Accepted Manuscript A recent Mw 4.3 earthquake proving activity of a shallow strike-slip fault in the northern part of the Western Desert, Egypt
Mohamed Ezzelarab, Mohamed O. Ebraheem, Jiří Zahradník PII:
S1464-343X(17)30495-8
DOI:
10.1016/j.jafrearsci.2017.12.021
Reference:
AES 3102
To appear in:
Journal of African Earth Sciences
Received Date:
20 October 2017
Revised Date:
17 December 2017
Accepted Date:
18 December 2017
Please cite this article as: Mohamed Ezzelarab, Mohamed O. Ebraheem, Jiří Zahradník, A recent Mw 4.3 earthquake proving activity of a shallow strike-slip fault in the northern part of the Western Desert, Egypt, Journal of African Earth Sciences (2017), doi: 10.1016/j.jafrearsci.2017.12.021
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
1
A recent Mw 4.3 earthquake proving activity of a shallow strike-slip
2
fault in the northern part of the Western Desert, Egypt Mohamed Ezzelarab1,2, Mohamed O., Ebraheem3 and Jiří Zahradník 4
3 4
1. Sultan Qaboos University, Earthquake Monitoring center, 50 Alkouhd , Postal code 123,
5
Muscat, Sultanate of Oman.
6
2. National Research Institute of Astronomy and Geophysics, Seismology Department,
7
Helwan, Cairo, Egypt.
8
3. Assiut University, New Valley Branch, Faculty of Science, Geology Department, Assiut,
9
Egypt
10
4. Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic.
11
Abstract:
12
The Mw 4.3 earthquake of September 2015 is the first felt earthquake since 1900 A.D
13
in the northern part of the Western Desert, Egypt, south of the El-Alamein City. The available
14
waveform data observed at epicentral distances 52–391 km was collected and carefully
15
evaluated. Nine broad-band stations were selected to invert full waveforms for the centroid
16
position (horizontal and vertical) and for the focal mechanism solution. The first-arrival travel
17
times, polarities and low-frequency full waveforms (0.03-0.08 Hz) are consistently explained
18
in this paper as caused by a shallow source of the strike-slip mechanism. This finding indicates
19
causal relation of this earthquake to the W-E trending South El-Alamein fault, which developed
20
in Late Cretaceous as dextral strike slip fault. Recent activity of this fault, proven by the studied
21
rare earthquake, is of fundamental importance for future seismic hazard evaluations, underlined 1
ACCEPTED MANUSCRIPT 1
by proximity (~65 km) of the source zone to the first nuclear power plant planned site in Egypt.
2
Safe exploration and possible future exploitation of hydrocarbon reserves, reported around El-
3
Alamein fault in the last decade, cannot be made without considering the seismic potential of
4
this fault.
5 6
Key words: South El-Alamein Fault; waveform inversion; source mechanismposition and time grid search; Jackknife test.
7
8
1. Introduction
9
On September 3rd, 2015, at dawn (local time 03:44, i.e. UTC time 01:44) an earthquake
10
of 4.5 mb occurred in northwestern Egypt. The epicenter of the quake was approximately 164
11
km southwest of the Egyptian coastal city of Alexandria (European Mediterranean
12
Seismological Centre [EMSC], (Fig. 1). The event was felt in Alexandria and Cairo as well.
13
The recent earthquake catalogue and seismicity maps compiled since 1900 AD (Mohamed et
14
al. 2012; Ezzelarab et al. 2016 and Sawries et al. 2016) indicate that this event is the first felt
15
earthquake which took place at the South El-Alamein fault region, thus calling attention for
16
considering this fault in seismic hazard assessment of the surrounding area. Particularly, if we
17
take into consideration that this zone is situated close (~65 km) to the area which was propsed
18
for the first nuclear power plant in Egypt. Additionally, from economical point of view,
19
numerous hydrocarbon discoveries have been reported around El-Alamein fault in the last
20
decade. It is well-known that faults in the Western Desert play a significant role in the trap of
21
hydrocarbon and its migration pathway compartmentalization (Moretti et al. 2010 and Abdel-
22
Fattah et al. 2014).
2
ACCEPTED MANUSCRIPT 1
Hypocenter of the current event was reported by several seismic agencies (Fig.1 &
2
Table 1) with significant uncertainty, which might negatively affect reliability of the source
3
mechanism determination (Šílený et al. 1996). Therefore, the aim of the current work to present
4
precise analysis of the source position and focal mechanism using waveform inversion
5
technique. Waveform inversion technique is adopted because the size of studied event is
6
relatively small and epicentral distances of the stations is large which increase the possibility
7
of distortion the first-arrive and accordingly the failure of the first arrive methods for retrieving
8
the source mechanism (Šílený et al. 1996, Benetatos et al. 2013 and Fojtíková and Zahradník
9
,2014). Also, waveform modeling is particularly useful in constraining earthquake depths (e.g.
10
Zhu and Helmberger 1996).
11
In the frame of ISOLA code (Zahradnik et al. 2008. Sokos and Zahradnik, 2008,
12
Fojtíková et al. 2010; Sokos et al. 2012, Sokos and Zahradnik , 2013) the waveform inversion
13
is preformed and the position of source focal mechanism is horizontally and vertically grid
14
searched. The stability of the resulted source mechanism is tested using Jackknife test.
15
Waveforms data of the Egyptian National Seismic Network (ENSN) and European Integrated
16
Data Archive (EIDA) were used.
17
2. Tectonic setting and seismicity
18
Generally, tectonics of the northern part of the Western Desert is structurally controlled
19
by relative motions between the African, Arabian and Eurasian Plates (Fig.2). In particular,
20
the separation of North Africa and Arabian Plate during the Jurassic age formed several E-W
21
trending rift basins (Dolson et al. 2001, Bosworth et al., 2008). The dextral convergence
22
between African and Eurasian Plates during the Late Cretaceous was associated with
23
compressional pluses system that affected the entire African Plate (Bosworth et al., 2008).
3
ACCEPTED MANUSCRIPT 1
Based on geophysical studies (Hantar, 1990), the normal faults in the northern part of
2
the Western Desert were characterized by strike-slip movements during part of their history.
3
These strike-slip movements were caused by lateral movements that the African plate
4
underwent during Jurassic (sinistral strike slip) and Late Cretaceous (dextral strike slip). The
5
analysis of Landsat images (Elshazly et al. 1976) mapped several trends of fractures (NW-SE
6
and NNW-SSE, ESE-WNW and ENE-WSW, NNE-SSW and NE-SW). They declared that the
7
NW-SE set is characterized by right-lateral strike slip fault.
8
The recent instrumental seismicity (Fig.2) shows that seismic activity concentrates
9
along specific trends; along the Hellenic and Cyprian Arcs, Northern Red Sea rift, Gulf of
10
Aqaba and a few inland earthquakes. There is no previous seismic activity in south El-Alamein
11
area and the studied event is the first recorded one.
12
3. Data
13
In this study, a data set of 31 seismic stations of the Egyptian National Seismic Network
14
was investigated. Additionally, data of 2 stations were downloaded from European Integrated
15
Data Archive (EIDA) that is initiated within ORFOUS (http://www.orfeus-eu.org/data/eida/).
16
However, only 9 stations with tri-axial long-period seismometers (i.e., STS-2 and Trilum-240
17
seismometers) could be selected to retrieve the moment tensor; their epicentral distance being
18
152–391 km (Fig.3). The selection of these stations was based on checking quality of the
19
waveform data (to be free from long-period disturbances; Zahradnik and Plesinger, 2010) and
20
evaluating the signal-to-noise ratio (SNR) based on amplitude spectra. The intention was to
21
employ relatively low frequencies, less affected by uncertainties of the velocity model, and to
22
work at epicentral distance not much greater than 10 shear wavelengths. Accordingly, the
23
proper frequency band for inversion was selected at 0.03 to 0.08 Hz. The 1-D velocity model
24
of Makris et al. (1979), based on deep seismic profiling was used in this study (Table 2). 4
ACCEPTED MANUSCRIPT 1
4. Methods
2 3
4.1 Source centroid location
4
minimizing the difference between the observed and synthetic seismograms based on the least-
5
squares method. Position and time of the point source (centroid) are grid searched in vicinity
6
of the hypocenter.
Inversion of full waveforms by ISOLA code enables seismic source to be modelled by
7
The earthquake epicenter reported by seismic agencies is highly uncertain (Fig.1).
8
Thus, a horizontal grid search for series of trial source positions around the epicenter position
9
of EMSC is employed to define the source centroid location based on the maximum correlation.
10
In total, 64 trial sources were tested with a 6km horizontal increment, covering an area of 48km
11
× 48 km.The horizontal increment is selected to be equal to the minimum difference between
12
the reported epicenters. Green functions are calculated by the discrete wavenumber method
13
(Bouchon, 1981 and Coutant, 1989). The ground-motion data is restituted by deconvolving
14
records with transfer function of the used seismometers. The same causal Butterworth band-
15
pass filter between 0.05 and 0.1 Hz is applied to both the observed and synthetic data.
16
The correlation between the observed (u) and synthetic (s) displacement seismograms
17
is given by the square root of variance reduction (VR), corr =√ (VR) (Krizova et al. 2013). The
18
global variance reduction is defined as
19
20
𝑉𝑅 = 1 ‒
∑(𝑢 ‒ 𝑠) 2
2
(1)
∑𝑢
where summation takes place over all stations, components and time samples.
5
ACCEPTED MANUSCRIPT 1
The resulting correlation is demonstrated in Fig.4. It shows that the maximum
2
correlation is located close to the epicenter reported by HLW and located at 30.4018 (N°) and
3
28.44 (E°).
4
To check that the centroid position is not biased by spatial distribution of the seismic
5
stations involved in the inversion, jackknifing tests (Hicks and Rietbrock, 2015; Horalek and
6
Sileny, 2013 and Horalek et al. 2010) are performed. This is done by recalculating the inversion
7
by repeatedly removing one station from the data set. The results confirm stability of the
8
defined centroid horizontal position (Fig. 5).
9 10
4.2 Source Mechanism and Centroid Depth
11
15 stations were used as input to FOCMEC (Snoke, 2003) with zero allowed misfit polarity,
12
however as shown in Figure (6) many different solutions satisfy the data equally well.
13
Therefore, we turned to moment tensor inversion utilizing ISOLA code, using vertical search
14
below the optimum horizontal position (indicated in the previous paragraph). The vertical grid
15
search was performed in terms of 25 trial sources with a 1 km vertical separation, spanning
16
depths from 5 to 29 km. Time grid search was also performed to deal the time uncertainty. The
17
investigated time uncertainty was within ±4 sec from the origin time, using 0.12 s time step.
For determination of earthquake source mechanism, P-wave first motion polarities at
18
The waveform inversion showed a very stable focal mechanism over all the trial
19
centroid depths (Fig.7). The optimal solution was found between 9 and 20 km, formally at 13
20
km focal depth, featuring the waveform correlation as high as 80%, and the double-couple
21
percentage of 87%. These features confirm plausibility of the employed instrumental
22
responses, epicenter position and velocity model. The time grid search clarified that the
23
optimal centroid time of the studied earthquake is close (+ 0.21 s) to the origin time (Fig. 8).
24
The waveform match (VR= 0.648) is shown in Figure (9); it can be characterized as quite 6
ACCEPTED MANUSCRIPT 1
satisfactory if we take into account that epicentral distances up to ~13 minimum shear
2
wavelengths are used (the wavelength is estimated as a product of a typical shear velocity 3
3
km/s and the shortest inverted period, 10 seconds). The inverted moment tensor indicted that
4
the mechanism of the South El-Alamein earthquake is strike-slip type with a minor reverse
5
component and its parameters are listed in Table 3.
6
The obtained mechanism well agrees with the P-wave first motion polarities at 15
7
seismic stations (Fig.10). Stability of the obtained source mechanism was tested again by
8
jackknife test. The test was performed to access the bias of the retrieved source mechanism due
9
to the spatial distribution of the observed data. The results showed almost constant strike, while
10
dip was varying from 63° to 89°, and rake from 163° to 168° or from 3° to 5°. The centroid
11
depth varied from 11km to 13km (Fig. 11).
12
6. Discussions and Conclusion
13
The broad-band records of the South El-Alamein earthquake have been collected and
14
analyzed to estimate the centroid moment-tensor parameters. The full-waveform moment
15
tensor inversion was carried out from the carefully selected stations situated at epicentral
16
distances 152–391 km, using frequency band of 0.03 to 0.08 Hz. These data guarantee good
17
quality of waveforms and resolvability of focal mechanism using the available 1-D velocity
18
model. The centroid position was estimated by mean of horizontal and vertical grid search. It
19
is close to location of the event by HLW network operated by NRIAG. The inverted focal
20
mechanism, consistent with polarities, is a strike-slip with minor reverse component. The
21
reliability and stability of the inverted mechanism was tested in terms of Jackknifing tests and
22
correlation with polarities of P-wave first motion. Due to proximity of the event to the
23
previously known South El-Alamein fault, which developed in Late Cretaceous as dextral
24
strike slip, the authors believe that the studied event ruptured along its E-W nodal plane. 7
ACCEPTED MANUSCRIPT 1
It is worth to note that, based on maps of Egyptian Geological Survey and Mining
2
Authority the length of South El-Alamein fault is about 14 km. considering the fault ruptures
3
in its whole length (worst case scenario) and according to the empirical relationships of Wells
4
and Coppersmith, 1994 the released moment magnitude could be 6.4 Mw.
5
Finally, this earthquake rings an alarm for considering it in the future planes and
6
knowledge of the recent activity of the fault is important for the ongoing earthquake hazard
7
analyses in Egypt, as well as for hydrocarbon explorations in the region.
8
Acknowledgment
9 10
The authors are grateful to the editor and the anonymous reviewers for their suggestions which have greatly helped to improve the manuscript.
11 12
References:
13
Abdel-Fattah, M., Gameel, M., Awad, A. and Ismaila, A., 2014. Seismic interpretation of
14
the Aptian Alamein Dolomite in the Razzak oil field, Western Desert, Egypt, Arab J
15
Geosci., doi: 10.1007/s12517-014-1595-4.
16
Benetatos, C., Málek, J., Verga, F., 2013. Moment tensor inversion for two micro-
17
earthquakes occurring inside the Háje gas storage facilities, Czech Republic. J Seismol
18
17:557–577. doi:10. 1007/ s10950-012-9337-0
19
Bosworth, W., A.S. El-Hawat, D.E. Helgeson, and K. Burke, 2008. Cyrenaican “shock
20
absorber” and associated inversion strain shadow in the collision zone of northeast
21
Africa: Geology, Vol. 36, p. 695-698.
22 23
Bouchon, M., 1981. A simple method to calculate Green’s functions for elastic layered media, Bull. Seismol. Soc. Am. 71, 959–971. 8
ACCEPTED MANUSCRIPT 1 2 3 4 5 6
Coutant, O., 1989. Program of numerical simulation AXITRA, Tech. Rep., LGIT, Grenoble, France (in French). Dolson, J., Shann, M., Matbouly, S., Harwood, C., Rashed, R., Hammouda, H., 2001. AAPG Memoir 74, chapter 23: the petroleum potential of Egypt, 453–482. Egyptian Geological Survey and Mining Authority, EGSMA., 1981. Geologic map of Egypt, 1:2000000.
7
El Shazly, E., Abdel Hady, A., El Ghawaby, Khawasik, S. and El Shazly, M., 1976.
8
Geological interpretation of landsat satellite images for the Qattara Depression area.
9
Egypt. OSU- remote Sensing Center, Cairo, 54.
10
Ezzelarab M., Shokry M.M.F., Mohamed A.M.E., Helal A.M.A., Abuoelela A. Mohamed,
11
El-Hadidy M.S., 2016. Evaluation of seismic hazard at the northwestern part of Egypt.
12
Journal
13
http://dx.doi.org/10.1016/j.jafrearsci.2015.10.017.
of
African
Earth
Sciences.113,
114-125.
14
Fojtíková, L., and Zahradník, J., 2014. A new strategy for weak events in sparse networks:
15
the first-motion polarity solutions constrained by single-station waveform inversion.
16
Seism. Res. Letters 85, 1265-1274; doi: 10.1785/0220140072.
17
Fojtíková, L., Vavryčuk, V., Cipciar, A., Madarás, J., 2010. Focal mechanisms of micro-
18
earthquakes in the Dobrá Voda seismoactive area in the Malé Karpaty Mts. (Little
19
Carpathians), Slovakia. Tectonophysics 492:213–229
20 21
Hantar G.,1990. North Western Desert. In: Said R (ed) The geology of Egypt. AA Balkema, Rotterdam, pp 293–319
9
ACCEPTED MANUSCRIPT 1 2
Hicks, S., and Rietbrock., A., 2015. Seismic slip on an upper-plate normal fault during a large subduction megathrust rupture. nature geoscience. 8. DOI: 10.1038/NGEO2585.
3
Horalek, J. and Sıleny, J., 2013. Source mechanisms of the 2000 earthquake swarm in the
4
West Bohemia/Vogtland region (Central Europe). Geophys. J. Int. (2013). doi:
5
10.1093/gji/ggt138.
6
Horalek, J., Jechumtalov, Z., Dorbath, L., and Sıleny, J., 2010. Source mechanisms of
7
micro-earthquakes induced in a fluid injection experiment at the HDR site Soultz-sous-
8
Forets (Alsace) in 2003 and their temporal and spatial variations. Geophys. J. Int. 181,
9
1547–1565. doi: 10.1111/j.1365-246X.2010.04506.x.
10 11
Křížová, D., J. Zahradník, and A. Kiratzi, 2013. Resolvability of isotropic component in regional seismic moment tensor inversion. Bull. Seism. Soc. Am. 103, 2460-2473.
12
Makris J, Stofen B, Vees R, Allam A, Maamoun M, Shehata W., 1979. Deep seismic
13
sounding in Egypt. Part I: The Mediterranean Sea between Crete- Sidi Barani and the
14
coastal area of Egypt. Unpublished report, National Research Institute of Astronomy
15
and Geophysics, Helwan, Egypt.
16
McClusky, S., et al. 2000. GPS constraints on plate motions and deformation in the Eastern
17
Mediterranean: implications for plate dynamics. J. Geophys. Res. 105, 5695-5719,
18
2000
19
Mohamed, A. E.-E. A., El-Hadidy, M., Deif, A., and Abou Elenean, K., 2012. Seismic
20
hazard studies in Egypt: NRIAG Journal of Astronomy and Geophysics, v. 1, no. 2, p.
21
119-140. DOI.org/10.1016/j.nrjag.2012.12.008.
10
ACCEPTED MANUSCRIPT 1
Moretti, I., Kerdraon, Y., Rodrigo, G., Huerta, F., Griso, J.J., Sami, M., Said, M and Ali,
2
H., 2010. South Alamein petroleum system (Western Desert, Egypt). Pet Geosci
3
16(2):121–132
4
Sawires, R., Peláez, J. A., Fat-Helbary, R. E., and Ibrahim, H. A., 2016. An earthquake
5
catalogue (2200 B.C. to 2013) for seismotectonic and seismic hazard assessment studies
6
in Egypt. In S. D’Amico (Ed.), Earthquakes and their impact on society (pp. 97–136).
7
Berlin: Springer.
8 9
Šílený, J., Campus, P., Panza, G.F., 1996 Seismic moment tensor resolution by waveform inversion of a few local noisy records - I. Synthetic tests. Geophys J Int 126:605–619.
10
Snoke, J.A., 2003. FOCMEC: FOcal MEChanism determinations. In: Lee WHK, Kanamori
11
H, Jennings PC, Kisslinger C (eds) International handbook of earthquake and
12
engineering seismology. Academic Press, San Diego, Chapter 85.12
13
Sokos, E. N., J. Zahradník, A. Kiratzi, J. Janský, F. Gallovič, O. Novotný, Kostelecký, J.,
14
Serpetsidaki, A. and. Tselentis.G, 2012. The January 2010 Efpalio earthquake
15
sequence in the western Corinth Gulf (Greece), Tectonophysics 530–531, 299–309.
16
Sokos, E., Zahradník, J., 2008. ISOLA a FORTRAN code and a Matlab GUI to perform
17
multiple-point source inversion of seismic data. Comput. Geosci. 34, 967-977.
18
Sokos, E., Zahradník, J., 2013. Evaluating Centroid-moment-tensor Uncertainty in the New
19
Version of ISOLA Software. Seismol. Res. Lett. 84, 656–665.
20
Zahradník, J., and Plešinger, A., 2010. Toward understanding subtle instrumentation effects
21
associated with weak seismic events in the near field, Bull. Seismol. Soc. Am. 100, 59–
22
73, doi: 10.1785/ 0120090087. 11
ACCEPTED MANUSCRIPT 1
Zahradník, J., Janský, J., Plicka, V., 2008. Detailed waveform inversion for moment tensors
2
of M~4 events; examples from the Corinth Gulf, Greece. Bull. Seismol. Soc. Am. 98,
3
2756-2771.
4
Zhu, Lupei and Helmberger, D. V., 1996. Advancement in Source Estimation Techniques
5
Using Broadband Regional Seismograms, Bull. Seismol. Soc. Am., 86, No. 5, 1634-
6
1641, 1996.
12
ACCEPTED MANUSCRIPT
List of Figures
Fig.1. Location of the studied earthquake near the city of El-Alamein. The inset is zoomed on the South El Alamein region. The stars refer to the epicenter labeled by its reported seismic agency as in table (1).
ACCEPTED MANUSCRIPT
Fig.2. Tectonic setting and seismicity in and around the northern Western Desert. (Tectonic elements are compiled from McClusky et al., 2000 and Bosworth et al., 2008). The seismicity map represents earthquakes from 1900 to2013. The inset is zoomed on the South El-Alamein region.
ACCEPTED MANUSCRIPT
Fig.3. Seismic stations (triangles) used for inverting the focal mechanism solution of the South El-Alamein earthquake (star).
ACCEPTED MANUSCRIPT
Fig.4. Interpolation of the correlation between observed and synthetic waveforms for the South El-Alamein earthquake obtained at various trial locations by the horizontal grid search.
ACCEPTED MANUSCRIPT
Fig.5. Interpolations of the correlation between observed and synthetic waveforms for the South El-Alamein earthquake based on station jackknifing tests. Panels are named by the station removed in each inversion and ordered based on the station distance.
ACCEPTED MANUSCRIPT
Fig.6. Fault plane solutions based on first-motion polarity. Uncertainty of the focal mechanism is very large.
ACCEPTED MANUSCRIPT
Fig.7. Correlation values and focal mechanism of South El-Alamein earthquake obtained at various trial depths by the vertical grid search method. The double-couple percentage (DC %) of beachballs is scaled on the right side. Note strong stability of the focal mechanism, and preferred centroid depths at ~9-20 km.
Fig.8. Correlation values and focal mechanisms at the optimal depth for the South El-Alamein earthquake through the time grid search.
ACCEPTED MANUSCRIPT
Fig.9. Waveform match between observed (black) and synthetic (red) displacement waveforms in the frequency range 0.03-0.08 Hz. Station codes appear at the right. The number in the top right corner of each box shows variance reduction of the corresponding component.
ACCEPTED MANUSCRIPT
Fig.10. Comparison of the focal mechanism (waveform inversion) and the P-wave firstmotion polarities.
ACCEPTED MANUSCRIPT
Fig.11. The jackknife tests. (a, b,c and d) show the histograms for values of strike, dip , rake and depth respectively. (e) Shows the stereographic projection of the nodal planes and the pressure and tension axes.
ACCEPTED MANUSCRIPT
Highlights 1. The first felt earthquake at South El-Alamein City is discussed. 2. Accurate analysis for the source position and focal mechanism is carried out. 3. The event is located close to the previously known South El-Alamein fault. 4. The inverted focal mechanism is a strike-slip with minor reverse component. 5. The studied event is important for the ongoing earthquake hazard analyses in Egypt.
ACCEPTED MANUSCRIPT
List of Tables
Table 1: The hypocenter parameters of the South Alamein fault earthquake. Seismic Agency European-Mediterranean
UTC Origin Time (hh:mm:ss) 1:44:40
Latitude (°N)
Longitude (°E)
Depth (km)
Magnitude
30.5100
28.4400
10
mb 4.5
1:44:37
30.6200
28.2800
10
mb 4.7
1:44:41
30.6200
28.3800
10
mb 4.7
1:44:41
30.6290
28.5210
13.9
mb 4.5
1:44:42
30.4584
28.4426
14
ML 4.3
Seismological Centre (CSEM) Geophysical Survey of Russian Academy of Sciences (MOS) Helmholtz Centre Potsdam GFZ German Research Centre For Geosciences (GFZ) National Earthquake Information Center (NEIC) National Research Institute of Astronomy and Geophysics, Helwan, Cairo (HLW)
ACCEPTED MANUSCRIPT
Table 2: Crustal structure used for moment tensor inversion (Makris et al. 1979). Horizontal layers with constant parameters are assumed. Depth (km) 0 7 18 30
Vp (km/s) 4.5 6.0 6.5 8.0
Vs (km/s) 2.52 3.37 3.65 4.49
Density (g/cm3) 2.6 2.8 3 3.2
Table 3: Source parameters of the South Alamein fault earthquake Strike (°) 272
Dip (°) 87
Rake (°) 163
Mw 4.3
Mo (Nm) 3.8212 x 1015