Introduction

Egypt is one of the very few countries in the world that is subjected to different seismotectonic activities. The seismicity of Egypt is mainly controlled by the active opening or the Red Sea rift on its the eastern boundary (Freund 1970; Girdler and Styles 1974; Cochran 1983; Girdler 1985), subduction in the Mediterranean Sea on the northern boundary (McKenzie 1972; Papazachos and Comninakis 1971; Quennell 1984; Papazachos et al. 1993), and some seismotectonic provinces inside the Egyptian plateau such as Faiyum, Aswan, and Abu Debbab (Kebeasy 1990; Woodword Clyde Consultant 1985).

After the disastrous 1992 earthquake (Ms = 5.2), Egypt came to be considered a high-risk country for earthquake activities. As a result, a new earthquake network for strong motions (acceleration and velocity) was installed by the National Research Institute of Astronomy and Geophysics.

Shear wave seismic velocity (Vs) is a very important measure for determining the site response of earthquake energy. It may even be considered the basic parameter that defines the dynamic properties of soils. It is very useful in the evaluation of soil stiffness, rigidity, hardness, strength, and liquefaction potential or foundation settlements (Richart et al. 1970; Seed and Idriss 1970; Schnabel et al. 1972; Walter 1981; Sykora and Stokoe 1983; Burland 1989; Sasitharan et al. 1994; Shibuya et al. 1995; Kramer 1996; Andrus and Stokoe 1997; Wills and Silva 1998; Mayne et al. 1999; Dobry et al. 2000; Lehane and Fahey 2002; Seed et al. 2003; Stewart et al. 2003; McGillivray and Mayne 2004; Idriss and Boulanger 2004; Holzer et al. 2005; McGillivray 2007; Akin et al. 2011; Cavallaro et al. 2018; Grasso et al. 2021).

We can say that earthquake resistance design, soil structure interaction, soil amplification, and damping depend on the Dynamic shear modulus (G) or on shear wave seismic velocity (Vs) and density (ρ) of the underlying soil.

$$\mathrm{G }=\uprho {{\mathrm{V}}_{\mathrm{s}}}^{2}.$$
(1)

Many studies have been carried out to determine the empirical relations for Vs soils (Ohba and Toriumi 1970; Imai and Yoshimura 1970; Fujiwara 1972; Ohsaki and Iwasaki 1973; Imai 1977; Ohta and Goto 1978; Seed and Idriss 1981; Imai and Tonouchi 1982; Sykora and Stokoe 1983; Jinan 1987; Lee 1990; Sisman 1995; Iyisan 1996; Kayabalı 1996; Jafari et al. 1997, 2002; Pitilakis et al. 1999; Kiku et al. 2001; Andrus et al. 2006; Hasançebi and Ulusay 2007; Hanumantharao and Ramana 2008; Dikmen 2009; Castelli et al. 2018).

Microseismic zonation map provides the basis for site-specific risk analysis, which can help in the mitigation of earthquake damage such as building collapses and liquefaction of soil and estimation of other response of soil layers under earthquake excitations (Grasso and Maugeri 2009, 2012). It can be used for evaluating site-specific risk analysis, which is essential for critical structures like nuclear power plants, subways, bridges, elevated highways, sky trains, and dam sites.

In this study, two site specific equations were deduced for each geological zone, one for Vs/depth variation and the other for N30/depth variation. As it is very difficult to obtain the values of Vs and SPT (N30) simultaneously at the same depth level, the empirical relations between Vs and SPT (N30) were not calculated directly. They were calculated by solving the two equations of Vs and SPT (N30) together as they are both function of depth. The collected data shows good quality, and this appears in the similarity of the Vs and SPT (N30) slopes with depth for the same zones. The site-specific equations for Vs/D or SPT (N30)/D and hence Vs/SPT (N30) are expected to serve as a reference for new developing projects in Egypt.

Brief description of seismotectonic provinces in egypt

The tectonics and seismotectonics of Egypt are a result of the interaction between three major tectonic plates: African, Eurasian, and European. A summary of the active seismic source zones in Egypt is given below. Figure 1 provides an illustration of these zones.

Fig. 1
figure 1

Instrumental and historical earthquakes affecting Egypt from 2200 B.C. to 2020 from different sources. Some historical earthquakes are put with felt areas. The dates of some of the important earthquakes are written beside its earthquakes (integrated from Makropoulos and Burton 1981; Maamoun et al. 1984; Ben-Menahem 1979; Woodward-Clyde Consultants 1985; Riad and Meyers 1985; Shapira 1994; NEIC and USGS 1964–2020)

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Subduction in the mediterranean sea (northern boundary)

The African lithosphere is subducting underneath the European lithosphere in the Hellenic Arc beneath Cyprus and the Crete Islands (McKenzie 1972; Papazachos and Comninakis 1971; Quennell 1984; Papazachos et al. 1993). This source produces the biggest earthquakes, with magnitudes that can reach Ms = 8. However, this zone is relatively far from urban areas in Egypt (Fig. 1).

Obduction in the red sea (eastern boundary)

The opening of the Red Sea is separated on two motions. These are a transform motion at the Gulf of Aqaba-Levant transform fault system and an opening of the Gulf of Suez (Freund 1970; Girdler and Styles 1974; Cochran 1983; Girdler 1985). This seismic source produces relatively large earthquakes that can reach Ms = 7 and affect certain urban areas in Egypt like east Cairo (Egypt’s capital), Sinai, and the Red Sea zone east of Egypt.

Some active seismotectonic provinces (within the egyptian plate)

These are some geographical regions inside the Egyptian plate that are known for producing seismic activities but for tectonic reasons that are not yet well understood. Some of these active seismic trends are the “Suez-Cairo-Alexandria” trend, the Pelusium trends (East of Cairo and Faiyum), Aswan, and the Abu Debbab area (Maamoun et al. 1984; Neev 1982; Kebeasy 1990; Meshref 1990).

The seismic sources from these zones produce moderate earthquakes of magnitude Ms = 3–6; however, they are potentially the most destructive seismic sources affecting Egypt because of their vicinity to crowded cities like Cairo, Faiyum, and Aswan.

Classification of geological zones

The following is a brief description of the geology of different zones in Egypt:

The eastern desert

The Eastern Desert is bound to the west by the Nile. The western part of Eastern Desert is covered by Cretaceous sandstone and the north by Eocene limestone and then Miocene clastics (EGSMA 1981) (Fig. 2). The desert is mainly composed of igneous and metamorphic rocks dating back to the Precambrian age. The basement rocks found here include granite, gabbro, rhyolite, andesite, basalt, schist, slate, and gneisses. These rocks are covered by Quaternary deposits of sand and gravel, raised beaches, and Miocene covers of gypsum and carbonates.

Fig. 2
figure 2

modified from EGSMA, 1981) showing the locations of the 26 test sites for both Vs and SPT (N30) values

Geological map of Egypt (

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The western desert

Most of the central part of the Western Desert is composed of thick Eocene marine limestone with minor clay beds. The depressions of Farafra and Bahariya are covered by Cretaceous limestones and layers of Paleocene shale, while the northern part has a Miocene age basal clastic section overlain by carbonate units. The southern part of the Western Desert is covered by Cretaceous sandstone (Nubia sandstone) and some Jurassic clastic rocks, and the south-east and east of the Western Desert are covered by Quaternary sand, forming the Great Sand Sea (EGSMA 1981) (Fig. 2).

The nile valley and delta

The Nile deposits consist of loose or semi-consolidated Quaternary sediments, sand, silt, clay, and gravel with several changes in deposition sequence due to the Nile’s course changes over time. The Nile Delta is covered by cultivated lands of the Quaternary age. The Nile brachiates into two branches before it meets the Mediterranean Sea (Rashid and Damietta). A lot of sabkhas exist in these areas especially in the lagoon areas of Lake Bardawil, Lake Burulus, and Lake Manzala. The sabkha deposits are a very famous composition characterized by the existence of large layers of salt, clay, sand, silt, and sometimes organic matter (EGSMA 1981) (Fig. 2).

Sinai

The Sinai Peninsula forms a portion of the Arabo-Nubian Shield and occupies a wedge-shaped block at the northern end of the East African Rift System, bound by the Gulf of Aqaba to the southeast and the Gulf of Suez to the southwest.

South Sinai is mainly composed of Precambrian igneous and metamorphic rocks caped by Quaternary wadi and Playa deposits, while in the north, it consists of sand dunes, Quaternary wadi and Playa deposits, Eocene beds of thick limestone, and/or Cretaceous fossiliferous clastics, with Jurassic coals around the Gabal Maghara. Most of the lowlands in the north and northeast are covered by Quaternary sand (EGSMA 1981) (Fig. 2).

Based on the regional geology and data collected from 26 sites as well as the Vs and SPT (N30) values, this study classified the following nine different geological zones: (1) the North Coast, (2) the Western Desert, (3) Sabkha and Lagoons, (4) the Gulf of Suez, (5) the Red Sea, (6) the Delta, (7) the Nile Valley, (8) the zone surrounding the Nile Valley, and (9) the Sinai Peninsula. About 71 drilling in-situ SPT (N30) value tests and about 82 Vs tests (Downhole, Crosshole, ReMi, or MASW) were used to introduce microseismic zonation maps for Egypt (Fig. 3).

Fig. 3
figure 3

The main geological zones classified for this study using the Vs and SPT data, namely, (1) The North Coast, (2) The Western Desert, (3) Sabkha and Lagoons, (4) The Gulf of Suez, (5) The Red Sea, (6) The Delta, (7) The Nile Valley, (8) The Surrounding Nile Valley, and (9) The Sinai Peninsula

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Data collection and methodology

Standard penetration test (SPT) method

SPT is considered a high-strain in-situ test designed to provide information on the geotechnical engineering properties of the soil. This test uses a thick-walled sample tube driven into the ground (45 cm) by hammering blows on its top with a slide hammer. The first 15 cm are considered seating for the tube while the last 30 cm are recorded as the soil resistance value SPT (N30). If 50 blows are not sufficient to advance it through a 15-cm interval, the engineers consider this a refusal (ASTM D1586–99 1999). The Vs test is considered a low-strain test since it involves exciting the soil by sound to determine its dynamic properties by generally using a sledgehammer.

A total of 82 locations were chosen to account for all the geological variations in Egypt. About 71 boreholes were drilled, and 82 Vs tests were conducted varying between Downhole, Crosshole, ReMi, and MASW tests (Fig. 3 and Table 1). The depths of these boreholes ranged from 20 to 60 m. Undisturbed samples were taken at nearly 1-m depth intervals, to obtain as continuous a soil profile as possible. Further, samples for the soil SPT blow counts were taken at 1-m depth intervals in most of the boreholes in the sandy layers. The ground water level for the studied areas varied between 1 and 20 m (Table 1) and is shown as encountered.

Table 1 Locations of the Vs tests and the drilled boreholes for determining the SPT (N30) values
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The following is a brief description of the tests used to determine the Vs.

Downhole seismic test method

A Downhole seismic test was conducted in this study with a three-component triaxial geophone 4HZ or 10 HZ (Vp, Vsh, and Vsz). The shear waves were generated using a wooden shear beam source of 15-cm diameter and 2.5-m length loaded by vehicle (Table 1 and Fig. 4). Both S + ve and S − ve shear waves were generated by opposite hitting on the wooden plate to generate the shear wave reversals (Fig. 4). The source used was a 10-kg impulsive hammer. The instruments used for this study were either the ES-3000 12 channel Geometrics, USA or the Geode 24 Channel from the same company (ASTM D7400 2017).

Fig. 4
figure 4

Schematic diagram showing the downhole seismic test procedure used in this study (ASTM D7400 2017)

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Crosshole seismic test method

A Crosshole seismic test was done using a shear wave seismic hammer placed inside the terminal borehole that was treated as the source borehole (ASTM D4428 2014) (Fig. 5). The seismic hammer was lowered to a certain depth. Thereafter, the two receiving geophones were lowered and located at the same depth using the cable connected to the geophones. Subsequently, a tension was applied to the clamps to achieve firm contact between the source, the geophones, and the borehole perimeter (Fig. 5).

Fig. 5
figure 5

source used (middle) and the 10HZ triaxial geophone (right; ASTM D4428 2014)

Schematic diagram showing the Crosshole seismic test procedure used in this study (left). The Crosshole hammer

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The ReMi method

The ReMi method is a passive technique. It relies on the measurement of ambient noise carried out with linear seismic arrays to obtain information about the dispersion curve (Vs versus frequency) (Coccia et al. 2010). Practically, the ReMi method is based on a 2-D slowness-frequency (p-f) transformation of a microtremor record. This transformation separates the Rayleigh waves from other seismic waves (Gamal and Pullammanappallil 2011; Strobbia et al. 2015) (Fig. 6). The main advantages of using this technique accrue regardless of the triggered source and it works best with seismicity noise, e.g., traffic and different vehicles (Louie 2001; Pullammanappallil et al. 2003; Gamal and Pullammanappallil 2011). This method previously proved useful in estimating the average Vs for several sites in Egypt (Gamal and Pullammanappallil 2011). In this study, we have used this method several times for drilling boreholes, especially in remote areas like Sinai, but not for drilling a downhole or crosshole (Fig. 6 and Table 1).

Fig. 6
figure 6

Example of p-f image, dispersion curve excluded for the ReMi profile conducted at Sinai city (left) and corresponding Vs (right) (Louie 2001)

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The MASW method

The MASW technique (Park et al. 1999) has been used several times in this study (Table 1). It was developed in response to the shortcomings of the spectral analysis of surface wave method in the presence of noise. The simultaneous recording of 24 or more receivers at a short distance (2–5 m) was used with an impulsive or hammer source to measure the variation of Vs with depth (Fig. 7). The advantage of this method is that it provides a relatively greater depth of penetration that sometimes reaches half of the survey depth (Park et al. 1999; Miller et al. 2000) (Fig. 8).

Fig. 7
figure 7

Acquisition parameters used for MASW profiles for this study with 2–5-m geophone spacing and forward and reverse shots at 2, 6, 12, and 24–48 from both terminals of the profile

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Fig. 8
figure 8

Example of dispersion image and dispersion curve excluded for MASW profile conducted at Ras Gharib city (left) and corresponding Vs (right) (Park et al. 1999; Miller et al. 2000)

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Results and discussion

The following is a description of the data collected for each of the geological zones.

The north coast zone

This zone includes the cities of Alexandria, Almeen, and Sidikrir and represents the northern coast of Egypt. This zone is composed mainly of silty clay, silty sand, sandy clay, claystone, limestone fragments, gypsum stone, and limestone rock (Fig. 9A).

Fig. 9
figure 9

Soil parameters for the North Coast zone. A Representative borehole logs, B Vs curves, C SPT (N30)/depth best-fit equations, and D velocity/depth best-fit equations

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The original Vs curves are drawn in Fig. 9B. The entire SPT (N30) data is illustrated in Fig. 9C and ranges from 10–150. The value of SPT (N30) > 50 is calculated by extrapolation. Thus, for example, 50 blow counts driving the spoon 10 cm is converted into SPT (N30 cm) = 150. The entire data for Vs is illustrated in Fig. 9D and ranges from 220–600 m/s.

The SPT (N30)/depth best-fit equations and those for 0–20 m and 20–50 m are presented in Table 2 and Fig. 9C, while the velocity/depth best-fit equations and those for depths 0–20 m and 20–50 m are presented in Table 2 and Fig. 9D.

Table 2 Best-fit equations for Vs versus the depth and standard penetration SPT (N30) value versus the depth for different geological zones excluded from this study
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The western desert zone

This zone is represented by the cities of Badr and East Baherya. This area is composed mainly of sand with silt and gravel, silty sand, and sand (Fig. 10A).

Fig. 10
figure 10

Soil parameters for the Western Desert zone. A Representative borehole logs, B Vs curves, C SPT (N30)/depth best-fit equations, and D velocity/depth best-fit equations

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The original Vs curves are drawn in Fig. 10B. The entire SPT (N30) data is illustrated in Fig. 10C and ranges from 50–150. The entire data for Vs is illustrated in Fig. 10D and ranges from 200–700 m/s.

The SPT (N30)/depth best-fit equations and those for 0–10 and 10–20 m are presented in Table 2 and Fig. 10C, while the velocity/depth best-fit equations and those for depths 0–10 m and 10–20 m are presented in Table 2 and Fig. 10D.

The sabkha and lagoon zone

This zone includes the cities of Rasheed, Al Burlos, Dameitta, Port Said, and El Gameil. It represents the sabkha and lagoon zones of Egypt. This area is composed mainly of silty sand, silty clay, sand with silt, sandy clay, and fat clay (Fig. 11A).

Fig. 11
figure 11

Soil parameters for the Sabkha and Lagoons zone. A Representative borehole logs, B Vs curves, C SPT (N30)/depth best-fit equations, and D velocity/depth best-fit equations

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The original Vs curves are drawn in Fig. 11B. The entire SPT (N30) data is illustrated in Fig. 11C and ranges from 3–150. The entire data for Vs is illustrated in Fig. 11D and ranges from 120–270 m/s.

The SPT N30/depth best-fit equations and those for 0–20 m and 20–60 m are presented in Table 2 and Fig. 11C, while the velocity/depth best-fit equations and those for depths 0–20 m and 20–60 m are presented in Table 2 and Fig. 11D.

The gulf of suez zone

This zone includes the cities of Ferdan, Hurgheda, and Ismailia. This zone is composed mainly of sand, sand with silt and gravel, sand with silt, sand traces, silt, traces of iron oxides, silty sand, sandy silt, clayey sand, silty clay, and thin layers of clay (Fig. 12A).

Fig. 12
figure 12

Soil parameters for the Gulf of Suez zone. A Representative borehole logs, B Vs curves, C SPT (N30)/depth best-fit equations, and D velocity/depth best-fit equations

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The original Vs curves are drawn in Fig. 12B. The entire SPT (N30) data is illustrated in Fig. 12C and ranges from 25–150. The entire data for Vs is illustrated in Fig. 12D and ranges from 200–600 m/s.

The SPT (N30)/depth best-fit equations and those for 0–20 m and 20–40 m are presented in Table 2 and Fig. 12C, while the velocity/depth best-fit equations and those for depths 0–20 m and 20–40 m are presented in Table 2 and Fig. 12D.

The red sea zone

This zone is represented in our study by the cities of Hamraween, Elsokhna, and Ras-Gharieb, and this area is composed mainly of sand with gravel, sandstone with limestone, silty sand, sand with gravel, sandy silt, silty clay, fat clay, and gravel (Fig. 13A).

Fig. 13
figure 13

Soil parameters for the Red Sea zone. A Representative borehole logs, B Vs curves, C SPT (N30)/depth best-fit equations, and D velocity/depth best-fit equations

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The original Vs curves are drawn in Fig. 13B. The entire SPT (N30) data is illustrated in Fig. 13C and ranges from 25–150. The entire data for Vs is illustrated in Fig. 13D and ranges from 400–800 m/s.

The SPT (N30)/depth best-fit equations and those for 0–20 m and 20–40 m are presented in Table 2 and Fig. 13C, while the velocity/depth best-fit equations and those for depths 0–20 m and 20–40 m are presented in Table 2 and Fig. 13D.

The delta zone

This zone is composed mainly of fat clay, silty sand, and sand with silt and is represented in this study by the Kafr El Sheikh, Mahmoudia, and Nubaria cities (Fig. 14A).

Fig. 14
figure 14

Soil parameters for the Delta zone. A Representative borehole logs, B Vs curves, C SPT (N30)/depth best-fit equations, and D velocity/depth best-fit equations

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The original Vs curves are drawn in Fig. 14B. The entire SPT (N30) data is illustrated in Fig. 14C and ranges from 25–100. The entire data for Vs is illustrated in Fig. 14D and ranges from 150–350 m/s.

The SPT (N30)/depth best-fit equations and those for 0–20 m and 20–40 m are presented in Table 2 and Fig. 14C, while the velocity/depth best-fit equations and those for depths 0–20 m and 20–40 m are listed in Table 2 and Fig. 14D.

The zone surrounding the nile valley

The zone surrounding the Nile Valley is represented in this study by the cities of Quena, Sohag, Elmenya, and Beni Suef. This zone is composed mainly of silty clay, claystone, siltstone, cemented sand, sand, dolomite limestone, crystalline limestone, silty sand with gravel, siliceous limestone, and gravel (Fig. 15A).

Fig. 15
figure 15

Soil parameters for the surrounding Nile Valley zone. A Representative borehole logs, B Vs curves, C SPT (N30)/depth best-fit equations, and D velocity/depth best-fit equations

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The original Vs curves are drawn in Fig. 15B. The entire SPT (N30) data is illustrated in Fig. 15C and ranges from 50–200. The entire data for Vs is illustrated in Fig. 15D and ranges from 200–900 m/s.

The SPT (N30)/depth best-fit equations and those for 0–12.5 m and 12.5–40 m are listed in Table 2 and Fig. 15C, while the velocity/depth best-fit equations for depths 0–12.5 m and 12.5–40 m are listed in Table 2 and Fig. 15D.

The nile valley zone

The Nile Valley zone is represented by the cities of Assuit, Aswan, and Benha. This zone is composed mainly of silty clay, silty sand, sand and sand with silt, and gravel (Fig. 16A).

Fig. 16
figure 16

Soil parameters for the Nile Valley zone. A Representative borehole logs, B Vs curves, C SPT (N30)/depth best-fit equations, and D velocity/depth best-fit equations

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The original Vs curves are drawn in Fig. 16B. The entire SPT (N30) data is drawn in Fig. 16C and ranges from 40–120. The entire data for Vs is drawn in Fig. 16D and ranges from 120–600 m/s.

The SPT (N30)/depth best-fit equations and those for 0–20 m and 20–40 m are presented in Table 2 and Fig. 16C, while the velocity/depth best-fit equations for depths 0–20 m and 20–40 m are presented in Table 2 and Fig. 16D.

Relations between Vs and SPT (N30)

The SPT (N30) for the previously mentioned zones were compared with their corresponding Vs values to estimate the empirical relations between the Vs and SPT (N30) for the different soil zones. These relations were estimated using the following steps:

  1. a)

    Plot the SPT (N30) versus the depth (D) for different zones (Figs. 9C–16C).

  2. b)

    Plot the Vs versus the same range of D used in step (a) for the same locations and zones (Figs. 9D–16D).

  3. c)

    Calculate the best-fit equations for Vs versus D and SPT (N30) versus D.

  4. d)

    As D is the same for both the equations, by solving both equations we can get the relation between Vs and SPT (N30) for the different soil zones as can be seen in Table 3.

Table 3 Vs SPT (N30) relations for different soil zones in Egypt
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The obtained relations are then compared with the different existing sources to show their consistency with these other sources (Kanai 1966; Imai and Yoshimura 1970; Ohba and Toriumi 1970; Fujiwara 1972; Ohsaki and Iwasaki 1973; Imai et al. 1975; Ohta and Goto 1978; Imai and Tonouchi 1982; Jinan 1987; Yokota et al. 1991; Athanasopoulos 1995; Sisman 1995; Kiku et al. 2001; Jafari et al. 2002; Hasançebi and Ulusay 2007; Hanumantharao and Ramana 2008; Dikmen 2009). This comparison has been shown in Fig. 17.

Fig. 17
figure 17

Example showing the consistency of the obtained equations for the SPT (N30) versus Vs with different sources (Kanai 1966; Imai and Yoshimura 1970; Ohba and Toriumi 1970; Fujiwara 1972; Ohsaki and Iwasaki 1973; Imai et al. 1975; Ohta and Goto 1978; Imai and Tonouchi 1982; Jinan 1987; Yokota et al. 1991; Athanasopoulos 1995; Sisman 1995; Kiku et al. 2001; Jafari et al. 2002; Hasançebi and Ulusay 2007; Hanumantharao and Ramana 2008; Dikmen 2009). A The Delta zone, B the Gulf of Suez zone, C the North Coast zone, and D the Red Sea zone

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Microseismic zonation maps for Egypt based on NEHRP (Vs 30) and refusal depth SPT(N30)

Two important microseismic seismological quantities were introduced for this study in one map. These quantities were the Vs for the upper 30 m and the refusal depth map for the upper 50 m. The Vs 30 was calculated for the different Egyptian soil zones using the following equation (Kanlı et al. 2006):

$${{\mathrm{V}}_{\mathrm{s}}}_{ }30= \frac{30}{\sum \left(\frac{{\mathrm{d}}_{\mathrm{i}}}{{\mathrm{V}}_{{\mathrm{s}}_{i}}}\right)}$$
(2)

where di is the thickness of layer i and \({\mathrm{V}}_{{\mathrm{s}}_{i}}\) is the shear wave velocity of layer i. The Vs 30 is calculated for the different soil zones and a Vs 30 map for the different Egyptian soils is produced from these calculations (Fig. 18).

Fig. 18
figure 18

Microseismic zonation map for Egypt based on geological classification, NEHRP seismic site classification, and the refusal depth D50 (SPT(N30) = 50 blows)

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To produce the refusal depth map for different Egyptian soils (D50 map), we have used the depth at which the spoon penetrates 30 cm inside the soil after producing 50 blows (SPT (N30) = 50) or D50 as referred to on the map (Fig. 18). This was done using the SPT (N30) versus (D) equations for each of the zones given in Table 2. These different depths for different Egyptian soil zones were used to produce an SPT (N30) D50 map of the different Egyptian soils, which is a function of the strength of the soil (Fig. 18).

The NEHRP/(Vs 30) seismic site classification was then used to determine the soil classes for the different Egyptian soils. The different Egyptian soils were then arranged from weak to strong using the NEHRP (2007) classification and refusal depth (D50) as shown in Fig. 18 and Table 4.

Table 4 The Egyptian soil NHRP classification and refusal depth values
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The Vs calculated for the mountains at Saint Katherine area was used for the mountains of the Red Sea (Fig. 18). The general trend that can be seen from the microseismic zonation map is that of an increase in the Vs 30 as the strength of the soil increases, and a simultaneous decrease in the refusal depth except in the northern coast zone that is affected by the weak soil of the EL Almeen site. This site is affected by the presence of cavities, which make the soil here extremely weak (Fig. 9A).

Detection of seismic risk and warnings

This microseismic zonation map is the first attempt at a seismic hazard assessment of Egypt’s soils (to the best of our knowledge). A regional assessment for the extent of hazard is believed to be very helpful for any new project to be taken up in Egypt. With the help of this map, we can get information about the NEHRP zone’s classification, the Vs 30 value, and know the strength of the soil in terms of the SPT (N30) refusal depth D50 (Fig. 18). However, in this regard, there are three important warnings for the people and decision-makers of Egypt:

  1. 1)

    Warning for the people in the weakest zone of “sabkha and lagoons”:

    This zone is classified as zone “E” by the NEHRP seismic classification and is composed of soft clayey soil. The Vs detected in this zone is 133 m/s, while the refusal depth for SPT (N30) is very deep at about 28 m (Fig. 18). Unfortunately, this zone has been seismically active since 1698 A.D. (Fig. 1). The seismic activities that caused the earthquakes of 1870 and 1955 were located offshore, under the sea. During the 1955 earthquake (Ms = 6.2; maximum intensity = VIII (Woodward and Clyde Consultant 1985)), 22 lives were lost, and many were injured. The weak soil in this zone, especially in Rashid and Idku, caused more than 300 buildings to be damaged. There was damage to infrastructure as well as pipelines to a large extent. Hence, a warning is given for this zone as many big projects and petroleum sector investments such as the liquefaction of natural gas in Rashid and Idku are being initiated in this zone.

  2. 2)

    Warning for the people living in the “Delta Zone and the Capital Cairo”:

    It became evident that the eastern part of Cairo and the Delta are active seismic zones that have experienced historical earthquakes that date back to 2200 B.C. (Fig. 1). The main reason for these activities is most probably the opening of the Gulf of Suez (Ben Menahem and Aboodi 1971). This area has the second weakest soil based on the Vs and NEHRP code (195 m/s and Zone “D”) (Fig. 18). This is most probably due to the accumulation of the alluvium deposits of the Nile inside the Nile Valley and the Delta (Fig. 18). The main hazard in this area is manmade and arises from the existence of a lot of houses with failed engineering. These houses have been documented to be a reason for loss of life after intermediate earthquakes because of the soil and non-engineering constructions, especially ceilings made of wood. The intermediate earthquakes such as the Faiyum earthquake of 1992 (M = 5.8) caused the death of hundreds of people and damaged hundreds of houses. This destruction has always occurred in the case of the old brick houses built in villages and non-engineering houses built in Cairo, particularly those in the Dowiqa area (Woodward and Clyde Consultant 1985).

  3. 3)

    Warning for the people living in the Villages of the Nile Valley:

    The Nile Valley zone is the third weakest zone in Egypt, most probably due to the weak alluvium deposits of the Nile valley from Cairo to Aswan (Fig. 18). This zone possesses Vs of 232 m/s and is classified as NEHRP zone D. This zone consists of poor villages as well as many old brick and non-engineering houses. This zone has been affected by several earthquakes that have been documented since 27 B.C. The “High Dam” in Aswan, which is one of the most important strategic constructions, also exists in this zone. Hence, care should be taken to warn the people living in this zone. The future projects in this zone should also be duly warned given this zone’s relative weakness compared to other geological zones in Egypt.

Conclusions

A total of 71 drilling boreholes and about 82 Vs velocity tests varying between Downhole, Crosshole, ReMi, and MASW were used to prepare the microseismic zonation maps for Egypt. The geological map of Egypt was used to extrapolate the seismic and engineering data to determine 10 different geological zones, namely, (1) Sabkha and Lagoons, (2) The Delta, (3) The Nile Valley, (4) The Gulf of Suez, (5) The Northern Coast, (6) Sinai, (7) The Western Desert, (8) The Surrounding Nile Valley, (9) The Red Sea, and (10) The Mountains. The new empirical evidence for the Vs versus depth and SPT (N30) versus depth that has been introduced by this study for each zone will help engineers and seismologists obtain the desired dynamic properties of the soil such as refusal depth (SPT (N30) = 50 blows) or the dynamic shear modulus G.

This study introduced new relations for each geological zone between the Vs versus SPT (N30) by solving equations of Vs 30 and SPT (N30) with depth.

To the best of our knowledge, this microseismic zonation map is the first such map in Egypt that can be used to know about the value of Vs 30 and the NEHRP seismic zone class. This map will be very helpful for future projects in Egypt to evaluate the number of hazards that they projects might face.

This study has given important warnings based on the historical seismicity and NEHRP classification of certain seismic zones. In particular, warnings have been given to the three weak zones identified by this study, i.e., the sabkha and lagoons zone, the “Delta zone and the Capital Cairo,” and the zone comprising the “villages of the Nile Valley.” These zones have been found to be seismically weak for a long time and are suffering from weak NEHRP seismic zone classification ranging between Zone “E” to Zone D. These zones have many poor residents, and the people live in non-engineering constructions composed of old brick and wooden ceilings in areas such as “El Dowiqa.” Any seismic activity can cause the ceiling to collapse, causing a huge loss of lives. A high risk has also been detected for future petroleum projects and oil rigs initiated in Idku and Rashid or in offshore Alexandria, where a wide destructive hazard may come from an offshore earthquake such as the earthquake of 1955 (M = 6.2). The reports regarding this historical earthquake have indicated that it had destroyed much of the construction and pipelines and caused many deaths in Idku and Rashid (Woodward and Clyde Consultant 1985).