New research into earthquakes suggests that they are both affected by and affect tectonic plates
At 3am on 17 August 1999, the earth beneath Izmit shook for 37 seconds. The largest earthquake in Turkey in the past 80 years, and one of the deadliest ever recorded, registered a magnitude of 7.6. It caused a motorway to buckle as buildings as far away as Istanbul, 80 kilometres northwest, collapsed. It also changed the movements of a tectonic plate.
It’s widely accepted that earthquakes are the result of a one-way mechanism. As plate boundaries move against each other in a stick–slip motion, energy builds up and releases in the form of seismic waves: earthquakes. But new research into the Izmit earthquake, published in Geophysical Journal International, suggests that the relationship between plate motion and seismicity actually goes both ways.
The development of GPS networks over the last few decades has allowed researchers to test this theory. In Turkey, where networks with sufficient geographical coverage were already developed by the 1990s, data could be collected from before and after the 1999 earthquake. ‘Typically, GPS stations are installed near plate boundaries, because this is where the deformation occurs,’ explains Juan Martin de Blas, co-author of the report. ‘But in our type of study, we were actually measuring how the rigid motion of the plate behaves before and after an earthquake, so we needed to use stations that are far away from the boundaries.’
The Anatolian microplate lies mainly beneath Turkey, where it’s squeezed between the Eurasian Plate to the north and the Arabian Plate to the south, inching slowly westwards at a rate of about two centimetres a year. It’s sufficiently small that the force associated with a magnitude-7.5 earthquake could match the force needed to alter its movement. After analysing all of the data, the researchers discovered that the Izmit earthquake changed the trajectory of the whole continental plate, influencing the frequency of earthquakes since then.
Martin de Blas admits that the paper met with some resistance during the review process. ‘It tests one of the central tenets of plate tectonics – that plate motions are the cause of earthquakes, but that earthquakes do not alter plate motions,’ he says. However, some of the reviewers, interested in the new findings, have contributed to strengthen the paper.
It’s still early days, he acknowledges, and further studies will be needed to establish whether similar dynamics apply in different tectonic settings. ‘In this particular case we study earthquakes occurring in strike–slip faults that move laterally, but another possibility is to explore plates that are connected to subduction zones or plates that are slightly bigger,’ he says. ‘Every tectonic area has its own particular characteristics.’
Even so, the discovery could have implications when it comes to assessing the risk from future earthquakes.