By Mukange, BA; Katwika, C; Jalum, B; Zana, NA; Tondozi, KF (2023). Greener Journal of Geology and Earth Sciences, 5(1): 76-108.
Return to Issue
Full text – PDF
Full text – HTM
Full text – EPUB
or highlight the text below to listen.
Table of Contents
Greener Journal of Geology and Earth Sciences
ISSN: 2354-2268
Vol. 5(1), pp. 76-108, 2023
Copyright ©2023, Creative Commons Attribution 4.0 International.
http://gjournals.org/GJGES
1Mention Physics, Faculty of Sciences and Technology, University of Kinshasa, Kinshasa, DR Congo.
2Departement of internal Geophysics, Center of Research in Geophysic (CRG), Kinshasa, DR Congo.
3 Department of Mining, Polytechnic Faculty , University of Lubumbashi, Lubumbashi, DR Congo.
Type: Research
Full Text: PDF, PHP, HTML, EPUB, MP3
The main objective of this work is to highlight the fine structure of the Rift zones using the model which exploits the notion of seismic species and the precise location of the main underground faults by positing the hypothesis that: “the main faults are located in places where the module of seismic activity is at its peak”. We are interested in the rift zone (30°E-35°E. 6°N-14¨S). This zone includes the square grid zones A41, A42, A42, A44, A51, A52, A52 and A54 with a side of 5° each. The demonstration of their fine structure consists of subdividing them into squares of side 1° each by calculating certain parameters deduced from the fundamental seismic parameters covering the period from 1975 to 2013. This model, introducing new concepts, led to the results following, some of which go beyond the known: Regarding the fine structure, we note:
that the rift zone (25-35°E), formerly homogeneous, is now subdivided into two distinct sub-zones:
The area located between 25 and 30°E (A42, A43 and A44). However, we notethat the structure of zone A44 (Upemba rift zone in the upper Katanga region) straddles between A42T (Virunga-Lake Kivu zone) and A43T (Tanganyika zone).
The zone between 30 and 35°E (A51, A52, A53 and A54). However, zones A42T (Kivu zone) and A54T (Malawi zone) have some similarities,
The rate of resemblance between the two sub-zones is 25%: the first zone is less seismic than the second, one having a form factor (III), the other (IV),
that the structure of the entire established DRC is almost identical to that of the A42B” zone (Virunga zone at a depth exceeding 30km).
The exploitation of the aforementioned hypothesis and our model made it possible to locate the main escaped faults and to note that:
These faults are exactly located in areas with intense seismic activity; going deeper, these faults change position: the shape is no longer vertical or rectilinear, but wavy and serpentine.
from the surface to a depth of 20 km, the faults of Kivu (A42) and Tanganyika (A43) zones are located near the rift, to move away from it beyond the depth of 20 km (corresponding to the position average of the Conrad discontinuity),
While the position of the main faults at layer G (0-10 km) is located at A5 for seismic zone A42, these faults are located at zone A3 for A43 for the same layer (G) and the opposite at the layer C (10-20 km) and are all located at A1 beyond 20 km.
Published: 30/12/2023
Prof. Mukange Besa Anscaire
E-mail: anscairbesa@ yahoo.fr
The map in figure (2) from previous studies (Mukange, 2016; Mukange, 2021a,b) carried out on the characterization of the seismicity of the DRC and its surroundings reveals that:
Seismic activity in the DRC is subdivided into two large zones:
A non-rift zone (10°E-25°E) characterized by low seismicity,
A rift zone (25°E-35°E) of high seismicity,
A degree of heterogeneity of 65% and 55% following respectively the horizontal and vertical subdivision, i.e. an average degree of heterogeneity of 60%,
Each grid zone (square with side 5°) has a degree of heterogeneity of 0%, i.e. homogeneous at 100%.
Seismic activity is concentrated in the main faults,
The main objective of this work is to highlight the fine structure of the Rift zones. To do this, thanks to the design of the unified characterization scale based on the notion of seismic species, it will be necessary, on the one hand, to calculate the following parameters: the rate or degree of heterogeneity of the grid zones, their rate resemblance and conservation of seismic species and on the other hand, to precisely locate the main underground faults by positing the hypothesis that: “the main faults are located in places where the module of seismic activity is at paroxysm.” The judicious exploitation of these parameters would allow better monitoring of geological phenomena and geodynamics, with an opening towards indirect geological prospecting.
The East African Rift System appears as a continental extension of the global system of lithospheric fractures which wind through the middle of the Atlantic and Indian Oceans and which extend into the eastern part of the African Continent via the Gulf of Aden and the Red Sea ( Mukange, 2016; Boden et al. 1988; Bantidi, 2014a). This system of fractures is made up of two branches, namely:
The eastern branch which, from the Afar triangle, crosses Ethiopia and Kenya to the northern Tanzanian divergence (Figure 1a); Mukange, 2012).
The western branch is made up of a system of fractures which cross the garland of the Great Lakes, that is to say, from Lake Albert (617 m altitude) passes through Lakes Edouard (912m), Kivu (1462m), Tanganyika (780m), Rukwa (782m), Malawi (460m) and continues south to Mount Beira in Mozambique and southwest to Lake Kariba, Zimbabwe. This branch therefore covers most of the eastern provinces of the DRC from latitude 4°N to latitude 8°S. From the Red Sea to the Zambezi the East African Rifts cover more than 6000km long and 40 to 60km wide. The two branches split in two at the level of the Aswan lineament and join at the level of Lake Malawi (Figure 1a).
The seismic activity of the DRC also includes that characteristic of intra-plate fractures which thus affects the entire Congolese basin known as the “Congolese craton”.
Figure 1a: The East African rift system (Mavonga, 2009)
Fig. 1b: Distribution of epicenters in the DRC (1972-2008)
This point is presented in two sub-points:
2.1. Analysis data
We analyze data collected through various sources (www.usgs.org and www.isc.ac.uk), covering the period from 1975 to 2013 over the geographical area between 6° North and 14° South latitude and between 25° East and 35° East longitude. This area includes the Rift grid zones A41, A42, A42, A44, A51, A52, A52 and A54 (figures 1b-2). Particular emphasis will be placed on zones A42, A42 and A44 located in the Congolese rift (Figure 1b).
2.2 Analysis method
Achieving our objectives requires the design of the unified characterization scale. This scale must integrate various classic parameters that we will have to calculate in each sub-zone; these are the following parameters:
The total number of earthquakes,
The total energy released by earthquakes,
Maximum Magnitude,
The maximum depth of the hypocenters,
Surface area of each sub-zone,
Volume of each subzone,
Density of earthquakes,
Energy density,
The b-value (Lay and al., 1995) and the “d-value”,
The degree of heterogeneity of the area,
The rate of resemblance of species,
The conservation rate of species.
For explanations not provided here relating to the calculation of certain parameters, we invite the reader to consult the literature (Mukange, 2021a-b; Mukange 2023a-bc); this is particularly the case for the rate of heterogeneity.
Figure 2: Seismic zoning in the Democratic Republic of Congo for seismic hazard assessment (Mukange, 2021b)
Figure 3 a: structural signature of the seismicity of the DRC
The map results above lead to obtaining the figure below called the structural signature of the seismic zone.
The characterization work will be carried out on each grid zone A41, A42, A43, A44, A51, A52, A52 and A54. Particular emphasis will be given to zones A42 and A43 due to the fact that one contains Lake Kivu in the Virunga volcano-seismic region, the other contains Lake Tanganyika in the Tanganyika seismic sub-zone (Mavonga, 2009; Figure 1).
The work of characterizing the internal structure of each of these zones therefore consists of:
Subdivide each grid zone (Aij) according to depth (Table 3a); this slice is called depth zone,
Subdivide each depth zone into vertical (Ai) and horizontal (Bj) sub-zones of one degree width (Table 2). Since each sub-zone Aij is a square of side 5°, by subdividing it thus, it will therefore be composed of twenty-five square sub-zones of side 1° each,
Calculate the aforementioned characterization parameters in each Ai and Bj,
Search for seismic species in each subzone Ai and Bj,
Assign the seismic level and the appropriate color to each Ai and Bj,
Calculate the module and assign the appropriate color to each grid zone Aij (Ai, Bj),
Discuss and interpret the results,
Draw the general conclusion and perspectives.
Table 1: Boundaries of the DRC rift grid zones under study
Table 2: Subdivision of zones (Aij) into vertical (Ai) and horizontal (Bj) sub-zones
ZONE A42 (25°E-30°E; 1°N-4°S)
Table 3: Subdivision of zones (Aij) into depth zones for A42 and A43
A42C”
3.1. Presentation of the results
The values of the various calculated parameters are contained, for illustrative purposes for zone A42, in the table below
Table 4: Illustration of the parameters calculated in each sub-zone of zone Aij, case of A42
Areas
maximum
0.024
3.2. Discussion of results
The discussion of the results is carried out in two main stages: the design of the unified characterization scale leading to the attribution of a seismic species to each sub-zone and the discussion itself (interpretation).
3.2.1. Design of the characterization scale
The characterization of the seismic activity of an area involves the design of a unified characterization scale which can reasonably integrate all the calculated parameters (Table 4). For this purpose, a characterization scale is developed consisting of five parameters and defined as follows:
X is the “form factor” which can take the value O, I, II, III, IV or V, with:
0 if the area is aseismic,
I if the maximum magnitude recorded is between,
II if the maximum magnitude recorded is between,
III if the maximum recorded magnitude is between,
IV if the maximum recorded magnitude is between,
V if the maximum magnitude recorded is between,
The group of numbers (1; 2; 3; 4) in subscript constitutes the “structure factor”, defined as follows:
the number 1 relates to the volume density of the seismic energy released in each sub-zone:
if this density is ≤ 50%, then the number 1 takes the index “a”,
if this density is between 50% and 100%, then the number 1 takes the index “b”,
if this density is greater than 100%, then the number 1 takes the index “c”,
the number 2 relates to the volume density of the number of earthquakes in each sub-zone:
if this density is ≤ 50%, then the number 2 takes the index “a”,
if this density is between 50% and 100%, then the number 2 takes the index “b”,
if this density is > 100%, then the number 2 takes the index “c”,
The number 3 relates to the λb-value parameter:
If λb-value >2, then it takes the index “a”,
If 1< λb-value≤ 2, then it takes the index “b”,
If λb-value <1, then it takes the index “c”,
Note that λb-value = 2.204 b-value. This parameter measures seismic activity.
The number 4 relates to the λd-value parameter:
If the λd-value >0.2, then it takes the index “a”,
If the 0.1< λd-value≤ 0.2, then it takes the index “b”,
If the λd-value <0.1, then it takes the index “c”,
Note that λd-value = 2.272d-value. This parameter is related to the structure of the soil.
3.2.2. Presentation of seismic species
The application of this scale to the various zones generates seismic species and levels (Tables 5-10). The seismic level is next to each seismic species. This scale also contains the seismic species discovered in the USA and Indonesia zone.
Table 5: Presentation of species and seismic level of each sub-zone
Table 6: Presentation of species and seismic level of each sub-zone (continued)
A2
Table 7: Presentation of species and seismic level of each sub-zone (continued)
A3
B5
Table 8: Presentation of species and seismic level of each sub-zone (continued)
28
Table 9: Presentation of species and seismic level of each sub-zone (continued)
B4
Table 10: Presentation of seismic species and level of each sub-zone (end)
3.2.3. Calculation of the similarity rate
The rate of resemblance between two zones is done by comparing the respective seismic species or by setting a species taken as a unit of measurement (reference). The calculation is carried out as follows:
The characterization scale, giving rise to a seismic species, is written X1224. Let zones A and B have respective seismic species XA1A2A2A4A and XB1B2B2B4B. The resemblance rate is calculated based on the form factor (X) and the structure factor (1, 2, 3. 4) using the following formula:
Form factor
If , ???????? − ???????? = 0, then the resemblance rate is 50%,
If , ???????? − ???????? = 1, then the resemblance rate is 40%,
If , ???????? − ???????? = 2 then the resemblance rate is 30%,
If , ???????? − ???????? = 3, then the resemblance rate is 20%,
If , ???????? − ???????? = 4 , then the resemblance rate is 10%,
If , ???????? − ???????? = 5 , then the resemblance rate is 0%,
Structure factor
If , 1???? = 1???? , then the resemblance rate is 15%, otherwise 0%
If , 2???? = 2???? , then the resemblance rate is 15%, otherwise 0%
If , 3???? = 3???? , then the resemblance rate is 10%, otherwise 0%
If , 4???? = 4???? , then the resemblance rate is 10%, otherwise 0%
We see that the total is 100%.
Example: calculate the rate of resemblance between the zones characterized by the following species IVccba and Icbbc then calculate it by taking the species IVcccc as a unit of measurement (reference). The first rate is called the relative resemblance rate, the other is the absolute resemblance rate.
Example of calculating the relative similarity rate
IVA − IB = 2, then the resemblance rate is 20%,
Like 1A(c) = 1B(c) , then the resemblance rate is 15%,
Like 2A(c) ≠ 2B(b) , then the resemblance rate is 0%,
Like 3A(b) = 3B (b) , then the resemblance rate is 10%,
Like 4A(a) ≠ 4B(c) , then the resemblance rate is 0%,
The total similarity rate is 45% (20%+15%+0%+10%+0%)
Example of calculating the absolute resemblance rate (we leave the task to the reader)
3.2.4. Results interpretation
The interpretation of the results focuses on the parameters below.
3.2.4.1. Seismic species, seismic levels and color
Observation of the results of tables (5-10) shows that:
In total, we identified 89 distinct seismic species,
There are 28 (42%) seismic species common to all areas,
There are 17% of species exclusive to zones A51, A52, A52 and A54, zones between 30 and 35°E,
There are 17% of seismic species exclusive to zone A42, subdivided into depth zones (A42G, A42B, A42B’, A42B”, A42C, A42C’, A42C”) according to table (3),
There are 17% of species exclusive to zone A43, also subdivided into depth zones (A43G, A43B, A43B’, A43B”, A43C, A43C’, A43C”).
The statistics indicate:
The first zone (A4j) is less seismic than the second (A5j): one having a form factor (III), the other (IV); As a result, we see that their intersection is empty (figure 3b),
The resemblance rate, based on structural factors, between these two zones is 17%: there is one common element out of a total of six (Figure 3c),
The structure of the DRC (25°-30°E) is more stable than that of Malawi-Zambezi (30°E-35°E),
Figure 3b: comparison of seismic species between the two zones of the A4j rift (25°E-30°E) and A5d (30°E-35°E)
Figure 3c: comparison of the structure factors between the two zones of the A4j rift (25°E-30°E) and A5d (30°E-35°E)
3.2.4.2. Seismic zoning of grid zones
The notion of grid zones (Aij) is related to that of vector representation (Mukange, 2021a-b)
The module (c) of each subzone Cij is calculated using the formula (2.12)
Or, and correspond respectively to the seismic levels of the vertical subzones (Ai) and (Bj) contained in tables (5-10).
Table 11: Color code relating to module slice and quantum level
The application of this code to each zone, and for illustration purposes to zone A42, leads to the results contained in the Table below.
Table 12: Illustration of the results from the previous table for the A42T zone
DRC A42T
42
20
The results in the table above and others in the appendix lead to the highlighting of zoning maps, representing the chromatic structure of seismic zones.
NB: Each sub-zone delimited by color is a square with a side of one degree (1°).
Figure 4a: Seismic zoning map of the DRC A42T zone
Figure 4b: Seismic zoning map of the DRC A43T zone
Figure 4c: Seismic zoning map of the DRC A44T zone
Figure 4d: Seismic zoning map of the DRC A51T zone
Figure 4e: Seismic zoning map of the DRC A52T zone
Figure 4f: Seismic zoning map of the DRC A53T zone
Figure 4g: Seismic zoning map of the DRC A54T zone
Figure 4h: Seismic zoning map of the DRC A42G zone
Figure 4i: Seismic zoning map of the DRC A42B zone
Figure 4j: Seismic zoning map of the DRC A42B’ zone
Figure 4k: Seismic zoning map of the DRC A42B” zone
Figure 4l: Seismic zoning map of the DRC A42C zone
Figure 4m: Seismic zoning map of the DRC A42C’ zone
Figure 4n: Seismic zoning map of the DRC A42C zone
Figure 4o: Seismic zoning map of the DRC A43 G zone
Figure 4p: Seismic zoning map of the DRC A43B zone
Figure 4q: Seismic zoning map of the DRC A43B’ zone
Figure 4r: Seismic zoning map of the DRC A43B” zone
Figure 4s: Seismic zoning map of the DRC A43C zone
Figure 4t: Seismic zoning map of the DRC A43C’ zone
Figure 4u: Seismic zoning map of the DRC A43C’ zone
Depending on the color arrangements, we observe that all these 21 zones are grouped into two shapes:
A symmetrical shape of the colors in relation to zone A3; these are zones A51T, A52T, A53T and A54T located between 30° and 35°E,
A bipolar form (two groups of colors) for all areas located between 25° and 20°E; these are the A42T, A43T, A44T and their derivatives.
3.2.4.3. Calculation of the degree of heterogeneity and the rate of resemblance
The degree or rate of heterogeneity is calculated by taking the ratio, as a percentage, of the total number of colors identified in the area to ten colors retained in the color code (Figures 4). The resemblance rate is calculated according to the formula indicated in point (2.2.2). However, we distinguish two similarity rates (TR):
The first, called absolute (TR1): this is the resemblance between the maximum seismic species taken as a reference and the maximum species observed among the Ai and Bj of the zone where we want to evaluate the rate (Tables 5-10 ; Tables 13-14),
The second, called relative (TR2) or cumulative calculated according to the depth of the layers going from the surface downwards (Tables 15-16).
Table 13: Calculation of the absolute resemblance rate for A42 zones
Table 14: Calculation of the absolute resemblance rate for A43 zones
Table 15: Calculation of the relative similarity rate for A42 zones
Table 16: Calculation of the relative similarity rate for A43 zones
60%
The figure below shows the distribution of similarity and heterogeneity rates for each zone.
Figure 5: Distribution of the absolute heterogeneity and resemblance rate according to the zones
Figure 6a: Distribution of absolute (TR1) and relative (TR2) resemblance rates according to zones A42 and A43
From these curves, the following observations emerge:
With a few exceptions, there is a correlation between the absolute resemblance rate (TR1) and the relative resemblance rate (TR2), (Figure 6);
With a few exceptions, except at A42G and A42B, there is a correlation between the absolute or relative rate of resemblance and the rate of heterogeneity (Figure 5);
There is a correlation between the number of curves and the rate of heterogeneity; in fact, we see that the number of curves decreases with the rate of heterogeneity: at less than 50% of this rate, there are at most three structural curves (Figure 5 and Figures 7).
As a result, another heterogeneity rate can be calculated based on the number of visible curves (out of five in total) on the structural curves (geo-seismic signature, figures 7a-l)
Figure 6b: Heterogeneity rate calculated based on geo-seismic signatures
The figure above groups the zones into four classes, made up of zones with a rate of 40%, 50%; 60% and 70% whose distribution of zones is represented by the graph below.
Figure 6c: Weight of each class made up of zones according to the level of heterogeneity rate of geoseismic signatures
We see that classes (1 and 3, odd) are dominant; they cover 76% of the areas
3.2.4.3. Species conservation rate
Calculating the conservation rate of species (Tables 17-20), inverse of the disappearance rate, consists of comparing the common species between two zones. Each zone has, at most, ten seismic species (Tables 5-10)
Table 17: Calculation of the absolute conservation rate of the species for A42 zones
IIaaab
Table 18: Calculation of the absolute conservation rate of the species for A43 zones
The tables above calculate the conservation rate of the zones step by step (relative rate)
Table 19: Calculation of the relative conservation rate of the species for A42 zones
0%
Table 20: Calculation of the relative conservation rate of the species for A43 zones
These tables indicate an average conservation rate of 5%, 5%, 2.5% and 5% for figures (17-20) respectively. We conclude that species are rarely preserved as a function of depth.
3.2.4.4. Structural curve (geo-seismic signature)
The results in table (12) and others in the appendix, in particular, have been transformed into the curves below, called “geo-seismic signatures” or “structural signatures”. The geodynamics of an area can be monitored based on the variation of the signature over time.
Figure 7a: Structural curves (signature) of zones A42T and A43T
Figure 7b: Structural curves (signature) of zones A44T and A51T
Figure 7c: Structural curves (signature) of zones A52T and A53T
Figure 7d: Structural curves (signature) of zones A54T and A42G
Figure 7e: Structural curves (signature) of zones A42B and A42B’
Figure 7f: Structural curves (signature) of zones A42B” and A43G
Figure 7g: Structural curves (signature) of zones A43B and A43B’
Figure 7h: Structural curves (signature) of zones A43B” and A42
Figure 7i: Structural curves (signature) of zones A42C’ and A42C”
Figure 7j: Structural curves (signature) of zones A43C and A43C’
Figure 7k: Structure curves (signature) of A43C” zones
The observation of these signatures groups them into five modes (Table 21).
Table 21: classification of zones according to the modes or shapes of the structural curves
(negative)
Concavity curve turned downwards
Statistics indicate that:
The majority (41%) of structures are in the category of downward-facing concavity curves,
Zones A42 (G and C) and A52T are singularities; remember that zone A42 is located in the Virunga-Lake Kivu volcanic region and that zone A52 is to its right (Figure 2).
3.2.4.5. Comparison of some structures
Among the structures above, there are those that attract our attention; in fact we see that:
The two structures below have the same shape or mode with concavity facing upwards, but symmetrical: the final part of the curves of one is worth the initial part for the other and vice versa; simply turn one over and superimpose it on the other to have identical structures.
Figure 8a: comparison of the structural curves of zones A43B’ and A43B”
What has just been observed above is also valid for the two structures below.
NB: A42 (Virunga-Kivu) and A43 (Tanganyika).
Figure 8b: comparison of the structure curves of zones A42T and A43T
We observe that the structure of zone A44 (Upemba rift zone, Haut-Katanga region) straddles A42T and A43T. These nuances are also observed in relation to the orientations of the main faults (Figure 9).
Figure 8c: comparison of the structural curves of zones A44T and A51T
Overall its two structures below are the opposite of one (concavity upwards) of the other (concavity downwards);
Figure 8d: comparison of the structure curves of zones A52T and A53T
The following two structures are so similar that we can affirm that the entire structure of the DRC (10-35°E, 6°N-14°S) is dictated by that of the A42B” zone (Virunga zone at depth exceeding 30km);
Figure 8e: comparison of the structure curves of the A4B” zones and the DRC (all)
These two structures below are similar, yet we observed that their total structures (A42T and A43T, figure 8b) are opposite or symmetrical; this shows that the orientations of the main faults of these two zones are sometimes parallel, sometimes crossed at certain depths (Figure 9).
Figure 8f: comparison of the structural curves of zones A43G and A42B
Indeed, the geological overview of the DRC shows that this territory has the following characteristics (Figure 1 and 9):
In the eastern part of the western branch of the East African Rift system, a network of main faults winds from north to south, from Lake Albert in the north to Lake Tanganyika in the south. From the southern end of Lake Tanganyika, the fault system extends in a southwest direction towards Lake Moero and Lake Upemba, then, in a southeast direction towards Lake Rukwa and Malawi (Figure 2.3),
Other faults have been highlighted in the north-western part of the DRC, in the territory of Ubangi, in the province of Equateur, and extends into the Central African Republic, towards Bangui its capital,
In the Kongo central province(Bas Congo), in the west of the DRC, the structural map of the DRC highlights the presence of faults; it is the same in the North-East of the DRC, in Orientale Province.
However, Lake Kivu is at the crossroads of two directions (South-West and South-East).
Figure 9: Structural map of the DRC (CRGM): the yellow lines represent the main faults
3.2.4.6. Comparison of zones based on horizontal and vertical subzones
The comparison uses the following histograms
Figure 10a: Distribution of seismic levels of horizontal zones (Bj) by seismic zone A43
Figure 10b: Distribution of seismic levels of vertical zones (Ai) by seismic zone A43
Figure 11a: Distribution of seismic levels of vertical zones (Ai) by seismic zone A42
Figure 11b: Distribution of seismic levels of horizontal zones (Bj) by seismic zone A42
From these figures, we draw the following conclusions:
A43T is similar to A43C based on vertical subzones (Ai),
A43T is similar to A43C’ based on the horizontal sub-zones (Bi),
A42T is similar to A42B based on vertical subzones (Ai),
A42T is similar to A42C based on horizontal subzones (Bi),
3.2.4.7. Final structures
The data in Table (12) and others in the appendix can be grouped into classes; These include statistics on the weight of each color (module) in the zoning maps (Figures 4). Table (21) therefore groups the zones according to classes (in steps of fifteen); we obtain six classes, one of which is empty
Indeed, we call the modulus gap the difference between the maximum and minimum modulus of the zone.
Table 21: classification of zones according to the gap relative to the weight of the colors
15-30
The six classes can again be grouped into three:
An exceptional group including the zones of classes 1 and 2 with 14% of the zones: this group only includes the derivatives of the zones of the Tanganyika region (A43 C”), Virunga-Lake Kivu (A42B’) and the rift of Upemba-Haut Katanga (A44T),
An intermediate group, 43%, made up of classes 4 and 5: there we find the zones derived from A42 and A43, as well as A42T and A43T,
A final group made up of class 6 which weighs 43%: this group includes all four zones between 30 and 35°E (A51T, A52T, A53T and A54T) and some derivatives of A42 and A43
Overall, these criteria highlight a clear distinction between the zones of the Congolese rift (A42, A43 and A44: 25-30°E) and that of the Malawi-Zambezi rift (A51, A52, A53 A54: 30-35°E ).
The combination of the similarity rate, heterogeneity and color gap parameters for each zone gives them the final structures presented below
Figure 12a: Distribution of parameters in legend by seismic zone
Observation of these structures (figure 10) groups them into three classes;
Table 22: classification of zones according to figure (10)
A42C,A42C’,A42C”,A43C,A43C’,A43C”
The analysis also integrating the heterogeneity rate parameter based on the geo-seismic signature (figure 10b) highlights two groups:
One, composed of areas located between 30 and 35°E (A51T, A52T, A53T and A54T),
The other consists of zones between 25 and 30°E (A42, A43 and A44T), in the Congolese rift.
However, depending on one or another parameter, these zones have some nuances:
The A44T zone (in the Upemba rift) tends to separate from A42T (Virunga zone) to get closer to A43T (Tanganyika zone),
The A42T zone, if not alone, is sometimes close to the A5j zones. The A44T area seems to act as a hinge or suture zone between the two groups ( ). The structural map (Figure 9) is in agreement with the aforementioned observations, particularly with regard to the orientations of the main faults (Figure 9).
3.2.4.8. Comparison of zones based on b-value and d-value parameters
These parameters will allow us to model the structures in order to follow the geodynamic evolution.
Remember that the b-value parameter measures the seismic activity of an area, while the d-value measures the internal structure (of the soil).
The search for the establishment of a correlation between these two parameters provided the following curves.
Figure 13 a: Correlation between the b-value and the d-value for zone A51T according to horizontal subdivisions
Figure 13 b: Correlation between the b-value and the d-value for zone A52T according to horizontal subdivisions
Figure 13c: Correlation between the b-value and the d-value for the A43T zone according to vertical subdivisions
Figure 13 d: Linear correlation between the b-value and the d-value for the A43T zone according to vertical subdivisions
Figure 13e: Correlation between b-value and d-value for zone A54T based on horizontal subdivisions
Figure 13f: Linear correlation between the b-value and the d-value for the A42T zone according to the horizontal subdivisions
Figure 13g: Correlation between the b-value and the d-value for the A42T zone according to vertical subdivisions
Figure 13h: Linear correlation between the b-value and the d-value for zone 42T according to horizontal subdivisions
Figure 13i: Correlation between the b-value and the d-value for the A53T zone according to vertical subdivisions
Observation of the shapes of these curves reveals two large families, one consisting of linear curves, the other of parabolic curves (Table 23)
Table 23: classification of zones according to the shape of the correlation curves
Positive concavity
A52T next Bj
A53T next Bj
A42T following Ai
A42T next Bj
Analysis of the results in this table highlights the following facts:
The zones located between 25 and 30°E, in the Congolese rift (A42, A43 and A44) have a linear shape
Those located between 30 and 35°E, in the Malawi-Zambezi rift (A51, A52, A53 and A54) have a parabolic shape,
However, zones A42T (Kivu zone) and A54T (Malawi zone) have nuanced trends (exceptions), therefore straddling the two previous groups.
3.3. Location of main faults
The location of underground faults is often done by exploiting the gravity and geomagnetic data of the region (Mbata, 2023; Ngindu, 2021; Mulopo, 2023; Tondozi, 2018). As far as we are concerned, the interest is focused on the location of said faults by exploiting the fundamental data of seismic activity.
To do this, this location will be based on the hypothesis that: “the main underground faults are located in places where the seismic activity module is at its peak”. This paroxysm corresponds in figure (7) to the zone where the modulus is the highest; which gives rise to the results contained in Figures (14-15) for zones A42 and A43.
Figure 14: Location of main and minor faults in the Aij zone
Reading Figure (14) responds to the legend above:
Red color: the main fault is located in layer G (0-10 km)
Yellow color: the main fault is located in layer C (10-20 km),
Green color: the main fault is located in layer C’ (20-40 km),
Blue color: the main fault is located in layer C” (>40 km),
Purple color: the main fault is located in the C’ and C” layer, therefore (>20 km),
White color: no main fault, but minor faults can be found.
The comparative analysis of these results reveals the following:
For both zone A42 and zone A43, no main fault was observed in zone A4; this zone is therefore the most stable.
While the position of the main faults at layer G (0-10 km) is located at A5 for seismic zone A42, these faults are located at zone A3 for A43 for the same layer (G) and the opposite at the layer C (10-20 km); this observation could be explained by:
The heterogeneity of the soil above 20 km,
The position of two zones: zone A42, including Lake Kivu, is located at a height of 1462 km and a depth of 485 km. The A43 zone, including Lake Tanganyika, is located at a height of 780 km and a depth of 1433 km,
The orientation of the faults in these two zones are opposite (Figure 1,2 and 9): NW-SE orientation for Lake Tanganyika (Bopili, 2009) and NE-SW for Lake Kivu .
We note a sort of alternation or compensation in the geodynamics between the two zones
towards regions close to the rift at a depth not exceeding 20 km,
Beyond twenty kilometers in depth, the main faults are almost, for both A42 and A43, located on A1; which means that :
From the surface to a depth of 20 km, the faults are close to the rift, moving away from it beyond the depth of 20 km (position of the Conrad discontinuity),
The soil structure is more homogeneous and denser beyond 20 km than above, in accordance with the literature,
In short, we conclude by saying that the characterization scale designed is reasonable and that the hypothesis put forward is also valid.
This research aims to highlight the fine structure of the seismic zones of the western branch of the East African rift system using the unified scale of characterization and the location of the main faults, leading to the following conclusions:
Regarding the fine structure, we note:
Statistics on seismic species discovered in the region show:
In total, we identified 89 seismic species,
There are 17% of species exclusive to zones A51, A52, A52 and A54, zones between 20 and 25°E,
There are 17% of seismic species exclusive to zone A42 subdivided into depth zones (A42G, A42B, A42B’, A42B”, A42C, A42C’, A42C”) in the table (),
There are 17% of species exclusive to zone A42 subdivided into depth zones (A42G, A42B, A42B’, A42B”, A42C, A42C’, A42C”).
The average conservation rate of species is 5%. We conclude that species are rarely preserved by going deep,
Generally speaking, all of these 21 zones align, depending on the color arrangements, on one of two shapes:
A symmetrical shape, these are A51T, A52T, A52T and A54T, therefore the zones located between 20° and 25°E,
A bipolar form (two groups of colors) for all areas located between 25° and 20°E; these are the A42T, A42T, A44T and their derivatives.
From these curves comparing the rate of resemblance to that of resemblance, the following observations emerge:
With a few exceptions, there is a correlation between the absolute resemblance rate and the relative resemblance rate;
With a few exceptions, except at A42G and A42B in the Virunga-Kivu region, there is a correlation between the absolute or relative rate of resemblance and the rate of heterogeneity,
There is a correlation between the number of curves and the rate of heterogeneity; in fact, we see that the number of curves decreases with the rate of heterogeneity: at less than 50% of this rate, we have fewer than three structural curves.
The average rate of heterogeneity is 49%, it is on average higher (55%) between 30 and 35°E and less between 25 and 30°E,
the combination of the three parameters (rate of resemblance, heterogeneity and conservation of the species) brings together 85% of the structures in the same class, the A42T zone (Virunga-Lake Kivu zone) being an exception and partially the Tanganyika zone ( A43B’),
We observe that the structure of the A44 zone (Upemba rift zone, upper Katanga region) straddles A42T (Virunga-Lake Kivu zone) and A43T (Tanganyika zone), with implication on the orientation failures ,
There is reason to affirm that the structure of the entire DRC is determined or predominated by that of the A42B” zone (Virunga zone at a depth exceeding 30km).
The analysis of the results based on the b-alue and the d-value, one measuring seismic activity, the other the soil structure, indicates that:
The zones located between 25 and 30°E, in the Congolese rift (A42, A43 and A44) have a linear shape with some particularities each,
However, zones A42T (Kivu zone) and A54T (Malawi zone) have trends
The resemblance rate, based on structural factors, between these two zones is 25%
The first zone is less seismic than the second, one having a form factor (III), the other (IV)
Each of these seven zones, whose heterogeneity rate was 38% (Figure 2), no longer resembles itself alone; therefore the heterogeneity rate increases to 100%
The location of the main faults is based on the hypothesis that: “the main faults are located in places where the module of seismic activity is at its peak”. The exploitation of this hypothesis and the structure curves have:
Made it possible to locate the main and secondary faults,
Shown that going deeper, these faults change position; the shape is no longer vertical nor rectilinear, but wavy and serpentine,
Showed that these results are in accordance with field observations and literature
allowed us to note that from the surface to a depth of 20 km, the faults of the Kivu (A42) and Tanganyika (A43) zones are located near the rift, to move away from it beyond the depth of 20 km ( corresponding to the average position of the Conrad discontinuity),
While the position of the main faults at layer G (0-10 km) is located at A5 for seismic zone A42, these faults are located at zone A3 for A43 for the same layer (G) and the opposite at the layer C (10-20 km) and are all located at A1 beyond 20 km,
In short, we say that our model based on the discovery of seismic species and the generation of the characterization scale is reasonable and that the hypothesis put forward is valid. Indeed, their exploitation has allowed a better characterization, both quantitative and qualitative, of the soil structure and its geodynamics using fundamental seismic parameters. These results go beyond what is known.
Bantidi M., Wafula M., Mavambou, Mukange B., Zana Nd., (2014a). Probabilistic assessment of seismic hazard in Lake Tanganyika Rift accounting for local geologies conditions. 2015. International Journal of Geology, Agriculture and Environmental Sciences. Vol.03 Issue 02 (April 2015), pp24-29.
Bantidi M., Mukange B., et Zana N., (2014b). Structure de la sismicité de la Branche occidentale des Rifts Valleys du système des Rifts Est-africains ; de 1954 à 2010, International Journal of Innovation and Applied Studies, ISSN 2028-9324 Vol. 9 No. 4 Dec. 2014, pp.1562-1581.
Biliki K;. and al.,(2021). Interpretation of Gravity Data and Contribution to the Study of the Geological Structure of the Province of Mai-Ndombe in DR. Congo: Implications in the Exploration of Hydrocarbons. International Journal of Innovative Science and Research Technology, Volume 6, Issue 4, April – 2021, pp213-921.
Borden J-P., (1988). Biologie-Géologie. Première S. Paris: Bordas
Bopili M.L., (2009). Etudes des fluctuations de la température et de la vitesse des vents au lac Tanganyika (une analyse par ondelettes). Thèse de doctorat : Université de Kinshasa, Faculté des Sciences, Département de Physique. pp. 9-28
Lay T., and Wallace T., (1995). Modern Global seismology. New-York : Academic Press
Mavonga Tuluka G., (2009).Seismic hazard assessment and volcanogenic seismicity for the Democratic Republic of Congo and surrounding areas, western Rift valley of Africa. Thèse de Doctorat: University of the Witwatersrand (Johannesburg), Faculty of Sciences.
Mbata A. and al., (2023). A comparative structural study of Southern region shallow basement of the North-Kivu Province (DR. Congo) by gravity and magnetic data analysis. Journal of Geoscience and environnement protection, Vol 11, pp 90-117.https:// doi.org/10.4236/gep.2023.119007.
Mukange B., Bantidi M., Zana Nd., (2013). Structure de la sismicité de la Branche orientale des Rifts Valleys du système des Rifts Est-africains ; de 1954 à 2010. Revue Congolaise des Sciences Nucléaires. vol.27, pp 151-169.
Mukange B., Bantidi M., Zana L., Wafula M., Zana Nd., (2015). The isoseismal map and their implication to underlining ground degree of heterogeneity (Kabalo quake’s case, September 11, 1992, magnitude 6.7, Upemba Rift). Greener Journal of Geology and Earth Sciences, vol. 3 (2), pp 030-042.
Mukange B., (2016). Conception d’un modèle physique pour la caractérisation et la surveillance de l’activité sismique et son implication géologique (Cas de la République Démocratique du Congo). Thèse de Doctorat : Université de Kinshasa, Faculté des Sciences. Département de Physique.
Mukange Besa, (2021). Cours de Géophysique générale. Université de Kinshasa, Faculté des Sciences.
Mukange B., (2021a). Design of a unified scale for the characterization of seismic activity. International Journal of Innovative Science and Research Technology, Volume 6, Issue 7 , July– 2021, pp.1407-1422. www.ijisrt.com
Mukange B., (2021b). Application of the unified scale to the characterization of seismic activity of the Democratic Republic of Congo and its surroundings (comparative study for Africa, Indonesia and the Pacific coast of Central America). International Journal of Innovative Science and Research Technology, Volume 6, Issue 7, July– 2021, pp.1516-1555. www.ijisrt.com
Mukange B., (2023a). Characterization of the volcano-seismic activity around Nyiragongo volcano and location of its crater by means of unified scale. Greener Journal of Geology and Earth Sciences, 5(1) December 2023: 28-51. http//gjournals.org/GJGES.
Mukange B., (2023b). Characterization of the volcano-seismic activity around Nyamulagira volcano and location of its crater by means of unified scale. Greener Journal of Geology and Earth Sciences, 5(1) December 2023: 52-75. http//gjournals.org/GJGES.
Mulopo, (2023). Analyse des corrélations entre les linéaments et les accidents tectoniques de la région de Lubumbashi-Kipushi : contribution à l’étude classique de la tectonique régionale en République Démocratique du Congo. Thèse de Doctorat : Université Pédagogique Nationale, Faculté des Sciences. Département de Physique
Musitu M. and al., (2023). Structural mapping of Kakobola and its surroundings by analyzing geomagnetic data. Journal of Geoscience and environnement protection, Vol 11, pp 64-89.https:// doi.org/10.4236/gep.2023.119006.
Ngindu D. and al., (2021). New Faults from the Geodynamics of South Katanga in D.R.Congo. International Journal of Innovative Science and Research Technology, Vol(6) Issue 1(January 2021),pp1596-1689.
Tondozi K. and al, (2018). Interpretation of gravity anomalies maps and contribution to the structural study of a sedimentary basin of major petroleum interest: Case of the Busira sub-basin in the Central basin of the DR Congo, IJIAS Vol. 24 N°1, p. 68-88.
Mukange, BA; Katwika, C; Jalum, B; Zana, NA; Tondozi, KF (2023). Highlighting the Fine Structure of the Seismic Zones of the Western Branch of the East African Rift System Using the Unified Characterization Scale and Its Geological Implication. Greener Journal of Geology and Earth Sciences, 5(1): 76-108.
Download [1.48 MB]
Your email address will not be published. Required fields are marked *
Comment *
Name *
Email *
Website
Save my name, email, and website in this browser for the next time I comment.
Post Comment
