Bauhaus University –

Weimar Earthquake Catalogues and

Seismicity models of Eritrea

Project Azure-II

Seismicity Map of Eritrea

Instructors: - Dr. G. Grünthal and Dr. Ing. J. Schwarz

Prepared by: - Simon Kahsay Hagos

Mt. Nr. 114899

July 03 2015

- 2 -

Contents

1. Introduction 2. Identification of hazard input parameters

2.1. Magnitude and intensities of historical Earthquakes (and exiting correlations)

2.2. Recent fault lines 2.3. Type of source mechanism (tectonic regime)

Source depths

3. Elaboration of basic data for seismicity models

3.1. Areal and fault line elements 3.2. Seismic zones

3.3. Location of major cities within active areas( selection of reference Sies)

4. Application of tools(provided by the lectures) to the

selected study area 4.1. Determination of completeness intervals for

relevant magmitude ranges 4.2. Assign completeness to the earthquake catalogue

4.3. Determination of seismic activity parameters by Gutenberg-Richter fit

4.4. Compute and plot annual rates versus magnitude 4.5. a and b values for non-cumulative and cumulative

rates by least square fit

- 3 -

1. Introduction

Eritrea is located in North East Africa (between 12° and 18° north, and 36°

and 44° east) and includes the Dahlak Archipelago and other islands along

the Red Sea coast. It is bordered by Sudan to the north and west, Ethiopia to

the south, Djibouti to the south-east and the Red Sea to the north and

north-east. With a land area of about 125.000 km2, Eritrea is about the size

of England, or the state of Pennsylvania in the USA. The coastline measures

around 1.200 km and off it there are over 350 islands, of which 210

comprise the area of the Dahlak Archipelago

Figure 1 - Map of Eritrea

2. Identification of hazard input papameters

The country is virtually bisected by one of the world's longest mountain

ranges, formed by the processes that formed the Great Rift Valley, with

fertile lands to the west and descending to a desert in the east. Eritrea, being

located at the southern end of the Red Sea, is the home of the fork in the rift.

- 4 -

The Afar Triangle or Danakil Depression of Eritrea is the probable location of

a triple junction where three tectonic plates are pulling away from one

another: - the Arabian Plate and the two parts of the African Plate (the

Nubian and the Somali plate) which are splitting along the East African Rift

Zone (USGS). The highest point of the country, Emba Soira, is located in the

centre of Eritrea at 3018 meters (9902 ft) above sea level.

Figure 2 - The Afar Triangle and the Great East African Rift Valley

2.1. Magnitude and intensities of historical Earthquakes (and exiting correlations)

Search for catalogues

Although local catalogues with good quality are preferable. It was not

possible to find one. Therefore, the ANSS (Advanced National Seismic

System) catalogue was used.

2.2. ANSS (Advanced National Seismic System) Catalogue

The ANSS (Advanced National Seismic System) composite catalogue is a

world-wide earthquake catalogue that is created by merging the master

- 5 -

earthquake catalogues from contributing ANSS institutions and then

removing duplicate solutions for the same event. The ANSS earthquake

catalog grew out of the efforts of the CNSS (Council of the National Seismic

System). It was previously called the CNSS earthquake catalog.

The ANSS catalog currently consists of earthquake hypocenters, orgin times,

and magnitudes. It is in plan to add phase and amplitude readings, as well

as first-motion mechanisms and moment tensors in the future.

The problem faced was that the Earthquake catalogue for the region Eritrea

is located starts only from 1961.

The search criteria used were:-

Catalogue = ANSS (Advanced National Seismic System)

Start time = 1961/01/01, 00:00:00

End time = 2010/06/28, 00:00:41

Minimum latitude = 12°N

Maximum latitude = 18°N

Minimum longitude = 36°E

Maximum longitude = 44°E

Minimum magnitude = 2

Maximum magnitude = 9

Minimum depth = 0 km

Maximum depth = 1000 Km

Some samples of the data from the Catalogue for 389 Events obtained from

ANSS for the search criteria above.

Year

Month

Day Time Lat.

Long.

Depth(Km)

Mag.

Magt. Nst SRC Event ID

1962 11 11 15:15:33.

60 17.2 40.7 34 4.6 Ms 24 NEI 1.96211E+1

1

1965 7 19 15:49:35.

90 12.1 42.6 33 4.3 Ms 5 NEI 1.96507E+1

1

1966 1 21 12:39:42.

50 12.1 43.8 33 4.7 Mb 7 NEI 1.96601E+1

1

1966 4 9 19:11:11.

20 14.5 40.6 33 4.7 Mb 9 NEI 1.96604E+1

1

1967 5 19 15:52:33.

20 14.566

40.167 2 5 Mb 51 NEI

1.96705E+11

1967 9 18 02:02:59. 15.7 39 33 4.8 Mb 14 NEI 1.96709E+1

- 6 -

80 1

1967 9 21 18:36:26.

10 17.9 40 16 4.4 Mb 7 NEI 1.96709E+1

1

1967 11 16 02:22:03.

10 15.1 39.8 33 5.1 Mb 7 NEI 1.96711E+1

1

1968 5 23 23:36:06.

40 14.747

40.217 33 4.8 Mb 13 NEI

1.96805E+11

1969 3 29 18:30:42.

20 12.001

41.376 33 4.6 Mb 8 NEI

1.96903E+11

1969 4 5 02:18:29.

90 12.153

41.198 17 6.2 Mb 46 NEI

1.96904E+11

1969 4 5 20:14:35.

80 12.025

41.473 33 5.2 Ms 19 NEI

1.96904E+11

1969 4 6 16:51:45.

50 12.028

41.12 20 5.4 Ms 39 NEI

1.96904E+11

1969 9 26 04:54:35.

70 16.428

40.983 25 5.3 Ms 42 NEI

1.96909E+11

1971 4 25 17:42:16.

50 12.0

2 43.584 33 3.9 Mb 9 NEI

1.97104E+11

Table 1 - Sample of the data from the Catalogue for 389 Events obtained

from ANSS for the search criteria above.

Fig.3 Historical Earthquake in Eritrea by Magnitude (ANSS Catalouge)

Since regional relations for conversion of Mb (Body Magnitude) and Ms

(Surface Magnitude) in to Mw (Moment Magnitude) is not available. The

global relations provided by (Suckale and Grünthal, BSSA 99/4, 2108-2126,

2009) for USGS catalogues was used, accordingly:-

- 7 -

Mw = 1,2690Ms – 1.0436 for USGS catalogue

Mw = 0.7813Mb + 1.5175 for USGS catalogue

Where;

Mw = Moment Magnitude

Mb = Body Magnitude

Ms = Surface Magnitude

(Suckale and Grünthal, BSSA 99/4, 2108-2126, 2009)

2.3. Recent Fault lines

Physio graphically Eritrea can be conveniently subdivided in to four entities:

- the Plateau, the rift margins (Escarpment), the Afar rift, and the Red sea

rift. In each of these Physiographical subdivisions there are well developed

tectonic structures which are manifestations of the tectonic phenomena

which started at the end of the Mesozoic or Early Cenozoic era (65 million

years ago) and still continue up to this time.

Fig. 4 recent fault lines in Eritrea

2.4. Recent Fault lines Tectonic movements are the major causes of earthquakes in Eritrea. Eritrea is located in the vicinity of three well recognized seismically active tectonic

- 8 -

features, namely: - the major tectonic plate boundary which divides

continental Africa from the Arabian peninsula (the Red sea), the major break in the earth’s crust due to the East African rift systems and the Gulf of Aden.

As a result Eritrea, along with countries in the Red sea and East African rift region, has experienced the effects of earthquakes through out their histories

a. The Plateau:-

The Eritrean plateau bears signs of structures of structures developed both

during the late Precambrian and in the Cenozoic. Some of the major linear

structures of Precambrian age are the Adobha strike-slip shear zone and the

Barka lineament. Moreover, rocks of the plateau are affected by strong

Cenozoic faulting trending parallel to the Red sea. That is NW-SE. Examples

are the Afabet-Himbol faults and Anseba faults.

b. The Rift Margin:-

Consists of the strip between the escarpment of the high plateau and the

Red sea margin in the northern sector of Eritrea, and the area between the

escarpment and the flat rift floor of the Afar depression. It is characterised

by a steep faulting and warping producing a total displacement of about

2000 metres.

The western margin of the rift consists of The Sabarguma plains in the

centre and the salt plains of Badda in the south. West of the Gulf of Zula,

there are at least four major transverse dislocations of the Afar rift-plateau

boundary. These faults have NW-SE trend as a result of which the margin of

the Eritrean plateau is displaced east ward going south (Mohr, 1967). The

dislocations occur along:-

The Ghindae-Irafaile fault, up throw SW.

A similar and parallel fault about 15-20kms further north.

Dekemhare – Dandero fault.

The ENE-WSE trending

c. The Afar Rift (Depression):-

The Afar rift is roughly 800km sided triangular depressed region consisting

of the southern portion of Eritrea, NW part of Djibouti, and NE region of

Ethiopia. It is bounded to the west by the scarp of the Eritrean and

Ethiopian plateaux, to the south by the scarp of Somali plateau and to the

north-east by Denkalia horst.

- 9 -

Fig.5 Simplified tectonic map

d. The Red Sea Rift:-

The Red sea is one of the best examples where a divergent rift system cuts

through continents (Africa and Arabia). This has lead to the formation of the

new oceanic crust by sea floor spreading processes. The margins of the Red

sea are normal faults, and the floor of the Red sea is a trough bounded by

these faults. the actual divergent plate boundary along the central axis of the

Red sea is marked by a rift valley.

- 10 -

Figure 6. Outline of important earthquake-causative tectonic features in

Eritrea and the horn of Africa (After Gouin, 1976)

Most of the earthquake with magnitude about 6 and above occurred in the

western Margin of the Afar Depression. The seismic activity that occurs

along the deep axial trough of the Red sea however clearly suggests that a

sea floor spreading process also plays a significant role for many of the

earthquakes that occur in Eritrea.

The frequent occurrence of earthquakes in Massawa channel suggests that

the northern part of Massawa channel might be a seismic line of crustal

weakness.

Between 19.5°N and 21.0°n (North of the Eritrean Red Sea) there is a

concentration of epicentres and some of these might be associated with an

active transform fault. A similarly trending transform fault also occurs

within the Eritrean segment of the Red sea, connecting northern Afar to the

axial trough of the Red sea (Fekadu Kebede and kulhanek, 1991). An

earthquake of magnitude 6.6 (focal depth 7km) has occurred along this

transform fault on Dec. 28, 1977 (Fekadu Kebede and kulhanek, 1991).

As described above, most of the large earthquakes are associated with the

rift system. It should however be borne in mind that some of the largest ones

are also located well away from the regions of faulting indicating caution is

needed when assessing the seismic risk for a given region.

- 11 -

3. Elaboration of basic data for seismicity models

3.1. Seismic Zoning Map of Eritrea related to Intensity

Eritrea is divided in to Four Seismic Zones based on the corresponding Peak

ground acceleration and Intensity of the areas. Zone 1 = Zone of minor damage. This corresponds to intensities of V, and acceleration up to 0.04g.

Zone 2 = Zone of moderate damage having intensity of VI. The corresponding acceleration is 0.06g.

Zone 3 = Zone of moderate damage having intensity of VII. The corresponding acceleration is 0.08g.

Zone 4 = Zone of major damage where intensity is VIII or more, and

acceleration is 0.1g or greater.

Fig. 7 Seismic Zoning Map of Eritrea

- 12 -

Fig. 8 Peak Ground Acceleration Map of Eritrea(GSHAP)

3.2. Location of major cities within active areas (Selection of Reference

Sites

Fig. 9 National code PGA of Eritrea and Location of major cities in active area

- 13 -

4. Application of tools(provided by the lectures) to the

selected study area

4.1. Determination of completeness intervals for relevant magnitude

ranges and Elaboration of representative database

The catalogue was declustered by using the software “Catreg” written by

Ch. Bosse and Prepared by G. Grünthal, Ch. Bosse and R. Wahlström (GFZ

Potsdam) for the International Training Course on Seismology, Hazard

Assessment and Risk Mitigation.

The declustering process involves the removal of the dependent events

(foreshocks, aftershocks and events in an earthquake swarm). The presence

of dependent events leads to too many small events with respect to the large

ones in an earthquake catalogue, i.e. the b value becomes too high.

Therefore the earthquake catalogue should be cleaned before deriving a and

b values.

4.2. Assign completeness to the earthquake catalogue

Figure 10 - Completeness Graph for Earthquakes in Eritrea from 1965 to

2014

- 14 -

MW 3.5 4 4.5 5 5.5 6 6.5 7 7.5

Complete From 1999 2006 2002 1993 2003 1967 1969 1968 1967

Table 2 – Starting year of completeness for each range of MW

4.3. Determination of seismic activity parameters by Gutenberg-

Richter fit

In probabilistic seismic hazard assessment, the Gutenberg-Richter relation

is used to describe the Seismicity of the study area under consideration, i.e.

the values of a and b are derived empirically by regression on catalogue data.

But in order to have reliable values of a and b it is important to consider the

earthquake catalogue data within times it is complete.

4.4. Compute and plot annual rates versus magnitude

MW 3.5 4 4.5 5 5.5 6 6.5 7 7.5

Compl. From 1999 2006 2002 1993 2003 1967 1969 1968 1967

Compl. Time 12 5 9 18 8 44 42 43 44

MW start 3.25 3.75 4.25 4.75 5.25 5.75 6.25 6.75 7.25

Count 1 9 10 16 2 0 2 0 0

Rate 0.083333 1.8 1.111111 0.888889 0.25 0 0.047619 0 0

Cum. Rate 4.180952 4.097619 2.297619 1.186508 0.297619 0.047619 0.047619 0 0

Log(rate) -1.07918 0.255273 0.045757 -0.05115 -0.60206 0 -1.32222

Log(cum.

Rate) 0.621275 0.612532 0.361278 0.074271 -0.52634 -1.32222 -1.32222

Fit 3.794733 2.052535 1.110196 0.600494 0.324802 0.175682 0.095025 0.051

398 0.027801

Cum. Fit 8.487549 3.510124 1.451653 0.600348 0.248281 0.102679 0.042464 0.017

562 0.007263

Table 3 – Computations for the plot below

- 15 -

Figure 11 - Plot of Annual rate, Cum. Rate, Fit, Cum. Fit versus Magnitude

4.5. a and b values for non-cumulative and cumulative rates by

least square fit

b a

Rate -0.47176

1.983273

Cum. Rate -0.71776

3.247635

Table 4 – a and b values for non-cumulative and cumulative rates

Therefore, the Gutenberg-Richter relation that can be used to characterize

the Seismicity of the region Eritrea is located, i.e. to describe annual number

of earthquakes as a function of magnitude is:-

(a) Non-cumulative Gutenberg-Richter relation

Log N(M) = a – bM

(b) Cumulative Gutenberg-Richter relation

Log N*(M) = a* – b*M

- 16 -

Where,

a = describes seismic activity = log N (M = 0 ± ½ ∆M )

b or b* = indicates the proportion of large to small earthquakes and

characterizes the seismic

regime of a certain region

a* = logΣN(Mj) (0 < j < Mmax) or

a* = logN*(M ≥ (0 - ½ ∆M)

N* = Cumulative frequency, i.e. the number of Earthquakes with M - ½ ∆M

But for N = 3500 and larger, the parameters of the cumulative and non-

cumulative relations are not substantially different.

- 17 -

References:-

Software “Catreg” written by Ch. Bosse and Prepared by G. Grünthal,

Ch. Bosse and R. Wahlström (GFZ Potsdam) for the International Training

Course on Seismology, Hazard Assessment and Risk Mitigation.

Suckale and Grünthal, BSSA 99/4, 2108-2126, 2009

For the Earthquake catalogue:-

Advanced National Seismic System (ANSS)

http://earthquake.usgs.gov/monitoring/anss/

Northern California Seismic Network, U.S. Geological Survey, Menlo Park

Berkeley Seismological Laboratory, University of California, Berkeley