This document analyzes seismic, gravity, and magnetic data from Northern Tunisia to interpret the complex fault systems created by the convergence of the African and Eurasian plates. It finds that many earthquakes occur along NE-SW trending faults associated with this convergence zone. Areas with the highest gravity anomalies coincide with thinning crust in the Tellian Atlas and Algero-Provencale Basin. High magnetic anomalies along the coast are interpreted to indicate volcanic rocks related to plate thinning. The data overall represents the complex active fault systems generated by the ongoing Cenozoic convergence in this region.

1. Seismic, gravity, and magnetic analyses of Northern Tunisia: Complex faulting
systems as a result of Cenozoic convergence
Andrew J. Hillier
University of Akron, Department of Geosciences, Undergraduate Senior in Geology, Akron, OH
Abstract
Data from several data repositories, including the Incorporated Research Institutions for Seismology’s Seismiquery, the International
Gravimetric Bureau’s 2012 World Gravity Map, and the International Association of Geomagnetism and Aeronomy’s World Digital Magnetic Anomaly
Map, were analyzed and processed to show changes in multiple geophysical parameters (seismic, gravity, and magnetic). Data from these repositories, for
each geophysical parameter, were mapped and spatially analyzed in relation to each other in order to determine similarities and differences in the trends
of each. Seismotectonic data (earthquake epicenter, earthquake magnitude, and active fault geometry) were the more general focus of this study, while
gravity and magnetic anomaly data were used to back up the interpretations drawn from the seismotectonic data analysis. It was observed that a large
number of the observed earthquakes occur along the NE-SW faults present along the Maghrebian-Apenninic-Alpine convergence front. The areas of
highest Bouguer anomaly coincide with the upper Tellian Atlas and Algero-Provencale basin. High spherical isostatic and surface free-air anomaly is
present along the Maghrebian-Apenninic-Alpine convergence front, over the Northern Atlas. Magnetic data presents relatively high intensity anomalies
along the northern coast of Africa and in the Algero-Provencale Basin. This data is interpreted as representing complex quaternary fault systems caused
by Cenozoic convergence of the Corsica-Sardina-Petite Kabylie plate and the Tunisian continental margin. This active fault system is of importance to
scientists because of the many moderately large magnitude earthquakes produced by it, which could be of civil, economic, and geologic importance.
1 Introduction
Active Northern African faults are of importance to society as they cause seismic activity, causing possible damage, economic problems,
and civil issues. According to T. Mourabit et al., “earthquake hazard in Northern Africa constitutes a constant threat to human life” (2013).
Improving our understanding of fault systems from Northern Tunisia would thus benefit scientists who are working to prevent or prepare for
these issues, as well as scientists who wish to study the geological implications of the modern convergence between Africa and Europe. By
analyzing changes in seismotectonics, gravity anomalies, and magnetic field anomaly, we can start to correlate the regional variation in these
geophysical parameters to the way in which modern convergence creates active faulting systems. By analyzing the effects of active faults on
Northern Tunisia, we can create better models and maps of faults that will allow us to better predict future earthquakes, and prepare for any civil
or economic issues they may pose.

2. 2 Methods
2.1 Seismotectonic Spatial Analysis
The Incorporated Research Institutions for Seismology’s (IRIS) Seismiquery earthquake event database was used to list earthquake
event parameters. Earthquake event data from the IRIS Seismiquery database were extracted for an area defined by geocentric coordinates 7° to
12°E, 35° to 38°N that occurred between January 1960 and December 2013, as ACSII-format text, imported to a spreadsheet, and space
delimited. The data was labeled, exported to a tab delimited text file, and imported to GIS software. Northern Tunisia was plotted with topology,
bathymetry, and hillshade as a basemap. Geocentric latitudes and longitudes of the extracted data were plotted on the basemap as points using the
WGS 84 (World Geodetic System, 1984) global positioning system. A 4 class, size dependent point classification system was classified using the
maximum and minimum magnitude values and applied to the plotted points, where larger points correspond to larger magnitudes, and conversely
smaller points to smaller magnitudes. Faults from the ESRI ArcAtlas Global Faults layer and the European Database of Seismogenic Faults were
plotted as vector line shapefiles. The final map had been saved as a PNG (portable network graphic) image file, such that the final map could be
spatially analyzed.
2.2 Gravimetric Spatial Analysis
The International Gravimetric Bureau (IGB) and Commission for the Geological Map of the World’s (CGMW) 2012 World
Gravity Map (WGM 2012) extraction tool was used to plot three 2’ x 2’ resolution regional gravity anomaly grids and one 2’ x 2’ resolution
regional topology grid. Gravity data from the WGM 2012 global model was extracted using the IGB’s extraction tool, for an area defined by
geocentric coordinates 7° to 12°E, 35° to 38°N. Extracted geocentric latitudes and longitudes of the extracted data were plotted on a 2’ x 2’ grid,
with a 14 to 21 class color ramp and hillshade classified using the maximum and minimum gravity anomaly/topology values. The final maps
produced by the extraction tool had been saved as a PNG (portable network graphic) image file, such that each gravity anomaly type and
topology could be spatially analyzed.
2.3 Magnetic Anomaly Spatial Analysis
The International Association of Geomagnetism and Aeronomy (IAGA) and Commission for the Geological Map of the
World’s (CGMW) World Digital Magnetic Anomaly Map (WDMAM) was used to extract global magnetic field anomaly data. Magnetic field
anomaly data from the WDMAM was downloaded as an ACSII grid file, and imported to GIS software. Northern Africa was plotted with
hillshade as a basemap. Geocentric latitudes and longitudes from the downloaded ASCII grid were plotted on the basemap as tiles using the
WGS 84 (World Geodetic System, 1984) global positioning system. A 12 class color ramp was classified and applied to the tiles using the
maximum and minimum field anomaly intensity values. Faults from the European Database of Seismogenic Faults were plotted as vector line
shapefiles. The final map had been saved as a PNG (portable network graphic) image file, such that the final map could be spatially analyzed.

3. 3 Results
3.1 Seismotectonic Data Analysis
A large number of the observed earthquakes occur along the Maghrebian-Apenninic-Alpine convergence front and along active faults
(Figure 1). Less earthquakes are observed north of the convergence zone in the Algero-Provencale basin (Figure 1). A majority of those
earthquakes are magnitude 3.0 or greater (Figure 1). A large number of earthquakes occur in the Mediterranean Sea off of Tunisia’s eastern coast
(Figure 1). A majority of those earthquakes are magnitude 3.0 or greater, with several event-dense regions of 4.0 and greater earthquakes present
(Figure 1).
3.2 Gravimetric Data Analysis
Areas with the greatest spherical Bouguer anomaly occurred along the Tellian Atlas in the northernmost section of the Atlas Mountains
in Tunisia, as well as in the Algero-Provencale Basin (Figure 2), while lesser values were found inland. More regional Bouguer anomaly
variations are present over the land surface, and fade out along the coast (Figure 2). The greatest surface free-air anomaly occurred directly over
the Northern Atlas and Tellian Atlas, with a similar geometry (Figure 2). The greatest spherical isostatic anomaly occurred over the Northern
Atlas, following the Maghrebian-Apenninic-Alpine convergence front, as wellas in the Algero-Provencale Basin (Figure 2). More regional
isostatic anomaly variations are also present over the land surface, and fade out along the coast (Figure 2).
3.3 Magnetic Anomaly Data Analysis
Magnetic data presents relatively high intensity anomalies along the northern coast of Africa, especially around the Tellian atlas and the
Algero-Provencale Basin, where fluctuations tend from red (257 nT) to blue (185 nT) (Figure 3).

4. Figure 1. Seismotectonic map of Northern Tunisia showing earthquake epicenters with size as a proxy for magnitude as well as active and supposed fault geometry, from data
extracted from the Incorporated Research Institutions for Seismology’s Seismiquery earthquake event database, as well as from vector shapefiles extracted from the
ESRI ArcAtlas Global Faults layer and the European Database of Seismogenic Faults

5. Figure 2. 2’ x 2’ resolution Spherical Bouguer gravity anomaly, surface free air gravity anomaly, spherical isostatic gravity anomaly, and topography maps of Northern Tunisia,
showing color variation as a proxy for changes in value, from data extracted from the 2012 World Gravity Map.

6. Figure 3. Magnetic Anomaly Map of the Mid-Western Mediterranean Coast showing color variation as a proxy for magnetic anomaly intensity in nanotesla and active faults in the
region, from data extracted from the International Association of Geomagnetism and Aeronomy’s World Digital Magnetic Anomaly Map.

7. 4 Interpretation
4.1 Seismotectonics
The large numbers of relatively high magnitude earthquakes observed following the Maghrebian-Apenninic-Alpine convergence front
(Figure 1) are interpreted to be caused by active quaternary faults created by or reactivated by the modern convergence of the Corsica-Sardina-
Petite Kabylie plate and the Tunisian continental margin. The higher density of relatively high magnitude earthquake events off of Tunisia’s
eastern coast (Figure 1) is interpreted as being caused by a supposed secondary N-S fault or fault system that runs along the coast (Figure 1).
4.2 Gravity Anomaly
The high spherical Bouguer anomaly along the Tellian Atlas and Algero-Provencale Basin relative to the anomaly inland are interpreted
to be due to the thinning of the crust northward (Figure 2). The more regional variations seen over Tunisia around the Tellian Atlas and Northern
Atlas (Figure 2) are interpreted as being caused by different geometries of and the heterogeneity of sediment and lithologic densities in the upper
crustalmaterial, caused by structures such as folds, faults, grabens, salt diapirs, etc. The high surface-free air anomalies observed over the Tellian
Atlas and Northern Atlas, of similar geometry to the terrain (Figure 2), are likely to be present due to the difference in elevation observed due to
the presence of mountains. The high isostatic anomaly along the Northern Atlas (Figure 2) is likely due to the relative thickening of the crust
there due to the convergence zone. The regional isostatic variations are almost congruent in geometry to the variations observed in the Bouguer
anomaly map, and are also interpreted as different geometries of and the heterogeneity of sediment and lithologic densities in the upper crustal
material, caused by varying structures.
4.3 Magnetic Anomaly
The high magnetic anomaly along the northern African coast, especially in the Algero-Provencale basin (Figure 3), are interpreted as
being large scale deposits of magnetic-rich bodies found in volcanic crustalmaterial and crystalline basement, such as ferromagnetic magnetite,
which is known to form in igneous bodies. Rather than deposits of material, this could also be interpreted as areas of volcanism caused by the
thinning of the crust in the present convergence zone, where magnetic minerals with thermal remnant magnetism are present in the produced
plutonic and extrusive bodies.

8. 5 Discussion
5.1 Seismotectonics
The large numbers of relatively high magnitude earthquakes observed following the Maghrebian-Apenninic-Alpine convergence front
are caused by active NE-SW quaternary fault systems created by or reactivated by the modern convergence of the Corsica-Sardina-Petite Kabylie
plate and the Tunisian continental margin, as is shown by studies of quaternary and focal seismic mechanisms performed by N. Bahrouni et al.
(2013). The higher density of relatively high magnitude earthquake events off of Tunisia’s eastern coast is interpreted as being caused by a
supposed secondary N-S fault or fault system called the N-S axis that runs along the coast, where a marine carbonate barrier extends along the
coastline and records tectonic deformations in the area (N. Bahrouni et al., 2013). According to N. Bahrouni et al., the N-S structures present
south of the study area were caused by the area’s first major compressionalevent known as the Tortonian Phase, while the NE-SW faults
observed in our study area are caused by the second compressionalevent, known as the Post-Vallafranchian Phase, and NE-SW Mio-Pliocene
extension (2013; Dhahri et al., 2015)
5.2 Gravity Anomaly
The high spherical Bouguer anomaly along the Tellian Atlas and Algero-Provencale Basin relative to the anomaly inland are due to the
thinning of the crust northward, as is shown in studies performed on the crustal-mantle boundary by Jallouli et al. (2002). The more regional
variations seen over Tunisia around the Tellian Atlas and Northern Atlas are likely caused by different geometries of and the heterogeneity of
sediment and lithologic densities in the upper crustalmaterial, caused by structures such as folds, faults, grabens, salt diapirs, etc. (Jallouli et al.,
2013). The high surface-free air anomalies observed over the Tellian Atlas and Northern Atlas, of similar geometry to the terrain are likely to be
present due to the difference in elevation observed due to the presence of mountains, as is predicted to be seen due to the lack of terrain and
Bouguer anomaly correction. The high isostatic anomaly along the Northern Atlas is likely due to the relative thickening of the crust there due to
the convergence zone. According to Jallouli et al., this higher anomaly could be in part due to plate subduction (2013). The regional isostatic
variations are almost congruent in geometry to the variations observed in the Bouguer anomaly map, and are also likely different geometries of
and the heterogeneity of sediment and lithologic densities in the upper crustalmaterial, caused by varying structures (Jallouli et al., 2013).
5.3 Magnetic Anomaly
The high magnetic anomaly along the northern African coast, especially in the Algero-Provencale basin are interpreted as being large
scale deposits of magnetic-rich bodies found in volcanic crustalmaterial and crystalline basement, such as ferromagnetic magnetite with thermal
remnant magnetism, which is known to form in igneous bodies. Rather than deposits of material, this could also be interpreted as areas of active
volcanism caused by the thinning of the crust in the present convergence zone, where magnetic minerals with thermal remnant magnetism are
present in the produced plutonic and extrusive bodies. This claim is supported by the presence of volcanic activity in the area (Cohen, C.R.,
1980).

9. 6 Conclusions
By analyzing changes in seismotectonic activity, gravity anomalies, and magnetic field anomaly, we can correlate the regional variation
in these geophysical parameters to the way in which modern convergence creates active faulting systems. By analyzing the effects of active
faults on Northern Tunisia, we can create better models and maps of faults that will allow us to better predict future earthquakes, and prepare for
any civil or economic issues they may pose. It was observed that a large number of the observed earthquakes occur along the NE-SW faults
present along the Maghrebian-Apenninic-Alpine convergence front. The areas of highest Bouguer anomaly coincide with the upper Tellian Atlas
and Algero-Provencale basin. High spherical isostatic and surface free-air anomaly is present along the Maghrebian-Apenninic-Alpine
convergence front, over the Northern Atlas. Magnetic data presents relatively high intensity anomalies along the northern coast of Africa and in
the Algero-Provencale Basin. This information is all interpreted as representing structures and events caused by the Cenozoic convergence of
Africa and Europe, or more specifically in this study, the Corsica-Sardina-Petite Kabylie plate and the Tunisian continental margin.

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