During 5 microzonation projects in Italy, we had access to 46 estimates of Vs30 (30 obtained with down-hole or
cross-hole measurements, plus 10 velocity profiles obtained with surface techniques, 6 sites were considered A
without drilling, being rock sites). In all the sites we performed HVNR measurements to verify the presence of a
resonant frequency and in 34 sites we installed seismic instrumentation to record earthquakes and estimate site
amplification using HVSR. It is important to note that we did not pre-selected the sites, but just followed the
requests of two Regional governments (Marche and Basilicata) to study a set of localities that were chosen for
reasons other than geo-morphological ones (previous earthquakes, pilot studies, design of new infrastructures).
The comparison between site seismic amplification and Vs30 showed that this last parameter is not a good proxy
of observed site effects. The reason why in Italy Vs30 does not provide satisfactory estimates is linked to
peculiar geological settings that are widespread in our country. The main problems encountered are
underestimations by Vs30 at sites with velocity inversions and overestimations on deep basins. Vs30 seems to
work fine only if a site has a strictly monotonic velocity profile increasing with depth and a strong impedance
contrast in the first dozen meters. Further data will be available thanks to an ongoing national project funded by
the Civil Defence Department that is focusing on Vs30 estimates in the presence of velocity inversions, fractured
rock masses, landslides and karst areas

Gallipoli, M. R.

Mucciarelli, M.

Earth-prints Repository

First European Conference on Earthquake Engineering and Seismology
(a joint event of the 13th ECEE & 30th General Assembly of the ESC)
Geneva, Switzerland, 3-8 September 2006
Paper Number: 270      
COMPARISON BETWEEN VS30 AND OTHER ESTIMATES OF SITE 
AMPLIFICATION IN ITALY
Marco MUCCIARELLI1, Maria Rosaria GALLIPOLI2 
SUMMARY
During 5 microzonation projects in Italy, we had access to 46 estimates of Vs30 (30 obtained with down-hole or 
cross-hole measurements, plus 10 velocity profiles obtained with surface techniques, 6 sites were considered A 
without drilling, being rock sites). In all the sites we performed HVNR measurements to verify the presence of a 
resonant frequency and in 34 sites we installed seismic instrumentation to record earthquakes and estimate site 
amplification using HVSR. It is important to note that we did not pre-selected the sites, but just followed the 
requests of two Regional governments (Marche and Basilicata) to study a set of localities that were chosen for 
reasons other than geo-morphological ones (previous earthquakes,  pilot studies, design of new infrastructures). 
The comparison between site seismic amplification and Vs30 showed that this last parameter is not a good proxy 
of  observed site  effects.  The reason why in Italy Vs30 does not  provide satisfactory estimates is  linked to 
peculiar  geological  settings  that  are  widespread  in  our  country.  The  main  problems  encountered  are 
underestimations by Vs30 at sites with velocity inversions and overestimations on deep basins. Vs30 seems to 
work fine only if a site has a strictly monotonic velocity profile increasing with depth and a strong impedance 
contrast in the first dozen meters.  Further data will be available thanks to an ongoing national project funded by 
the Civil Defence Department that is focusing on Vs30 estimates in the presence of velocity inversions, fractured 
rock masses, landslides and karst areas.
1. INTRODUCTION
The average of shear wave velocity in the first 30 m (Vs30) has been internationally adopted since the NEHRP 
classification in the USA. The original work of Borcherdt (1992) was based on data from Western USA, and so 
were the first  papers discussing pro (Anderson et al,  1996) and cons (Wald & Mori,  2000) of the method. 
Outside the region where the method was developed, some doubts arose about the capability of Vs30 to predict 
amplification in deep basins (Park & Hashash, 2004), in other tectonically active regions (Stewart et al., 2003) or 
in presence of a velocity inversion (Di Giacomo et al., 2005).
The Italian seismic code, updated in 2003 largely following the provisions of EuroCode8, has taken for granted 
the fact that Vs30 can be used without modification also in Italy. The aim of our work is to verify if:
1. Vs30 is a good proxy of site amplification in a country with complex geology
2. The grouping of Vs30 in the Italian Seismic Codes in different soil classes and relevant spectra is 
adequate
We want to point out that we did not pre-selected the sites where the study was carried out, but we just followed 
the requests of two Regional governments (Marche and Basilicata) to study a group of sites that were chosen for 
reasons other than geo-morphological ones (previous earthquakes,  pilot studies, design of new infrastructures).
We took into account 30 estimates of  Vs30 obtained with down-hole  or  cross-hole measurements,  plus  10 
velocity profiles obtained with surface techniques, 6 sites were considered A without drilling, being rock sites). 
In all the sites we performed HVNR measurements to verify the presence of a resonant frequency and in 34 sites 
we installed seismic instrumentation to record earthquakes and estimate site  amplification. The HVNR was 
estimated using a digital tromometer (Micromed Tromino) and the same processing technique for all the sites. 
1 DiSGG – Università della Basilicata, Viale dell’Ateneo Lucano 10, 85100 Potenza Italy 
Email :  mucciarelli@unibas.it
2 IMAA – CNR, Contrada S.ta Loja, Tito Scalo (PZ) Italy 
Email: gallipoli@pz.cnr.it 
1
The  earthquakes  were  recorded  with  Lennartz  1  Hz  seismometers  and  24  bits  digitisers  at  11  sites,  with 
Nanometrics BB station at 4 sites and with Etna Kinemetrics at all the others. 
2. DATA ANALYSIS
All the parameters used in the subsequent analyses are reported in Tab. 1. The first column is the name of the 
site, the second reports the technique used for the estimate of the Vs profiles (H= down-hole or cross-hole, S= 
NASW), the third indicates the soil class, the fourth the fundamental frequency (if any) estimated as HVSR from 
earthquakes (associated  amplitude  in  the  fifth  column)  or  by HVNS where  earthquakes  were  not  available 
(associated amplitude in the sixth column)
Tab. 1 Data used in the study. See text for the details
2
The first analysis performed was the grouping of Vs30 profiles according to the soil classification scheme, where 
class A has Vs30>800 m/s, B has 360<Vs30<800 m/s, C has 180<Vs30<360 m/s and D has Vs30<180 m/s. In 
our  study  we  did  not  encounter  E  type  sites.  Fig.  1  reports  the  Vs  profiles  grouped  by  soil  class,  with 
superimposed the interpolated profiles according to power increase of velocity as a function of depth.
Fig. 1 Vs profiles grouped by soil class. Red lines are interpolated profiles.
Three things are worth noting: the larger variability of sites belonging to class B due to larger velocity range, the 
fact that only 5 sites reach 800 m/s (i.e. bedrock) within the first 30 m, and that the two parameters of the 
interpolating  equations  are  inversely  correlated  as  suggested  by  other  authors  (Albarello,  2006;  personal 
communication). We then analysed the scatter of amplification as a function of soil category (Fig 2). 
Fig. 2 Distribution of maximum amplitude of HVSR and HVNR grouped by soil classes.
3
Both average and median are increasing with soil category but the median of class A and median of class B are 
almost  the  same.  There  are  several  sites  belonging  to  class  B  and  few of  class  C  sites  that  do  not  show 
amplification (HVSR<2). Finally, there are few sites in class D for a meaningful statistic, but they show less 
amplification than class C.
Fig. 3 is a pie diagram reporting the percentages of sites showing expected or unexpected behaviour. 
A without ampl
A with ampl
B, C,D without ampl
B, C,D with ampl @<1Hz
B, C,D with ampl @>1Hz
Fig. 3 Distribution of observed cases according to compliance or failure.
In 1 out of 5 cases there are amplifications where not expected or vice-versa. If we include the occurrence of 
amplification at low frequencies where not expected, the percentage of sites with problems reaches 36%.
Fig 4 shows the distribution of amplitude vs. frequency for the different soil classes.
0,0 0,5 1,0 1,5 2,0 2,5 3,0 3,5 4,0 4,5 5,0 5,5 6,0
2,0
2,5
3,0
3,5
4,0
4,5
5,0
5,5
6,0
6,5
Soil A
Soil B
Soil C
Soil D
Frequency
Am
pl
ifi
ca
tio
n
Fig. 4 Distribution of amplitude vs. frequency for different soil classes.
4
There is no clear correlation between frequency and amplification for different soil classes. Large amplifications 
at low frequencies occur at sites that are located within large sedimentary basins. This is surprising, because 
being HVSRs, these amplifications should be connected with 1-d effects and not with 2-d effects.
At this point we tried to answer the second question outlined in the introduction: in the Italian Seismic Code, is 
the grouping of Vs30 in different soil classes and relevant spectra adequate?
We then plotted the observed amplification vs. Vs30, without any grouping a priori (Fig. 5).
Fig. 5 Correlation between Vs30 and amplification, for D-H only. 
We performed this analysis just for the best quality data, that is borehole profiles for class B and C only. There is 
a slight correlation between Vs30 and amplification, with a Spearman Rank Correlation = 0.239, that passes the 
significativity test at just 80% confidence level.
For  the  same  reduced  data  sample,  we perform a  comparison  between  the  observed  spectra  and  the  ones 
provided by the Code. To perform this check, we normalised the observations determining a statistic ε defined as
ε =[(Code spectrum B/ Code spectrum A)-mean observed B]/S.D. observed B                                                   (1)
Fig. 6 reports the result of the comparison.  For soil category C, the building code underestimates the average 
observed amplification in the range 0.3-5 Hz, never exceeding 1 standard deviation of the distribution of the 
observed amplification functions. For frequencies higher than 5 Hz, the code is over-conservative, exceeding 1 
standard deviation in the range 7.5-8.5 Hz
For soil category B, the building code underestimates the average observed amplification in the range 0.3-10 Hz, 
exceeding 1 standard deviation between 2.5-3.5 Hz.
5
Fig. 6  Plot of the statistic estimated according with eq. 1.
3. DISCUSSION AND CONCLUSIONS
The general explanation why Vs30 may fail to correctly predict site amplification at a site was already given by 
Wald & Mori (2000): “The disparity between observed and predicted amplifications appears to be a result of  
oversimplification inherent in the amplification estimation methods, such as... the smoothing effect of using an  
average velocity, limiting the properties considered to the uppermost 30 m of material, and complexities in the  
wave  propagation  that  are  not  addressed  by  these  methods”.  We  tried  to  identify  if  there  are  common 
characteristics for sites in Italy where Vs30 works fine or not. 
The sites classified as Class A that show unexpected amplification are not affected by topographic effect, being 
mostly on hill flanks. A more probable cause is the possible presence of different degrees of rock fracturation, 
also induced by faults. A good example of this kind of situation is given by Martino et al. (2006) for Cerreto di 
Spoleto (Italy). A case of unexpected amplification is due to velocity inversion. This is a widespread situation in 
Italy. Ancient settlements took place on slices of rigid material overlying soft sediments: those remnants were 
the most  suitable place to build a  village.  In southern Tuscany and Umbria and northern Latium, strata  of 
quaternary volcanic products rest atop softer sediments; in the Apulia foredeep, well-cemented conglomerates 
overlay deep strata of clay and sand. The rigid top may well exceed 30 m, making the Vs30 estimate both 
difficult to obtain and meaningless at the same time.
The sites B and C that do not show any amplification are located on very deep basins with a probable low 
impedance contrast at the sediment-bedrock interface. In some cases the fundamental frequency is lower than 
expected by the Code, so the general pattern is an underestimation by the Code at frequency below 5 Hz. In 
either instances, the presence or not of a clear fundamental frequency with a strong amplification is due to 
cemented strata somewhere in the stratigraphic sequence, with very little importance of the Vs in the first meters. 
The Po plain, the Sant’Arcangelo Basin and the Adriatic foredeep are places were this phenomenon may occur. 
We have seen the best example of it in the city of Senigallia: all the site are classified as C, but some show 
amplification higher  than expected while  others  have  flat  spectrum. The quaternary clay and sand deposits 
overlay Pliocene marls with almost no velocity contrast at 600 m/s. The limestones are more than 1.000 m deep. 
The amplification is due to the presence or not of compacted gravels strata in the first 30 m.
We would advise practitioners to be cautious when using Vs30. The Italian Building Code recommends to 
perform microzonation studies and to use Vs30 if no other data are available. Especially for public buildings, we 
recommend to try to record weak motions at the site of interest. Italy is a country with a diffused seismicity 
6
allowing for short recording periods, and the cost of a basic seismometer or accelerometers equals two 30 m 
drillings plus down-hole test performed by contractors. At even lower cost, HVNR should be performed in sites 
where “rocky” appearance might suggest avoiding very expensive drillings thought to be useless. 
We expect to obtain more data by ongoing projects that should double the data base of observation in the next 
two years, including sites on unstable slopes and karst areas, other two situations very common in Italy.
4. ACKNOWLEDGEMENTS
This work was funded by a DPC-INGV grant (project S3). We thank the Marche Regional Gov., Civil Protection 
Office and the Basilicata Regional Gov, Environment and Land Use Dept for funding the projects that made the 
data available.
5. REFERENCES
Anderson J. G., Y. Lee, Y. Zeng and S. Day (1996). Control of Strong Motion by the Upper 30 Meters, Bull.  
Seism. Soc. Am., Vol. 86, 1749-1759.
Borcherdt  R.  D.  (1992).  Simplified site  classes  and empirical  amplification factors  for  site-dependent  code 
provisions,  in  Proc.  NCEER,  SEAOC,  BSSC  Workshop  on  Site  Response  during  Earthquakes  and 
Seismic Code Provisions, November 18-20, University of Southern California, Los Angeles, California.
Di  Giacomo D.,  M.  R.  Gallipoli,  M.  Mucciarelli,  S.  Parolai,  and  S.  M.  Richwalski  (2005).  Analysis  and 
modeling of HVSR in the presence of a velocity inversion: the case of Venosa, Italy, Bull. Seism. Soc. 
Am. 95, 2364-2372.
Martino S., A. Minutolo, A. Paciello, A. Rovelli, G. Scarascia Mugnozza and V. Verrubbi (2006). Evidence of 
amplification effects in fault zone related to rock mass jointing, Natural Hazard, in press.
Park D. and Y.M.A. Hashash (2004). Probabilistic seismic hazard analysis with non linear site effects in the 
Mississippi embayment, Proc. 13th World Conf. Earthq. Eng, Vancouver, CD-Rom Edition, paper n. 1549.
Stewart J. P., A. H Liu. and Y. Choi (2003). Amplification Factors for Spectral Acceleration in Tectonically 
Active Regions, Bull. Seism. Soc. Am., 93, 332–352.
Wald L. A. and J. Mori (2000). Evaluation of Methods for Estimating Linear Site-Response Amplifications in 
the Los Angeles Region, Bull. Seism. Soc. Am, 90, S32–S42
7


Comparison between VS30 and other estimates of site amplification in Italy

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Comparison between VS30 and other estimates of site amplification in Italy

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