|September 14th, 2005, 08:04 PM||#1|
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Earthquake in Makkah
Makkah Quake a Minor Event, Says Geologist
Roger Harrison & Essam Al-Ghalib, Arab News
JEDDAH/MAKKAH, 14 September 2005 — The earthquake that shook Makkah’s Al-Otaibiyyah neighborhood, about two kilometers from the Grand Mosque, at 2 a.m. on Monday has engendered a sense of shock and vulnerability in the citizens of Makkah.
There were no casualties and no damage to buildings, although residents reported dogs barking just before the shocks and a sound of rumbling or what sounded like an explosion.
The tremors, which are variously reported as occurring for between one and nine hours after the initial shock, kept tension high in the city. Suggestions that the tremors were caused by construction work going on in the city have been discounted.
Twenty-three-year-old Siraj Muhammad Omar, a resident of the Al-Otaibiyyah neighborhood, was thankful that there was no damage. He said he felt the earth move four times, with the first and fourth shakes being the strongest. He and seven other family members were scared after the first jolt and rushed to the roof of the building.
Muhammad Baid, a 22-year-old Moroccan, said he left the house soon after feeling the first shake and went back when everything returned to normal.
Seventeen-year-old Aiyman Al-Maihmadi was at the controls of his PlayStation when he heard a low rumble, followed by a soft knock and then a hard knock. He said people rushed on to Shara Al-Jazayer, where many people milled around wondering what had caused the shakes.
Makkah Governor Prince Abdul Majeed has formed a committee to establish the exact circumstances and details of the event.
Dr. Ian Stewart, geophysical technical adviser to the Saudi Geological Society (SGS), said the scale of the shock in Makkah was “not worth worrying about.”
The earthquake was monitored in Madinah, 300 km away, at about 3.7 on the Richter scale.
“Anything which is moderately well built shouldn’t suffer anything at all. It would hardly raise an eyebrow in California,” he said.
California is an extremely active earthquake zone.
The energy released by an earthquake of magnitude 3-3.5 on the Richter scale is roughly equivalent to 30-70 tons of TNT exploding. They are often felt but rarely cause anything other than minimal damage. It is estimated that 50,000 or so take place worldwide each year.
“I imagine, and this is before we have analyzed the data, that the quake was extremely shallow,” said Dr. Stewart.
The Red Sea and the edges of the Arabian tectonic plate, which comprises the whole of Saudi Arabia and the Arabian Peninsula, is an active seismic region. Earthquakes are the result of seismic waves that are generated by sudden movement in masses of rocks, most commonly from slippage along fault lines. These most frequently occur at the margin of tectonic plates — a piece of the Earth’s crust (or lithosphere).
The surface of the Earth consists of seven major tectonic plates and many more minor ones; all are in constant movement. As the Arabian plate is largely contiguous with the borders of the Kingdom, there is considerable seismic activity along the borders.
Jeddah and Makkah lie close to the edge of the Arabian plate and are therefore subject to a surprising number of earthquakes, albeit minor ones, every year.
Dr. Stewart said the center of the Red Sea is quite active with oceanic sea floor spreading taking place. “There are a number of faults running inland and a number of big dyke systems as well. There is quite a hefty patch of activity in the middle — it’s pretty well continually active. I’m not overly worried about living here.”
Jeddah and the west coast of the Kingdom are protected from the shocks of the seismic activity in the Red Sea by a fortunately positioned shock absorber. Running at a depth of some hundreds of meters below the coastline from Obhur Creek all the way to Jizan is a thick layer of evaporite — a rock composed of minerals laid down by the evaporation of a solution; in this case seawater leaving behind wet salt.
This layer behaves, according to Dr. John Robool, geologist at the SGS, “like a large rubber tire acting as a shock absorber. The shocks from the seismic activity under and near Jeddah are absorbed by this layer. It does not, however, extend inland as far as Makkah.”
Most of the fault movements from the Red Sea are in the center and the further away toward the Saudi shoreline, they become minimal. One particularly active center is located in the Suakin Deep, located 70 only kilometers or so west of Jeddah.