The 2005–2022 Variations in the Slope of the Recurrence Curve in the Tonga Subduction Zone

The Tonga-Kermadec subduction zone lies between the Pacific and Australian plates. The location shows the highest rate of subduction for the Pacific plate and a dominant tension. Two great earthquakes occurred in the region in 2006 and 2009 whose magnitudes were M_w = 8.0 and 8.1. There are about 170 islands around the Tonga subduction zone; these islands are volcanic centers that have been regularly in eruption during the last several decades. The present study presents the results from the determination of time-dependent variations in the slope of the recurrence curve (the b-value) at the Tonga subduction zone during 2005–2022, and depth-dependent variations in b. The time-dependent variations in b reflect the general tendency of great earthquakes occurring upon the background of lower b-values in the surface layer only, at depths of 0–100 km. The comparison between the depth-dependent variations in b and the tectonic model for the Toga subduction zone implies the hypothesis that lower b-values may reflect greater stresses in the upper part of the plunging plate due to its bending. Higher b-values seem to be connected to tensional mechanisms. A region of higher b-values at depths of 90–100 km has been identified for the Tonga subduction zone, as well as for other subduction zones, which can be related to the presence of a magmatic front at these depths, which is related to active volcanism.