The local geologic and soil conditions at a site have a profound influence upon the characteristics of earthquake ground motion and the corresponding response spectra. Subsoil structures controls the variation of intensity of seismic motion over relatively short distance. This suggests the necessity to consider site effects, especially for seismic hazards analysis for cities and towns in earthquake prone areas.
The cities of Arusha and Dodoma and Babati town are located along the Eastern branch of East Africa Rift System (EARS) in Northern Tanzania. The eastern branch of the EARS is characterized is characterized by moderated to high seismicity. Several destructive earthquakes motion data showing substantial magnitude and intensities were recorded in eastern branch. The eastern branch of east African rift system resembles that of mid-oceanic ridges with central rift valleys acting as depositional basins (Midzi, 1999). The geology of eastern branch is divided into two zones of northern and southern region, the northern part which corresponds to Arusha and Babati is mainly formed of metamorphic rocks of Paleo-Proterozoic Mozambique belt, sedimentary rocks and volcanic rocks of tertiary age (4.8 Ma) to recent, whereas the southern region, which corresponds to Dodoma town, is covered mainly covered by rocks of Dodoman supergroup including granite and sedimentary rocks.
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The surface of soils over the bedrock in the eastern branch of east African rift system were generally formed by fluvial actions and volcanic processes, and these are represented as alluvial soils and volcaniclastic soils respectively. Based on these geomorphological and geological characteristics of the eastern branch of east African rift system, the three representative study areas of the cities of Arusha and Dodoma and Babati town were selected for assessment of the seismic site effects.
The city of Arusha and Babati town are mainly composed of alluvial soils and volcaniclastic soils, and has abundant destructive earthquake events such as Natron event 5.9 magnitudes in July 17th 2007 and Eyasi event of magnitude 6.4 in 1964, where as the city of Dodoma is principally composed of alluvial soil and has small and moderate frequently occurring occasional seismic events.
Both study areas, the cities of Arusha and Dodoma and Babati town are vulnerable to earthquake hazards since their historical and instrumental earthquake events reflect the frequent seismic activities and high potential future earthquakes. The bedrocks of three study areas are mainly composed of metamorphic rocks and granitic rocks. Over the bedrock, the alluvium and volcaniclastic deposits are distributed at plain zones embracing river and basins. These geological features and their proximity to the seismic source of study area are expected to enhance the vulnerability of these cities and town to the seismic hazards.
To address the issue of seismic hazard in the cities and towns along the eastern branch of east African rift system the research in seismic site effects based on strong ground motion is important. The aim of the research is to provide information on the local site condition and site response of soil due to ground motion for different locations in the pilot study areas (cities of Arusha and Dodoma and Babati town).
STATEMENT OF THE RESEARCH PROBLEM
It has long been known that, a quantitative strong ground motion prediction has been recognized to be a key for assessing and mitigating the seismic hazards. The level of earthquake strong ground motions depend on factors such as seismic source, path and site effects. Among three factors, site effects have been recognized to play an important role on the level of strong ground motion.
The site effects refer to the modification of the amplitude, frequency content and duration of the ground motions from an earthquake due to the variations of geological structures and their composition near the surface of the Earth. These modifications can be manifested as amplification and/or deamplification of ground motion at all frequencies induced by the impedance contrast, attenuation of seismic waves and the resonance of an equivalent layer or layers on half space.
The cities of Arusha and Dodoma and Babati town had to face up to seismic risk being located in the seismogenic zone and closely to the seismogenic district which experienced strong earthquakes during recent times. Several destructive earthquakes with magnitude 5.6 to 6.4 have occurred in these areas. The latest of them registering a magnitude 5.9, struck the city of Arusha in 2007 and shock most of towns in East Africa including Kampala and Nairobi. Its epicenter was located about 150 km west of the city in Lake Natron basin. These destructive earthquakes have provided the opportunity to study the vulnerability of these cities and towns along east Africa rift system to earthquake events. These cities and towns are vulnerable to earthquake events since most of its infrastructure had been constructed with standards that did not account for local site effects associated with quaternary unconsolidated sediments with lateral heterogeneities distributed over crystalline bedrocks.
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For all these reasons, it is necessary to carry out seismic site effect studies, since there is no study on the effect of the local geologic and soil condition on the ground motion has been done. The aim of this research is to determine the influence of the local geologic and soil condition on the ground motion characteristics. This research will consider a number of variables such as geological data, the variation on physical properties of the stratigraphic units at the site and predicted bedrock ground motion at the site.
OBJECTIVES OF THE STUDY
The general objective of this study is to determine the ground motion characteristics for a specific site from bedrock to the ground surface taking into consideration of local geology and soil condition during possible earthquakes for the cities of Arusha and Dodoma and Babati town.
In addition to the general objective, this study aims to achieve the following specific objectives:
To establish the local geological conditions above the bedrock of the study area.
To determine the amplitude content of the strong ground motion..
To determine the frequency content parameters.
To determine the duration of strong ground motion.
To determine the transfer function from the bedrock to the surface based on the local geological conditions.
SIGNIFICANCE OF THE STUDY
This study will allow determination of site natural periods and assessment of ground motion amplification. Results from this study will provide structural engineers with various parameters, primarily response spectra, for design and safety evaluation of structures. Furthermore, the information on response of soil to ground shaking will contribute as an input on evaluation of potential liquefaction, seismic stability evaluation for slopes and embankments in cities of Arusha and Dodoma and Babati town.
6.5.1 Geological and geomorphological settings
The cities of Arusha and Dodoma and Babati town are located along the Eastern branch of East Africa Rift System (EARS) in Northern Tanzania. The eastern branch of the EARS is characterized by broad zone of blocking faults and half graben structures, which were formed during rifting episodes in tertiary times (Schlutter, 1997). The morphology of the eastern branch of the EARS resembles that of mid-oceanic ridges with central rift valleys acting as depositional basins (Midzi, 1999). The geology of eastern branch is divided into two zones of a northern and southern region, the northern part which corresponds to Arusha and Babati is mainly formed of metamorphic rocks of Paleo-Proterozoic Mozambique belt, sedimentary rocks and volcanic rocks of tertiary age (4.8 Ma) to recent, whereas the southern region, which corresponds to Dodoma town, is covered mainly covered by rocks of Dodoman supergroup including granite and sedimentary rocks. In view of geomorphology, the eastern branch is also classified into the northern region with steep and high mountains and southern with low mountains and alluvial plains.
The city of Arusha lies within the northern region of eastern branch of the east African rift system in Tanzania, which is mainly covered by various volcaniclastic deposits and metamorphic rocks of Paleo - Proterozoic Mozambique belt. The mountains and hills in Arusha are formed with volcaniclastic soils and rocks (various types of tuffs); erupted by mount Meru volcanoes by intruding Paleo - Proterozoic metamorphic rocks during Cenozoic age. The surficial deposits consist of quaternary fans deposits derived primarily from volcanic deposits. These fans are composed of clays, silts, sands, gravels and some volcanic rocks. The thickness of surficial deposits and volcaniclastic to the basement does not exceed 200m (Ongór, 2006).
The city of Dodoma, the capital of Tanzania is located near the Kondoa fault zone and Bahi fault zone, which are seismically active and have records of abundant historical earthquakes events such as series of earthquakes that occurred in the year 2000-2002. Dodoma with its suburb extents towards N-NE direction towards Makotopora basin, the area is characterized by small volcanic intermountain basins filled with continental sediments. The geology of Dodoma is mainly covered by intrusive granite and partially by recent sediment deposits. The recent sediments deposits are composed of mbuga clay and sandy soils in varying proportions are widely distributed underlain by granite. Depth of the basins in Dodoma city has still to be investigated, but it is reasonably estimated through geophysical and geotechnical studies. These studies have shown that the thickness of the sediments for the city of Dodoma does not exceed 10 m (Sudian, 2007).
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The town of Babati, the capital of Manyara region is broad and structurally delineated by fault zones, Balangida fault zone in the northwest, Manyara basin fault zone in the north and Kondoa fault zone in the south. These fault zones are seismically active and producing frequently occurring small and occasional moderate seismic events. Babati town is located in volcanic tuff ridge situated between minor volcanic hills. The dark brown soils of an area characterized by occasional float of lava and tuffs. The basement rock of Babati town constitutes the Paleo-Proterozoic Mozambique. The surficial deposits consist of quaternary fans deposits derived primarily from volcanic deposits. These fans are composed of clays, silts, sands, gravels and some volcanic rocks. The thickness of the surficial deposits varies with topography, and it has still to be investigated.
The cities of Arusha and Dodoma and Babati town have experienced numerous earthquakes since their foundation. Many of these earthquakes caused moderate damage in this area with exceptional to those which occurred in the Lake Eyasi fault zone. From seismotectonic point of view the broader area is characterized by tensional stress field approximately horizontal, striking in an almost N-S direction. The geotectonic and stress field of the fault zones had been extensively studies by geological observation (Nyblade et al 1996) and fault plane solutions of strong and weak earthquakes (Msabi,2009;Gabriel,2007).
The knowledge of the seismic activity in the eastern branch of East African Rift System is based on the historical and recent acquired records. Worldwide seismic networks and available data from few stations around the East African Rift System provide information with encouraging consistency.
Usually, the East African Rift System is considered to be of very high seismic activity. The oldest historical record dates back to 1910 when the strong Kasanga earthquake (Ms=7.3) of 13 December 1910 shook the southern part of the Tanzania and caused a significant damage (Midzi, 1998). The most recent strong seismic events are Chenene hills, Dodoma earthquake of July 2002, magnitude 5.6 and Natron basin earthquake of July 2007, magnitude 5.9. These earthquakes were induced by normal faulting events with slip direction orthogonal to the rift segments locally. Micro-earthquake studies in the northern Tanzania have indicated that high level seismic activity is associated with NE-SW trending faults (Nyblade et al., 1996). Based on the given data, the maximum seismic potential of east African rift system in the northern part of Tanzania is estimated to be not less than magnitude of 5.9.
6.5.3 Site effects
The crucial role of the surface geology in the amplification has been demonstrated by many authors both from theoretical and empirical point of view (Borcherdt, 1990; Borcherdt and Glassmoyer, 1992 and Bard 1993 and many others). Site effects can be due to the heterogeneity of the subsoil materials (Impedance contrast) or due to irregular geometry (amplification due to topography) (Chavez- Garcia, 2007). In next sections, important site effects such as local amplification related to soft surface layers, surface topography and basin effects are illustrated.
18.104.22.168 Effects of soft surface layers
It has been recognized for a very long time that earthquake damage is generally larger over soft sediments than on firm bedrock outcrops. The fundamental phenomenon responsible for the amplification of motion over soft sediments is the trapping of seismic waves due to the impedance contrast between sediments and the underlying bedrock (Sanchez-Sesma and Luzon, 1996).
When the structure is horizontally layered (which will be referred to in the following as 1-D structures), this trapping affects only body waves travelling up and down in the surface layers. When the surface sediments form a 2-D or 3-D structure, i.e., when lateral heterogeneities such as thickness variations are present, this trapping also affects the surface waves which develop on these heterogeneities, and thus reverberate back and forth. The interference between these trapped waves leads to resonance patterns, the shape and the frequency of which are related with the geometrical and mechanical characteristics of the structure. While these resonance patterns are very simple in the case of 1-D media (vertical resonance of body waves), they become more complex in the case of 2-D and a fortiori 3-D structures.
22.214.171.124 Topographic effects
It has been often reported after destructive earthquakes that buildings located at hill tops suffered much more intensive damage than those located at the base. The influence of surface topography has been noted in several earthquake reports and demonstrated in instrumental studies and results may be found in Géli et al. (1988), Faccioli (1991) and Finn (1991). However, there is not enough number of instrumental studies to derive a correlation between topographic effects to studies dealing with soft soil amplification. Currently, it is not possible to develop a statistical relationship of changes in frequency and amplitude of strong ground motion and topography. However, theoretical and numerical models predict a systematic amplification of seismic motion at ridge crests, and, more generally, over convex topographies such as cliffs. Correspondingly, they also predict de-amplification over concave topographic features, such as valleys and the base of hills.
126.96.36.199 Basin effects
An increase of significant effects has been evidenced near the edge of the soft basins (Rovelli et al, 1995; kawase, 1996) particularly if bordered by faults that can induce abrupt amplification. The shape of the basin and the type of sediments deposits in the basin play major role on the amplification of ground motion. If the shape of the basin is curvature and filled with soft sediments which can trap, some of body waves and transform to surface waves. Surface waves can create stronger shaking and make longer duration of shaking.
6.5.4 Methods for estimating site effects
Estimation techniques for site effects can be divided into two main categories; experimental which necessitates records and numerical which uses soil information (Lacave et al, 1999). However, numerical studies are well advanced of experimental ones regards the understanding of the effects of geological settings. Most of experimental studies of site effects emphasize the effect of soft soils directly under recording stations i.e.1-D site effects (Dimitriu et al, 1998), whereas detailed numerical studies are concentrated on reflecting the difficulties of evincing the effect of lateral heterogeneities in data sets that are obtained with an insufficient number of stations or using arrays that are not dense enough for the dominating wavelength.
Experimental methods try to clarify the response on the surface using outcomes of seismic records. The method uses the spectral ratios between the two components (Horizontal and vertical) of seismic records. The most common methods are Standard spectral ratio (SSR) and H/V ratio (Lacave et al 2002).
Standard spectral ratio (SSR)
The spectral ratio technique is common useful way to estimate empirical transfer function to evaluate site effects in the region of moderate to high seismicity (Borcherdt, 1970; Borcherdt and Gibbs, 1976). This approach considers the ratio between the spectrum at the site of interest and spectrum at reference site which is usually a nearby rock site. Usually, the intense S-wave part of record is used to estimate spectral ratio relatively to reference station, whose records are assumed representative of incident free field to the topographic or geological irregularities which are investigated. These spectral ratios constitute a reliable estimate of site response if the "reference site" is free of any site effect and is located near the soft soil site. The proximity of the reference station must be measured in terms of dominant wavelength (Chavez - Garcia, 2007).
The spectral ratio method however has important limitation to estimate site effect. The spectral ratio method depends on the availability of an adequately reference site but in certain cases it may be very difficult to find convenient place for reference station. Another limitation is that the choice of reference site may pose large difficulties due to fact that surface rock sites are inevitably affected by amplification at frequencies as low as 4 to 5 Hz, because of thin weathered layer that is always present at rock sites (steidl et al, 1996).
To overcome difficulties in citing of reference site, a new technique which is nonreference site dependent technique was introduced by Nakamura (1989). This technique ,originally applied to microtremors (Field and Jacob,1993;Lachet and bard,1994), has been subsequently extended to weak motions ( Field and Jacob 1993;lachet et al 1996; Riepl 1998) and, in some cases , strong motion data are used (Lermo and Chavez-Garcia,1993; Theodulidis and Bard ,1995). This technique uses the ratio between the Fourier spectra of the horizontal and vertical (H/V) spectra of S wave window for each site (Lermo and Chavez-Garcia, 1993).
Various sets of experimental data (Lermo and Chavez- Garcia 1993; Field and Jacob, 1993b; gitterman et al, 1996; fah, 1997) have shown that the H/V procedure can be succefful applied to identifying the fundamental resonance frequency of the sedimentary deposits. These observations are supported by several theoretical investigations (Field and Jacob, 1993b; Lachet and Bard, 1994; Lermo and Chavez- Garcia, 1994; Cornou, 1998), showing that synthetics obtained with randomly distributed, near surface sources lead to horizontal-to-vertical ratios sharply peaked around the fundamental S-wave frequency, whenever the surface layers exhibit a sharp impedance contrast with the underlying stiffer formations. However, the absolute level of site amplification does not correlate with the amplification obtained from more traditional methods.
For arbitrary and general heterogeneous media, experimental methods are no longer valid. Therefore, numerical techniques had to be developed. They all based on the wave equation and many different models have been proposed to take account of various aspects of site effects. However, the numerical approach requires a good knowledge of local structures responsible for site effects. Both geometric and mechanical properties have to accurately model the seismic response of the soil. The next paragraphs presents some useful models that have been used in account various aspects of site effects.
In 1D model, the transfer function is calculated for horizontal stratified medium and vertically incident S-wave using the reflectivity method (Heskel, 1960; Kennett 1983; muller, 1985). These models have been long developed and become most classical with known SHAKE- code (Schnabel, 1972). These analyses can be performed considering either linear or non linear behavior of soils (Lacave et al, 2002).
I-D models have been the preferred choice because of their simplicity, reliability and possibility of generalizing results. However, it has been shown by different researchers (Riepl et al 2000; Makra et al 2004) that in complex local subsurface geometries, 1-D models tends to underestimate the amplification pattern.
2D and 3D models
2D and 3D models go one step further to allow variation of properties along horizontal dimension of layers in basin by using more elaborate mechanical algorithms such as finite elements( Gomez et al,1999) ,finite difference (Moczo,1989), Discrete elements methods (Sincranian and oliveira,2002) and discrete wave number (Aki and Larner,1972).
One of most used method for 2D is discrete wave number by Aki and Larner (1970) and improved by Bard and Bouchon (1980) and Bard and Ganel, 1986). This technique allows accounting for one irregular interface separating the underlying hard rock from sedimentary basin fill. The transfer functions due to vertically SV-wave are calculated considering strong and very low anelastic attenuation.
Although all numerical methods have the same base, - i.e. the wave equation - many different models have been proposed to investigate several of the various aspects of site effects, which involve complex phenomena Typically, these advanced methods may be classified into four groups:
Analytical methods, which can be used only for very simple geometries, are extremely valuable as benchmarks.
Ray methods, which are basically high frequency techniques. It is uneasy to use them when wavelengths are comparable to the size of heterogeneity, a situation which is generally the most interesting one.
Boundary based techniques (including all kinds of boundary integral techniques, or those based on wave function expansions), which are the most efficient when the site of interest consists of a limited number of homogeneous geological units.
Domain based techniques (such as finite-difference or finite-element methods), which allow to account for very complex structures and material behaviour, but are very heavy from a computational viewpoint.
Although these methods need heavy computational processes, their main advantage rests in their flexibility and versatility (combined with their cheapness on standard computers), which have lead to significant breakthroughs in the understanding of site effects during the last two decades. Not only they allow to carry out phenomenological and parametric studies, they can also be used to assess the uncertainty in a site's seismic response, given the imperfect knowledge regarding the mechanical and geometrical characteristics of the considered site.
The so called hybrid method use the combination of different techniques or adaption of approaches originally devised to study other problems. The hybrid method has been used by different researchers (eg, Panza, 1993; Fah, 1994). The technique uses the mode superposition of the earth composed by concentric layers to propagate seismic motion from source to neighborhood boundary around the site and then uses the finite elements to propagate this motion from boundary to the site considering local soil 2-D characteristics.
The following are the hypothesis to be tested in order to understand the complex nature of local site effect and hence the induced seismic amplification of the study area;
The site effects that induce seismic amplification are mostly associated with geologic site condition.
Peak ground motion parameters are the same for rock and consolidated soils and highest for shallowest unconsolidated soft soils.
Resonance frequency of a soil media differs depending upon its physical nature and depth of the bedrock.
The duration of the strong shaking in unconsolidated soils within the basin are higher than duration of shaking of unconsolidated soils at hills.
Transfer function is specific for a particular site and will give the response for a specific site.
7.0 METHODOLOGY AND MATERIALS
The properties that require characterization for most of ground response analysis and dynamic geotechnical problems include stratigraphy, density, degree of saturation, water table, soil plasticity, water content and void ratio, modulus and modulus degradation with strain, damping and damping increase with strain and finally undrained strength.( Pitilakis,1998)
The mentioned goals of this research will be achieved through combinations of literature review, geotechnical investigation, geophysical investigation, seismic data and geological field work.
A detailed determination of geometry and dynamic properties of soil will be obtained using different and complementary prospecting techniques including borehole seismic tests, P and SH refraction, P reflection and surface wave inversion. The seismic prospecting campaigns will be followed by extensive geotechnical in situ laboratory testing program including drilling, sampling groundwater table measurements, SPT, CPT, cycling triaxial and resonant column test.
The most important parameter for selection for collection of shear wave velocity is selection of site. In this research, the selection of the site will be based on local geology, topography and availability of open spaces. Cities geology map are available but some literature are not available regarding local geology. To find out the open space in these cities and towns Google earth and remote sensing data will be used as a basic tool that will verified with field investigation to locate the open space.
SITE EFFECTS ESTIMATES
In this research, the site effect will be estimated by both experimental and numerical analysis. When the two results coincide, their mutual strengthen the confidence with which the result can be used for seismic hazard analysis (Chavez - Garcia, 2007).
Regional and Local Geology
The initial stage in a seismic hazard analysis is the geological investigation of the region to determine the geological settings. The first purpose is to identify the tectonic formations that may produce future earthquakes (Barka & Kadinsky-Cade, 1988; Barka, 1991, Barka, 1992; Jackson, 2001, Yeats, 1997). As observed in some recent major earthquakes (Kobe 1995, Spitak 1988), one of the reasons declared for the excessive damage was inadequate estimation of earthquake characteristics for the region. In evaluating the seismicity of a region, two factors need to be considered. The first factor is the tectonic and geologic formations that can produce earthquakes in the region and the second factor is the seismic history. To understand
the possible mechanisms that can generate earthquakes, detailed geological and seismological
studies are necessary (Working Group on California Earthquake Probabilities, 1995; Bolt, 1999).
Geophysical investigations are performed to determine stratigraphic details of the site. Seismic techniques have long been used for characterization of subsurface geotechnical information (Telford et al,1990).There are various techniques available for seismic surveys .All of these techniques are based on the measurements of travel time of seismic waves. Refraction tests using variable geophones spacing and different sources may provide good picture of the surficial and/or deep soil stratification including the bedrock.
The Spectral analysis of surface waves (MASW) a newly developed technique by Stokoe et al (1999),Raptakis (1995) have been used succefful in many study to provide a detailed velocity profiles comparable to crosshole measurements. The main advantage of the technique is that it can easily differentiate between signal, noise, and separate noise at the time of data processing.
For this research work, to get the shear wave velocity, ( ) method will be used at different sites of the cities and town to observe the variation of shear wave velocity distribution around the study areas.
Resistivity tests using modern technology will be used to complement the geological information specifically regarding water table and upper surface bedrock.
The second need for detailed geological and geotechnical investigation is related with identification of local site conditions and their variation both in vertical and horizontal directions (Pitilakis, 1999). As shown by many researchers (Borcherdt & Gibbs, 1976, Iglesias, 1988; Gazetas et al., 1990; Seed et al., 1991; Ansal et al., Tertulliani, 2000;) geotechnical site conditions could play a dominant role in damage distribution as well as in the recorded strong motion records (Aki, 1993, 1998; Bard, 1994). The determination of geotechnical site conditions requires identification of the soil stratification and properties of soil layers based on detailed in-situ tests, borings and sampling, and laboratory tests on recovered soil and rock samples (Pitilakis et al., 1992, 1998).
In order to accomplish the whole work, the following materials will be required as the field and study gear to facilitate the study.
Geological maps, topographic maps, landsat images and aeromagnetic maps.
Compass, global positioning system (GPS), geological hammer.
Field boots, gumboots, sun goggles, raincoat and hard field clothes.
Field notebook, pen, pencil, colored pencil and camera.
One desktop computer and laptop, printer and scanner, stationeries and geological drawing sets.