Camilla Penney, COMET Research Student (Cambridge)
Camilla is a postgraduate student at the University of Cambridge studying continental tectonics. She uses information from earthquakes, satellite imagery and GPS to study how continents move and change shape over time.
Combining datasets has helped COMET scientists to understand one of the world’s most enigmatic subduction zones.
The Makran subduction zone, which runs for 1000 km parallel to the southern coasts of Iran and Pakistan, is a difficult place to access. As a result, there are many outstanding questions about the deformation of the region, particularly the onshore part of the overriding plate.
By working with Iranian and Pakistani colleagues, and combining data from GPS, seismology and satellite imagery, we have been able to study the kinematics of the Makran, and to understand how deformation in the region varies in both space and time.
The key question in terms of hazard is whether the Makran megathrust is accumulating elastic strain, which could be released in a large, probably tsunamigenic earthquake. An M8 earthquake in 1945 demonstrated that the eastern (now Pakistani) part of the megathrust can produce large earthquakes. However, there is no record of historical earthquakes in the western (Iranian) Makran, although this may be because of the region’s sparse population.
Using new GPS data collected by colleagues in Iran, it was found that shortening observed in the Iranian Makran is consistent with elastic strain accumulation above a locked megathrust. This result suggests that the western Makran may move in large earthquakes, posing a risk to the rapidly growing megacity of Karachi, as well as the Omani capital, Muscat, and industrialising ports in Iran and Pakistan.
This elastic strain accumulation might also provide the solution to another long-standing tectonic question. The Sistan Suture Zone, to the north of the Makran, accommodates right-lateral motion between central Iran and Afghanistan on a series of north-south striking faults. These faults don’t continue through the Makran, raising the question of how right-lateral motion is accommodated at the southern end of the Sistan Suture Zone.
It may be that this motion is transferred across the Jaz Murian depression onto right-lateral faults at the western end of the Makran. This requires the Jaz Murian depression to be bounded by normal faults. By looking at the geomorphology, it can be inferred that the depression is bounded by active dip-slip faults, with no evidence of thrust faulting. However, no shallow normal-faulting earthquakes have been observed around the Jaz Murian. This leads to the suggestion that the faults bounding the Jaz Murian are held in compression in the megathrust interseismic period and may then move in normal-faulting earthquakes after a megathrust earthquake in the western Makran.
As the world’s largest exposed accretionary prism, the Makran also provides an interesting insight into how strain in accretionary prisms varies in space and time. The majority of onshore shallow earthquakes in the Makran are strike-slip and several, including the 2013 Minab and Balochistan earthquakes, show evidence of thrust faults being reactivated in a new strain regime. This suggests that the Makran accretionary prism has reached the maximum elevation which can be supported by the underlying megathrust, allowing us to make inferences about the frictional properties of the megathrust.
Undoubtedly, the Makran has many more insights to offer into the kinematics of subduction zones and accretionary prisms. Continued international collaboration, along with combining a wide range of techniques, should further understanding both of this region and of subduction tectonics globally.
Penney, C. et al. (2017) Megathrust and accretionary wedge properties and behaviour in the Makran subduction zone, Geophys. J. Int. doi: 10.1093/gji/ggx126