Introduction

Jizan City is largely affected and controlled by the geodynamic processes of the Red Sea rifting system. Consequently, the Red Sea is the main effective source of large earthquakes that could affect Jizan area, which has experienced significant earthquakes (magnitude > 6) throughout both historical and recent periods. Heterogeneities in the topsoil profile play an effective role in the widespread severe damage and foundation failure that occur in coastal areas. Ground motion conditions have become one of the devastating types of natural disasters currently faced throughout the world. Burland (2006) stated that establishing the soil profile is the critical factor during site investigations. Moreover, the presence of sabkha soil with a considerable thickness accelerates the geotechnical implications of ground failure from land subsidence due to loose soil and/or building collapse.

Recently, several studies have examined problems related to ground motion conditions where Abu-Hajar and Hossain (1991) investigated soil profiles in Jeddah City. Dhowian and Erol (1993) investigated the ground motion conditions and associated structural problems in the Jizan and Tabuk regions. Matsah and Hossain (1993) studied the ground motion conditions in Al-Madinah Al-Munawarah, Saudi Arabia. Anbazhagan and Sitharam (2006) evaluated the soil profiles in Bangalore City. Shaaban et al. (2013) tested variations in the ground subsidence near the City Star Shopping Mall in Cairo, where a large building was tilted. Roy and Bhalla (2017) inspected the effect of soil geotechnical properties on civil engineering structures. Ravi (2017) conducted an intensive geotechnical survey under severe ground motion conditions in India. Abdelrahman et al. (2020) evaluated the ground motion conditions in Makkah al-Mukarramah City, Saudi Arabia. Alhumimidi (2020) studied the hazardous impacts of shallow deposits throughout Jizan City in terms of their thickness and certain geotechnical parameters. The competence of rock or soil materials is widely applied (Sjogren et al. 1979) for geotechnical purposes, such as the N value, concentration index, stress ratio, and allowable bearing capacity (Imai 1975; Abd El Rahman and Abd El Latif 1990; Abd El Rahman et al. 1991).

Jizan City is located along the southwestern coast of Saudi Arabia (Fig. 1). The old city was built on an uplifted salt dome surrounded by a low-lying flat area. Jizan City has recently experienced rapid growth and a significant number of economic, urban, and industrial projects. Due to heterogeneities in the thicknesses and lithofacies, with significant variations over short distances, the city suffers from a number of geotechnical problems, including land subsidence; foundation failures; and damage to buildings, roads, and infrastructure, especially in the old town, which have destroyed many facilities. This study aims to delineate the geological conditions of Jizan City and evaluate the dynamic and geotechnical properties of the entire city following the flowchart in Fig. 2.

Fig. 1
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Jizan area with borehole locations

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Fig. 2
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Flowchart describing the methodology of the work

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Geological setting and geotechnical problems

The coastal plains of the Jizan region are entirely flat, except for the Jizan salt dome, which rises 50 m above sea level (Fig. 3). The old Jizan area is composed of salt rocks of gypsum, anhydrite, shale, and coralline limestone. These rocks are overlain by 6- to 15-m-thick fine and loess sand. However, sabkha extends beneath Jizan urban extensions. Geological cross-sections through the study area (Fig. 2) illustrate the lateral and vertical heterogeneities in the lithology.

Fig. 3
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Local geologic setting of Jizan City

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Mahfouz (2016) elucidated that the soil outline composed of soft clayey silt in Jizan City occurs between two silty sand layers at depths between 5 and 11 m. The soft clayey silt has a high water content, low SPT-N values, and a low bearing capacity. The following soil conditions were established based on field studies. Two types of geological units have been identified in and around Jizan City, i.e., sand dunes and sabkha. Information from 320 borings have been analyzed to establish the subsurface conditions in these two zones. The thickness of sand dunes reaches 20 m, underlined by salt rocks. The sand layer is constituted by fine quartz grains and volcanic rock fragments, with a fine content ranging from 7 to 30% (average = 19%) (Al-hadad et al. 2001). The natural water content of the sand layer varies between 4 and 20%. The density of the sand fluctuates significantly from loose at shallow depths (0–2 m), loose to moderately consolidated at intermediate depths (2–10 m), and consolidated below 10 m. Standard test results for the penetration resistance vary from site to site and range from 4 to 18 for the topsoil (5 to 10 m).

More recent dense sediments covering the flat coastal plain, including soft/loose organic saline soil, are locally known as sabkha. The sabkha soil profile consists of three main units (Table 1), with groundwater at a depth of 1 m. The average layer thickness varies from 1 to 2 m and consists of fine-grained sediments (silty sand and/or fine sand with silt and gravel), with SPT-N values from 4 to 30. The second layer, which is 8- to 12-m-thick, is soft and/or loose deposits consisting of soils that range from non-plastic fine sands to silt and clay with very low SPT-N values (1 to 4). The basal layer is composed of fine–coarse and dense–very dense sand (Hodgson et al. 1989, Dhowian et al. 1987; Al-Farraj 2008).

Table 1 Generalized coastal sabkha profile, Red Sea Coast
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Seismicity map construction for the Jizan area

A number of studies have recorded and analyzed the seismicity of the southwestern region of Saudi Arabia. According to Merghelani and Gallanthine (1981), five portable seismic stations were installed at 17 different locations between January and February 1978. No significant earthquakes were detected in the Red Sea trough, but an unexpectedly high level of local seismicity was recorded. The magnitude of these earthquakes ranged from 1.4 to 2.5, and their aerial distribution suggests that these earthquakes do not lie along a single active structure. These earthquakes suggest a general level of tectonic activity on a number of structures in this region.

Historical seismicity (AD 200–1964)

According to published compilations of historical earthquakes for the Middle East (Poirier and Taher 1980; Ambraseys et al. 1994), the historical seismicity related to volcanic eruptions along the western Arabia. Based on these characteristics, we can observe the following: (1) most earthquakes have located around Sana’a–Aden along the Red Sea, due to the population density in Arabia. (2) Some historical earthquakes have occurred offshore; these earthquakes were felt on land, reflecting the continuation of marine tectonics onto land. Approximately 22 earthquakes occurred from 1900 to 1964 with magnitudes ranging between 4 and 6.3. (3) Most historical earthquakes appear to correlate with the general tectonics of the region (El-Isa and Al Shanti 1989).

Instrumental seismicity (1964–2017)

Recent events were collected from various local and international catalogs and publications related to El-Isa and Al-Shanti (1989). The earthquakes affecting the study area through 1964–2017 vary in magnitude 2 ≤ mb ≤ 6.9. The epicentral distribution of the instrumental seismicity in the study area shows the following characteristics: (1) the general trend of earthquakes agrees with the major structures of the area. A concentrated area of epicenters south of Sana’a correlates well with a regional Tertiary and Quaternary volcanic plateau. (2) The instrumental activities show a noticeably higher activity in the southern Red Sea and central Red Sea compared with other areas. (3) The seismicity of this region is characterized by both mainshock–aftershock and swarm type activity (El-Isa and Al Shanti 1989). The swarm type activity may occur along strike-slip and normal faults, and may or may not have a volcanic origin.

Earthquake data sources and seismotectonic source zones

The Arabian plate is affected by various plate boundaries, approximate plate convergence vectors, and principal geologic features (Fig. 4). To assess the earthquake activities affecting Jizan Area, an earthquake catalog has been compiled by gathering the available data from several data sources around the study area. The obtained datasets cover the historical (AD 112–1964) and instrumental periods (1964 to December 2019). Through this study, the sources of earthquake data are (1) two regional catalogs (Ambraseys and Adams 1988; Ambraseys et al. 1994) and (2) the Saudi National Seismic Network (SNSN) maintained by the Saudi Geological Survey (SGS). Then, the collected data were precisely reviewed, re-analyzed, and refined from duplicated events. These international resources include the International Seismological Center (ISC) on-line bulletin and the European Mediterranean Seismological Center (EMSC). Then, a new seismicity map has been constructed (Fig. 5). Moreover, four seismic source zones were identified based on the homogeneity of the datasets. Figure 6 shows the location and geometries of the selected seismic source zones, and a brief description of these zones is provided below. Earthquake activity in the southern Red Sea is accompanied by volcanic activity; for example, basaltic eruptions occurred at Jabal At Tair and on the Zubair islands in 2007, 2011–2012, and 2013 (Jónsson and Xu 2015).

Fig. 4
figure 4

Simplified map of the Arabian Plate, with plate boundaries, approximate plate convergence vectors, and principal geologic features (after Stern and Johnson 2010)

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Fig. 5
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Distribution of the earthquakes from 1900 to 2019 around Jizan City

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Fig. 6
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The identified earthquake source zones affecting Jizan City

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Yemen earthquake source zone (Z1)

This zone has experienced damaging historical earthquakes (e.g., Ambraseys et al. 1994). On December 13, 1982, a large area in the Dhamar Province of Yemen experienced a destructive earthquake (mb = 6.0, MS = 5.7; epicenter at 14.7° N and 44.2° E). This moderate shallow event occurred in a densely populated region approximately 70 km south of Sana’a and resulted in 2500 deaths and 1500 injured (Ambraseys et al. 1994; Arya et al. 1985). More than 70,000 dwellings were damaged and 500,000 people were affected where major losses have been recorded in highly populated villages and their houses were constructed of rubble masonry followed by some of widely felt aftershocks (mb = 5.1; Langer et al. 1987).

Southern Red Sea zones (Z2, Z3, and Z4)

This segment is located near the Afar Triple Junction and is characterized by high seismic activity. The Red Sea axial trough becomes less pronounced here (Salem et al. 2013), and earthquakes are distributed throughout the area. Broad submarine shelves exist immediately offshore of the Red Sea margin. Islands, such as the Farasan Group near Jizan, exhibit raised Quaternary reefs that overlie sediments containing a Miocene-layered evaporite sequence. The Saudi Arabian side of the shelf has been the target of hydrocarbon exploration by ARAMCO. Evaporite deposits migrate downslope to build accumulations of up to 6-km-thick where salt domes and faults form in the overlying sediments. Near the median trough, salt flows emerge and flow into the trough. The accumulation of small magnitude earthquake data with a high-quality hypocenter definition may allow the separation of the zone into several distinct parts, e.g., the northern, central, and southern areas.

Estimation of the shear wave velocity to 30 m depths (V s30)

Soil profile characterizations

Ninety-one geotechnical boreholes were collected throughout Jizan City, as illustrated in Fig. 1, while Figs. 7 and 8 display two examples of the soil logs at two of the selected locations.

Fig. 7
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Geotechnical parameters for the streaked borehole in New Jizan

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Fig. 8
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Geotechnical parameters for the borehole at the King Fahd road extension

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Geotechnical borehole (SPT) data correction and results

Recent building codes have added key parameters of Vs30 and SPT-N value (Borcherd 1994; Borcherdt and Glassmoyer 1994; Dobry et al. 2000) into site soil classification scheme. The average (Vs30) and N (30) were calculated with Eqs. (1) and (2), and then applied for the site soil conditions as follows:

$$ {V}_s(30)=\frac{\sum \limits_{i=1}^n{d}_i}{\sum \limits_{i=1}^n\frac{d_i}{v_{si}}} $$
(1)
$$ N(30)=\frac{\sum \limits_{i=1}^n{d}_i}{\sum \limits_{i=1}^n\frac{d_i}{N_i}} $$
(2)

where Vs30 = ∑di is the cumulative depth in m for an average depth of 30 m, Vsi is the shear wave velocity (m/s), Ni is the SPT-N value as measured, and di represents the thickness of each layer from 0 and 30 m depth.

Significant variations exist among the various published relations, which are partially caused by differences in the geology, but also by errors in the measurement of N and Vs. In this study, particular attention was given to the functional form used to relate Vs to N60. Almost every relation directly represents Vs as a function of N without considering the vertical effective stress, including early and more recent relations (e.g., Dikmen 2009; Hasancebi and Ulusay 2006). The subsoil profiles were grouped into zones for the different districts in the Jizan area. N values measured in the field via the SPT procedure (N60) were corrected for different parameters (Seed et al. 1983; Skempton 1986; Sitharam et al. 2005; Anbazhagan et al. 2007) using Eqs. (3) and (4):

$$ {N}_{60}=N\times \left({C}_N\times {C}_E\times {C}_B\times {C}_S\times {C}_R\times {C}_{\mathrm{fines}}\right) $$
(3)
$$ {N}_{60}=1.6{E}_{\mathrm{m}}{C}_{\mathrm{b}}{C}_{\mathrm{r}}\mathrm{N} $$
(4)

where N60 is the corrected N, Em is the hammer efficiency (for US equipment, Em is 0.6 for a safety hammer and 0.45 for a doughnut hammer), Cb is the borehole diameter correction (Cb = 1.0 for boreholes 65–115 mm in diameter, 1.05 for a diameter of 150 mm, and 1.15 for holes with a diameter of 200 mm), Cr is the rod length correction (Cr = 0.75 for up to 4 m of drill rods, 0.85 for 4 to 6 m of drill rods, 0.95 for 6 to 10 m of drill rods, and 1.0 for drill rods in excess of 10 m), and N is the measured N value. Figure 9 shows the corrected SPT-N value for the two selected boreholes in Figs. 7 and 8.

Fig. 9
figure 9

SPT corrections for two boreholes throughout the Jizan study area. a The borehole in Fig. 5. b The borehole in Fig. 6

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Calculation of V s30 parameter and site classification

The Vs30 was calculated using the Hasancebi and Ulusay (2006) relationship as follows:

$$ Vs=104.79\ {\left({N}_{60}\right)}^{0.26} $$
(5)

Moreover, the Vs30 was then calculated according the Boore (2004) procedure, for shallow boreholes where their depth is less than 30 m. Accordingly, the average values of the Vs30 ranged from 196.086 to 589.931 m/s (Fig. 10). Furthermore, site soil has been classified. The resulting map helps cities assess their earthquake shaking vulnerabilities and prepare mitigation strategies where the generated classes are listed in Table 2. The Vs30 map (Fig. 10) exposed two soil classes (classes C and D) covering the study area (Fig. 11).

Fig. 10
figure 10

Distribution of the average Vs30 all over the study area

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Table 2 The site soil classification of the Jizan study area
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Fig. 11
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Map of the NEHRP site classification for the Jizan area

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Identifying site response

The Strong Motion SIMulation (SMSIM) software (Boore 2003) has been applied for site response estimation considering the values of Vs30, predominant frequency, and the corresponding amplitude amplification factor.

Synthetic ground motion

The synthetic model (SMSIM-program; Boore 2003) has been applied to generate the acceleration-time histories (Atkinson and Boore 1995; Hwang and Huo 1997) where the site Fourier amplitude spectrum can be expressed based on:

$$ \mathrm{Ground}\ \mathrm{motion}\ (f)=\mathrm{Source}(f)\times \mathrm{Path}(f)\times \mathrm{Site}(f)\times \mathrm{Type}(f) $$
(6)
$$ C= R\theta \phi FH/4\pi \sigma {\beta}^3r $$
(7)

where C is the scaling parameter; Rθϕ signifies the radiation pattern for a range of azimuths, θ, and takeoff angles, ϕ; F accounts for the free surface effect; H is the reduction factor, which accounts for the partitioning of energy into two horizontal components; r is the hypocentral distance; and σ and β represent the crustal density and shear wave velocity, respectively. Moreover, the ω-square model (Brune 1970) has been used where:

$$ \mathrm{Source}\ (f)= Mo/\Big(1+\left(f/ fc\left){}^2\right)\right. $$
(8)

where Mo and fc represent the seismic moment and corner frequency. The value of fc can be obtained as follows:

$$ {f}_c=4.9\times {10}^6\beta {\left(\Delta \sigma / Mo\right)}^{1/3} $$
(9)

The path (f) can be understood using both geometrical spreading factor and frequency-dependent quality factor, Q(f),

$$ \mathrm{Path}\ (f)=\mathrm{GSP}(r)\exp \left(-\pi fr/Q(f)\beta \right) $$
(10)

Site (f) is assumed as frequency-dependent amplification where A(f), and diminution, D(f), factors,

$$ \mathrm{Site}\ (f)=A(f)\times D(f) $$
(11)

where A(f) can be defined as follows:

$$ A(f)={\left\{\rho \times \beta /\rho (z)\mathrm{avg}\times \beta \mathrm{avg}(z)\right\}}^{1/2} $$
(12)

where ρ(z)avg and βavg(z) are the average density and average Vs to the depth of a quarter wavelength, respectively. The diminution factor, D(f), is defined as follows:

$$ D(f)=\exp \left(-\pi {k}_of\right) $$
(13)

where ko is the distance-independent high frequency attenuation operator (kappa factor). Type (f) is a filter used to shape the spectrum:

$$ \mathrm{Type}\ (f)={\left(2\pi f\right)}^s $$
(14)

where s is 0, 1, and 2 for acceleration, velocity, and displacement, respectively.

Input parameters

The whole-path attenuation was controlled by the frequency-dependent quality factor, Q. The geometrical spreading factor is equal to 1/r for r < 100 km and 1/10 √r for r > 100 km (Singh et al. 1999), where r is the hypocentral distance. The distance-dependent duration is a function of the path and source

$$ T=1/{f}_c+ br $$
(15)

where 1/fc is the source duration; br is a distance-dependent account for dispersion. The value of 0.05 to variable b has been assigned (Herrmann 1985; Boore and Atkinson 1987). The Vs30 and rock-type densities applied through this study were obtained from 91 sites in the Jizan area. Site amplification [A(f)] at the assumed engineering bedrock level was determined by the quarter wavelength method for the 91 geotechnical boreholes in Jizan City (Fig. 12), while the fundamental frequencies were also calculated (Fig. 13) at the same borehole locations.

Fig. 12
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The amplification factor distribution throughout the Jizan area

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Fig. 13
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Variation of the predominant frequency (fo) within the Jizan study area

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According to Fig. 12, the values of the ground motion amplification factor vary between 1.6 and 18.2 throughout the study area. This significant variation reflects the thickness difference in boreholes. Figure 12 indicates that higher amplification values extend along a north–south strip westword with very soft/loose sediments. In contrast, Fig. 13 illustrates that higher fundamental frequency values (≥ 1 Hz) cover the Jizan salt dome and extend due north parallel to the coast until the Ash-Shati and Ar-Rawabi districts that represent compacted sediments and salt dome that underlie these areas.

Chandler et al. (2006) collected a database of estimated ko values for numerous regions of the world using the average Vs30. The ko was calculated as follows:

$$ {k}_o=\frac{0.057}{V_{s30}^{0.8}}-0.02 $$
(16)

The resulted average ko for the bedrock site in the Jizan area was 0.019.

The bedrock ground motion parameters were considered for the 91 boreholes in the Jizan area using the SMSIM software resulting from 6.25 Mw earthquake (Fig. 14). Here, the horizontal motion, which corresponds to a peak value of acceleration at the bedrock level, was synthetically generated. Furthermore, the PGA was used to generate an ArcGIS contour map (Fig. 15). According to Fig. 15, the values of the PGA at the bedrock are low, varying between 4.8 and 10.7 cm/s2.

Fig. 14
figure 14

Ground motion simulated time series for PGA, PGV, and PGD at the bedrock level

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Fig. 15
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The simulated PGA at the bedrock level all over the study area

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Ground surface response spectra

The input motions at the bedrock level were produced for all of the borehole locations considering the hypocentral distance to each borehole site. Then, the estimated values of Vs30 at each borehole location were used to calculate the site response effects throughout the study area (Figs. 16 and 17).

Fig. 16
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The simulated ground motion time series at the ground surface

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Fig. 17
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Variation of the peak ground acceleration (PGA) at the ground surface

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According to Fig. 17, the values of the PGA at the ground surface level vary between 7.7 and 32 cm/s2 oriented north–south through the western zone of the Jizan study area. The response spectra are determined at a critical damping of 5% (Fig. 18).

Fig. 18
figure 18

Examples of site response spectra in Jizan area

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Depending on the response spectra, Fig. 18 reveals that a sharp peak (103 cm/s2) occurred at 1.0 Hz in borehole 41 while two peaks appeared at site 3, where the first peak was 21 cm/s2 at 2 Hz and the second peak was 28 cm/s2 at 10.5 Hz. Moreover, site 21 reveals two peaks in the PGA of 21 and 18.5 cm/s2 at frequencies of 1 and 10 Hz, respectively. Borehole 91 shows a clear and sharp peak of 17.3 cm/s2 at 10 Hz. The same phenomenon was recorded at 10 cm/s2 at 1.5 Hz. Furthermore, two peaks were recorded at 25 and 19 cm/s2 at frequencies of 1 and 10 Hz, respectively. Thus, regions in zones with a PGA larger than 103 cm/s2 are more seismically vulnerable than others. However, a very high PGA that lasts for only a very short period with very high frequencies may cause damages to structures. Hence, an improved estimation for regions characterized by high seismic vulnerability can be prepared by identifying which areas are susceptible to higher rock motion amplification.

Discussion and conclusions

Jizan City is located along the southwestern coast of Saudi Arabia; the old city was built on an uplifted salt dome surrounded by a low-lying flat area. Jizan City has recently experienced rapid growth and significant economic, urban, and industrial projects. Due to heterogeneities in the thicknesses and lithofacies, with significant variations over short distances, the city suffers from geotechnical problems, including land subsidence; foundation failure; and damage to buildings, roads, and infrastructure, especially in the old town, which have destroyed many facilities.

The distribution of earthquakes affecting the Jizan area is related to Red Sea floor spreading and in-land volcanism. One of the largest events, at a magnitude of MS 6.5, occurred on January, 11 1941, near the current Saudi Arabia-Yemen border (epicenter at 16.4° N and 43.5° E, epicentral intensity of MSK VIII). This earthquake caused several hundred deaths and damaged 1700 houses, among which 300 were destroyed and 400 were damaged beyond repair (Ambraseys et al. 1994). On January 23, 2014, an earthquake sequence was recorded approximately 50 km northeast of Jizan City; the magnitude of the largest event was estimated at ML 5.1 (El-Hadidy 2015; El-Hadidy et al. 2015).

Ninety-one geotechnical boreholes were drilled throughout Jizan City. The maximum penetrated depth of these holes was approximately 30 m. The subsoil profiles were grouped into zones for the different districts in the Jizan area. After this procedure, the Vs30 was calculated for boreholes and varied from 196.086 to 589.931 m/s. Moreover, the site soils classified into two soil classes (classes C and D) cover the study area. The estimated amplification factor varied between 1.6 and 18.2 throughout the study area. This reflects a significant variation in the thicknesses of the soft soil sediments within the boreholes. It is noticed that the higher amplification values extend along a north–south strip through the western part indicating very soft/loose sediments characterized by a substantial thickness. In contrast, the higher fundamental frequencies (≥ 1 Hz) extend due north parallel to the coast until the Ash-Shati and Ar-Rawabi districts that represents an extension of compacted sediments.

The bedrock-generated PGA is contoured where it varies between 4.8 and 10.7 cm/s2. While, ground surface PGA ranges from 7.7 to 32.0 cm/s2. The higher PGA extends north–south to the west of the Jizan area. Furthermore, site response spectra have been estimated at the ground level with 5% critical damping. The pseudo-spectral acceleration (PSA) values at frequencies of 1.5, 3, 5, 8, and 10 Hz for a 5% critical damping ratio have been computed for 91 locations and mapped using ArcGIS package (Figs. 19, 20, 21, 22, and 23) where this frequency range represents the natural frequencies of tall to low-rise buildings. According to Figs. 19, 20, 21, 22, and 23, the higher PSA values at the lower frequencies of 1.5, 3.0, and 5.0 Hz cover the western zone of the Jizan area to the east of the Jizan salt dome. For the frequencies of 8.0 and 10.0 Hz, the higher PSA values cover a wide area in the western zone, including the presence of Jizan salt dome. The presence of higher PSA values reflects a high seismic risk compared with the lower values.

Fig. 19
figure 19

Distribution of the simulated PGA at 1.5 Hz throughout the Jizan area

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Fig. 20
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Distribution of the simulated PGA at 3.0 Hz throughout the Jizan area

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Fig. 21
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Distribution of the simulated PGA at 5.0 Hz throughout the Jizan area

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Fig. 22
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Distribution of the simulated PGA at 8.0 Hz throughout the Jizan area

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Fig. 23
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Distribution of the simulated PGA at 10.0 Hz throughout the Jizan area

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The maximum PSA and corresponding frequencies of all 91 borehole sites were computed and displayed using ArcGIS (Figs. 24 and 25). The values of the maximum PSA ranged between 17 and 103 cm/s2, covering the western zone of the Jizan study area (Fig. 24). These results must be considered by the engineering administration of the Jizan municipality before initiating land use planning and establishing residential, educational, developmental, and health facilities whether for governmental or private purposes.

Fig. 24
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Distribution of the maximum PSA throughout the Jizan study area

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Fig. 25
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Distribution of the maximum frequencies throughout the Jizan study area

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