Citation: Fordjour, B., S. Nakai (2022), Microzonation map of seismic site condition and amplification ofGreater Accra Region, Ghana, Synopsis of IISEE-GRIPS Master’s Thesis.

MICROZONATION MAP OF SEISMIC SITE
CONDITION AND AMPLIFICATION OF GREATER
ACCRA REGION, GHANA
Benjamin Osei Fordjour1
MEE21712

Supervisor: Shoichi NAKAI2
ABSTRACTThe capital city of Ghana, Accra, is located within the most seismically active region of the country.
This study seeks to assess the seismic site condition and amplification based on the shear wave velocity
for the regional capital by the topography as a proxy method. Point slope measurements are extracted
and subjected to seismic site condition correlation of the NEHRP for stable regions. The correlative
seismic site condition within a limiting slope boundary corresponding to a specific soil class is obtainedusing regression analysis. We first analyzed available single-point microtremor measurements to know
the fundamental soil frequency for the study area, after which some inversion parameters based on
diffusion assumption were considered concerning the soil frequencies to understand the terrain within
the study area. The inversion outcome could not support the actual representation of soil layers since it
did not include dispersion curves to aid in generating profiles with relatable soil layers. The obtained
Vs30 is then further subjected to the linear site amplification equation for stable regions by Stewart etal. Scaling ground condition for linear soil response over an average oscillator period of 0.4 to 2 seconds.
An interpolation using the inverse distance weight method was applied to the calculated seismic site
conditions and the site amplification data sets to generate a representative map at a square kilometer arearaster. A comparison will then be made between the global seismic site condition map to understand thechanges and possible similarities. The newly generated map could serve as an improved first-orderapproximation of Vs30 for the capital region.
Keywords: Microtremor, Topographic slope, Seismic site condition (Vs30), Amplification.
1. INTRODUCTION
Seismic microzonation is the procedure of estimating the response of soil layers under earthquake
excitation and thus the variation of ground characteristics on the ground surface about displacement,
velocity or acceleration. Site amplification or excitation increases the intensity of ground vibration andresponse due to local geological conditions depending on soil properties that differ significantly in local
geological environments. The local soil is a filter that influences the seismic waves from the subsurface
bedrock to the ground surface. Good seismic-resistant design of structures would thereby be resistant to
the ground amplification intensity during earthquakes.
Several research works establish the southern part of Ghana as the most seismically active
zone, though the country is far from the global tectonically active region. Kutu et al. (2013) establishthat the seismicity of southern Ghana, as known in past and present, is due to tectonic fault activities of
the St. Paul's and Romanche transform-fracture zone systems offshore Ghana.

1
2

Ghana Geological Survey Authority, Ghana.
Professor Emeritus, Chiba Univ., and Visiting Research Fellow, Building Research Institute.

1

The probabilistic seismic hazard study by Ahulu et al.
(2017) also cemented the reactivation scenario with
explanations of work done in six critical cities in thesouth on generating a 10% probability of exceedanceof peak ground acceleration in 50 years. Findings from
Ahulu et al. highlight that the highest level of seismic
hazard in Ghana is within the capital city, Accra andTema, the industrial city of Ghana, with peak groundacceleration of about 0.2g, diminishing with distanceaway from the south. Amponsah et al. (2008) computed
the seismic ground motion along four profiles acrossgeological ground conditions to access the hazard
zones and extract engineering parameters helpful indesigning, using a deterministic hybrid approach.
Movement along the Transform fault andFracture zone continuously causes ongoing seismicity
Figure 1. Seismicity Map of Ghana indicating in southern Ghana hence the recent reoccurring oftremors within the south. Assessment of seismic
records of earthquakes both past and present
hazards and microzonation of the capital region of
Ghana will enable us to correlate the potential seismic
areas with relatable exposures to ground shaking hazards, mainly earthquakes
2. DATAThe relief contour elevation data set of the Greater Accra Region from the Geoscientific Information
System Division of the Ghana Geological Survey Authority will be subjected to data processing using
the ArcGIS pro Software. Also available for comparison is the ground conditions map relative to thegeology and stratigraphic map of the capital region. Single point microtremor measurements werecarried out sporadically from May to June 2019 within the study area using the 4.5-hertz Trillium 120PA
broadband seismometer, which will also serve as query data for estimating the average shear wave
velocity. Finally, an extracted global seismic site condition map will be used as supplementary data forcomparison and improvement.
3. METHODOLOGY
We will extract point attributes from the global seismic site condition map with the corresponding the
average shear wave velocity in the upper thirty meters of soil depth (Vs30) regionally to ascertain ourstudy's regional perspective of ground response. The Vs30 regional point extraction will be comparedto the intended Vs30s to understand the influences of relatable topographic measurements on theregional Vs30 map from the USGS. The USGS site condition extracted data set and the estimated seismic
site condition data set will be subjected to the average PGV amplification model to understand andcompare the ground behavior. A seismic site condition and a site amplification map representing the
average PGV of the study area will be generated using these two data sets, USGS Data Set and theestimated seismic site condition data set.

3.1. Planar Method
The slope of the study area was determined based on the planar method adopted in the ArcGIS Pro
software. The slope is defined as the rate of changes of the surface in the horizontal (dz/dx) and vertical(dz/dy) directions from the center cell or point elevation to each adjacent cell or point elevation
multiplied by a shortened description of the result from 180 divided by 𝜋.

2−1𝑠𝑙𝑜𝑝𝑒(°) = tan

(√(

𝑑𝑧 2
𝑑𝑧 2
) + ( ) ) × 57.26578
𝑑𝑥
𝑑𝑦

(1)

3.2. Slope Based Seismic Site Condition (Vs30) Method
The estimated topographic slope at any point is grouped within the subdivided slope range limits usedby the National Earthquake Hazard Reduction Program (NEHRP) to classify the ground conditions into
soil classes based on regression analysis using the equation below:
𝐸𝑠𝑡𝑖𝑚𝑎𝑡𝑒𝑑 𝑉𝑠 30 (m/s) = (𝑆𝑙𝑜𝑝𝑒 (Rad) × 𝑀) + 𝐼𝑛𝑡𝑒𝑟𝑐𝑒𝑝𝑡(𝐶)(2)

M is considered the gradient of the regression equation within the limits of a known NEHRP slope class.
3.3. Microtremor Horizontal to Vertical Spectrum Ratio
As proposed in several studies, the horizontal to vertical spectrum ratio (HVSR) in three directions of asingle-point measurement is very useful in determining the ground frequency of a location. It is also avigorous means of estimating the site’s fundamental frequency. The proposed equation for calculating
the HVSR is as follows:
𝐻𝑉𝑆𝑅 =√𝐹NS (𝜔)2 + 𝐹EW (𝜔)2
𝐹UD (𝜔)(3)

Where 𝐹NS (𝜔)2 , 𝐹EW (𝜔)2 and 𝐹UD (𝜔) are the respective Fourier amplitude spectra of the north-south,
east-west, and up-down parameters of the site record, the frequency that corresponds to the peak
spectrum transfer ratio relates to the fundamental frequency for each site location (Nakamura, 1989).
3.4. Inversion of HVSR Measurements of Rayleigh Waves with Clear Peaks
Using the diffuse field theory, we will perform our single-point measurement inversion of the HVSR.
With the hypothesis of a diffuse seismic field (noise), agreeing with Sanchez-Sesma et al. (2011), the
HVSR can be considered like the imaginary part of the Green’s function on a horizontally layered
medium. Considering a diffuse seismic field of microtremors, the HVSR is as expressed in the equation
below as directional energy densities:
𝐸1 (𝑥, 𝜔) + 𝐸2 (𝑥, 𝜔)[ 𝐻 / 𝑉 ] (𝜔) = √𝐸3 (𝑥, 𝜔)(4)

The directional energy densities, 𝐸1 , 𝐸2 and 𝐸3 , referring to the horizontal and vertical degrees of
freedom at the point x is like the expression of equation (3). Generating the HVSR with the imaginary
part of the Green's function (GF) as an unknown parameter, as in equation below for a horizontallylayered structural design made of isotropic elastic layers. Where the 𝐺11 (𝐱, 𝐱, 𝜔) is the displacement
GF in the direction 1 at a point x due to the application of a unit point force in the same direction appliedat the same point.
Im[𝐺11 (𝐱, 𝐱, 𝜔)] + Im[𝐺22 (𝐱, 𝐱, 𝜔)][ 𝐻 / 𝑉 ] (𝜔) = √Im[𝐺33 (𝐱, 𝐱, 𝜔)]3(5)

3.5. Site Amplification Map.
In this study an alternative site amplification model by Stewart et al. (2017) based on Vs30 as the sole
predictive variable for site response will be used to enable us to generate an ergodic site amplification
map from Vs30 values concerning stable crustal regions. The simulated model becomes very compatible
with the soil site class categories adopted in the NEHRP, with soil class boundary dependent on thelimits of Vs30. The soil class boundary of this model is related to the assumed model used in the Building
Code Ghana for site class classification or condition.
The linear model has two main measurable parameters, the Vs30 scaling term (𝐹𝑉 ) and theamplification at Vs30 reference condition of outcrop map where Vs=3000m/s.( 𝐹760) as a modifier. Dueto the unavailability of ground motion intensities,𝐹760 is said to be negligible.
𝐹𝑙𝑖𝑛 = 𝐹𝑉 (𝑉𝑠30, 𝑇) + 𝐹760 (𝑇)(6)The trilinear log to log scale Vs30 scaling model is shown below as:
𝑉1
),
𝑉𝑟𝑒𝑓
𝑉𝑠30
𝑐 ∗ ln (
),
𝑉𝑟𝑒𝑓
𝑉2
𝑐
𝑉𝑠30
𝑐 ∗ ln (),) + ∗ ln (
𝑉𝑟𝑒𝑓
2
𝑉2
𝑐 ∗ ln (𝐹𝑉 ={𝑉𝑠30 ≤ 𝑉1
𝑉1 < 𝑉𝑠30 ≤ 𝑉2

(7)

𝑉𝑠30 > 𝑉2

Where c is the average slope in the log-log space for the central portions between the corner velocities
𝑉1 and 𝑉2 as indicated in the coefficient to the period graph based on the electronic supplement provided
by Stewart et al. (2017). 𝑉𝑟𝑒𝑓 represents the velocity at which 𝐹𝑉 =0 thus it is taken as 760 m/s. All thesecoefficients are dependent oscillator periods.
4. RESULTS AND DISCUSSION
4.1. The microtremor measurements
The values estimated correspond well with results from Amponsah et al. (2008) within the geology
polygon constraints, though it could not indicate the layer difference with depth. Most of the available
measurements showed no clear peaks, with some showing no peaks. The flatness of most of thesemeasurements gave an initial impression of stiffness within the study area with just about a few with
two or more peaks. With the few available clear peaks, the estimated results are as below. The absenceof the Rayleigh dispersion curve in performing the joint inversion modelling proved the HVSR data to
be undependable due to the constraining parameters being prone to increasing ambiguity.
Using the equation used in the building code, CEN (2004), we evaluated the Vs30 on theclear peaked HVSR curves. Regarding this outcome, the southern area could be classified as Soil TypeD with a Vs30 range between 180m/s and 360m/s using the NEHRP classification and C with a similar
Vs30 range regarding the country’s building code.
4.2. The Topography Proxy Method
This method proves that physical soil properties that influence shear modulus are equally a vital
influence on shear properties. In general, void space and effective mean stress dominate shear modulus
changes since density variations tend to be smaller in soils. This explains the good correlation between
the lower shear wave velocity and lower slope topography. The study also proved the influence of the4shear wave velocity on stiff interlocking geological units like rocks, based on hardness and fracture
spacing, given that they are more resistive to erosion: a steeper slope and higher shear wave velocity.
Estimating the shear wave velocity by regression analysis showed good distribution overthe slope range within the study area and correlated well with the global Vs30 by the USGS. The
estimated Vs30 ranged between 100 m/s in the low-lying flat surface within the southern part towards
the east of Greater Accra to about 1530m/s towards the north of the study area and the western, northeast,and southwest topographic relief trends. Estimated Vs30 within some parts of the study area was lower
than that of the global Vs30 by the USGS, with some regions having higher estimations than theworldwide map.

(A)

(B)

Figure 2. Map of estimated Vs30 (m/s) (A) and USGSVs30 (B) maps for Greater Accra Region. The
values range between 130 to 1530 m/s for the Estimated map and 180 to 900 m/s for the USGS map.
4.3. The Vs30 Site Amplification.
With coefficients estimated from a given electronic supplement for the linear amplification model,
without considering the non-linear component, a relational Vs30-based site amplification map for theGreater Accra region is developed. The total amplification is the sum of the Vs30-scaling term and the𝐹760 term, representing the amplification at Vs30=760m/s site condition relative to the 3000m/s
reference condition. Generally, building codes use 𝐹𝑉 factors at an average period ranging from 0.4 to
2.0 s as a moderate site amplification. When used, the cap 𝐹𝑣 factor is a modifier for ground motions ingenerating hazard maps with site conditions referenced at 760m/s for Vs30.

(B)

(A)

Figure 3. Final site amplification map generated based on estimated Vs30 (A) and USGS Vs30 (B), fromslope map. Ranging between 1.794 to 0.624 and 1.794 to 0. 927.respectively.
Finally, a site amplification factor map about Vs30 is generated from the site amplification
model equation (7) above. The developed map using the model above shows some relations with the5highest and minimum amplifications at 1.79 and 0.62, respectively, relating to high topographic reliefs
and flat-lying areas in the study area. About sixty per cent of the study area shows site amplifications
relating to the high amplification levels, with about forty per cent showing minimal amplification. The
increased amplification signals attest to the continuous record of seismicity in an area like Weija.
5. CONCLUSIONS
The map generated was characterized by different site values of Vs30 estimations ranging between aminimum of 100 m/s and 1530 m/s. The newly estimated Vs30, compared to the globally determined
Vs30 map for the study area, showed some similarities and differences, which further proves the update
expected for this study due to developmental changes within the study area.
We then tried to estimate Vs30 from available single-point microtremor measurements
within the study area. Results from this evaluation were non-relatable due to unavailable parameters indelineating layer thickness and assuming that the ground condition is homogeneous.
We then evaluated the region's site amplification map based on the estimated Vs30 and theextracted Vs30 from the global Vs30 map using the Vs30 scaling method over an average oscillator
period from 0.4 to 2 seconds. The final map gives a surface distribution of the PGV amplification factor
over the study area, comparing the estimated and USGS values. The estimated site amplification andseismic site condition maps are better representation maps that relate well to the capital city's groundmotion and land condition. The maps will serve as a guide to improving the land use management
practices of stakeholder institutions.
ACKNOWLEDGEMENTS
I express deep and sincere gratitude to Professor Shoichi Nakai, my research supervisor and Dr Hiroto
Nakagawa, my academic advisor, for their excellent guidance and professional tutelage throughout thisperiod. Their dynamism, vision, sincerity and motivation have deeply inspired me. You have equipped
me with the best research skills and methodology throughout a meaningful, practical, andunderstandable presentation. I am incredibly grateful for what you have offered me.
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