This is the approximate location of the epicentre of the earthquake that struck Northeastern UAE on 11th March 2002. It is an approximate location due to the uncertainty over exact epicentre location as, at the time, there were no local seismic monitoring stations capable of providing a location. The GZ used is that of the United States Geological Survey determination of the Epicentre.
Whilst the area around the quake is not densely populated, the quake was strong enough to be felt throughout the Northern Emirates including Ras Al Khaimah, Sharjah and as far away as Dubai.
The main earthquake was registered as being 5.1 on the Richter scale, with approximately 25 smaller tremors being felt in the following weeks, although most of these tremors were found to originate on the Iranian side of the Arabian Gulf, unlike the initial earthquake. “Foreshocks” were also felt in the months leading up to the earthquake.
A further earthquake of 5.1 on the Richter scale was registered in early April 2002 with an epicentre close to the one on 11th March, however it was at a much greater depth.
It’s all about the plates…
Tectonically, the UAE is situated in the South-Eastern part of the Arabian plate. The earth is made up of major plates (each having an area greater than 10 million km2), minor plates (less than 10 million km2 but more than 1 million km2) and microplates (less than 1 million km2). The Arabian plate covers roughly 5 million km2 and is one of the youngest plates that make up the surface of the earth.
The Arabian plate has slowly been on the move – Northward. The plate is moving around 20-25mm per year, colliding with the Eurasian plate. The plate was once part of the African plate, however the collision with the Eurasian plate led to the formation of the Zagros fold belt which, in addition to raising the elevation of the Zagros mountains, causes earthquakes.
The Zagros belt is one of the most active fault zones in the world. Whilst the vast majority of recorded earthquakes occur along the fault line primarily in Iran as can be seen below. One recent example with a high casualty rate was the earthquake in Bam (Iran) in 2003 that registered 6.6 on the Richter Scale and killed 26, 271 people.
Mapped Earthquakes in the Arabian Gulf © ISC Data
Geology and the Earthquake
By conducting waveform modelling, an estimation of the direction of the slip of the rock – causing the earthquake, known as the “focal mechanism”. This direction was consistent with pre-existing faults in the area that were caused by the obduction of the Semail ophiolite.
The Semail ophiolite block is separated into 12 “blocks”, all of which are separated by faults. Two of these blocks are located almost entirely within the UAE and it is one of these blocks, the “Khor Fakkan block” that the tectonic movement was centred on. As a result of the modelling, it was considered likely that the earthquake was caused by reactivation of an older fault
Seismology
In the weeks following the earthquake, more than 25 earthquakes were recorded in the region with a strength of between 4 and 4.8 on the Richter Scale. These were located along the Zagros Fault in Iran and were then followed in April by another 5.1 strength earthquake in the same region near Masafi, although at a greater depth to the previous quake. The area was previously not thought to be seismically active, however monitoring stations were installed after the earthquake with some seismicity resulting in small and moderate sized earthquakes having been recorded since as seen below:
Following such monitoring, it has been deduced that the earthquake that struck the Masafi region was caused by a Northeast normal fault with a small shear component. Since the monitoring stations have been active, seismologists have identified several Northeast faults linking two major right lateral strike slip faults. The shear component is due to the difference in convergence rates between:
- The continental collision of the Arabian and Eurasian Plates along the Zagros fault region
- The oceanic–continental convergence in the Makkran region.
The Masafi earthquake of 2002 thankfully did not cause any injuries or largescale damage to infrastructure. It did, however, enable a greater understanding of the seismic activity within the UAE and enable a greater understanding of the risk of future earthquakes and their causes.
Questions:
1) Is the Arabian Plate categorised as a major plate, minor plate or micro plate? Explain why.
2) Explain the link between the Zagros mountains in Iran and the earthquakes experienced in the UAE.
3) Why was the earthquake in April not felt by residents, unlike the one in March, despite being the same strength on the Richter scale?
4) Looking at the map of earthquakes in the Arabian Gulf, would you rather construct a tall building in Fujairah or Abu Dhabi? Why?
5) When at GZ looking in a South Westwards direction what do you see? Why would this have prompted initial suspicions that the earthquake had had a “man-made” cause?
6) When at GZ, describe the size of the rocks under your feet. Are they a mass of rock, small boulders or fine grains? In case of an earthquake, how do you think the rock composition could impact on the track below?
7) OPTIONAL: Take a photograph of yourself approaching or at GZ.
Note: Please email your answers to the email address on my geocaching profile (geocyberinkblot@gmail.com)
You are free to log your visit prior to approval of submitted answers. Incorrect answers will be returned to enable further details to be submitted prior to any logs being deleted. I endeavour to provide a response to all submissions within 7 days however this cannot always be guaranteed due to work assignments often taking me out of Internet range for periods of time.
References include:
- https://workspace.imperial.ac.uk/geotechnics/public/PhD%20Thesis%20-%20G%20Aldama%20Bustos%20-%20Final%20version.pdf
- http://adsabs.harvard.edu/abs/2003EAEJA.....1700A
- http://www.researchgate.net/publication/226345564_Scenarios_of_Seismic_Risk_in_the_United_Arab_Emirates_an_Approximate_Estimate
- http://www.iitk.ac.in/nicee/wcee/article/13_1008.pdf
- http://www.researchgate.net/publication/261712914_Improved_Characterization_of_Local_Seismicity_Using_the_Dubai_Seismic_Network_United_Arab_Emirates