Although earthquakes that strike in the interior of tectonic plates can inflict widespread damage, the processes that drive this type of seismicity are still poorly understood. This is partly due to the lower rates of deformation occurring in these regions compared to those at plate boundaries. Researchers have proposed that intraplate deformation is concentrated along ancient faults inherited from earlier cycles of tectonic activity. But exactly how these inherited structures influence modern seismicity remains a topic of vigorous debate.
Now Tarayoun et al. have quantified the impact of inherited structural features on the deformation occurring within eastern Canada’s Saint Lawrence Valley, a region that has experienced two full cycles of ocean basin inception and closure during the past 1.3 billion years. Using new episodic and continuous GPS data acquired from 143 stations, the team calculated surface deformation rates across the region and compared them to the rates predicted by models of glacial isostatic adjustment (GIA), the main process controlling deformation in the valley today.
The results indicate that within the Saint Lawrence Platform—the geological province paralleling the Saint Lawrence River that is riddled with inherited, large-scale faults—the rates of deformation average 2 to 11 times higher than those measured in the surrounding provinces. And although the GPS-derived and GIA-predicted deformation rates generally agree in the surrounding provinces, the GPS-calculated rates are, on average, 14 times higher than those predicted by GIA models within the Saint Lawrence province. This result strongly suggests this zone of inherited structures concentrates modern surface deformation.
This research offers compelling evidence that the Saint Lawrence Valley represents a zone of high intraplate deformation, controlled by forces linked to the region’s postglacial rebound and amplified by inherited structures from earlier tectonism. As the first study to quantify the impact of structural inheritance on surface deformation, this groundbreaking research will help unravel the processes that control deformation, as well as the poorly understood earthquakes that occur in the center of tectonic plates. (Journal of Geophysical Research: Solid Earth, https://doi.org/10.1029/2017JB015417, 2018)
—Terri Cook, Freelance Writer