National Centre for Earthquakes and Volcanoes
Tectonics of the Arabian Plate
The crust of the earth is composed of many tectonic plates, and most of the major earthquakes occur at the plate boundaries. The eastern and northern margins of the Arabian plate consist of the Zagros and Makran Mountains in Iran, and the Taurus Mountains in southern Turkey, and these form a convergent zone where the Arabian plate collides with the Eurasian plate. The Dead Sea transform fault system in the northwestern part of the Arabian plate extends from the northern part of the Red Sea to the Taurus Mountains in southern Turkey, passing through the Gulf of Aqaba and the Dead Sea. The Owen fracture zone on the south-east edge of the Arabian plate extends from the eastern part of the Gulf of Aden to the Makran Mountains. The Arabian plate moves in a northeasterly direction between the Owen fracture zone and the Dead Sea fault (see diagram), with widening of the Red Sea and Gulf of Aden, and collision or subduction with the Makran, Zagros, and Taurus Mountains.
Earthquakes in Saudi Arabia
Apart from the seismicity along the axis of the Red Sea and along the Gulf of Aden, considerable activity occurs along the Dead Sea transform fault system, and many earthquakes also occur due to collision at the subduction zone along the Zagros Mountain belt. In the central and western part of the shield some of the Cenozoic volcanic areas are still potentially active, and some seismicity is associated with this low-level volcanism.
As far as Saudi Arabia is concerned, the most active area is along the Gulf of Aqaba (Dead Sea transform fault), where the left lateral movement relative to Sinai is 4 to 5 mm per year. The region of the Gulf has active sinistral transform faults with associated pull-apart basins (the deeps in the Gulf of Aqaba), and hence is an area where large damaging earthquakes occur quite regularly. The last major event was the 1995 Haql earthquake in the Gulf of Aqaba (magnitude 7.3) which caused significant damage on both sides of the Gulf and was felt hundreds of kilometers away. Earthquakes of magnitude 6 are common along the spreading axis of the Red Sea but generally they are not felt onshore and appear to pose little risk to infrastructure. The figure here shows earthquake epicentres greater than magnitude 2 in the SGS catalogue for all years up to 2009.
On 19 May, 2009, 19 earthquakes of M4.0 or greater took place in the volcanic area of Harrat Lunayyir to the north of Yanbu, including a M5.4 event that caused minor damage to structures in the town of Al Ays (40 km to the SE). This event produced the spectacular ground cracks seen in the photograph below. The maximum actual dip-slip offset on the fault in the hard rock in the nearby hills was about 90 cm.
Seismology at SGS
In accordance with the council of ministers decision number 228 dated 13/8/1425H (2003 AD), SGS is now responsible for all earthquake monitoring within the Kingdom of Saudi Arabia. Permanent stations from the other previous seismograph networks have now been integrated with the SGS national network and upgraded using new broad-band instrumentation and satellite telemetry. Most earthquakes greater than magnitude 2 within the Kingdom are now routinely located and a comprehensive earthquake data base has also been established for earthquake research. The stations (about 75 at present, see map) are concentrated in western Saudi Arabia, where most of the seismicity and hence the risk occurs. Eventually when the network of about 100 stations is completed the coverage will enable earthquakes as small as magnitude 2 to be detected and located anywhere within the Kingdom.
The SGS seismograph stations use a standard arrangement, with equipment manufactured by Nanometrics Inc. in Canada. Each broad-band seismometer is on bedrock in an insulated concrete vault about 2 m in depth, and is insulated to minimize temperature variations. Data from remote sites are transmitted via satellite to the SGS processing centre in Jeddah and satellite channel usage is managed via commands from the SGS seismic center.
Typical records of seismic waves arriving at 2 SGS stations from an earthquake in eastern Saudi Arabia are shown. The first wave to arrive is the direct P (compressional) wave, followed by a succession of P and S (shear waves). From the arrival times of these waves we can work out the location of the earthquake.
The processing software from Nanometrics used for phase and amplitude picks, plots, and magnitude calculations is Atlas for local events and Hydra for teleseismic earthquakes. Waveforms are viewed on the screen and the arrival times of earthquake waves can be picked manually at each seismograph station and filtered to enhance the signal-to-noise ratio. Earthquake epicenter plots are also viewed in Atlas and magnitudes are calculated. Standard models for the velocity of seismic P waves in the crust and upper mantle are then used in a triangulation procedure to invert the data to give the location, including source depth, origin time and magnitude. Generally events within most of western Saudi Arabia are located with an accuracy of just a few kilometers with the present seismograph network. The locations are then plotted automatically and the data is added to the earthquake database.
The risk of damage from earthquakes is quite low over most of Saudi Arabia, the main areas of risk being near the Gulf of Aqaba and Jizan, with lower risk in the west near the Red Sea and in some of the harrats. The database is used to estimate the expected recurrence rate for earthquakes of different magnitudes in areas of interest, from which statistical estimates of risk are derived. The location can then be placed in the correct zone or level in the building code so that appropriate methods of construction are used. This is particularly important for large infrastructure projects. We undertake risk studies for other government departments as well as for the private sector using our unique catalogue of earthquake activity.