Introduction

The northern Red Sea and the southern portion of the Gulf of Suez are an extensional stress system, whereas the tectonic deformation of the Gulf of Aqaba–Dead Sea fault system is regulated by a shear stress system (Abdel-Fattah et al. 2006). In the northern Red Sea and the Gulf of Suez, geodynamic extension is prominent and results in normal faults, while oblique extension results in sinistral strike-slip and dip-slip faults along the Aqaba–Dead Sea fault. The majority of earthquakes in the northern Red Sea are low to moderate. On March 31, 1969, an ML = 6.8 earthquake, the maximum recorded to date, was detected in the northern region of the Red Sea, accompanied by many small and moderate earthquakes with magnitudes of < 6.0 (Daggett et al. 1986; Badawy 1999). An earthquake of moderate local magnitude (\(M_{{\text{L}}}\) = 5.4) occurred on June 16, 2020 at 14:30 GMT in the northern Red Sea, eastern Egypt. Previous studies and findings about earthquake source characteristics and faulting mechanisms in the northern region of the Red Sea have revealed that in the northern Red Sea and the southern part of the Gulf of Suez, shear stress results in normal movement, with a minor strike-slip component and nodal planes oriented NW–SE, whereas the oblique extension and tectonic deformation of the Gulf of Aqaba–Dead Sea fault system results in strike-slip movement, with minor dip-slip motion and NNE–SSW-oriented nodal planes (e.g., Badawy 1999; Abdel-Fattah et al. 2006; Hussein et al. 2013; Abdel-aal and Yagi 2017; Badreldin et al. 2019; Saadalla et al. 2020). This moderate earthquake was felt in many cities on the western Red Sea coast, including Hurghada city (a highly populated city and a main tourism center). Therefore, a comprehensive study of its mechanism and source features and their spectral parameters is important for future ground motion modulation studies and earthquake hazard analyses. The source movement mechanism, spectral features, and characteristics of the main earthquake and its significant aftershock sequences were estimated. The main earthquake was analyzed using the full waveform moment tensor inversion method (Herrmann 2013), and the movement type of the causative fault was determined. The main earthquake and its significant aftershock sequences were comprehensively investigated using the spectral inversion method, and the spectral parameters were calculated (i.e., seismic moment, moment magnitude, corner frequency, source radius, and stress drop).

Background geology and structure

Three tectonostratigraphic phases are found in the northern Red Sea stratigraphic section (Fig. 1A): pre-rift, syn-rift, and post-rift. The Red Sea is still rifting, so post-rift rocks are only present in the Suez Rift (Said 1990; Khalil and McClay 2001). Gulf of Suez and northern Red Sea pre-rift rocks comprise platform sedimentary strata unconformably overlying Precambrian crystalline basement rocks (Saleh et al. 2006). The Proterozoic-aged Precambrian basement rocks of the Gulf of Suez and northern Red Sea are visible on the rift flanks and in the eroded-up dip regions of tilted fault blocks inside the rift, including many rift blocks in the vicinity of the northwest coastal Red Sea (Khalil and McClay 2016). These Precambrian basement rocks are unconformably overlain by non-metamorphosed to gently metamorphosed intermediate to silicic volcanics (Dokhan volcanics) and the clastic sedimentary rocks of the Hammamat Group. They are composed of serpentinites, metavolcanics, metasediments, and metagabbros aligned with gneisses in structural highs (El Gaby et al. 1990). Tectonically, both the Gulf of Suez and the Red Sea formed by continental rifting during the early stages of Arabia splitting from Africa. According to several tectonic theories, the rifting spread from the Red Sea into the Suez Rift during the Middle Miocene, eventually becoming an abortive continental rift in the Precambrian shield south of Sinai (Ghebreab 1998; Bosworth et al. 2005). The active Gulf of Aqaba was formed as the Arabian plate split from the Sinai sub-plate, and it has remained active since the Middle Miocene orogeny (Mart and Horowitz 1981; Mart and Rabinowitz 1986). The three areas (Red Sea, Gulf of Suez and Gulf of Aqaba) form a triple junction at the southern extremity of the Sinai Peninsula (Red Sea, Gulf of Suez and Gulf of Aqaba) (Fig. 1B). The rifting process began in the Late Oligocene to Early Miocene and has continued ever since (Bosworth and McClay 2001; Cochran 2005). Four mega-half grabens with exact reverse tilt patterns have been identified in the Suez Rift and the north western region of the Red Sea. The positions and alignments of the accommodation regions of these mega-half grabens were regulated by pre-rift structures (Fig. 1C). According to several focal mechanism and tectonic investigations, right-lateral shearing forces dominate the northern section of the Gulf of Suez, while approximately NNE–SSW-oriented extensional strain completely dominates the northern Red Sea region (Hempton 1987). The Red Sea is a relatively young and active continental rift system that was initiated in the Late Oligocene–Early Miocene (Coleman 1993; Bosworth et al. 2005). It formed in response to the northeast separation of the Arabian plate from the African plate (Hempton 1987). By the late Middle Miocene, the opening of the Red Sea was linked to sinistral strike-slip displacements along the Gulf of Aqaba–Dead Sea transform fault system. The Suez Rift and the northwest Red Sea rift are characterized by a zigzag fault pattern, composed of NW–SE and NS to NNE–SSW striking extensional fault systems on the rift borders and within the rift basins (Meshref 1990; Patton et al. 1994; McClay et al. 1998). Other subsidiary fault trends (WNW–ESE and NE–SW) also exist in the Gulf of Suez and the northwest Red Sea.

Fig. 1
figure 1

A Geology map for the northern Red Sea region (from EGS, 1994). B Plate tectonic setting of the Red Sea and its surroundings modified after Khalil and MacClay (2002) and Bosworth et al (2005). C Fault pattern system for the northern Red Sea region modified after (Bosworth and McClay 2001; Cochran 2005; Khalil and MacClay 2009; Bosworth et al. 2017). The 16 Jun, 2020 earthquake location shown as blue star at each map

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Seismicity and the dataset

The number of seismic stations in Egypt has increased, and existing stations have been updated with highly sensitive seismic sensors. As a result, earthquakes are being detected more accurately. The majority of earthquakes in the northern Red Sea are of low-to-moderate magnitude. In comparison, multiple moderate- to high-magnitude earthquakes are concentrated along the rift axis in the southern Red Sea and Afar (Al-Ahmadi et al. 2014). A seismicity map created for the study area includes historical and instrumental seismic events with Mw values of < 1 to 7.3 for the period up to 2020. The data were collected from Maamoun (1979), Maamoun et al. (1984), Kebeasy (1990), Ambraseys et al. (1994), Abou Elenean (1997), Badawy (1999), Riad et al. (2004), Badawy et al. (2010), Sawires et al. (2016), and the Egyptian National Seismic Network (ENSN) (Fig. 2A). The distribution of the earthquake epicenters indicates a significant level of activity along the entrance to the Gulf of Suez, and the relative movement of the African and Arabian plates in the Sinai has been ascribed to this activity (Daggett et al. 1986). The most recent maximum earthquake (local magnitude \(M_{{\text{L}}} = 6.8\)) was recorded on March 31, 1969. It was situated 80 km NE of Hurghada (Daggett et al. 1986; Badawy 1999). According to Daggett et al. (1986), the adjustment of motion at the triple junction of the Arabian plate, the African plate, and the Sinai sub-plate is what causes the high rate of seismicity in the northwest Red Sea. The data set was composed of the June 16, 2020 main shock and its significant aftershock sequences (Fig. 2B). The selected earthquakes were monitored by the broadband three-component seismic stations of the ENSN. In addition, a few seismic stations from international networks, such as GEOFON (http://www.iris.edu) were included to ensure good azimuthal coverage of the main earthquake (Fig. 2B). The data set was relocated using the HYPOINVERSE-ATLAS package (Klein 1978), and the relocated origin parameters are summarized in Table 1. The studied aftershock sequences are mostly of a smaller magnitude \(2 \le M_{{\text{L}}} \le 3.4\), while the magnitude of the main earthquake was \(M_{{\text{L}}} = 5.4.\) Spectral inversion was applied to the main earthquake and its significant aftershock sequences, while full moment tensor inversion was only performed for the main shock. Table 1 lists the origin parameters of the studied events.

Fig. 2
figure 2

A Seismicity of the northern Red Sea, Gulf of Suez and Gulf of Aqaba based on the compiled earthquake catalogs. B The selected dataset and the recorded seismic stations

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Table 1 The relocated origin parameters of the 16 Jun, 2020 earthquake and its significant aftershocks sequences
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Methods

Moment tensor analysis

Applying moment tensor analysis using full waveform required some specific preparation of the recorded earthquake data. The waveform data were stored in SEED format, with a sampling frequency 100 Hz. The waveform data were processed as follows. First, the original recordings of the main earthquake were prepared by identifying and removing the mean and linear trends, the waveform was resampled, and traces were tapered and corrected by removing the instrument response (Fig. 3A). The displacement seismograms were then rotated and filtered (Fig. 3B). Only earthquakes with a good signal-to-noise ratio were used for the analysis (Fig. 3C). Second, the wavenumber integration (Herrmann and Wang 1985) method was used to compute the synthetic Green’s functions for the northern Red Sea using Hosny et al.’s (2009) velocity model (Table 2). The calculated elastodynamic Green’s functions and the observed seismograms were filtered by the same frequency band in the range of 0.02 to 0.06 Hz to suppress and remove bad signals from the data. This frequency band range is reasonable for moderate earthquakes, as recommended by Havskov and Ottemoller (2010) and Zhang et al. (2016). Finally, full inversion of the moment tensor was applied to selected traces based on an algorithm in a seismology computer program written by Herrmann (2013). The best double-couple mechanism was performed assuming a point seismic source, a displacement trace \(d\) in direction \(n\) at an arbitrary position \(x,\) and time t, equated as follows (Stump and Johnson 1977):

$$d_{n} = M_{ij} \left[ {G_{,j} *s\left( t \right)} \right],$$
(1)

where \(M_{ij}\) is the moment tensor, the synthetic seismograms are represented by \(G_{,j}\) in \(j\) direction, \(s\left( t \right)\) is the source time function, and the time convolution is indicated by the asterisk assuming a delta function of \(s\left( t \right)\) and a synchronized seismic source. Displacement \(d_{n}\) is a linear function of the elements of the moment tensor, and the computed Green’s function leads to linear inversion in the frequency domain, as expressed in Eq. (2),

$$d_{n} \left( {f,x} \right) = M_{kj} \left( f \right)G_{nk,j} \left( f \right).$$
(2)
Fig. 3
figure 3

A The corrected waveform data, B the rotated and filtered waveform, and C example of the obtained signal-to-noise ratio at KAT seismic station for the main shock of the 16 Jun, 2020 earthquakes sequences

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Table 2 Velocity model for the northern Red Sea after Hosny et al. (2009) utilized for calculation of the synthetic seismograms and spectral parameters
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The inversion is solved as a least-square problem (Jost and Herrmann 1989), as formulated in Eq. (3):

$$d = Gm,$$
(3)

where \(G\) is a matrix containing the synthetic seismograms and \(m\) is a vector with the six components of the moment tensor. The best-fitting depth, moment magnitude, scalar seismic moment, and fault movement mechanism were selected based on the fitting goodness between the calculated and observed waveforms.

Spectral analysis

The spectral parameters were estimated for the earthquakes by modeling the spectra of the three-component waveform recordings with clear phase arrivals. As a consequence, the waveforms were selected from time windows with a of length 2.56 s starting 0.5 s before the manually picked S-phase arrivals. The 2.56-s window length was selected to avoid contamination with other phases on the seismograms. The spectra were then calculated using a fast Fourier transform (FFT), which was smoothed using a Hanning window length of 5.0 Hz, averaged over the total number of the used station for each event to obtain the average displacement spectra. The corrected source spectra were typically extracted from the observed seismogram after removing the attenuation and site effects using Andrews’s (1986) inversion technique, extended by Iwata and Irikura (1988). The inversion technique was performed based on the assumption that the reference station had the smallest site effect and had an amplification factor of around 2.0 for the whole frequency range, considering the free surface effect. See Saadalla et al. (2019, 2020) for a further description of the inversion technique. First, three seismic stations (i.e., Kathreen [KAT], HRG, and BST), which showed relatively stable horizontal-to-vertical ratio (H/V) curves, were tested. Namely, the spectral inversion was carried out three times using one of the three stations as an alternative reference site, with its H/V curves as the site effect for the reference site. Figure 4A shows an outside view of the KAT seismic station and its surface geology, while Fig. 4B shows the preliminary result of the H/V horizontal-to-vertical spectral ratio at the KAT station using the earthquake’s coda-wave. The KAT station was found to have the most reasonable site amplification effect, so it was used as a reference site. The obtained spectra from all available recordings for each event were modeled individually based on the displacement spectrum model for S-waves (Brune 1970) as follows:

$$M_{0} = \frac{{4\pi \rho v^{3} }}{{F \times R_{\theta \varphi } \times G\left( {r,h} \right)}}\Omega_{0} ,$$
(4)

where \(\rho\) is the density, \(v\) is the seismic wave velocity, \(F\) is the effect of the free surface, \(R_{\theta \varphi }\) is the radiation pattern, and \(G\left( {r,h} \right)\) is the geometrical spreading effect. The velocity model, after Hosny et al. (2009) was used to calculate the S-wave travel time. For a radiation pattern correction, we used 0.63 for the S-wave (Aki and Richards 2002). The corner frequency was calculated for each source spectra using Andrews’s (1986) automated method. Under the assumption of a circular plane fault and a double-couple source, scalar seismic moment \(M_{{\text{o}}}\), source dimension \(r\), and stress drop Δσ can be determined from Ω0 and \(f_{{\text{c}}}\) using the following relationships (Eshelby 1957; Brune 1970):

$$r = \frac{2.34\beta }{{2\pi f_{{\text{c}}} }},$$
(5)
$$\Delta \sigma = \frac{7}{16}\left( {\frac{{M_{{\text{o}}} }}{{r^{3} }}} \right).$$
(6)
Fig. 4
figure 4

A Outside view of the location site of the Kathreen (KAT) seismic station; B the horizontal-to-vertical spectral ratio (H/V SR) curve obtained for KAT reference site

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Results

Moment tensor results

Full moment analysis and the best-fitting double-couple mechanism for the June 16, 2020 Red Sea main shock revealed that strike-slip movement with minor normal components dominated, with NNE–SSW and ESE–WNW trending fault planes. The obtained parameters associated with the best-fitting, such as moment magnitude, seismic moment, source depth, best double-couple solution, and fitting variance, are summarized in Fig. 5A. The relocated parameters especially the source depth of the mainshock are consistent with the optimum fitting source depth obtained in this study (Fig. 5B). The aftershocks selected in this study revealed that the plane oriented NNE–SSW toward the Gulf of Suez is the fault plane (Fig. 5C). The mainshock and the aftershocks are relocated, the average root mean square (RMS), the average horizontal error (ERH), the average vertical error (ERZ) are 0.17 s, 1.47 km, 0.98 km, respectively (Table 1). The main shock is located at 4 km depth, where the aftershocks are located at depth range from 8.8 to 28.6 km toward the north western side of the 16 Jun, 2020 main event. In general, the aftershocks sequences distribution did not coincide with the 16 Jun, 2020 main earthquake, neither in map view nor with respect to depth. The fault plane solution of the 16 Jun, 2020 main earthquake is compared with previous finding and solutions (e.g., Saadalla et al. 2020; Badreldin et al. 2019; Mohamed et al. 2015; Hussein et al. 2013; Megahed 2004; Abou Elenean 1997) as shown in Fig. 5D. Figure 6A, B shows examples of the waveform correlation and the location error of the inverted earthquake. To summarize, the correlation between the observed and calculated seismograms was 95% fitting goodness, with a time shift equal to 1.4 s, and data with high signal-to-noise ratios indicated that our results are accurate and robust.

Fig. 5
figure 5

A The best-fitting fault mechanism of the 16 Jun, 2020 earthquake. B The correspondence best depth fitting, and C the aftershock clustered and distribution with the best double-couple solution of the mainshock; D the focal mechanism distribution obtained by previous studies

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Fig. 6
figure 6

A Synthetic and observed seismograms correlation of the current main shock of 16 Jun, 2020 northern Red Sea. B The time shift and location error of the inverted event

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Spectral and source parameter results

All the obtained spectra were flat in the frequency range from 0.8 Hz to each corner frequency and decreased rapidly at frequencies of < 10 Hz. The smoothed displacement spectra at each station of each event (Fig. 7A) were inverted, and their average source spectra and standard deviation were calculated (Fig. 7B). Figure 8 shows an example of the good fitting between Brune’s (1970) theoretical omega-square model and the observed displacement source spectra of the main earthquake with a moment magnitude of 5.0 and one of the aftershock earthquakes with a moment magnitude 3.0. Based on Brune’s (1970) modulation and Andrews’s (1986) automation analysis, the seismic moment and corner frequency, respectively, were determined for the studied events. The estimated seismic moment and corner frequency of the main earthquake and its aftershock sequences varied from 8.293E+18 to 7.541E+22 dyn·cm and 2.2 to 8.2 Hz, respectively (Table 3). Determining the seismic moment and the corner frequency enabled us to estimate the dynamic source parameters for the studied earthquakes, including the moment magnitude, source radius, and stress drop. Table 3 lists the source parameter results for the main earthquake and its significant aftershock sequences. Our results revealed that the estimated value for the moment magnitude Mw, source radius \(r\) (m), and stress drop \(\Delta \sigma\) (MPa) ranged from 2.4 to 5.0, 171 to 633 m, and 0.02 to 13 MPa, respectively.

Fig. 7
figure 7

A The corrected and smoothed waveform at each station of the June 16, 2020 event. B The inverted Fourier spectra with their average spectra and standard error

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Fig. 8
figure 8

A The average inverted spectra of the 16 Jun, 2020 main shock and its fitting with the theoretical model of Brune. B An example of the inverted spectra of the 20 Jun, 2020 aftershock event and its fitting with the theoretical Brune model

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Table 3 The calculated spectral parameters of the 16 Jun, 2020 earthquake and its significant aftershocks sequences
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The assigned local magnitude catalog derived from the ENSN shows a linear relationship with the determined moment magnitude Mw and the logarithm seismic moment \(M_{{\text{o}}}\) of the earthquakes investigated in this study (Fig. 9A, B). The determined corner frequencies decayed with increasing seismic moment, while the estimated source radius increased with increasing seismic moment (Fig. 10A, B). The investigated earthquakes were tectonic interplate earthquakes. The stress drop value obtained for the main shock earthquake (ML = 5.4) is 13 MPa, which is much higher than the stress drop values obtained for the aftershock sequences (\(M_{{\text{L}}}\) ≤ 3.3). The determined stress drop values plotted against the logarithm seismic moment (Fig. 10C) indicate that the stress drop values increase with the increasing seismic moment of the current earthquakes’ sequences. The dependence of the obtained source parameters on earthquake depth was also investigated, and our results reveal that the obtained corner frequencies, seismic moments, and stress drops of the main aftershocks of the June 16, 2020 earthquake may be depth dependence (Fig. 11A–C).

Fig. 9
figure 9

Regression analysis between the local magnitude assigned by the ENSN and the logarithm of the seismic moment (A) and their linear relation with the moment magnitude obtained for the current events (B)

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Fig. 10
figure 10

The dependence of the seismic moment in logarithmic scale on the essential spectral parameters including (corner frequency, source radius, and stress drop)

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Fig. 11
figure 11

The dependence of the spectral parameters such as: seismic moment (A), corner frequency (B), and stress drop (C) on the earthquakes depth included in this study

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Discussion and conclusion

Generally, the northern Red Sea covers a broad area of active tectonics and is characterized by zigzag fault patterns, composed of NW–SE and N–S to NNE–SSW striking extensional fault systems (Meshref 1990; Patton et al. 1994; McClay et al. 1998). Extensional strain, oriented approximately NNE–SSW, completely dominates the northern Red Sea region (Hempton 1987). The majority of the earthquakes in the northern Red Sea are of low-to-moderate magnitude (Badawy 1999). A moderate earthquake (\(M_{{\text{L}}} = 5.4\)) occurred on June 16, 2020, about 80 km NE of Hurghada city in the northern part of the Red Sea. Several aftershocks followed the main event in the same area from June 16, 2020 to August 23, 2020, with magnitudes of < 3.5. The main earthquake was found to have strike-slip faulting, with minor dip-slip motion and nodal NNE–SSW and ESE–WNW oriented planes. The obtained fault plan solution revealed that the T-axis and P-axis were directed ENE–WSW, which is consistent with the general tension movement along the Red Sea. The aftershocks sequences distribution did not coincide with the 16 Jun, 2020 main earthquake, neither in map view nor with respect to depth. The studied aftershocks are extended and clustered at the north western side of the main shock which may indicated a secondary fault plane activation, or suggested that the seismicity in the north western side were activated by the 16 Jun, 2020 mainshock due to stress triggering (Donner et al. 2013). The best double-couple mechanism determined using the full waveform moment tensor analysis for the current earthquake was consistent with previous findings (e.g., Abou Elenean 1997; Megahed 2004; Hussein et al. 2013; Mohamed et al. 2015; Badreldin et al. 2019; Saadalla et al. 2020). The main objective of the current work was to comprehensively study recent seismicity and its spectral parameters in addition to understanding the tectonic setting of the northern Red Sea and the interaction of its extensional movement with the Gulf of Aqaba and Gulf of Suez for seismic hazard assessment and risk management modulation.

The focal depth of the June 16, 2020 earthquake was very shallow (3 km) compared to the 4 km obtained by the relocation analysis. The investigated earthquakes, including the main earthquake and its significant aftershock sequences, were processed using the generalized inversion technique constrained by a reference rock site. The corrected displacement source spectra obtained for the earthquakes’ sequences were modulated based on Brune’s (1970) theoretical model, and the advanced spectral parameters (i.e., seismic moment, moment magnitude, corner frequency, source radius, and stress drop) were extracted. The isolated source spectra modulated very well with Brune’s spectra. Based on the shear-wave spectral inversion and Brune’s modulation, the spectral parameters using the isolated S-wave displacement spectra were determined. The calculated range of the seismic moment and the corner frequency were reasonable for the magnitude size of the current earthquake sequences. The estimated moment magnitude was slightly higher than the assigned local magnitude reported by ENSN. Source radius and stress drop values estimated in this study have a linear relationship with the magnitude of the current earthquakes. The calculated stress drop values are in good agreement with the stress drop range of small-to-moderate interplate earthquakes, from 0.1 to 100 MPa (Kanamori 1994), and have depth dependence (the stress drop relatively increased with increasing the earthquakes source depth). As a result, comprehension of the northern Red Sea's tectonic context and the connection of its extensional movement with the Gulf of Aqaba and Gulf of Suez might aid in the evaluation of seismic hazards and the improvement of risk management.