FEASIBILITY STUDY OF VS20-BASED DESIGNSPECTRA FOR THE URBAN AREA OF MANAGUA,NICARAGUA
Jorge Vigarny Rojas Gonzalez
MEE17717

Supervisor:

Shoichi NAKAI 

ABSTRACTI proposed design spectra considering the non-linear site response base on the amplification factor of 40
velocity profiles located in the urban zone of Managua, Nicaragua. The method applied the multichannel
surface analysis (MASW) of 22 different places and 18 different reference points provided by Rojas etal. (2016), Escorcia et al. (2013), and Facciolli et al. (1973); to provide the shear wave velocity profileand propose a soil classification map based on the Vs-20 analysis of the study area. The non-linear
response spectra were developed from the backbone curve using the linear equivalent and hyperbolicmodel fitted with the dynamic deformation laboratory test data provided by Facciolli (1973). The soil
amplification factors and design response spectra were defined using the soil classification map and thenon-linear response spectra.
The results showed that the zones with the lower shear velocity have larger soil
amplification factors and were located in the northern zone on the coast of Managua Lake. Second, thecalculation of the reference points using Vs-20 had the best fitted distribution made by geology and land
use constraints. Finally, the developed design spectra for this study proposed a soil classification that
included a soft soil design spectra and moderate soft soil with three differences range limits in the
spectral acceleration branch.
Keywords: Design Response Spectra, Amplification Factor, MASW method, Seismic Micro-zonation.

1. INTRODUCTIONManagua, the capital of Nicaragua, is the city with the highest seismic activities in the country due tothe location on the Pacific coast that is mainly affected by the subduction zone. The Pacific coast ofNicaragua is part of the ring of fire and; this zone is characterized by the subduction of the Coco plate
beneath the Caribbean plate, and the principal consequences of this are the volcanic activity and thehistorical large earthquakes in shallow and intermedia depths. In addition, the city is affected by theshallow inland earthquakes product of the effect of the geological settings, geological faults and the soft
soil conditions established by an alluvial fan located in the city.
The national seismic code of Nicaragua (RNC-07) presents a dynamic analysis using thelinear elastic system, unlike the inelastic analysis. Therefore, knowing the inelastic soil conditions andpredicting the site response through seismic hazard studies is a very important task. A greater knowledgeof soil conditions allows reducing the uncertainty in soil conditions and facilitates the prediction of theprobable affectations that the structures may suffer during a seismic event. The purpose of this study is
to estimate the amplification factor based on the design response spectra proposed using the shear wave
velocity profiles for the urban area of Managua and define a seismic micro-zonation shear wave velocity
map.



Institute of Geology and Geophysics, National Autonomous University of Managua, Nicaragua.
Professor Emeritus, Chiba University, and Visiting Research Fellow, IISEE, BRI, Japan.



12. DATAIn this study, the earthquake records were used to calculate the frequency domain analysis of the soil
profile structures. In total 30 records data were selected to satisfy the following criteria: a) recorded ata rock site, b) period range between 0.1 and 1.0 secs, c) cover a wide range of peak acceleration values(0.6 to 1.2 g).
Table 1. Record of Strong Motion data by PEER Catalog.
Country
Nicaragua
El Salvador
MexicoCosta Rica
Imperial Valley
Loma Prieta
Coyote Lake
Mammoth Lakes
Parkfield
Chi-Chi, Taiwan
L'Aquila, Italy
Iwate, JapanNew Zealand

No.
3
3
8
2
3
2
2
2
1
1
1
1
1

Year
1972
1986 - 2001
1985-2010
1991
1940
1989
1979
1980
1966
1999
2009
2008
2011

Mag.
5.00 - 6.24
7.50 – 7.70
5.40 - 7.20
7.50
5.01
6.93
5.74
5.69
6.19
7.62
6.30
6.90
6.20

The type of earthquakes
was selected according to the probabilityof occurrence and similar tectonics
conditions of Nicaragua. In consequence,the strong motion records have been
selected in Central America, Mexico andCalifornia Area (Table 1). The nonlinear
seismic analysis was considered for thedesign ground motion in base on thehazard map with a return period of 475
years by RNC-07. Therefore, for eachsoil category and for five levels of
expected intensity in rock site (PGA rock> 0.1g, 0.1g > PGA > 0.2 g, 0.2g > PGA> 0.3 g, 0.3g > PGA > 0.4 g, 0.4g > PGA> 0.5g) spectral acceleration factors at
the surface were determined for theamplification factor and an intensity of
0.3 g in rock site for Managua.

3. THEORY AND METHODOLOGY
3.1. Frequency domain analysis
The response of the soil spectra to the base rock motion is calculated using the frequency domain
analysis. This method uses the ground motions associated with the vertical propagation of shear waves
between two layers. The methodology used the development of transfer functions for multi-stratified
soils; using N horizontal layers, of which each layer defines the properties in the half space as the bottom
layer. The calculation starts with the assumption that the soil only has a vertical propagation of shear
velocity and that it originates only a horizontal displacement in Eq. (1).

u=u(x,t)(1)

The complex shear modulus formula was defined in terms of the critical damping ratio
instead of the viscosity; therefore, the critical damping ratio assumes the independence of the frequency
using the Eq. (2). Finally, Eq. (3) shows the transfer function defined between half-space rock strong
motion and the free surface.

G *  G. .1  2 2  j 2 1   2Am 22e1 ( )2eN ( )



(2)

(3)

3.2.

Shear wave velocity

The shear wave velocity calculated for this study used a database of 40 reference points with 6 SPT, 12
reference points collected from other researches and 22 using MASW method. The MASW analysis canobtain a shear velocity model defining the variation of near-surface soil layers around the linear
arrangement. In order to complement the information available for the city of Managua, the shear
velocity profile of the data processing using the MASW analysis is calculated with the following
procedure:
i.

ii.
iii.

Surface Data wave measurement: the waves will be registered with a roadside linear
arrangement. The arrangement consists of eight vertical seismometers of 2.0 Hz, spaced eachother with 2 m, these geophones measured the surface waves generated using an artificial
resource located at 16 m. The total length of the arrangement was 80 m.
Dispersion Curve analysis: using the MATLAB code was define the dispersion curve in termsof phase velocity (c) and frequency (ω) using the sum of all the phase-shifted traces (N).
Inversion Model: This analysis used the iteration process to estimate the theoretic soil profile
collected by tested synthetic data. I apply the dispersion curve to define the theoretical modelusing the program Dinver/Geopsy.

4. RESULTS AND DISCUSSION
4.1.

Hybrid vs20 mapping strategy

The strategy to develop a soil classification map used the Vs-20 and common indicators variables todefine the micro-zonation analysis for the study area. The calculation of the reference points using Vs20 had depth with the best fitted distribution made using the indicators the parameters present in theconstraint map. I consider two different approaches depending on the common observational constraints:
the geology and land use correlation into a single map of Vs-20. The combination of geology and landuse take into account to generate a micro-zonation that considers the main characteristics of theseresources.
First of all, the categorical predictorof geology proposed is an extract compiled fromthe geological map collected by Nicaraguan
Institute of territorial studies (INETER) andCzech development agency (CzechAid) (INETER,
2002); this map summarizes the general geology ofManagua and surrounding areas. This map shows
the geological units generated in this proposal tocalculate the zones of influence for the microzonation of the urban area of Managua. The main
geological units of Managua are divided in:
Artificial Fill, Holocene, Pleistocene, PlioMiocene and Tertiary.

Figure 1. Micro-zonation map of the urban area
in Managua using Vs-20.

Second, I calculate a restructured land use base in the generalized map of Managua
proposed by INETER in 2010, which is divided into urban and rural areas. For this study, the land use
map was complemented the urban zones to make a subdivision using the urban model show in the
topographic map of the city and present a classification with a with direct influence on the soil profiles.
The new areas of influence were divided in: house class A, B and C, commercial zones, education
centers, residential and industrial zones.

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Finally, the trend estimate tocalculation process is used to define theinterpolation of the soil classification map for the
study area. From this relationship of terms between
the predictive category of Geology and Land use,
the study area was divided into four different zones
which are shown in Figure 1.
Zone 1 represents the residential
houses located in the northwest area of thedowntown, including type A, B, and C houses; thispart have a dense deposits of alluvial fan
originating from las Sierras formation. Zone 2, Figure 2. Soil classification map based on Vswhich covers the education centers, and 20 analysis of Managua.
commercial area that is located northeast, iscomposed mainly of the dense deposits of alluvial
fans. Zone 3 is composed of ashes, alluvium fans and volcanic rocks from Masaya volcano; this region
represents the marginal area in the south of the city that includes the forest, open space and agriculturalareas. Zone 4 is represented by the coastal area and industrial zone in the east of the downtown, mainlycomposed of artificial fill and alluvium fans.
4.2. Non-linear behavior of the Soil Profile
The performance of the hyperbolic model and the equivalent linear model was calculated in the profile
corresponds to the National Autonomous University of Nicaragua station in the south part of thestudy area. The equivalent linear analysis was obtained from theoretical dynamic curves proposed byElectric Power Research Institute (EPRI, 1993) and the non-linear behavior fitted with the hyperbolicmodel using the study elaborated by Faccioli, Santoyo V., and León T. (1973). The shear modulus anddamping values derived from the data collected by Faccioli (1973) at the ESSO station, located about
10 meters below the ground with a confining pressure of 1,156 kg / cm².
(B) Damping ratio(A) Normalize shear modulus
1.000040.0
30.0
Damping

G/Gmax0.75000.50000.25000.00000.0001NL
LE

0.0100γ (%)

1.0000

NL
LE

20.0
10.0
0.0
0.00010.0100γ (%)

1.0000Figure 3. Comparison between normalized shear Figure 4. Comparison between damping ratio
modulus fitting using the data test by Facciolli reduction curve fitting using the data test by(1973).
Facciolli (1973).
The use of the hyperbolic model overestimates the spectral acceleration in the short rangesof shear strain (0.002 to 0.2) compare with to the linear model. On the contrary, the damping ratio in thelong strain ranges with the hyperbolic model has a 5% of underestimation compared with the equivalentlinear analysis. Finally, it can be seen that the line model presents a curve with maximum stress of 23kPa.
The surface response spectra using the strong motion database (Table 1), with 0.1 and 0.3
PGA was defining for the linear and non-linear analyses. For 0.1 PGA a smaller difference can beappreciated (3.33%), the maximum acceleration is found in 0.1 sec. Contrarily, with 0.3 PGA the values
in the acceleration increased, but the inelastic method begins to reduce its acceleration proportionally,
the difference between the two models was 19.81%.

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4.3.

Design response spectra analysis

In this chapter, I proposed an improvement in the design response spectra for the different soil conditions
based on the analysis of soil profiles generated for this study, which are defined with the characteristicsof the seismic ground response. I used the database of soil profiles calculated in the urban area ofManagua and database collected (Table 1). Emphasizing in the proposed model using the non-linear
hyperbolic method, I have taken into account the amplification produced by the thickness of soil deposits,
the impedance ratio affected in soil behavior and the degradation curve that verifies the variation of thenormalized shear modulus reduction of the soil profile in Managua.

Spectral Acceleration(a/g)(A) Soil IV: Soft soil
2.0
D

1.5
1.0
0.5
0.0
0.01

0.10

1.00

10.00

Period (sec)(B) Soil III: Moderately soft soil
Spectral Acceleration
(a/g)

2.0
C3
C1
C2

1.5
1.0
0.5
0.0
0.01

0.10

1.00

10.00

Period (sec)Spectral Acceleration(a/g)(C) Soil II: Firm Ground
2.0

B
RNC-07

1.5
1.0
0.5
0.0
0.01

0.10

1.00

10.00

Period (sec)Figure 5. Comparison between the proposed
design response spectrum and RNC-07.

Figure 5 shows the maximum
acceleration proposed in the ground surface of all the
influence zones in the seismic classification map; as
the reference, the elastic response spectra proposed
by RNC-07 was calculated except the soft soil (shear
velocities between 180 to 270 m/s. In these values, it
can be seen the average response spectra have a
direct agreement with the reference spectrum.
First, soft soil (shear velocity between
180 to 270 m / s) is shown in Figure 5(A); it did nothave its amplification values in the seismic code ofNicaragua. The maximum value of the response
spectra had 1.06 g, the lower load capacity in the
micro-zonation areas; this is due to the shear strain
shown by said profiles that can cause the soil profile
to exceed the limits values of shear stress presented.
This soil profile is more likely to present liquefaction
effects, due to the presence of water table at surface
level, the contact with the Managua coast and thehigh clay content shown throughout the area.
Second, Figure 5(B) show the design
response spectra proposed for moderately soft soil
conditions, which had overestimation comparedwith the RNC-07, being an adequate safety factor,
but it is necessary to make an adjustment in thelimited periods that will be proposed in thisdocument.
Finally, Figure 5(C) shows the
spectrum for firm ground, and the maximum valueof the response spectrum had underestimation of
19%. Therefore, the spectral amplification
parameters were redefined to adjust to the maximum
acceleration in the design spectrum.

The calculated values for high shear velocity show the maximum variability from the curve
proposed by RNC-07. On the contrary, the proposal of the acceleration response spectra in themoderately soft soil (Soil C1, C2, C3) has small values of the national seismic code. Therefore, theRNC-07 has a good behavior with a safety factor of 9% with a reasonable amplitude of periods for the
spectral acceleration branch. Finally, Figure 5(C) shows the proposal for soft soil; since this type of soil
is not evaluated in the RNC-07, this model will be an initial proposal spectrum, with an acceleration of
1.10 g, with a period range of the spectral acceleration branch between 0.15 and 0.75 sec.

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5. CONCLUSIONS
The feasibility study of vs20-based design spectra was conducted for the urban area of Managua using
the soil profile of the downtown calculated with the MASW method to calculate the non-linear analysis
to estimate the response spectra and the soil classification map. The following conclusions were made:
i.
ii.

iii.

iv.

The calculation of the reference points using Vs-20 had depth with the best fitted distribution
made by geology and land use constraints.
The areas with the highest velocities are located in the east part of the city with values greaterthan 540 m/s. Instead, the lowest velocities are found in the north zone, near the coast ofManagua lake, with values lower than 200 m/s.
The moderate soft soil was included as Subdivision in the classification of the response spectra(C1, C2, C3), which had similar spectral acceleration conditions, but with a different range ofthe limits in the spectral acceleration branch.
The design spectrum Proposal for soft soil had a low value of spectral acceleration with a wide
range of the limits in the spectral acceleration branch.

6. RECOMMENDATION
Although the research has tried to study the effect of the site in the urban area of Managua with theavailable data, these results should be taken as an initial proposal, therefore an extension of the local site
effects at site response should be done, using data from earthquakes with different conditions and newly
proposed reference sites. According to the above reasons, I recommend the following suggestions:
i.

ii.
iii.

To confirm site amplification proposed in this paper, it is necessary to compare the soil profiles
with reference points applying calculations using other microtremor methods and drilling tocheck the accuracy of the soil profiles.
To find a wide range of the amplification factor is recommended to complement with highfrequency strong motion data (frequencies greater than 1 sec).
Add points in an area with higher elevation in such a way that you can make a comparative
analysis with a topographic slope map.

ACKNOWLEDGEMENTS
I would like to express my sincere gratitude to the supervisor Dr. Shoichi NAKAI and my advisor Dr.
Toshihide KASHIMA for their continuous support, valuable suggestion and instruction during my study.

REFERENCES
Electric Power Research Institute (EPRI), 1993, Guidelines for Determining Design Basis Ground
Motions Palo Alto, vol. 1–5, EPRI TR-102293.
Faccioli, E., Santoyo, E., and Leon, J., 1973, Microzonation criteria and seismic response studies for thecity of Managua, Proceedings of the conference on the Managua, Nicaragua Earthquakes.
Kramer, S., 1996, Geotechnical earthquake engineering, Nueva Jersey, U.S., Prentice-Hall.
MTI, 2007, Reglamento nacional de construcción (RNC-07), Managua, Nicaragua: Ministerio deTransporte e Infraestructura.
Pitilakis, K., Gazepis, C., and Anastasiadis, A., 2004, Design response spectra and soil classification forseismic code provisions, 13 World Conference on Earthquake Engineering, Vancouver, Canada.
Wald, D., McWhirter, L., Thompson, E., Hering, A., 2011, A new strategy for developing vs30 maps,
Colorado, U.S., 4th International IASPEI/IAEE Symposium, California.

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