The Gulf of Aqaba area is considered one of the most terrific touristic areas in the Middle East. The aim of the present work is to determine the amount of seismic hazards that the constructions may suffer due to seismic activities. This is done by determining the design response spectrum for this area from available earthquake response spectra, then taking into consideration the soil response for some Egyptian and Jordanian soils. The main shock of the November 22, 1995, the Gulf of Aqaba and its aftershocks were mainly used in producing the design response spectrum. This earthquake was considered as the biggest earthquake that hit this area since 160 years. Its magnitude was determined as Mw = 7.2. Thousands of aftershocks with intermediate magnitude followed the main shock, such as the aftershock that occurred on November 23, 1995 with a local magnitude of M L = 5.4. The best estimate of the focus location was determined in the area between Dahab and Nuweiba cities. This great earthquake was felt in Lebanon, Syria and Israel in the North and Egypt, Saudi Arabia and Sudan in the South. The touristic areas surrounding the Gulf of Aqaba were mostly affected. Different accelerograms for this great earthquake were collected and soil responses spectra for Sinai Peninsula and some Jordanian soils were calculated. The design response spectrum shows an average spectral acceleration of about 250 cm/sec 2 for frequency range between 1 - 10 HZ. Soil Amplifications were then calculated using Microtremors site response technique and maximum spectral accelerations filtered by the soil were in range between 120 - 450 cm/sec 2 for the study area. The analysis presented here is intended to be used in the future to allow reducing the seismic risk, help in proper structural design and detailing of buildings and structures to account for beam-column connections and shear reinforcement.
It is well known that after the earthquake occurs, the areas that experienced the maximum peak ground acceleration (PGA) are not necessarily showing the maximum damage. This is simply because acceleration is modified and amplified by soil and then again by structure. So one of the most important parts in making seismic hazard analysis is to make assessment for the soil. The soil response may increase or decrease the effect of the earthquake based on its composition. The present work is very important for two reasons: one is that it uses the spectral acceleration which is very rare in this area to accurately show acceleration values carried on which frequencies, second to make soil frequency analysis to know the amount of energy that will enter this soil and consequently the structures. Such work is very important for engineers to know the specific effect that their design will suffer especially when they know its resonance frequency. So the fundamental natural frequency of the soils and structures is very important in seismic hazard assessment. For this reason, the concept of the response spectra was introduced. This important engineering quantity is determined from the original earthquake ground motion by using narrow band-pass filters that acts like simple oscillators or structures [
Simple systems such as simple pendulum can be used for simulating the performance of simple structures during earthquake excitation [
The main types of displacement, velocity or acceleration responses are:
1) Relative Displacement (RD)
2) Relative Velocity (RV)
3) Pseudo Relative Velocity (PSRV)
4) Absolute Acceleration (AA)
5) Pseudo Absolute Acceleration (PSAA)
Where,
RD: is the maximum value of relative displacement of the simple system during vibratory Motion,
RV: is the true relative velocity of oscillator,
PSRV: is the maximum velocity relative to its base, of the center of mass of resonant simple structure.
AA: is the true absolute acceleration of oscillator and
PSAA: is the measure of maximum elastic spring force per unit of mass.
PSAA is actually quite close to AA but PSRV can be quite different from RV.
The soil independent procedure is based on the use of standard spectrum shapes. The standard spectrum shapes are considered to be independent regardless of the characteristics of the site because the original seismograms from which the spectra were derived depict ground motions for a wide range of geological and seismological conditions. Housner 1959 [
Another technique used is the relation between damping of response spectrum for a certain range of frequencies and the amplification factors which is shown in
damping, the peak ground motion values are shifted or amplified and smoothened to give the design response spectrum at the specific site. The derived response spectrum discussed by Newmark and Hall [
In 1973, another important “soil-independent response spectra” was introduced by the U.S. Atomic Energy Commission (AEC) in which the same idea of normalizing amplification factors was used by (Newmark and Hall, 1969) [
The “soil dependent response spectrum” was introduced before the “soil independent response spectrum”. The soil dependent response spectrum uses similar seismograms recorded over soils having the same geological and seismological conditions. However, it is very difficult to find a set of ground motion data having the same or similar source focal mechanism, attenuation path and soil conditions to determine soil dependent response spectrum. Seed et al., 1976 [
a) Rocky soils (28 accelerograms),
b) Stiff soils (31 accelerograms),
c) Cohesionless soils (30 accelerograms)
d) Soft and Medium clayey soils (15 accelerograms).
[
This was found to be in a good agreement with [
frequency when compared with rocky or stiff soils (
Damages of the Aqaba earthquake were distributed in a large area and many cities ranging from Jordan in the North to Sudan in the South. This is most probably due to the large magnitude of the earthquake [
Most of the severe damages were occurred in Egypt for old and deteriorated buildings which were not properly designed to resist earthquake loads. Severe damages were occurred in many cities from Nuweiba (about 35 km from focus) until Cairo (About 370 km from the epicenter). The majority of the damage was occurred at the city of Nuweiba on the Gulf of Aqaba (
Nuweiba city, including failure of the port quay-wall due to liquefaction. The three-story Paracoda hotel was shattered completely and some other hotels were suffered damages (e.g. Dolphin village, Coral, Helnan). Other very important
structures such as the distillation plant and electrical power plant were subjected to damages causing loss of electricity. The damages were also observed along the roads between Dahab and Nuweiba by stones falling from hills.
In the town of Aqaba one poorly constructed building was collapsed [
One person was died from a heart attack and several others were injured. In the port city of Eilat, damage in the Sport Hotel on the gulf beach was reported [
One person was killed and two were slightly injured. At El-Durra customs office (~90 km), a complete collapse of a free standing shed was occurred. Another collapse of a concrete roof of the passenger terminal at the same facility was reported. The concrete beams carrying the water tank at the Haql border guard headquarters were also damaged [
Different response spectra were obtained for this earthquake. The effect of the great Aqaba 1995 earthquake continued for about 490 km along the Jordanian axis. The recording stations of the earthquake are listed in
Recording station AQA2 (
At a further distance where Amman recording station exists, 390 km from earthquake epicenter, the effect becomes very weak (
The effect of the earthquake at 450 km is showing a weak spectral acceleration of about 20 cm/sec2 for Yarmouk station at about 2 sec (0.5 Hz) (
On February 21st, 1996, an aftershock of magnitude 4.7 for the same earthquake was recorded by the Egyptian Geological Survey at Dahab and Nuweiba (~35 km from epicenter). Both stations are initiated over basement rocks (
Station | M | Date | Ref. | Soil Type | Max. PGA (cm/sec2) |
---|---|---|---|---|---|
Mokat | 7.1 | 22/11/95 | CU | Limestone | 8.5 |
AQA1 | 7.1 | 22/11/95 | JSL | Alluvium | 66.5 |
AQA2 | 7.1 | 22/11/95 | JSL | Sand | 157 |
MA’N | 7.1 | 22/11/95 | JSL | Sand | 19.5 |
Amman | 7.1 | 22/11/95 | JSL | Sand | 2.8 |
Yarmouk | 7.1 | 22/11/95 | JSL | Alluvium | 4.6 |
Nuweiba | 4.7 | 21/2/96 | EGS | Basement | 22.6 |
Dahab | 4.7 | 21/2/96 | EGS | Basement | 56.2 |
Nuweiba | 3.9 | 26/2/96 | EGS | Basement | 35.2 |
Dahab | 3.9 | 26/2/96 | EGS | Basement | 29.3 |
CU: Cairo University; JSL: Jordan Seismological Lab; EGS: Egyptian Geological Survey.
Again on 26/2/1996, an aftershock of magnitude 3.9 for the great Aqaba earthquake was recorded by the same stations [
Average design response spectrum for Aqaba region was determined using the main shock recorded on November 22, 1995 over the Jordanian soil at stations AQA1 (Alluvium) and AQA2 (Sand). We used also the recorded aftershocks for the same earthquake occurred on November 21, 1996 (M = 4.7) and November 26, 1996 (M = 3.9) at the city of Dahab and Nuweiba over the basement rocks. These are considered as moderate earthquakes affected the Gulf area. The spectral acceleration obtained was normalized to average damping value of 5% (
The Microtremors site response method [
for the Jordanian soils (
1) Recording 15-min of Microtremors at a fixed reference station (representing soil base) and another mobile station moving among variable Jordanian sites simultaneously (both stations work together and synchronized in time),
2) Zero correction to the total 15-min. Microtremors noise at time domain was applied,
3) We then subdivided each 15-min. Microtremors signal into fifteen 1-min sub windows, each of these series was tapered with a 3-sec hanning taper and converted to the frequency domain using a Fast Fourier transform,
4) We then smoothed the amplitude spectrum by convolution with 0.2 Hz boxcar window,
5) Soil response of a given site location is derived by dividing the average spectrum of the mobile station for all processed 15 sub windows at each site; over the response of the reference station recorded over the nearest bedrock (best rocky site nearby the recording station).
6) After that, we smoothed the final response curves by running average filter for better viewing. A complete description to the methodology can be found in [
For the Egyptian soils like Umm Baraqa and Ras Mohamed, the response was determined using [
Soil frequency map was introduced using the fundamental natural frequency of vibration for the Jordanian and Egyptian soils surrounding the Gulf of Aqaba. Different soil composition were found in the gulf area such as the clay deposits found at Umm Baraqa or basement rocks found at Saint Catherine region (
Maximum Spectral acceleration Map for Sinai and surrounding Jordanian soils was then determined using the generated design response spectrum (
The fundamental natural frequency of vibration for the Sinai Peninsula and some Jordanian soils was determined using microtremors site response [
Design response spectrum for the Gulf of Aqaba region was calculated using four response spectra recorded at four stations. Two stations recorded the main shock AQA1 (M = 7.1, 1995); AQA2 (M = 7.1, 1995) and another two stations recorded moderate values aftershocks, Dahab (M = 4.7, 1996) and Nuweiba (M = 4.7, 1996). The average design response spectrum showed an average value of 220 cm/sec2 for frequency range 1.5 - 10 HZ.
Maximum Spectral acceleration Map for Sinai and surrounding Jordanian soils was then determined using the generated design response spectrum, soil
Site | Distance from Gulf of Aqaba (Km) | Fundamental Resonance Frequency (HZ) | Amplification Factor |
---|---|---|---|
S1 | 0 | 2.5 | 1.1 |
S2 | 25 | 2.3 | 1 |
S3 | 50 | 3.1 | 0.65 |
S4 | 75 | 3.2 | 0.85 |
S5 | 105 | 2.9 | 0.85 |
S6 | 126 | 3.4 | 0.85 |
S7 | 145 | 2.2 | 0.8 |
S8 | 165 | 1 | 1.35 |
S9 | 192 | 0.8 | 0.9 |
S10 | 212 | 0.9 | 0.9 |
S11 | 232 | 0.9 | 0.85 |
S12 | 252 | 0.9 | 1.2 |
S13 | 285 | 0.8 | 0.85 |
S14 | 305 | 3.1 | 0.9 |
S15 | 325 | 1.5 | 0.75 |
S16 | 343 | 0.9 | 0.9 |
S17 | 360 | 1 | 1.35 |
S18 | 380 | 1 | 1.1 |
S19 | 394 | 0.9 | 1.4 |
frequency map and soil amplification values determined for each site in this study. The Maximum spectral acceleration map splits Sinai into two parts: the western part which is showing maximum spectral acceleration between 120 - 250 cm/sec2 and the Eastern part which is showing maximum spectral acceleration between 280 - 440 cm/sec2. The Jordanian soil in the path Aqaba-Amman is showing maximum spectral acceleration between 150 - 250 cm/sec2. The Gulf area is surrounded by maximum spectral acceleration between 300 - 440 cm/sec2.
Although the distance between the epicenter of the great earthquake of Aqaba 1995, and both capitals of Jordan and Egypt are nearly the same (~380 km), the spectral acceleration recorded in the direction of the capital of Jordan axis is showing a higher rate of attenuation (5 - 10 cm/sec2, Amman) rather than in the direction of the capital of Egypt (~40 cm/sec2, Cairo). This is mainly due to the attenuation path and the nature of the overlying soil which is alluvium and sandy in the direction of Amman, while basement rocks are abundant in the direction of Cairo city at the Sinai Peninsula.
Most buildings found in the region of the Gulf of Aqaba are of few floors < 7 floors (in resorts and touristic areas or small villages). These buildings have
natural frequencies of vibration between 5 - 10 Hz (0.1 - 0.2 Sec). It may be concluded that they will be suffering from high spectral acceleration of about 220 cm/sec2 over the bedrock (Sites like Saint Catherine and rock sites where no amplification exist).
Based on the maximum spectral acceleration map and the damage reports in the cities surrounding the Gulf of Aqaba, an important conclusion is that, the spectral acceleration will exceed the values of the maximum recorded accelerations to reach the level of 440 cm/sec2, especially over the soils of thick and soft
deposits. This is in good agreement with the damage reports for the area surrounding the gulf such as Dahab and Nuweiba which showed that almost all buildings had suffered from damages without shattering. This occurred in the main shock recorded on November 22nd, 1995.
It is recommended to increase the level of the maximum expected acceleration in the Gulf area to ³ 440 cm/sec2. This should be taken into consideration especially when initiating new structures, resorts and hotels. Without taking into consideration these high levels of spectral acceleration, the proper structural design and detailing of shear reinforcement will be affected. This may cause construction instabilities and ultimately complete collapse as was observed in 1995 Aqaba earthquake for buildings in cities of Dahab and Nuweiba in Egypt.
It is important also to realize that the natural frequencies of vibration in this study are not sufficient to account for all soil changes. So other elaborated studies should take into consideration this note and make more extensive work to account for all soil variations.
The area of the Gulf of Aqaba need more spectral acceleration recordings in order to account for different soil dependent and soil independent response spectra and help making mare accurate design response spectrum. This can be done using at least 100 acceleration times history. This could be achieved when more acceleration stations is planted in this area especially in the Egyptian part.
We would like to thank the Jordan Seismological Observatory for giving raw data for Aqaba 1995 Earthquake, Dr. Amarat and all Jordan Seismological Observatory team. Also thanks for Drilling Geophysics Co. which provided raw data in the Jordanian part “Jordan-Gas-Pipeline project”.
The authors declare no conflicts of interest regarding the publication of this paper.
Gamal, M.A. and Abdelwahed, A. (2019) The Great Gulf of Aqaba 1995 Earthquake Design Response Spectra over Sinai Peninsula and Some Jordanian Soils. International Journal of Geosciences, 10, 463-480. https://doi.org/10.4236/ijg.2019.104027