directions at different azimuths.

Polar diagrams and Calculate the ellipticity

ratio

Using E-W and N-S components of the recordings to calculate the Arias Intensity on accelerograms rotated at 10° azimuth intervals. Polar diagrams show the directional variations of AI, normalized by its maximum value. Diagrams are given for a representative sample of events differing for magnitude, distance and back-azimuth, together with the average normalized AI values (NAIav), calculated in different directions for all the recorded events.

min

max

Ia

Ia

Research Method


amplification ：

1. The dynamic response of Car2 compare with Car1,an average relative amplif ication by a factor of 2.2 in total shaking energy.

2. At the site CAR2 constantly high ell ipt icity values were found, with all the Ia maxima oriented within a narrow azimuth interval around the local maximum slope direction.

PRELIMINARILY STUDY

Support：

Preliminary measurements of S-wave velocity with the technique of refraction microtremor analysis [Louie, 2001] gave values of 300–600 m/s for the landslide material and 1000–1500 m/s for the mudstone. At CAR3 a contribution to amplification likely derives from both topographic effect and impedance contrast between the carbonate breccias and underlying limestones.

Site Effects：

Hypothesis ：

The relative amplification at CAR2 was attributed to impedance contrast between the colluvial (landslide) deposits and the underlying mudstone.

Discussion and Conclusions

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


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

CAR1

(15 events)

CAR2

(14 events)

CAR3

CAR4

(7 events)

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

(19 events)

POLAR DIAGRAMS OF HVSR

Data Analysis


Site Eff ects ：

Directivity ：

Given the occurrence of directivity in landslides, fault zones and fault-bounded slopes, both with or without the presence of ground motion amplification

This phenomenon can be masked by directivity related to source eff ects, and hence its recognition requires the analysis of several recordings of events with sources located at diff erent station-epicentre back-azimuths and having diff erent focal mechanisms.

DIRECTIVITY


STRONG MOTION DATA

2009L’AQUILA EARTHQUAKE


PRELIMINARILY STUDY

c) Iamax CAR2/CAR5

0

1

2

3

4

5

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Magnitude

Am

plif

ica

tio

n

b) PHA CAR2/CAR5

0.0

1.0

2.0

3.0

4.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Magnitude

Am

plif

ica

tio

n

2009.04.06 01:32:26 East

-100

-50

0

50

100

0 20 40 60 80 100 120

Time (s)

Acc

eler

atio

n (

gal

)

CAR2CAR5

Site response relative amplifi cation: soft soil sites

Data Analysis


PHA amplification vs event magnitude

b) PHA CAR2/CAR5

0.0

1.0

2.0

3.0

4.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

MagnitudeA

mp

lific

ati

on

a) PHA CAR2/CAR1

0.0

1.0

2.0

3.0

4.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Magnitude

Am

plif

ica

tio

n

Data Analysis

PHA CAR2/CAR4

0

1

2

3

4

5

6

7

1.0 2.0 3.0 4.0 5.0 6.0 7.0

Magnitude

Am

plif

ica

tio

n


c) Iamax CAR2/CAR5

0

1

2

3

4

5

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

MagnitudeA

mp

lific

ati

on

b) Iamax CAR2/CAR1

0

1

2

3

4

5

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Magnitude

Am

plif

ica

tio

n

Arias intensity amplification vs event magnitude

Data Analysis

Iamax CAR2/CAR4

05

10152025303540

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Magnitude

Am

plif

ica

tio

n

22.6

3.1

2.5


CAR2

0

1

2

3

4

5

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Magnitude

Ia e

llip

tic

ity

CAR3

0

2

4

6

8

10

12

14

16

18

20

0.0 1.0 2.0 3.0 4.0 5.0 6.0 7.0

Magnitude

Ia e

llip

tic

ity

CAR3

0

2

4

6

8

10

12

14

16

18

20

0 20 40 60 80 100

Distance (km)

Ia e

llip

tic

ity

CAR2

0

1

2

3

4

5

0 20 40 60 80 100

Distance (km)

Ia e

llip

tic

ity

min

max

Ia

Ia

ELLIPTICITY VS EVENT MAGNITUDE &

ELLIPTICITY VS EPICENTRAL DISTANCE

Data Analysis

Source Effects


Arias intensity amplifi cation vs epicentre back-azimuth

Data Analysis

CAR3

0

5

10

15

20

25

30

35

Ia_max Azimuth

Nu

mb

er o

f eve

nts

CAR3 (without Aquilano events)

0

2

4

6

8

10

12

14

Ia_max Azimuth

Nu

mb

er o

f eve

nts

CAR2

0

5

10

15

20

25

30

35

Ia_max Azimuth

Nu

mb

er o

f eve

nts

CAR2 (without Aquilano events)

0123456789

10

Ia_max Azimuth

Nu

mb

er o

f eve

nts


HVSR RESULTS

CAR2



CAR2: comparison HVSR (Horizontal-to-Vertical Spectral Ratios)HVNR (Horizontal-to-Vertical Noise Ratios) SSR (Standard Spectral Ratios)



There is evidence that seismic ground motion on slopes covered by thick colluvia or by deep-seated landslides can be considerably amplified and that in some cases this amplification can have a pronounced directional character with maxima oriented along potential sliding directions.

The causes of the directivity phenomena are stil l unclear: possibly a combination of topographic, l ithological and structural factors acts to re-distribute shaking energy, focusing it on site-specific directions.



Limiting the observations to few nearby and small magnitude events can cause an underestimate of site response amplification.

The wide range of shaking energy amplification (1.4–36.4) observed for different events relatively to a reference station on rock highlights the difficulty in quantifying local amplification factors.


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

advances and problems in understanding the seismic response of potentially unstable slopes

Documents

unstable slopes

accelerometric network

landslide deposits

site effectsoctober

town of caramanico

caramanico termemt

carbonate breccia23

carbonate brecciaz quaternary