Improved Tools for Disaster Risk Mitigation in Algeria ITERATE

20 Pages·2017·5.92 MB·English

by  Ricardo Monteiro

ITERATE

Improved Tools for
Disaster Risk Mitigation in Algeria

ITERATE

Deliverable B.2

Updated source zone model

ECHO/SUB/2016/740181/PREV23 – ITERATE – Improved Tools for Disaster Risk Mitigation in Algeria
Project co-funded by ECHO – Humanitarian Aid and Civil Protection
ITERATE

Author CRAAG
Mohamed Hamdache
Abdelkarim K. Yelles
Date 20 August 2017

Review IUSS Pavia
Iason Grigoratos
Ricardo Monteiro
Date 12 September 2017

ECHO/SUB/2016/740181/PREV23 – ITERATE – Improved Tools for Disaster Risk Mitigation in Algeria
Project co-funded by ECHO – Humanitarian Aid and Civil Protection
ITERATE

Table of Contents
1. Table of figures .............................................................................................................................. 1
2. Table of tables ................................................................................................................................ 1
3. Introduction ................................................................................................................................... 1
4. Tectonic overview ........................................................................................................................ 2
5. Seismic data .................................................................................................................................... 4
5.1. Earthquake catalogue ....................................................................................................................... 4
5.2. Focal Mechanisms .............................................................................................................................. 4
6. Preliminary considerations regarding the seismogenic source zonation ................ 7
7. Updated seismogenic source zonation ............................................................................... 10
8. Conclusions .................................................................................................................................. 11
9. References .................................................................................................................................... 12

ECHO/SUB/2016/740181/PREV23 – ITERATE – Improved Tools for Disaster Risk Mitigation in Algeria
Project co-funded by ECHO – Humanitarian Aid and Civil Protection
ITERATE

1. Table of figures
Figure 1. Global tectonic sketch for the studied region including seismicity. Only shallow
earthquakes above magnitude M 4.0 are depicted. .................................................................... 3
w
Figure 2. Shallow depth (≤30km) focal mechanism data compiled in the region. The Frohlich
(1992) diagram is showed, and the Zoback (1992) classification is used (TF: thust faulting; TS:
thrust to strike slip; SS: strike slip; NS: normal to strike slip; NF: normal faulting). Sphere size
is proportional to magnitude. Black: Shallow-depth zones. White: Intermediate-depth zone.
Orange: NUVEL-1A plates model boundary (DeMets et al., 1994). .......................................... 5
Figure 3. Lower-hemisphere equal-area stereoplots of the selected focal planes for the defined
sources in North Algeria, and associated slip lines. Results are represented by the orientation of
the three principal stress axes (a turquoise dot surrounded by a circle for σ , a triangle for σ ,
1 2
and a square for σ ). The related SH and SH orientations are represented by large arrows
3 min max
outside the stereogram. The histogram represents the distribution of the misfit angle weighted
arithmetically according to the magnitude. .................................................................................. 6
Figure 4. Plot of the SH and SH results for the shallow and intermediate seismogenic zones.
min max
.................................................................................................... Error! Bookmark not defined.
Figure 5. Preliminary seismogenic source zonation by Hamdache et al. (2017). ............................... 9
Figure 6. Updated seismogenic source zonation by Pelaez et al. (2017). Black: Shallow-depth
zones. White: Intermediate-depth zone. Orange: NUVEL-1A plates model boundary (DeMets
et al., 1994). Only shallow earthquakes above magnitude M 4.0 are depicted........................ 10
w

ECHO/SUB/2016/740181/PREV23 – ITERATE – Improved Tools for Disaster Risk Mitigation in Algeria
Project co-funded by ECHO – Humanitarian Aid and Civil Protection
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2. Table of tables
Table 1. Stress inversion results for the defined sources in North Algeria. eq: number of focal
mechanism data; n: number of data explained by the stress tensor; nt: total population of fault
plane solutions; σi: principal stress axes; Az: azimuth; Pl: plunge; R: stress ratio; α: average
misfit angle; αmax: maximum misfit angle; F5: misfit function of Delvaux and Barth (2010);
R': stress regime index; SHmin and SHmax: minimum and maximum horizontal stress axes,
respectively; Q: quality rank. ....................................................................................................... 5
Table 2. Number of events (n) above magnitude M 4.0 and maximum observed magnitude, in the
w
defined sources. .......................................................................................................................... 10

ECHO/SUB/2016/740181/PREV23 – ITERATE – Improved Tools for Disaster Risk Mitigation in Algeria
Project co-funded by ECHO – Humanitarian Aid and Civil Protection
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3. Introduction
Seismogenic (also called seismic) source zones are areas that share common seismological,
tectonic, and geologic attributes, and that can be described by a unique magnitude-frequency
relationship. In other words, they represent a region of the earth’s crust in which future seismicity is
assumed to follow specified probability distributions for occurrences in time, earthquake sizes, and
locations. This deliverable presents an updated seismogenic source zone model for North Algeria,
which has been delineated taking into account the tectonics, geomorphology and inversed focal
mechanisms of previous earthquakes in the region. Since earthquakes can affect sites more than 100km
away, an extended version of the Maghrebian region has been analysed.
Different authors proposed previously seismogenic zone models for to carry out different
researches in the studied area. For example, Hamdache (1998), Aoudia et al. (2000), Hamdache and
Retief (2001), and Pelaez et al. (2003) developed zone models aimed to seismic hazard studies in
northern Algeria. The zone models by Hamdache (1998) and Hamdache and Retief (2001) cover only
the Tell region, while the Aoudia et al. (2000) zone model cover strictly the Algerian territory, not
including the nearby Moroccan, Tunisian and offshore seismicity. The proposed model by Pelaez et
al. (2003) was used later for additional seismic hazard studies (Pelaez et al., 2005; 2006; Hamdache
et al., 2012). In addition to these zonations, mention must be made to that included in the different
Algerian building codes (RPA, 1999; 2003), which consider only the seismicity level.
Finally, we should mention few models developed for the entire Maghrebian region. They are
those proposed in the works by Jimenez et al. (1999), Mourabit et al. (2014) and more recently by
Hamdache et al. (2017) (Chapter 6) and Pelaez et al. (2017) (Chapter 7). All of them were meant to
be used in seismic hazard studies.

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4. Tectonic overview
The Morocco–Algeria region, also named the Maghrebian region, is situated at the north-west part
of the African Plate (the Nubian Plate), in, what is referred to, as its continental crust. Its oceanic
crust extends to the Azores Islands. The Eurasian Plate is situated immediately to the north, whereas
between the Gibraltar Arc and the south of Italy lies a domain of intermediate complexity. This
domain was formed by some oceanic basins, as were the Algero-Provençal and the Tyrrhenian basins,
and by a former region, presently disintegrated, and now forming the Betic–Rifean Internal Zone, the
Kabylias (in Algeria), the Peloritani Mountains (Sicily) and the Calabrian area in Italy (Figure 1).
This region is called the AlKaPeCa domain. From the early Miocene, this area has been subjected to
the northward subduction of Africa and the subsequent opening of the small oceanic basins mentioned
before, accompanied by the disintegration of the AlKaPeCa domain. Presently, the convergence
between the Nubian and Iberian plates trends in an approximately NNW–SSE direction, with values
of the order of 3 to 5cm per year. The compression is accompanied, at least in the area of the Gibraltar
Arc (in the Alboran Sea), by a noticeable ENE–WSW trending tension, which, in some cases, is even
more important than the compression. For this reason, some extensional movements in the Alboran
area could be deemed important. The Maghrebian region is a complex area in which the Saharan
Shield, affected by the Pan-African orogeny (Precambrian to early Cambrian), is in contact with the
Atlas Mountains of mainly Alpine age. The Moroccan Meseta is situated to the north of the Atlas
Mountains, and to the west are the High Plateaus in Algeria, which, to the north, are in contact with
the Rif and Tell Mountains, which are typical Alpine mountain chains. The Saharan Shield forms part
of the Precambrian areas of Africa, clearly cratonised and generally not affected by later major
deformations. In fact, in the Maghrebian area, the Saharan Shield corresponds to a clearly stable area.
In Morocco, the so-called Anti-Atlas mountain range corresponds to a Precambrian and, mainly,
Palaeozoic area, forming a tectonic transition between the Saharan Shield and the Atlas mountain
range. The Atlas Mountains correspond to an intracontinental chain. To the west, in Morocco, the
High Atlas mountain range reaches the coast in the Agadir area and continues to the northeast and
east, passing, although at lesser heights, to the Saharan Atlas range, which crosses Algeria and reaches
the central part of Tunisia. These units can be considered as aulacogens and border the north part of
the Saharan Shield. To the north, the Middle Atlas mountain range in Morocco has a different
direction, NE–SW, separating the Moroccan Meseta and the High Plateaus, both formed by
Palaeozoic rocks, although with a Mesozoic and Tertiary cover that is well developed in some areas.
Overall, the Atlasic Mountains have been tectonically unstable from the Triassic period, and, during
the Alpine orogeny, were subject to major deformations. More recently, during the Quaternary, the
Atlas Mountains were also subject to major volcanism.
The Rif and Tell mountains that thrust southward toward the Moroccan Meseta and the High
Plateaus, and even in some places part of the Atlas Mountains, are formed by sedimentary external
zones (only slightly affected by metamorphism in some Moroccan places) and by the Internal Zones.
Mostly, the Internal Zones (divided into several tectonic complexes) are affected by Alpine
metamorphism and the existence of previous Palaeozoic and even older deformations. Their present
structure was formed during the Alpine orogeny. These Internal Zones appear mainly to the east of
Tetouan, in Morocco, and in the Kabylia region, in Algeria. These Alpine chains were formed from
the Cretaceous period to the Oligocene–Early Miocene. Later, numerous Neogene basins were
formed, in many instances clearly cutting previous structures. During this time, particularly from the
late Miocene to the present, a near N–S compression induced strike-slip faults (NE–SW, sinistral, and
NW–SE, dextral), and reverse faults, many of which trend N 70ºE to an E–W direction. In many

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ECHO/SUB/2016/740181/PREV23 – ITERATE – Improved Tools for Disaster Risk Mitigation in Algeria
Project co-funded by ECHO – Humanitarian Aid and Civil Protection
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instances, the mentioned strike-slip faults moved mainly as normal faults, practically perpendicular
to the compression, and, in the process, released the regional tension.

Figure 1. Global tectonic sketch for the studied region including seismicity. Only shallow earthquakes above
magnitude M 4.0 are depicted.
w

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Project co-funded by ECHO – Humanitarian Aid and Civil Protection
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5. Seismic data
The seismic catalogue is very important when establishing a seismic zonation, since it illustrates
the spatial distribution of the seismicity. Patterns which can be associated with active fault structures
can be also identified. In addition, focal mechanism data are essential in order to study the
seismotectonics of a certain region, and accordingly, to characterise the delimited seismotectonic
zonation (Sawires et al., 2016). Each zone should be homogeneous, hence contain events
predominantly from one focal mechanism.
5.1. Earthquake catalogue
To carry out this study, two unified earthquake catalogues for north Morocco (Pelaez et al., 2007)
and North Algeria (Hamdache et al., 2010) have been used. The Moroccan catalogue covers the area
within 27° to 37°N and 15°W to 1°E, during the period 1045 to 2005. The Algerian catalogue covers
the area between 32° to 38°N and 3°W to 10°E, during the period 856 to 2008. The Algerian catalogue
contains events from Spanish National Geographic Institute (NGI) network, the Algerian CRAAG,
International Seismological Centre (ISC), National Earthquake Information Center (NEIC)
Preliminary Determination of Epicentres catalogue (USGS) and the European-Mediterranean
Seismological Centre (EMSC-CSEM) catalogue (Godey et al., 2009). Hamdache et al., (2017)
merged these catalogues and updated them to June 2011, while carefully removing duplicated events
in the overlapping areas as well as the non-crustal events (events with a depth below 30km), with a
different seismic behaviour. Figure 1 shows the spatial epicentral distribution map of all the events
included in this catalogue.
5.2. Focal Mechanisms
The stress pattern for the delineated shallow and intermediate seismic sources model has been
computed from the available focal mechanism data and by applying the inversion method developed
by Delvaux and Sperner (2003) and Delvaux and Barth (2010). In the final assessment have been
used only earthquakes above magnitude 3.5. Earthquakes below this magnitude usually correspond
to aftershocks that can display local instead of regional stresses. Details related to the used
methodology are explained in Pelaez et al. (2017).
This compiled focal mechanism database includes information on the following earthquake
parameters: date, time, latitude, longitude, depth, magnitude (M ), and strike, dip, and rake for the
w
two fault planes. The hypocentral information was standardised, based on the earthquake catalogue
of the Spanish IGN. In order to obtain a unified moment magnitude, when necessary, the relationships
by CNIG (2013) and Heaton et al. (1986) were used. Pelaez et al. (2017) used the criterion of Custodio
et al. (2016) to prioritise the information during the merging focal mechanism data. Figure 2, displays
the spatial distribution of focal mechanism, assigning to each style a color based on the so-called
Frohlich triangle diagram (Frohlich, 1992).

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Project co-funded by ECHO – Humanitarian Aid and Civil Protection
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Figure 2. Shallow depth (≤30km) focal mechanism data compiled in the region. The Frohlich (1992) diagram
is showed, and the Zoback (1992) classification is used (TF: thust faulting; TS: thrust to strike slip; SS: strike
slip; NS: normal to strike slip; NF: normal faulting). Sphere size is proportional to magnitude. Black: Shallow-
depth zones. White: Intermediate-depth zone. Orange: NUVEL-1A plates model boundary (DeMets et al.,
1994).
Table 1. Stress inversion results for the defined sources in North Algeria. eq: number of focal mechanism data;
𝑛: number of data explained by the stress tensor; 𝑛 : total population of fault plane solutions; 𝜎: principal
1 3
stress axes; Az: azimuth; Pl: plunge; 𝑅: stress ratio; 𝛼: average misfit angle; 𝛼 : maximum misfit angle;
678
𝐹5: misfit function of Delvaux and Barth (2010); 𝑅′: stress regime index; 𝑆𝐻 and 𝑆𝐻 : minimum and
63= 678
maximum horizontal stress axes, respectively; 𝑄: quality rank.
𝜎 𝜎 𝜎
sou e 𝑛/ ? @ A stress
rce q 𝑛 A P A P A P 𝑅 𝛼 𝛼678 𝐹5 𝑅′ 𝑆𝐻63= 𝑆𝐻678 𝑄 regime
1 z l z l z l
T1 1 16 3 1 2 3 5 0 2 1 TF-Pure
52 A
8 / 36 20 4 22 0 72 6 .25 9.1 31.4 5.0 .25 42 comp
T2 8 76 3 1 8 0 2 1 TF-Pure
55 A
4 /168 25 5 55 2 68 5 .33 9.1 33.5 4.9 .33 45 comp
T3 3 28 3 2 1 6 0 2 1 TF-Pure
55 A
3 / 66 29 1 61 4 61 9 .62 7.1 26.5 4.9 .62 45 comp
T4 6 53 3 2 1 1 6 0 1 2 1 TF-Pure
50 A
3 /126 25 0 59 0 74 7 .54 0.4 25.5 6.3 .54 40 comp
T5 3 1 1 7 0 1 1 SS-Compl
86 C
5 5/ 10 56 6 91 3 87 4 .13 2.9 4.7 0.4 .87 76 strike-slip

T1 T2 T3

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ECHO/SUB/2016/740181/PREV23 – ITERATE – Improved Tools for Disaster Risk Mitigation in Algeria
Project co-funded by ECHO – Humanitarian Aid and Civil Protection