Deficit of large aftershocks and postseismic deformation

Inessa Vorobieva1, Peter Shebalin1, Sergey Baranov2

1Institute of Earthquake Prediction Theory and Mathematical Geophysics, Russian Academy of Sciences
2Kola Branch of Geophysical Survey of Russian Academy of Sciences


Earthquakes are often accompanied by postseismic deformation, which starts immediately after the event and are mainly caused by aseismic slip (afterslip) and viscoelastic relaxation. If the coseismal slip occurs almost instantly after the earthquake, the postseismic movement caused by relaxation processes can last for years. The contribution of aseismic slip and viscoelastic relaxation to postseismic deformations are currently being actively studied and some models were suggested, although the final theory has not yet been developed.

To clarify the role of after-slip in the different stages of postseismic relaxation we study aftershock sequences of the 4 great earthquakes in the subduction zones: Sumatra, 2004, Mw 9.1; Sumatra, 2005, Mw 8.6; Chili, 2010, Mw 8.8; Tohoku, 2011, Mw 9.1. The postseismic deformations due to these earthquakes have fast afterslip. The data was taken from ANSS ComCat provided by USGS. We used aftershocks with the magnitude M 4.7 and the depth H 70 km occurred in the subducting slab. The time-magnitude distributions for stacked aftershocks due to the 4 earthquakes shows a deficit of strong events in the subducting slab during the first 10 days after the four great earthquakes. This leads to bigger b-values estimated for the first 10 days comparing to those for 100-3000 days. At the same time, we revealed that commonly there is no deficit of large aftershocks after earthquakes with lesser magnitudes (8 M 8.5) from the subduction zones.

We explain this phenomenon by the relatively smaller rate of afterslip after smaller earthquakes, which is insufficient to prevent large aftershocks. Perhaps the smaller size of earthquake source plays a significant role and significantly smaller coseismic slip which captures less areas of low adhesion comparing to strong earthquakes. In other words, the areas of maximum displacement during strong earthquakes make up only a small part of the source, while postseismic deformations cover a much larger area and are traced by aftershock clouds. We also suppose that high rate of afterslip plays a major role at the beginning of postseismic relaxation, while viscoelastic relaxation is more important at later stages. This research was supported by Russian Science foundation, project Nu. 20-17-00180.