If you thought the ground never moved for you, you are wrong; it does — and in the case of Saudi Arabia about two centimeters a year in a generally north easterly direction, rotating counter-clockwise about a theoretical point located in the eastern Mediterranean sea. The whole of Saudi Arabia is slowly colliding with, and sliding under, Iran which is being pushed toward the Himalayas.
The Kingdom is not alone in its protracted peregrinations about the planet. All the world’s land masses are moving to some extent and at various speeds, smashing in slow motion into new continental partners and splitting apart from others in terpsichorean majesty conceivable only on a geological timescale.
The reason for this majestic progression of apparently immovable continents and land masses is that, in planetary terms, all the solid land we exist on is merely a minutely thin and fragmented crust floating on a ball of incredibly hot plastic and liquid rock — magma. The temperature of the layers beneath the surface varies, from a balmy 350 degrees Celsius at about 30 kilometers beneath old continents to a searing 1400 degrees Celsius or more at 150 kilometers beneath the surface.
The hot mantle is solid, down to about a depth of 2900 km, but it is not completely homogeneous and convects very slowly in huge cells. The driver of the convection is heat, both from the liquid (slushy) core and from radioactive decay within the mantle itself. While the mantle is essentially solid, there is a more plastic layer (the asthenosphere) below the crust and the uppermost part of the mantle, at around 100 — 200 km depth, where the temperature and pressure conditions are such that it is not completely solid. This effectively acts as the layer which transports the fragments of continents — tectonic plates — like leaves on the surface of a stream. The crust and the fractured pieces of crust — the tectonic plates — are really very thin. They average about 10 kilometers thick under the oceans and up to 50 kilometers thick under the continents. This is the only layer of the Earth that humans have actually seen.
Visualize it this way: If the 12,756 kilometer diameter of the earth was reduced to the size of a basketball — 250 mm — the continental crust that is Saudi Arabia would be just 1 mm thick.
On the ocean floors, this would reduce to 0.2 mm — about the thickness of this page. Or put another way, tectonic plates are relatively thinner than the orange part of an orange skin in relation to the diameter of the fruit. It puts a whole new spin on the idea of “fragile earth.”
Being merely a thin crust on this boiling mass of rock, the unnervingly thin plates crack and realign themselves occasionally. When these plates collide, or crack at the point where they move apart and are renewed by upwelling of magma from deep below the surface, then seismic activity abounds. As most seismic activity takes place at the edge of tectonic plates — and the Arabian Peninsula is a single plate — the edges of the Kingdom are very active seismic areas. Seismic activity is recordable and therefore in one sense, predictable. While no one can predict precisely when an earthquake will happen, rough time scales and probable locations can be deduced.
Earthquake prediction is far from an exact science. It is a discipline that follows Kierkegaard’s observation that “life is lived forwards but only understood backwards.”
Despite huge efforts over the last three decades or so, seismologists still cannot predict times or locations of earthquake with any degree of accuracy. Past events are only ever be a guide to future locations. As to when they will occur, at best seismologists have only a statistical idea of the frequency of occurrence. And that is precisely what a team of cheerful geophysicists at the Earthquake Center of the Saudi Geological Survey based in Jeddah was set up to do.
Currently four institutions operate seismographic networks which cover the Kingdom in varying degrees. They are the King Abdulaziz Center for Science and Technology, (KACST) King Saud University in Riyadh, King Abdul Aziz University in Jeddah and the Saudi Earthquake Center at the SGS in Jeddah. To some extent, they overlap and duplicate information. Monitoring of seismic activity in the Kingdom began in 1978. King Saud University set up a system in 1982 and KACST in 1998. SGS began its work in 2000.
Last year SGS installed 13 new stations around the Kingdom, six on the west coast between Jeddah and Haql and seven in the Eastern Province. These broadband stations — which can pick up a wide frequency of seismic waves from very long period ones to high frequency ones — have considerably extended the sensitivity and range of data available to the SGS.
The earth acts as a filter, attenuating the recordable seismic waves that an earthquake causes. The high frequency short waves are soon absorbed; the low frequency long waves — sometimes five or more seconds in duration — travel huge distances. The differences in distance and travel times of the seismic waves, coupled with the process of triangulation, allows the seismic center to locate the epicenter and magnitude of an earthquake with considerable accuracy.
“Very soon,” said Hani Zahran, the manager of the Earthquake Center, “the other systems will become rationalized under the control of SGS.” Some of the current equipment spread over the Kingdom is not working, some not compatible — there is a variety of recording methods are being used. “The whole lot has to be rationalized and optimized to get much wider, evenly distributed and efficient coverage.”
Sensing the seismic activity is one thing; getting the data back to the center for analysis is another. Seismic stations are, by their nature, very sensitive to vibrations and so cannot be located near roads where telephone lines are used for transmission as many of the older stations were. “We now use a satellite connection so we can put the stations anywhere,” said Zahran.
The seismic recording stations are independent units. Powered by a battery which is recharged using solar panels, the data is continuously collected and transmitted in what is very nearly “real time.” Events recorded on the seismometer are displayed on the screens in the SGS just 11 seconds later. The stations can be calibrated remotely; the only time they need a visit is if there is a problem with the equipment. The software analyzes the incoming data and presents it as a variety of images and graphs on the screen and gives an accurate location of the seismic event. With the broadband equipment, the entire range of shock waves can be recorded. The seismologist can zoom in and out of the seismic signal to get fine detail which gives a very accurate analysis of the traces.
The results can be located on a map, straight from the incoming data, resulting in an instant positioning. Combined with the recent introduction of the SGS’s digital mapping system, these seismic events can be overlaid onto other maps and related to current geological knowledge of the area in order to build up a comprehensive knowledge of the structure beneath the Kingdom.
Dr Ian Stewart, Geophysical Technical Advisor to SGS said that many of the earthquakes in the Kingdom are not located very accurately. “Many have been located by worldwide networks using structural models which are not perfect,” he said. “There could be errors of tens of kilometers. The new rationalized network will give us a greater density of recording points, give us detailed information and help us locate seismic epicenters much more accurately.”
It is the accuracy of location of seismic events and their potential for destruction that provide the two pillars supporting the center. “It is useful to know where the earthquakes are for geological studies so you can see what’s moving where and can relate it to other measurements,” said Dr Stewart, “for example, mapped geology. The core commercial and social reasons of course are so you can work out the earthquake risk for building projects, allowing low risk areas to be selected and earthquake-resistant structures to be built. That’s the big reason,” explained Dr Stewart. “That’s why it is done everywhere.”
He pointed out that by using the data collected by the seismic sensors, it is possible to establish the probability of suffering a magnitude six earthquake, for example, within a specified period of time. “We know earthquakes follow very well-defined statistics and we can work out the risk,” he said. “If you know that you are going to experience a ground acceleration rate of x centimeters per sec squared in 50 years, you can design buildings to allow for this.”
The Japanese, who live in a very seismically active region, have a huge problem with earthquakes but have great experience with dealing with them. Now they are building a number of tall structures; because these are designed to withstand the accelerations, they can resist most tremors. “The first one was the 30 floor Mitsui building which flexes a certain amount,” said Dr Stewart. “If it were made of plain reinforced concrete, it would fall down.”
Three years ago, a committee was set up in the Kingdom to establish building codes on a formal basis. Fire and safety codes exist, but according to several architects and civil engineers, currently no national codes exist concerning earthquake resistance. However, Hisham Tabbara, vice-president of Saudi Consult (Western Region) says that a code is in force for the western part of the Kingdom and for Jeddah, where the vast majority of seismic activity takes place. “There are codes being followed strictly by the Baladia,” he said “and by all consultants who design in Jeddah.”
Earthquake codes are numbered from 1 to 4; the code currently applied in Jeddah is number 2a. This covers earthquake resistance at various levels — but there is no guarantee that a particular level of code covers a particular level of earthquake on the Richter scale. The nature of earthquakes is such that, at best, buildings can be earthquake-resistant — using lateral reinforcement beams and designing the buildings to be flexible — rather than earthquake-proof.
Riyadh, which has the Kingdom’s tallest buildings, is about as far from seismic activity on the Arabian tectonic plate as it is possible to get. However, with sensitive enough seismographs, the very minor earthquakes that occur almost everywhere can be detected. In practical terms, significant earthquakes start at about three to four on the Richter scale; at three you can feel it. Makkah recently experienced a shock of 3.7 and very minor structural damage occurred. Even minor cracking — especially if it occurs in a supporting beam in a building — is a potential disaster.
At 4.5 — 5 on the Richter scale, a shock begins to create cracking and minor structural damage When the shock climbs to 6 you have to start worrying. The recent earthquake in Pakistan reached 7.6 and was followed up by two or three dozen aftershocks.
Dr. Salah Hadidy, a seismologist, pointed out that the biggest concentration of seismic activity in the area is a trough in the Red Sea between 70 — 100 km off shore due west of Jeddah in the Suakin Deep, another in the Gulf of Aqaba and the third to the south in the Gulf of Aden and along the Arabian Sea. “Slightly less active,” he said, “is the area along the coast of Iran up through to Turkey along the Zagros and Taurus Mountains.”
The trough in the Red Sea is a divergence zone — a point at which the sea floor is spreading and pushing the Arabian tectonic plate away from Africa and into Iran, colliding on the eastern side with Eurasian plate. “As a result of the seafloor spreading, there is considerable seismic activity just off Jeddah.”
The reason Jeddah suffers only very small tremors when it is relatively close to an active zone, which historically has produced shocks of Richter 6 and above, is in a word: salt. Running deep under the west coast, extending from Jizan in the south to Haql in the north is a layer hundreds of meters thick of salt — evaporate. This acts as a gigantic shock-absorber, attenuating the destructive force of the shocks emanating from the Suakin Deep.
Other places nearby are not so lucky; in December 1982 Yemen was hit by a Richter 6 earthquake which devastated Dhamar province leaving over 3,000 dead. More recently, the Gulf of Aqaba in 1995 was hit with a Richter 7.3 shock.
The risk to Jeddah is, said Dr Stewart, “Pretty low. The activity is a hundred kilometers away — and lots of it. The evaporate buffer under Jeddah, however, prevents the shocks reaching us. Here is minor risk — nothing catastrophic. All the activity is where the sea floor spreading occurs.” However, with things on the move as they always are, it’s only a matter of time, albeit geological time.
