Slow deformation event between large intraslab earthquakes at the Tonga Trench.

Slow deformations associated with a subducting slab can affect quasi-static displacements and seismicity over a wide range of depths. Here, we analyse the seismotectonic activities in the Tonga subduction zone, which is the world's most active area with regard to deep earthquakes. In our study, we combine data from global navigation satellite systems with an earthquake catalogue. We focus on the deep earthquakes that are below 400 km at the lower part of the Wadati-Benioff zone. We find that trenchward transient displacements and quiescence of deep earthquakes, in terms of background seismicity, were bounded in time by large intraslab earthquakes in 2009 and 2013. This "slow deformation event" between 2009 and 2013 may have been triggered by a distant and shallow M8.1 earthquake, which implies a slow slip event at the plate interface or a temporal acceleration of the subduction of the Pacific Plate. These findings provide new insights into the relationship between shallow and deep earthquakes in the subduction zone.