DB1-Library of Algerian hazard model characteristics

29 Pages·2017·6 MB·English

by  Ricardo Monteiro

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Improved Tools for
Disaster Risk Mitigation in Algeria

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Deliverable B.1

Library of Algerian hazard model characteristics

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

Review IUSS Pavia
Iason Grigoratos
Ricardo Monteiro
Date 8 May 2017
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Table of Contents
1. Table of figures .............................................................................................................................. 4
2. Table of tables ................................................................................................................................ 5
3. Introduction ................................................................................................................................... 1
4. Scope ................................................................................................................................................. 2
5. Tectonic overview ........................................................................................................................ 3
6. Declustering and completeness of the whole catalogue ................................................. 5
7. Spatially smoothed seismicity forecasting model for M ≥ 5.0 earthquakes in
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northern Algeria ........................................................................................................................................ 8
8. Site-specific seismic hazard results for the building code .......................................... 10
8.1. Methodology ..................................................................................................................................... 10
8.2. Uniform hazard spectra and design spectra .......................................................................... 13
8.3. Relationships between different intensity measures ......................................................... 16
9. Conclusions, identified gaps and future perspectives .................................................. 20
10. References ............................................................................................................................... 22
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1. Table of figures
Figure 1. Seismotectonic map for the Maghreb region (Morocco–Algeria–Tunisia). Different physio-
geographic tectonic domains are delineated, including their corresponding rupturing mode,
represented by the focal mechanisms of recent earthquakes (modified from Beauchamp, 1998).
...................................................................................................................................................... 4
Figure 2. Seismicity map, including a tectonic sketch for the main tectonic features and domains. .. 6
Figure 3. Cumulative number of earthquakes above magnitudes M 4.0, 5.0 and 6.0, obtained from
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the whole catalogue. ..................................................................................................................... 7
Figure 4. Smoothed number of events in the range M 4.0–4.9 since 1925. Single events with
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magnitude above M 5.0 in the used catalogue are also displayed. ............................................ 8
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Figure 5. Probability of exceedance per cell for earthquakes with magnitude above M 5.0 in the next
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10 years. Single events with magnitude above M 5.0 located in the last 10 years (2003– 2013)
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are also displayed. ........................................................................................................................ 9
Figure 6. Probability of exceedance per cell for earthquakes with magnitude above M 6.0 in the next
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10 years. Single events with magnitude above M 6.0 located in the last 10 years (2003– 2013)
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are also displayed. ........................................................................................................................ 9
Figure 7. Predicted values using Ambraseys et al., (1996) attenuation relationship for rock and
damped at 5%: (left) acceleration vs distance for M 7.0 earthquake and different periods; (right)
s
acceleration vs distance for a period of 0.5s and different magnitudes. .................................... 11
Figure 8. Seismic hazard map by Pelaez et al. (2006) with 10% probability of exceedance in 50 years
(i.e. 475 return period): (a) PGA at rock, (b) SA(0.2s) at rock, (c) SA(1.0s) at rock. ............... 12
Figure 9. Uniform Hazard Spectra by Hamdache et al. (2012) for different soil types damped at 5%
with 39.3 % probability of exceedance in 50 years (return period of 100 years), 10% probability
of exceedance in 50 years (return period of 475 years). ............................................................ 14
Figure 10. Uniform Hazard Spectra at rock versus period, damped at 5%: for a return period of 100
and 475 years. The graphs also show the plot of the proposed design spectra and the EC8
response spectra of type I for the two return periods (Hamdache et al., 2012). ........................ 15
Figure 11. Linear relation between PGA and PGA , and SA and SA . The plots show
475 100 max475 max100
the results for rock, stiff and soft soil (Hamdache et al., 2012). ................................................ 17
Figure 12. Graphs by Hamdache et al. (2012) displaying the relation between SA , SA(1.0s) and
max
SA(0.2s) vs PGA. Return period is 100 years (left) and 475 years (right). ............................... 18
Figure 13. General marine and onland tectonic framework of the Algiers region. The arrow with the
circle represents the direction of convergence from Africa towards Europe (Nocquet and Calais,
2004). L= length of the fault, s= horizontal shortening rate of the fault from bibliography (onland
and coastal faults) or from seismic profiles. KDF: Khayr al Din fault MAF= Mahelma fault.
ThF: Thenia fault. BF: Blida fault. SF: Sahel fault. ABF: Ain Benian fault. CF: Chenoua fault.
.................................................................................................................................................... 21
Figure 14. Tentative 3D view of the marine and terrestrial Digital Elevation Models showing a
possible interpretation of the tectonic framework and fault geometry at depth in the
offshore/onshore Algiers region. The black dashed line (suture zone) is assumed to be mostly
inactive. KADB: Khayr al Din bank, KADF: Khayr al Din fault, Th.F.: Thenia fault, A.M.:
Algiers Massif, S.A.: Sahel Anticline, S.F.: Sahel fault, C.M.: Chenoua Mount, C.F.: Chenoua
fault, B.F.: Blida fault, IZ: Internal Zones, EZ: External Zones. ............................................... 22
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2. Table of tables
Table 1. Past probabilistic seismic hazard assessment studies in Northern Algeria. ........................... 2
Table 2. Characteristic values at each site for a return period of 100 years and for rock, soft and stiff
soil. ............................................................................................................................................. 16
Table 3. Characteristic values at each site for return period of 475 years and for rock, soft and stiff
soil (Hamdache et al., 2012). ..................................................................................................... 19
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3. Introduction
This deliverable outlines key past studies related to the probabilistic seismic hazard assessment in
Northern Algeria, as well as the current state-of-knowledge regarding the seismicity in the region
(Hamdache et al., 2010; Pelaez et al., 2016; Hamdache et al., 2017). It also identifies gaps and possible
improvements that ITERATE should focus on. The analysis led to specific considerations that should
be taken into account in the next update of the Algerian building code. The current code was updated
in 2003 (RPA 99; 2003) and includes seismic loads. However, the input regarding seismic hazard is
based on expert opinion and not on specific scientific studies.
The first part of this deliverable is devoted to a tectonic overview of the studied region. Then the
latest earthquake catalogue for the region is presented. This catalogue was the basis for the
development of a spatially smoothed seismicity forecasting model for M ≥ 5.0 earthquakes in
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northern Algeria.
For site-specific seismic hazard, results are provided for 33 sites (Pelaez et al., 2006) in terms of
spectral acceleration (SA) for three different type of soils in the Algerian building code (rock, soft and
stiff) and for return periods of 100 and 475 years is compiled (5% damping). Uniform hazard spectra
(UHS) have been obtained for different return periods at different locations in northern Algeria, from
which UHS for different soil types and for return period of 100 and 475 years are proposed as design
spectra. The well-known Newmark-Hall approach is used with certain modifications; as proposed in
the most recent International Building Code (IBC, 2006).

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ECHO/SUB/2016/740181/PREV23 – ITERATE – Improved Tools for Disaster Risk Mitigation in
Algeria
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4. Scope
The Algerian Centre de Recherche en Astronomie Astrophysique et Géophysique (CRAAG) and
other institutions have developed several hazard models for Algeria in the past (Table 1). This
deliverable will present the results only of the latest one, i.e. Pelaez et al. 2006, in Chapter 8. It should
be noted that the study by Pelaez et al. (2006) has not benefited from the latest earthquake catalogues
for the region (Pelaez et al., 2007; Hamdache et al., 2010). These updated earthquake catalogues have
been analysed by Hamdache et al. (2017) and Pelaez et al. (2016), only in terms of magnitude-
frequency distributions and not in terms of intensity measure estimates (e.g. hazard maps or UHS).
Table 1. Past probabilistic seismic hazard assessment studies in Northern Algeria.
Intensity
Reference Return period (years) Notes
measure
Smoothed
Pelaez et al. (2006) SA(0-2s) 100, 475
seismicity
Smoothed
Pelaez et al. (2005) PGA 475
seismicity
Smoothed
Montilla et al. (2003) PGA 100, 475
seismicity
Naili and Benouar (2000) PGA 475 Only for Algiers
Jimenez et al. (1999) PGA 475 10 zones, GSHAP
Hamdache (1998) PGA 100, 200, 400 6 zones, 6 sites
Benouar et al. (1996) PGA, MSK 950 -
Benouar (1996) PGA 950 Only for Algiers

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5. Tectonic overview
The Morocco–Algeria region, also named the Maghrebian region (Figure 1), is situated at the
north-western 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. 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 5 cm 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 (Figure 1). 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 northern 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 N70ºE to an E–W direction. In many

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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. Seismotectonic map for the Maghreb region (Morocco–Algeria–Tunisia). Different physio-
geographic tectonic domains are delineated, including their corresponding rupturing mode, represented by the
focal mechanisms of recent earthquakes (modified from Beauchamp, 1998).

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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
ITERATE

6. Declustering and completeness of the whole catalogue
The latest available earthquake data file was obtained by merging two earthquake catalogues
compiled specifically for northern Morocco (Pelaez et al., 2007) and northern Algeria (Hamdache et
al., 2010). 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
catalog (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 30 km), with a different seismic behaviour. Figure 2 shows the
spatial epicentral distribution map of all the events included in this working data file.
After preparing the earthquake data file, Hamdache et al. (2017) proceeded to identify and delete
all non-Poissonian (clustered) events using the Gardner and Knopoff (1974) procedure, with space-
time windows adapted to the study region. Window sizes of 900 days and 100km were used for a
given M 8.0 event, and 10 days and 20km for a M 3.0 event. For in-between magnitudes,
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proportional values for distance and time were used. As directed by this method, the largest event
within a particular space-time window is considered the main shock.
The analysis of the magnitude completeness of the whole catalogue was performed using the
cumulative number of events with magnitude above a specified threshold. Figure 3 shows the
cumulative number of earthquakes above M 4.0, M 5.0 and M 6.0. Hamdache et al. (2017)
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considered complete above magnitudes M 5.0, starting from 1900, with a mean rate of 2.15
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events/year, and above magnitude M 6.0 since 1885, with a mean rate of 0.21 events/year.
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Earthquakes above M 4.0 are considered complete only since 2003, with a mean rate of 29.82 events
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per year. In the preceding period 1925–2003, the mean rate is only 7.71 events/year (Figure 3). It is
important to note that because the study region was not well covered by the Moroccan and Algerian
seismological networks, many of the earthquakes included in the Algerian and Moroccan catalogues
were in fact detected by the Spanish National Geographic Institute network (NGI), distant from the
southern and eastern parts of the study region (Peláez et al., 2007; Hamdache et al., 2010; Hamdache
et al., 2012). This explains why the earthquake data file only becomes complete above M 4.0 from
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the year 2003.

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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