Residents of Izmit, Turkey were sleeping when the ground began to shake on August 17, 1999. tremor recorded a magnitude of 7.4, triggered aftershocks and proved totally devastating, leaving over 17,000 dead, tens of thousands injured and billions of dollars in damage. But it was extraordinary for another reason: it affected the movement of a tectonic plate. The finding could prompt a rethink of how earthquake risk models are put together.
Scientists have long known that earthquakes occur when landslides occur along fault lines. These can be generated by stresses on the edges of tectonic plates, which are in constant motion on the Earth’s mantle. But researchers investigating the Izmit earthquake made unprecedented observations of plate motion and concluded that earthquakes themselves can affect plate motion, which could, in turn, affect subsequent earthquakes. “These results suggest the existence of a whole-plate kinematic signal associated with the stress released by large earthquakes,” they wrote in a paper. International Geophysical Journal study.
GPS data reveals movement change
Plate tectonics theory, generally accepted since the mid-20th century, assumes that large earthquakes do not disturb plate motion; instead, motion is the result of energy transfer via stress accumulated over long periods of time. The researchers reasoned that although this one-way relationship is the case for large plates, microplates may be more susceptible to the influence of earthquakes due to their reduced area spanning the boundary between the lithosphere and asthenosphere. .
The Anatolian Microplate offered a good opportunity to test this idea, not only because it is small, spanning most of Turkey and moving towards the Mediterranean Sea, but also because it underlies a region which has had good GPS data coverage for decades. GPS data is used in geodetic surveys, which analyze points on the Earth’s surface, including how they are affected by earthquakes. Comparing data over time can reveal plate movement.
The researchers calculated that the rotational force, or torque, needed to alter the trajectory of the tectonic plate was very close to the torque released by earthquakes. In an analysis of GPS data, they found that not only did the Anatolian microplate change direction, but the frequency of earthquakes around Turkey also changed after the Izmit earthquake.
“In our study, we observe a change in Anatolian kinematics from before to after the Izmit earthquake, and we find that the change in force required to alter Anatolian motion to the degree determined by geodetic observations is in agreement with the force change imparted by the Izmit earthquake,” said the co-author Juan Martin de Blaspostdoctoral researcher in geology at the University of Copenhagen.
“Earthquakes are not just the product of plate movements,” he added. “Instead, earthquakes and plate motions are linked in a feedback mechanism, meaning that large seismic events are able to alter the rigid motion of the plates. This link has further implications as it implies that plate motions may also vary over the years, or even decades, before an earthquake occurs.
A slew of new questions
This feedback mechanism could represent a major new insight into plate tectonics. The challenge to one of the central tenets of tectonics – that plate motions are fundamentally stable throughout the earthquake cycle – met with some resistance during the study’s review process. However, de Blas said, the team found open-minded reviewers who offered suggestions that helped strengthen the document.
The researchers believe this idea should be incorporated into computer models used to calculate earthquake risk. Instead of assuming that plate motions are constant, a feedback mechanism between earthquakes and plates should be taken into account, they argue. This calculation also includes greater use of data from GPS devices located far from the edges of the plate to give a better overall picture of plate dynamics, according to the team.
“Although scientists suspect that plate velocities may vary somewhat over decades or even millennia, this variation has not been documented as clearly as it has been in this study for plates as large as the Anatolian Plate,” said Richard Styron, a specialist in active faults with the Italian foundation Global Earthquake Model, who did not participate in the study. “Future research will be needed to better understand the mechanisms of this change, in particular the response of the viscoelastic lower crust and upper mantle that modulate plate motions and seismic cycle timing.”
Other unknowns include whether the change in plate motion is permanent, linked to broader regional tectonic evolution, or a transient change that will be reversed, said Styron, who hailed the work as “a remarkable study.”
Meanwhile, de Blas and his colleagues plan to investigate other areas to find evidence for the feedback mechanism. They are looking for geological settings where similar dynamics apply and will test whether the feedback mechanism also applies to the interseismic period, when energy builds up before being suddenly released by earthquakes.
—Tim Hornyak (@robotopia), science writer