A recent Mw 4.3 earthquake proving activity of a shallow strike-slip fault in the northern part of the Western Desert, Egypt

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

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A recent Mw 4.3 earthquake proving activity of a shallow strike-slip

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fault in the northern part of the Western Desert, Egypt Mohamed Ezzelarab1,2, Mohamed O., Ebraheem3 and Jiří Zahradník 4

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1. Sultan Qaboos University, Earthquake Monitoring center, 50 Alkouhd , Postal code 123,

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Muscat, Sultanate of Oman.

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2. National Research Institute of Astronomy and Geophysics, Seismology Department,

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Helwan, Cairo, Egypt.

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3. Assiut University, New Valley Branch, Faculty of Science, Geology Department, Assiut,

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Egypt

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4. Charles University, Faculty of Mathematics and Physics, Prague, Czech Republic.

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Abstract:

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The Mw 4.3 earthquake of September 2015 is the first felt earthquake since 1900 A.D

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in the northern part of the Western Desert, Egypt, south of the El-Alamein City. The available

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waveform data observed at epicentral distances 52–391 km was collected and carefully

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evaluated. Nine broad-band stations were selected to invert full waveforms for the centroid

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position (horizontal and vertical) and for the focal mechanism solution. The first-arrival travel

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times, polarities and low-frequency full waveforms (0.03-0.08 Hz) are consistently explained

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in this paper as caused by a shallow source of the strike-slip mechanism. This finding indicates

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causal relation of this earthquake to the W-E trending South El-Alamein fault, which developed

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in Late Cretaceous as dextral strike slip fault. Recent activity of this fault, proven by the studied

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rare earthquake, is of fundamental importance for future seismic hazard evaluations, underlined 1

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by proximity (~65 km) of the source zone to the first nuclear power plant planned site in Egypt.

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Safe exploration and possible future exploitation of hydrocarbon reserves, reported around El-

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Alamein fault in the last decade, cannot be made without considering the seismic potential of

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this fault.

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Key words: South El-Alamein Fault; waveform inversion; source mechanismposition and time grid search; Jackknife test.

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1. Introduction

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On September 3rd, 2015, at dawn (local time 03:44, i.e. UTC time 01:44) an earthquake

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of 4.5 mb occurred in northwestern Egypt. The epicenter of the quake was approximately 164

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km southwest of the Egyptian coastal city of Alexandria (European Mediterranean

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Seismological Centre [EMSC], (Fig. 1). The event was felt in Alexandria and Cairo as well.

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The recent earthquake catalogue and seismicity maps compiled since 1900 AD (Mohamed et

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al. 2012; Ezzelarab et al. 2016 and Sawries et al. 2016) indicate that this event is the first felt

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earthquake which took place at the South El-Alamein fault region, thus calling attention for

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considering this fault in seismic hazard assessment of the surrounding area. Particularly, if we

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take into consideration that this zone is situated close (~65 km) to the area which was propsed

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for the first nuclear power plant in Egypt. Additionally, from economical point of view,

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numerous hydrocarbon discoveries have been reported around El-Alamein fault in the last

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decade. It is well-known that faults in the Western Desert play a significant role in the trap of

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hydrocarbon and its migration pathway compartmentalization (Moretti et al. 2010 and Abdel-

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Fattah et al. 2014).

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Hypocenter of the current event was reported by several seismic agencies (Fig.1 &

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Table 1) with significant uncertainty, which might negatively affect reliability of the source

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mechanism determination (Šílený et al. 1996). Therefore, the aim of the current work to present

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precise analysis of the source position and focal mechanism using waveform inversion

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technique. Waveform inversion technique is adopted because the size of studied event is

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relatively small and epicentral distances of the stations is large which increase the possibility

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of distortion the first-arrive and accordingly the failure of the first arrive methods for retrieving

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the source mechanism (Šílený et al. 1996, Benetatos et al. 2013 and Fojtíková and Zahradník

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,2014). Also, waveform modeling is particularly useful in constraining earthquake depths (e.g.

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Zhu and Helmberger 1996).

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In the frame of ISOLA code (Zahradnik et al. 2008. Sokos and Zahradnik, 2008,

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Fojtíková et al. 2010; Sokos et al. 2012, Sokos and Zahradnik , 2013) the waveform inversion

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is preformed and the position of source focal mechanism is horizontally and vertically grid

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searched. The stability of the resulted source mechanism is tested using Jackknife test.

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Waveforms data of the Egyptian National Seismic Network (ENSN) and European Integrated

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Data Archive (EIDA) were used.

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2. Tectonic setting and seismicity

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Generally, tectonics of the northern part of the Western Desert is structurally controlled

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by relative motions between the African, Arabian and Eurasian Plates (Fig.2). In particular,

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the separation of North Africa and Arabian Plate during the Jurassic age formed several E-W

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trending rift basins (Dolson et al. 2001, Bosworth et al., 2008). The dextral convergence

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between African and Eurasian Plates during the Late Cretaceous was associated with

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compressional pluses system that affected the entire African Plate (Bosworth et al., 2008).

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Based on geophysical studies (Hantar, 1990), the normal faults in the northern part of

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the Western Desert were characterized by strike-slip movements during part of their history.

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These strike-slip movements were caused by lateral movements that the African plate

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underwent during Jurassic (sinistral strike slip) and Late Cretaceous (dextral strike slip). The

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analysis of Landsat images (Elshazly et al. 1976) mapped several trends of fractures (NW-SE

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and NNW-SSE, ESE-WNW and ENE-WSW, NNE-SSW and NE-SW). They declared that the

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NW-SE set is characterized by right-lateral strike slip fault.

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The recent instrumental seismicity (Fig.2) shows that seismic activity concentrates

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along specific trends; along the Hellenic and Cyprian Arcs, Northern Red Sea rift, Gulf of

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Aqaba and a few inland earthquakes. There is no previous seismic activity in south El-Alamein

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area and the studied event is the first recorded one.

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3. Data

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In this study, a data set of 31 seismic stations of the Egyptian National Seismic Network

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was investigated. Additionally, data of 2 stations were downloaded from European Integrated

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Data Archive (EIDA) that is initiated within ORFOUS (http://www.orfeus-eu.org/data/eida/).

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However, only 9 stations with tri-axial long-period seismometers (i.e., STS-2 and Trilum-240

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seismometers) could be selected to retrieve the moment tensor; their epicentral distance being

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152–391 km (Fig.3). The selection of these stations was based on checking quality of the

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waveform data (to be free from long-period disturbances; Zahradnik and Plesinger, 2010) and

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evaluating the signal-to-noise ratio (SNR) based on amplitude spectra. The intention was to

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employ relatively low frequencies, less affected by uncertainties of the velocity model, and to

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work at epicentral distance not much greater than 10 shear wavelengths. Accordingly, the

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proper frequency band for inversion was selected at 0.03 to 0.08 Hz. The 1-D velocity model

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of Makris et al. (1979), based on deep seismic profiling was used in this study (Table 2). 4

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4. Methods

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4.1 Source centroid location

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minimizing the difference between the observed and synthetic seismograms based on the least-

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squares method. Position and time of the point source (centroid) are grid searched in vicinity

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of the hypocenter.

Inversion of full waveforms by ISOLA code enables seismic source to be modelled by

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The earthquake epicenter reported by seismic agencies is highly uncertain (Fig.1).

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Thus, a horizontal grid search for series of trial source positions around the epicenter position

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of EMSC is employed to define the source centroid location based on the maximum correlation.

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In total, 64 trial sources were tested with a 6km horizontal increment, covering an area of 48km

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× 48 km.The horizontal increment is selected to be equal to the minimum difference between

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the reported epicenters. Green functions are calculated by the discrete wavenumber method

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(Bouchon, 1981 and Coutant, 1989). The ground-motion data is restituted by deconvolving

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records with transfer function of the used seismometers. The same causal Butterworth band-

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pass filter between 0.05 and 0.1 Hz is applied to both the observed and synthetic data.

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The correlation between the observed (u) and synthetic (s) displacement seismograms

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is given by the square root of variance reduction (VR), corr =√ (VR) (Krizova et al. 2013). The

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global variance reduction is defined as

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𝑉𝑅 = 1 ‒

∑(𝑢 ‒ 𝑠) 2

2

(1)

∑𝑢

where summation takes place over all stations, components and time samples.

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The resulting correlation is demonstrated in Fig.4. It shows that the maximum

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correlation is located close to the epicenter reported by HLW and located at 30.4018 (N°) and

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28.44 (E°).

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To check that the centroid position is not biased by spatial distribution of the seismic

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stations involved in the inversion, jackknifing tests (Hicks and Rietbrock, 2015; Horalek and

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Sileny, 2013 and Horalek et al. 2010) are performed. This is done by recalculating the inversion

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by repeatedly removing one station from the data set. The results confirm stability of the

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defined centroid horizontal position (Fig. 5).

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4.2 Source Mechanism and Centroid Depth

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15 stations were used as input to FOCMEC (Snoke, 2003) with zero allowed misfit polarity,

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however as shown in Figure (6) many different solutions satisfy the data equally well.

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Therefore, we turned to moment tensor inversion utilizing ISOLA code, using vertical search

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below the optimum horizontal position (indicated in the previous paragraph). The vertical grid

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search was performed in terms of 25 trial sources with a 1 km vertical separation, spanning

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depths from 5 to 29 km. Time grid search was also performed to deal the time uncertainty. The

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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

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The waveform inversion showed a very stable focal mechanism over all the trial

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centroid depths (Fig.7). The optimal solution was found between 9 and 20 km, formally at 13

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km focal depth, featuring the waveform correlation as high as 80%, and the double-couple

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percentage of 87%. These features confirm plausibility of the employed instrumental

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responses, epicenter position and velocity model. The time grid search clarified that the

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optimal centroid time of the studied earthquake is close (+ 0.21 s) to the origin time (Fig. 8).

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The waveform match (VR= 0.648) is shown in Figure (9); it can be characterized as quite 6

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satisfactory if we take into account that epicentral distances up to ~13 minimum shear

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wavelengths are used (the wavelength is estimated as a product of a typical shear velocity 3

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km/s and the shortest inverted period, 10 seconds). The inverted moment tensor indicted that

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the mechanism of the South El-Alamein earthquake is strike-slip type with a minor reverse

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component and its parameters are listed in Table 3.

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The obtained mechanism well agrees with the P-wave first motion polarities at 15

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seismic stations (Fig.10). Stability of the obtained source mechanism was tested again by

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jackknife test. The test was performed to access the bias of the retrieved source mechanism due

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to the spatial distribution of the observed data. The results showed almost constant strike, while

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dip was varying from 63° to 89°, and rake from 163° to 168° or from 3° to 5°. The centroid

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depth varied from 11km to 13km (Fig. 11).

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6. Discussions and Conclusion

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The broad-band records of the South El-Alamein earthquake have been collected and

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analyzed to estimate the centroid moment-tensor parameters. The full-waveform moment

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tensor inversion was carried out from the carefully selected stations situated at epicentral

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distances 152–391 km, using frequency band of 0.03 to 0.08 Hz. These data guarantee good

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quality of waveforms and resolvability of focal mechanism using the available 1-D velocity

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model. The centroid position was estimated by mean of horizontal and vertical grid search. It

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is close to location of the event by HLW network operated by NRIAG. The inverted focal

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mechanism, consistent with polarities, is a strike-slip with minor reverse component. The

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reliability and stability of the inverted mechanism was tested in terms of Jackknifing tests and

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correlation with polarities of P-wave first motion. Due to proximity of the event to the

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previously known South El-Alamein fault, which developed in Late Cretaceous as dextral

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strike slip, the authors believe that the studied event ruptured along its E-W nodal plane. 7

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It is worth to note that, based on maps of Egyptian Geological Survey and Mining

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Authority the length of South El-Alamein fault is about 14 km. considering the fault ruptures

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in its whole length (worst case scenario) and according to the empirical relationships of Wells

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and Coppersmith, 1994 the released moment magnitude could be 6.4 Mw.

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Finally, this earthquake rings an alarm for considering it in the future planes and

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knowledge of the recent activity of the fault is important for the ongoing earthquake hazard

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analyses in Egypt, as well as for hydrocarbon explorations in the region.

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Acknowledgment

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The authors are grateful to the editor and the anonymous reviewers for their suggestions which have greatly helped to improve the manuscript.

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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).

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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.

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Fig.3. Seismic stations (triangles) used for inverting the focal mechanism solution of the South El-Alamein earthquake (star).

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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.

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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.

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Fig.6. Fault plane solutions based on first-motion polarity. Uncertainty of the focal mechanism is very large.

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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.

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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.

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Fig.10. Comparison of the focal mechanism (waveform inversion) and the P-wave firstmotion polarities.

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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.

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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.

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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)

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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