EFFECT OF SURFACE GEOLOGY (CONSIDERINGNONLINEARITY OF SUBSOIL) ON GROUNDMOTION IN THE URBAN AREA OF MANAGUA,NICARAGUA
Delvin Martinez
MEE10515

Supervisor:

Hiroshi ARAIABSTRACTThe purpose of this study is to evaluate the site response on ground motion in the urban area ofManagua, Nicaragua, considering nonlinear behavior of the subsoil, using the program EERA, which
is based on the equivalent linear soil model. The site response models were constructed using shear
wave profiles, which were obtained by applying a geophysics method (MASW) in the target area,
geotechnical and geological data available at the sites. The numerical simulation programs used in thisstudy can be downloaded from http://gees.usc.edu/GEES/ . In this study, seven site response analyses
were carried out using the input ground acceleration recorded during the 1972 Managua Earthquake.
Based on the EERA analyses results, the ground motion in the target area is strongly affected by the
first ten meters of soil which are mainly composed of silty sand, classified as “moderate soft soil”,
with shear wave velocity between 180 m/s and 360 m/s. Comparing the response acceleration
spectrum obtained at each site with the design acceleration spectrum, provided by the Nicaragua
Seismic Code, the design spectrum for the Managua area can be considered as overestimated. In
addition, the liquefaction potential was evaluated in two cases with different water levels. In bothcases, the liquefaction is supposed to occur between the depths 2.1 and 5.6 meters, corresponding to
the layers composed mainly for silty sand. The severity of the liquefaction expected is classified asmoderate to minor liquefaction, according to the liquefaction potential index estimated.
Keywords: Ground response, bedrock motion, rock outcropping motion, liquefaction potential,liquefaction potential index.

1. INTRODUCTION
The distribution of earthquake damage is strongly influenced by the response of soils under cyclicloading. This response is controlled in large part by the mechanical properties of the soil. The
evaluation of ground response is one of the most important and most commonly encountered problems
in geotechnical earthquake engineering. Ground response analyses are used to predict ground surface
motions for the development of design response spectra, to evaluate dynamic stresses and strain for theevaluation of liquefaction hazards, and to determine the earthquake-induced forces that can lead to theinstability of earth and earth-retaining structures.
In this study, the computer program EERA (Bardet et al., 2000) is applied with the objective
to evaluate the effect of surface geology (considering nonlinearity of subsoil) on ground motion in theurban area of Managua, Nicaragua. Several soil profiles (shear wave velocity and geotechnicalprofiles) obtained around the urban area of the city will is used.


Geoscientific Research Center, Managua City, Nicaragua
National Institute for Land and Infrastructure Management, Tsukuba, Ibaraki, Japan.

1In addition, the liquefaction potential is estimated by means of the analysis of shear wave
velocities and the results obtained from ground response analysis performed at each site. The
evaluation of the liquefaction potential was performed in two cases with different water levels.
2. DATAIn this study, the input ground motion used is the record time history acceleration recorded during the1972 Managua earthquake (Figure 1), in the ESSO refinery seismic station (indicated by red star in
Figure 2), deconvolved to “bedrock”. The instrument in this site had a nominal range of 0.5g
(Engineering Report “1972 Managua Earthquake”, 1975). The maximum acceleration is 0.218g at
6.420 seconds. It was localized beneath the center of Managua, the capital of Nicaragua. The materials
in this site are predominately composed of sandstone, tuff, marl and shale (INETER, 2003) anddensity on the surface layer is 1.6 t/m3 (Faciolli et al., 1973)

Figure 1. Acceleration time record, EW component,
1975 Managua Earthquake.

Figure 2. Site simulated into
geological map of the Managua city
(INETER, 2003)

Seven sites (red points in Figure 2), distributed around the urban area of Managua city, were
simulated. The geological and geotechnical conditions (including dynamic properties of the soil at
each layer) were taken into account. This information was obtained from several studies performed in
the target area.
3. THEORY AND METHODOLOGY
3.1. Shear wave velocity profiles
The shear wave velocity profiles, for each site considered, were obtained by means of seismic surveys
performed with the geophysics method, called Multichannel Analysis of Surface Waves, MASW (Parket al., 1999). The field work was developed following the procedures established by Park et al.,(1999), using the conventionally arrays. This arrays permit to check in situ (to each instant) the qualityof each record and solve any problems that can be presented.
3.2. Ground response analysis
To perform the ground response analysis, the method applied was the “Equivalent Linear Method”
compiled by the computer program EERA. The software EERA, is a modern implementation of thewell-known concepts of equivalent-linear earthquake site response analysis, which was previously
implemented in original SHAKE and the subsequent version SHAKE91 (Schnabel et al., 1972; andIdriss and Sun, 1992), takes full advantages of the dynamic array dimensioning and matrix operationsin FORTRAN 90, input and output are fully integrated with the spreadsheet program Excel. That2

means that all the characteristic and numerical accuracy of the Excel are taken advantage, mainly on
inputting data like the acceleration files. Even the small values can be taken into account as far as thelower limit of the Excel program. On the other hand, in the case of the strain values (that are very
sensitive), it can be taken into account that the full spectrum of the numerical accuracy is given by
Excel, which have significant and important influence on the final ground response.
3.3. Evaluation of liquefaction potential
From the results of ground response analysis, the peak ground acceleration and shear wave velocities
are used to estimate the liquefaction potential, following the simple procedure by Seed and Idris(1971). The Safety against Liquefaction (FL) value is the criterion to apply, considering the simplicityof this method when the information from laboratory tests is limited.
 FL>1, liquefaction will not occur FL<1, liquefaction is supposed to occur under specified earthquake conditions
The liquefaction phenomenon is very close related with the depth to the water level. In thisstudy, the depth to the water level was assumed, through two possible scenarios that could be occur.
Firstly, was considered the worse scenario that could be presented, the water level reaches to thesurface and the soil layers are in saturated conditions. Secondly, the water level was assumed to adepth of 5 meters, considering that all the layers above to that depth are in dry conditions and the
below layers in saturated conditions. In both case the liquefaction potential was evaluated.
4. RESULTS AND DISCUSSION
4.1. Shear wave velocity
Based on the criterion established by the Nicaraguan Seismic Code (bedrock, any layer with Vs>750m/s), the shear wave velocity profiles determined, show that the bedrock on the sites simulated isfound 15 meters in depth. Table 1 shows the depth at which the bedrock was found at each site and its
geotechnical description. The depth to the bedrock varies between 15 meters (Baseball Stadium site)
and 44 meters (CIGEO seismic station site).
Table 1. Depth to the bedrock and its shear wave velocity respective for each site to simulate
Depth

Vs

(m)

(m/s)

44

780

15

750

20

750

25

755

Dense poorly graded gravel, density 1.91 t/m3

Cristo Rey

20

750

Poorly graded gravel, density 1.90

RUCFA

30

900

Poorly graded gravel, density 1.92 t/m3

UNI-IES

25

750

Poorly graded gravel, density 1.9 t/m3

Site

CIGEO seismic station
BaseballNational

Stadium
The Old Cathedral
Cerveceria
site

Nacional

Description
Poorly graded sand with silty sand, volcanic origin,density 1.5 t/m3.
Moderate dense poorly graded gravel, density 1.85 t/m3
Poorly graded gravel with mix with pieces of rock, density
1.90 t/m3

3

t/m3

According to these results, the depth to the bedrock in Managua urban area is variable, and it
is not possible to assume a uniform depth. At each site the bedrock was considered as elastic, withconstant shear modulus and critical damping ratio equal to 1% (Kramer, 1996). The shear modulus isconstant and depends on the shear wave velocity and density value in the bedrock. Basically, thebedrock in Managua city is composed of poorly graded gravel, with density around 1.9 t/m3, and shear
wave velocity about 780 m/s in average. This material, same as all the soil in Managua city, which isof volcanic origin, locally called “cantera”.
4.2. Shear strain analysis
Typically the shear strains below about 0.001% do not induce significant nonlinear stress-strain
behavior in the soil (Kramer, 1996). In this study, the range of the strain analysis considered was
between 0.0001% and 1.0% by means of the soil models used. The Table 2 shows the maximum shear
strain computed at each site, which values are less than 0.1%, which means that methodology used
(Equivalent linear method) is the appropriate to this analysis, and the results obtained at each site arereliable.
Table 2. Maximum shear strain computed at each site

The obtained results show,theeffectof nonlinearity in the(m)strain (%)
subsoil in the sites simulated can be
CIGEO seismic station
3.0
0.056
considered as not strong, which
could be related with the magnitude
Baseball National Stadium
8.4
0.028
of the input ground motion used
The Old Cathedral
8.7
0.033
(6.2MW), which is expected cannot
Cerveceria Nacional site
4.7
0.091
produce larges shear strain as wasfound. The layers that present the
Cristo Rey
8.7
0.028
bigger shear strain are mainly
RUCFA
2.5
0.044
composed for inorganic silt, silty
UNI-IES
7.0
0.043
sand and poorly graded sands, withshear wave velocity between 200 and
400 m/s, which indicate that is not possible to establish a relationship using the properties of the soil,
at least in these cases.
Depth

Site

Maximum shear

4.3. Peak ground parameters
Table 3. Depth to the bedrock and its shear wave velocity respective for each site tosimulate
Sites

PGA

T

f0 (Hz)

(g)

(gal)

(s)

1

CIGEO

0.516

506.060.40

2.4

Baseball Stadium

0.318

311.96

0.16

Old Cathedral

0.324

317.43Cerveceria Nacional

0.378

Cristo Rey

Amp

2

3

1

2

3

6

8.8

3.5

3.45

3.39

6.2

15

>25

3.33

2.95

-

0.18

5.6

14

23.80

3.32

2.73

1.89

370.23

0.20

5.4

-

-

3.76

-

-

0.322

315.40

0.19

5.6

13.6

22.4

3.41

2.86

2

RUCFA

0.415

406.83

0.23

5.2

10.6

24.1

3.97

3.74

1.61

UNI-IES

0.403

395.43

0.24

5

11.6

18

3.5

2.93

2.28

4

Table 3 shows
the peak groundacceleration(PGA) at thesurface layer, thefundamental
period (T), thepeak frequenciesof resonances (f0)andtheamplification
factor for each
vibration mode.

The peak ground acceleration values obtained at the top of the surface layer show relatively highvalues. The obtained fundamental periods which correspond to the first vibration modes at each site,vary between 0.16 and 0.40 seconds that can be related with moderate soft soil (RNC – 07). Threevibration modes were estimated, except in Ceveceria Nacional site, where it was possible to detect
only the first vibration mode.
The variation of the peak ground acceleration in depth(Figure 3) shows the strong influence of the soils deposited on
the first ten meters on the ground at each site, composed
mainly of silty sands and silt, which is classified as Soil type
III (moderate soft soil) in the Nicaraguan Seismic Code.

Figure 4. Response acceleration spectrum ateach site and design acceleration spectrum
for Managua city provided by RNC – 07 forstructures of Group A and B, in soil type III.
Figure 3. Computed maximum
acceleration at the top of each layer
within the entire soil column at thesites simulated

The response acceleration spectrum obtained ateach site compared with the design response acceleration
provided by the Nicaraguan Seismic Code (Figure 4)shows that the peak spectral values obtained are below tothe design values.

4.4. Liquefaction potential
Two cases were considered (Case 1 and Case 2), considering different depth to the water level (0 and 5
meters). The Figures 5
and 6, show the results
obtained at both cases. In
general, in both case, theliquefaction is supposed
to occur between thedepths 2.1 and 5.6 meters,
which correspond to thelayers composed mainly
for silty sand, with shear
wave velocity between
170 and 200 m/s. The
severityoftheliquefaction expected isFigure 5. FL value & depth
results, Case 1

Figure 6. FL value & depth results,
Case 25

classified as moderate to minor liquefaction, according to the liquefaction potential index estimated(PL).
5. CONCLUSIONS
The results of the measurements of shear wave velocity which were obtained at each site simulated,show that the depth to the bedrock in Managua urban area is variable and it is impossible to establish asingle depth to the entire city. The maximum shear strain computed at each site is less than 0.1%,
which means that the obtained results with the equivalent linear method are quite reliable. In generalthree vibration modes were found and the dominant period in the range from 0.16 seconds to 0.40
seconds, which are related with hard soils and moderate soft soils, with amplification factor values lessthan 4 times. The variation of the peak ground acceleration in depth shows the strong influence of thesoils deposited on the first 10 meters on the ground at each site is mainly composed of silty sands andsilt. From the Nicaraguan Seismic Code it is classified as moderate soft soil in the Nicaraguan seismiccode. Through the comparison of the response acceleration spectrum obtained at each site and thedesign response spectrum provided by the Nicaragua Seismic Code was found that for this earthquake
the design values are overestimated in the Managua case. The liquefaction is supposed to occur
between the depths 2.1 and 5.6 meters, which severity supposed can be classified as moderate to minorliquefaction, according to the liquefaction potential index values estimated.
AKNOWLEDGEMENT
I would like to express my sincere gratitude to my adviser Dr. T. Kashima of BRI for their continuous
support, valuable suggestion and guidance during my study.

REFERENCES
Bardet, J. P., Ichii, K. and Lin, C. H., 2000. EERA: A Computer Program for Equivalent-linear
Earthquake site Response Analysis of Layered Soil Deposits, University of Southern California,
Department of Civil Engineering, http://geoinfo.usc.edu/gees.
Engineering report on the Managua earthquake of 23 december 1972, 1975. National Academy ofsciences, University of Illinois.
Faccioli, E., Santoyo, E. and León J. T., 1973. Microzonation criteria and seismic response studies forthe city of Managua. Proceedings of the Conference on the Managua, Nicaragua Earthquake ofDecember 23, 1972, vol. II, San Francisco, C.A. 29-30 Nov. 1973, pp. 271-291.
INETER, Microzonificacion Sismica de Managua
Idriss, I. M. and Sun, J. I. 1992. User’s Manual for SHAKE91, Center for Geotechnical Modeling,
Department of Civil Engineering, University of California, Davis.
Kramer, S. L., 1996. Geotechnical Earthquake Engineering, Prentice Hall, Upper Saddle River, New
Jersey, pp.254 - 280.
Park, C.B., Miller, R.D., and Xia, J., 1999, Multichannel analysis of surface waves: Geophysics, v. 64,
n. 3, 800-808.
RNC, 2007. Reglamento Nacional de Construcción de Nicaragua. Ministerio de transporte e
infraestructura. Dirección general de normas de construcción y desarrollo urbano.
Schnabel, P. B., Lysmer, J., and Seed, H. B., 1972. SHAKE: A Computer Program for Earthquake
Response Analysis of Horizontally Layered Sites, Report No. UCB/EERC-72/12, Earthquake
Engineering Research Center, University of California, Berkeley, December, 102p.

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