COMET researchers have unravelled a complex seismic sequence using a combination of techniques, explaining not only the earthquake sequence itself but also the formation of the mountain range where it occurred.
On August 18 2014, an Mw 6.2 earthquake struck Murmuri, near Dehloran in the Zagros Mountains of South West Iran (Figure 1), and was followed by five aftershocks of Mw of at least 5.4. The largest of these was a Mw 6.0 aftershock which took place 16 hours after the main event.
These were the first large seismic events in the region since important developments in satellite Earth Observation (EO) have allowed us to study earthquakes in unprecedented detail, providing the potential to combine a range of satellite-based and seismological approaches.
The study team, which included COMET scientists from Cambridge and Oxford along with colleagues from Iran, Colorado, and Canada, saw this as an opportunity to shed light not only on the Murmuri earthquake but also on how the Zagros Mountains themselves are evolving.
Figure 1 below shows the distribution of earthquakes in the region along with the 2014 Murmuri event.
The depth of the earthquake-generating thrust faults in the Zagros Mountains has been the subject of debate for some time. Previous studies have disagreed on whether the faults break a thick sequence of sedimentary layers, or are confined to the underlying crystalline rocks.
Importantly, rather than using a single technique, the team combined satellite-based EO techniques with studies of the seismic waves generated by the earthquake and aftershocks. The first step was to identify their locations, which was crucial to understanding the relationship between the ground motions detected by InSAR and the fault planes that caused the earthquakes. They then used the results to generate models of the faults.
Modelling the seismic waves showed that all except one of the events were caused by thrust faults. The smooth signals in the InSAR interferograms meanwhile showed that the faulting which led to the earthquake was buried deep under the surface.
The next question was whether the earthquakes had been caused by a single or multiple faults. The interferograms showed distinct lobes extending to the east and southeast of the main affected area, suggesting that the displacements on the surface were caused by two if not three separate faults.
COMET’s Alex Copley, from the Department of Earth Sciences at the University of Cambridge, explained: “ We found that a single-fault model couldn’t reproduce the deformation patterns shown by the interferograms, so we investigated by applying multiple-fault models instead.”
The team modelled the faults to establish characteristics including their direction, length and angle. When they used the interferograms alone there were a wide range of different fault parameters that could produce models that matched the data, but by including the seismic data the team could narrow down these characteristics.
Dr Copley added: “The only way we managed to work out what actually happened was by using seismological techniques, and then using these results to interpret the satellite measurements.”
The results showed that the 18 August 2014 event involved significant slip on two planes, which produced a complex displacement pattern in the InSAR, and that there were two separate events big enough to produce surface deformation signals, hence the two lobes on the interferograms.
It also became clear that most if not all of the faulting took place in the sedimentary layers rather than the igneous rocks below, at depths of 3-9km. The faults were also found to be longer than they were deep, which is relatively unusual – most faults tend to be more or less equal in length and depth. This could be because changes in the mechanical properties of the rocks below stop the faults from extending any deeper.
As well as explaining the events at Murmuri, the results throw light on the large scale tectonics of the Zagros Mountains, showing which combination of tectonic forces and material properties of the rocks can give rise to the shape and deformation pattern of the mountain range.
Dr Copley summarised: “If we had used seismology or satellite measurements alone we would have failed to learn much that was new about this earthquake sequence. Instead, our approach allowed us to shed light not only on the formation of the Zagros, but also how similar fold-thrust belts form across the globe.”
The full paper is: Copley, A., Karasozen, E., Oveisi, B., Elliott, J.R., Samsonov, S., Nissen, E. Seismogenic faulting of the sedimentary sequence and laterally-variable material properties in the Zagros Mountains (Iran) revealed by the August 2014 Murmuri (E. Dehloran) earthquake sequence, Geophysical Journal International, 2015 doi: 10.1093/gji/ggv365