Subdued Red Sea seismicity due to thermal blanket effect of sediments and fluid overpressure?

What is it about?

We calculate the amount by which the Arabian and African plates are moving apart during earthquakes in the northern Red Sea is an order of magnitude smaller than in the southern Red Sea. This diminished seismicity represents a seismic gap and the historical evidence of Ambraseys suggests that it has been persistently quiet seismically. We suggest two main reasons why the seismicity could be reduced here. First, the evaporites (kilometres in thickness) and overlying Plio-Pleistocene hemipelagic sediments (hundreds of metres in thickness) cause a thermal blanketing effect, which raises the lithosphere's temperature by ~100˚K more than otherwise, weakening the brittle part of the lithosphere. Second, the impermeability of the evaporites may allow fluid over-pressures to develop, reducing effective stress in the shallower parts of the brittle lithosphere.

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Photo by Jose Antonio Gallego Vázquez on Unsplash

Photo by Jose Antonio Gallego Vázquez on Unsplash

Why is it important?

Where gaps in seismicity occur along major faults, they can represent areas of potential hazards from large earthquakes. If a M>7.0 earthquake were to occur in the northern Red Sea, it could cause ground shaking and tsunamis impacting coastal towns and ports. More generally the significance of seismic gaps is at the heart of seismic hazard uncertainty arising from the limited time over which seismometer reading have been available (are gaps waiting for a large earthquake to fill them or are they longer term areas of quiescence). In our case, the historical record and our explanations suggest that the northern Red Sea is a long-term gap without major potential. If the geological conditions that we describe are replicated elsewhere, this may offer some reassurance that a major earthquake is not a significant risk. In “seismotectonics”, earthquake properties are used to work out tectonic motions and is particularly important for submerged regions where GPS measurements are impossible. Some "seabed GPS" measurements are being made with acoustics and pressure sensors, though they are unlikely to be used widely because of cost. With around 70% of Earth’s surface underwater, most local tectonic differential motions are not monitored. Our results show that an area known to be deforming (from GPS measurements either side of the Red Sea) has very muted seismicity. Hence if our conditions are duplicated elsewhere there could be tectonic motions that have so far not been revealed by seismic methods. We don’t know how big this problem is, though it is one that the tectonics community should become more aware of as our knowledge of continental motions gets better with further GPS measurements but not the 70% of submerged Earth where GPS measurements are not possible.