estimation of local site effects using microtremor testing in

Estimation of local site effects using microtremor testing in

Vijayawada city, India

by

Manne Akhila, Dr D Neelima Satyam

in

Geotechnique Letters

Report No: IIIT/TR/2013/-1

Centre for Earthquake EngineeringInternational Institute of Information Technology

Hyderabad - 500 032, INDIANovember 2013

Estimation of local site effects using microtremor testing inVijayawada city, India

A. MANNE* and N. D. SATYAM*

The microtremor method is very useful in urban areas to estimate local site effects for microzonationpurposes. As seismic waves propagate from bedrock to the ground, their characteristics are sitespecific due to the heterogeneity of substrata. Source, path and site effects are prime parametersthat affect ground response. Many destructive earthquakes that occurred in the past (e.g. Bhuj 2001,Bingol 2003, Kashmir 2005, Haiti 2010, Tohoku 2011, Van 2011 and Yunnan 2012) clearly illustratethe effects of local soils on damage severity and pattern. The Indian sub-continent has more than400 major faults that influence seismic activity and India has experienced several devastatingearthquakes (Assam 1897 (M 5 8?7), Kangra 1905 (M 5 8?6), Bihar-Nepal 1934 (M 5 8?4), Assam-Tibet 1950 (M 5 8?7), Latur 1993 (M 5 6?4), Chamoli 1999 (M 5 6?8), Bhuj 2001 (M 5 7?6) and Sikkim2011 (M 5 6?9)). The state of Andhra Pradesh is located in the central part of peninsular India (zones IIand III according to Indian seismic code) and has a record of earthquakes along the coast. This studyconsidered Vijayawada (zone III), the third largest city in the state. Microtremor surveys were carriedout at 75 different locations in the Vijayawada urban area and analysis was carried out using theNakamura technique. Dynamic characterisation was carried out by considering the shape of theresponse curve, horizontal to vertical (H/V) amplitude, predominant frequency and the characteristicsoil profile at all the test sites. Based on this detailed analysis, a classification is proposed and apredominant frequency map of the study area was developed. It was found that areas in the northeastern and south eastern parts of the city, with silty and clayey sand formations, have comparativelyhigh predominant frequencies (> 4 Hz). The northern and western parts of the city, with high silty clayand silty sand, are characterised by moderate frequency values (2–4 Hz). Low (, 2 Hz) values ofpredominant frequencies were observed at a few locations. The H/V amplitudes are high (2–3) in theeastern and western regions.

KEYWORDS: earthquakes; ground movements; in situ testing; seismicity; vibration

ICE Publishing: all rights reserved

NOTATION

Ah amplification factor for horizontal motion of bodywave

Av amplification factor for vertical motion of body waveFEW Fourier amplitude spectrum of the east–west componentFNS Fourier amplitude spectrum of the north–south

componentFV Fourier amplitude spectrum of the vertical componentHb horizontal spectrum in the basement under the

surface groundHf horizontal spectrum in the basement on the surface

groundHs spectrum of horizontal direction of Rayleigh wavesTh amplification factor for vertical motion of surface

sedimentary ground based on seismic motion on theexposed rock ground near the basin

Tv amplification factor for vertical motion of surfacesedimentary ground based on seismic motion on theexposed rock ground near the basin

Vb vertical spectrum in the basement under the surfaceground

Vf vertical spectrum in the basement on the surfaceground

Vs spectrum of vertical directions of Rayleigh waves

INTRODUCTIONLocal site effects are phenomena caused by seismic wavepropagation in geological formations or irregularities nearthe surface. Damage to life and property during seismicshaking is governed by local site conditions, as exemplifiedin earthquakes worldwide (e.g. Mexico City 1985, LomaPrieta 1989, Northridge 1994, Kobe 1995, Bhuj 2001,Tohoku 2011 and Christchurch 2011). Local soil conditionsplay a significant role in the amplification of seismic waves(Street et al., 2001; Slob et al., 2002; Ansal et al., 2004).Amplification depends on the frequency of the groundmotion, and younger softer soils generally amplify theground motion more than older more competent soils orbedrock (Aki, 1993). Local amplifications of unconsoli-dated sediments were responsible for intensity variations ofabout two degrees on the modified Mercalli intensity scaleduring the San Francisco 1906 earthquake (Borcherdt &Gibbs, 1976) and the Loma Prieta 1989 earthquake (Seedet al., 1988). In the Bhuj earthquake in India in 2001, thedamage pattern was greatly influenced by loose soil coverand topographical features in the Rann area (Krinitzsky &Hynes, 2002). Apart from soft sediments and topography,the water table and bedrock depth also influence siteeffects. Earthquake damage induced by such effects thus

Manuscript received 21 March 2013; first decision 11September 2013; accepted 17 October 2013.Published online at www.geotechniqueletters.com on 13November 2013.*Earthquake Engineering Research Centre, InternationalInstitute of Information Technology Hyderabad, Hyderabad,India

Manne A. and Satyam N. D. (2013) Geotechnique Letters 3, 173–179, http://dx.doi.org/10.1680/geolett.13.00033

173

needs to be considered in seismic regulations, land useplanning or the design of critical facilities.

The current area of study, the Vijayawada urban area, islocated at 16?52uN latitude, 80?62uE longitude and is thethird most populated metropolitan area in the state ofAndhra Pradesh, India. Occupying an area of about73 km2, it is an important urban agglomeration in thecountry with considerable historical and agriculturalimportance and cultural heritage (Fig. 1). The city islocated along the stream of the Krishna River and iscovered by dominant amounts of silty clay (MH–CH) ofabout 76% and some silty sands and sandy silts (Manne &Satyam, 2011). The geology of the city is interestingand varied. Vijayawada is the only city in the world withtwo rivers and three canals. The lithological formationsvary from Khondalites, Peninsular Gneisses, Dharwarsand Proterozoic groups of rocks to sandstones of theGondwana group and coastal alluvia. Medium- to small-range hills cover the eastern and western parts of city. Withabout 22 seismic sources in and around the city (GoI, 2000)and prone to frequent tremors, the city (zone III (BIS,2002)) is seismically active (Manne & Satyam, 2011).Considering the seismic and geotechnical characteristics ofthe city it seems necessary to estimate site effects due toprobable seismic hazard. To estimate the seismic hazard ofVijayawada, microtremor testing (using the Nakamura(1989) method) was carried out and peak frequency andamplitude maps were generated.

THE NAKAMURA METHODThe Nakamura method, also termed the H/V (horizontalto vertical spectral ratio) method, is an experimentaltechnique to evaluate the characteristics of soil deposits.Microtremors are low-amplitude (micrometres) waves ofvibration caused by natural and artificial disturbances.From microtremor measurements, Nakamura (1989) foundthat site characteristics are related to a site transferfunction derived from the ratio of Fourier spectra of thehorizontal and vertical components of the microtremor

recording at the surface. The method facilitates recordingswithout the need for a reference site, and the ratio ofhorizontal to vertical spectra is evaluated to eliminate patheffects. The curve obtained by dividing horizontal andvertical spectra shows a peak at the resonance frequency ofthe site and the corresponding amplitude of vibration.

The H/V technique is very effective at estimating thenatural frequency of soft soil sites when there is a largeimpedance contrast with the underlying bedrock. The H/Vamplitude does not coincide with the amplitude of the soilcolumn, but the frequency coincides in almost all cases. Atypical geological structure for estimation of loose sedimentfrequency using microtremor testing is shown in Fig. 2.Nakamura (2000) deduced the quasi transfer spectra, orH/V, as follows.

Hf~AhHbzHs (1a)

Vf~AvVbzVs (1b)

Th~Hf=Hb (1c)

Tv~Vf=Vb (1d)

80.56 80.60

Gollapudi

Vidhyadharpuram

adepalli© 2013 Google

lmage © 2013 GeoEye

Indrakiladri hill

Budameru

Rvves canal

Gunadala hill

Bundar canalKrishna River

KrishnaDst.

Andhra Pradesh

India

canal

Vijayawada

80.64Longitude E

0 1 2 3 4 km

80.68

Latit

ude

N

16.50

16.52

16.54

16.56

Fig. 1. Location map of Vijayawada (Google maps)

Hr, Vr = Hb, Vb

Hf, Vf

Hb, Vb

Rock outcrop

Surface

Basement

Fig. 2. Typical geological structure of sedimentary basin(Nakamura, 2000)

174 Manne and Satyam

Th and Tv are the amplification factors for horizontal andvertical motion of surface sedimentary ground based onseismic motion on the exposed rock ground near the basin.

T�h~Th

Tv

~Hf=Vf

Hb=Vb

~H=V

Hb=Vb

(2)

where the horizontal to vertical spectral ratio H/V 5 Hf/Vf.According to measurements at rock sites, Hb/Vb > 1,therefore T�h > H/V.

In equations (1) and (2), Ah and Av are the amplificationfactors of horizontal and vertical motions of the bodywave, Hs and Vs are spectra of the horizontal and verticaldirections of Rayleigh waves and Th and Tv representamplification factors of the horizontal and vertical motionof surface sedimentary ground based on seismic motion onthe exposed rock ground near the basin. The spectra ofhorizontal and vertical motion in the basement under thesurface ground and on the surface ground are representedby Hb and Vb and Hf and Vf, respectively.

Microtremor studies provide a fundamental basis forground response analysis, particularly in densely populated

urban areas where there is difficulty in utilising conven-tional seismic techniques. The study area of Vijayawada isunderlain by loose sandy silts and silty clay, which makes itvulnerable to damage caused due to the ground motionamplification of the young and loose soil deposits in thearea. The city consists of low- to medium-range hills in thenorthern, north western and south western parts of the city,which give rise to topographic effects. The high water table(2–6 m) in these regions can be causative of large grounddisplacements and liquefaction effects even for low-frequency seismic waves. Hence, to estimate local siteeffects in Vijayawada city, H/V analysis of ambient noisemeasurements was considered.

ACQUISITION OF FIELD DATAThe microtremor setup consisted mainly of a recorder unit(MR2002-CE, SYSCOM (http://www.syscom.ch/)) andvelocity sensor (MS2003+, SYSCOM) (Fig. 3). The mea-surement of ambient vibration can be achieved using thecommunication software WINCOM (SYSCOM) with thissetup. To obtain reliable estimates of site effect parameters,experimental conditions have to be perfectly maintained toobtain good-quality data. After setting up the recorder andsensor, base line correction has to be performed to ensurerecording of a signal centred on zero.

Extensive ambient noise measurements were obtainedin Vijayawada, spreading from Enikepadu in the east,Jakkampudi in the north and the national highway in thesouth. Test sites such as flat ground areas, open spaces,low-noise localities and so on were thus selected to preventany interaction from traffic movement, constructionactivities, blasting, etc. Ambient noise measurements wereobtained at 75 locations (Fig. 4). Sample data were takenfor about 15 min to measure and setup trigger levels in allorthogonal directions to prevent errors in recordings. Thevibrations were recorded for a duration of 60 min with 1 seach of pre-event and post-event time. A GPS was used torecord the latitude and longitude of the test locations. Toensure noise levels were low, the data were collected in earlymorning and late evening.

Field laptop

Recorder

Connecting

Velocity sensorMs2003+

cables

MR2002

Fig. 3. Microtremor field testing setup

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48

49 50

51

4752

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6162

5826

75 6942 4344

684555

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46705667

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7274 14 31 66

6364

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4027

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30

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ude

N

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16.4880.58 80.60 80.62 80.64

Longitude E

0 1 2 3 4 km

80.66 80.68 80.70

Fig. 4. Location of microtremor test sites in Vijayawada city

Estimation of local site effects using microtremor testing in Vijayawada city, India 175

1.2 1.61.41.21.00.80.60.40.2

2.0

1.5

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0.40.2

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 9 10

1.61.41.21.00.80.60.40.2

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.50.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5Machavaram (I)

Patamata (I) Gurnanak Nagar (I)

Gunadala (I)Kondapalli (I)

H/V

Am

plitu

deH

/V A

mpl

itude

H/V

Am

plitu

deH

/V A

mpl

itude

H/V

Am

plitu

deH

/V A

mpl

itude

Maruthi Nagar (I)6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5 10.0

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0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0

0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5 5.0 5.5 6.0 6.5 7.0 7.5 8.0 8.5 9.0 9.5

1 2 3 4 5 6 7 8 9 10

1 2 3 4 5 6 7 8 9 10

Ibrahimpatnam (II) VMC Ground (II)

Bharathi Nagar (II)Gunadala (II)

Ayyapa Nagar (III)Frequency: HzFrequency: Hz

Bhavanipuram (III)

Fig. 5. H/V amplitude versus frequency plots


DATA ANALYSISThe H/V technique was applied to the recorded data toanalyse the behaviour of the ground under dynamicloading. H/V is computed as the average of two horizontalcomponent spectra divided by the vertical spectrum foreach window using View 2002 software (http://www.syscom.ch/). Care was taken to record the trigger levelsbefore actual recording was carried out at every test site,and signals erroneously recorded based on the trigger levelswere removed before carrying out the analysis.

The recorded 1 min files at a particular test site wereprocessed using the file group option. Fourier analysis(Bracewell, 1986) based on the H/V spectral ratio wasapplied for each location. The microtremor recordings weretransformed into the frequency domain using the fastFourier transform method (Walker, 1996). Fourier spectraof the filtered data were generated by vectorial summationof two horizontal component spectra (EW and NS) toobtain the resultant horizontal spectrum H. The H/Vspectrum was then obtained by dividing the horizontalspectral amplitude by the vertical component spectralamplitude at each frequency

H=V~F2

NSzF2EW

F2V

� �1=2

(3)

H/V is the horizontal to vertical component ratio and FNS,FEW and FV are the Fourier amplitude spectra of the north–south, east–west and vertical components, respectively.

The final plot produced by the program includes H/Vas the ordinate and frequency as the abscissa. For clearidentification of peak frequency, the spectrum is smoothedusing the moving average technique.

RESULTS AND DISCUSSIONField recordings of ambient noise were conducted inVijayawada city to estimate amplitude and frequencyparameters. As noted earlier, care was taken to preventany possible error in obtaining the data. The records wereprocessed using View 2002. Different peaks were observedin the plots of H/V versus frequency (Fig. 5). Only peaks in

the frequency range 0–10 Hz were considered for analysisof the 75 sites and a classification is proposed. All testlocations were classified into three categories (T1, T2 andT3) based on shape of the curve, soil characteristics, H/Vamplitude and predominant frequency. Most of the peaksare clear and often sharp, but a number of sites exhibitedpeaks of high amplitude (> 4 Hz).

The Fourier spectra of higher frequency (> 4?0 Hz) areclassified as type T1. The spectra in this classification arecharacterised by peaks starting from 1?0 H/V amplitudeand shifting towards the left in an increasing fashion.Most of the central and eastern parts (53% of the testedlocations) of the city are covered by such peaks. Theeastern regions of the city (Autonagar, Moghalirajapuram,Jakkampudi, Madhuranagar, Machavaram, Ramava-rappadu, Kasturibaipet, Governerpet, Prasadampadu,etc.) are covered by silty clay and silty sand. The areas ofMoghalirajapuram, Patamata and Madhuranagar hadalmost the same peak ground frequency. Figure 6 showsa map of fundamental frequencies of the sediments with adistribution in a wide range of 0?5 to 9?85 Hz.

In type T2, the frequency ranges from 2?0 to 4?0 Hz;35% of the locations fall under this category. Almost allthe western and southern parts of the city exhibit suchfrequencies, with Barati Nagar, Jakkampudi, Gunadala,Enikepadu, Pakirgudem, Brahmin Street, Bhavanipu-ram, Crombay, PNT colony, Vambay and Labbipetfalling into this classification. In this category, peaks areshifted towards the origin. The predominant soil in theselocations is sandy silt and silty clay, but clayey sand ispredominant (3?0–7?5 m) in a few locations (e.g.Labbipet). The spectra of type T2 have strikingly dif-ferentiable peaks.

Type T3 classification is for low frequency (, 2?0 Hz).These locations (MG Road, Brindhavan colony, Jak-kampudi, NH9, Ramalingeswara Nagar, Ayyapa Nagar)are few (12%) and have clayey sand and silty clayformations.

The spread of H/V amplitude throughout the city isshown in Fig. 7. It is interesting to observe that H/Vamplitudes are less than 2 for locations along the river andsurrounding the canals. These locations are covered by

16.54

16.53

16.52

Latit

ude

N

16.51

16.50

16.49Outcrop

RTC colony

SatyanarayanapuramSethannapeta

KasturibaipetMoghalirajapuram

PNT colony

Vambay colony

Madhura NagarMachavaram

Ramavarappadu

Currency NagarPrasadampadu

Bank colony

Loyola collegeGunadala

Kanuru PanchayatNew P&T colonyPatamata

MG Road

Pakirgudem

Labbipet Benz Circle

Ramalingeswara Nagar

GovernerpetIndhiragandhi municipal stadium

Bhavanipuram

Durga temple

Crombay

Water body

80.60 80.61 80.62 80.63 80.64 80.65 80.66 80.67 80.68 80.69

0.5 Hz

2 Hz

4 Hz

Frequency

Longitude E

0 1 2 3 4 km

Fig. 6. Predominant frequency map for Vijayawada city


sandy silt and silty sand. Higher amplitudes (H/V 5 2–4)were observed in the north western and eastern parts of thecity where most of the hills are concentrated. The locationscovered by such high amplitudes have dominant amountsof silty clay and clayey sand.

CONCLUSIONSThe microtremor technique is a rapid, cost-effective andefficient method for ground response study. It can be usedto estimate ground response in densely populated urbanareas where there are constraints in using conventionalseismic techniques. Microtremor investigations can be usedto survey a large area in a reasonable time at relatively lowcost in order to detect the potential danger of soil structureresonance.

Microtremor measurements were taken at 75 differentlocations in the Vijayawada urban region and theNakamura (H/V) technique was adopted for analysis ofthe results. Reliability of the results and rapidity of datacollection was ensured by H/V measurements fromambient noise recordings. The ambient noise data wereprocessed using the computer program VIEW 2002 togenerate an H/V plot for each site. Depending on theshape of the response curve and the estimated resonancefrequency, all the sites were classified into one of threecategories. The H/V approach thus provided a simplemeans of determining the predominant frequency of a soilsite. The H/V spectra revealed that most of the amplifica-tion peaks are in the frequency range 1?0–9?85 Hz. Thehigh-frequency range (7?5–9?0 Hz) is characteristic of thenorth western and south eastern parts of the city. Thenorthern part of the city is characterised by mediumsediment frequencies (4?5–6?0 Hz). The transition betweenlower frequencies in the western part and mediumfrequencies in the eastern part is 1 ?5–6 ?0 Hz.Amplification factors were also identified and representa-tive maps developed.

The Nakamura technique provided a simple means ofdetermining the predominant frequency of a soil site. Theresults can be used to establish seismic microzonation ofa city and hence this study can help assess the potentialparameters for seismic hazard estimation.

AcknowledgementThe authors gratefully acknowledge financial supportprovided by the Department of Science and Technology,Government of India (SR/FTP/ES-77/2009).

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80.60 80.61 80.62 80.63 80.64 80.65 80.66 80.67 80.68 80.69Longitude E

0 1 2 3 4 km

0.6

1

2

3

H/VCrombay

RTC colony

Bhavanipuram

Outcrop

Water body

SatyanarayanapuramSethannapeta

PNT colony Vambay colony

Madhura NagarMachavaram

GovernerpetIndhiragandhi municipal stadium

Ramavarappadu

Currency Nagar

PrasadampaduBank colony

Kanuru PanchayatNew P&T colonyPatamata

GunadalaLoyola college

Moghalirajapuram

Labbipet

KasturibaipetDurga Temple

Krishna River

MG Road

PakirgudemBenz Circle

Ramalingeswara Nagar

Fig. 7. H/V amplitude map for Vijayawada city


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