Abstract
After a large earthquake, many small earthquakes, called aftershocks, ensue. Additional large earthquakes typically do not occur, despite the fact that the large static stress near the edges of the fault is expected to trigger further large earthquakes at these locations. Here we analyse ~10,000 highly accurate focal mechanism solutions of aftershocks of the 2016 Mw 6.2 Central Tottori earthquake in Japan. We determine the location of the horizontal edges of the mainshock fault relative to the aftershock hypocentres, with an accuracy of approximately 200 m. We find that aftershocks rarely occur near the horizontal edges and extensions of the fault. We propose that the mainshock rupture was arrested within areas characterised by substantial stress relaxation prior to the main earthquake. This stress relaxation along fault edges could explain why mainshocks are rarely followed by further large earthquakes.
Highlights
After a large earthquake, many small earthquakes, called aftershocks, ensue
This stress is added to the stress that accumulated before the earthquake; if the fault and its surroundings were in a state of uniform stress before the earthquake, it is thought that the fault edges and extensions would subsequently be in a very unstable state, allowing earthquakes to be generated[2]
The red circle indicates the mainshock hypocentre determined by Japan Meteorological Agency (JMA). c Cross-section perpendicular to the fault strike. d Magnitude−frequency distributions of the aftershocks focal mechanisms of which were determined and manually picked. e Large scale map showing the aftershock region modified from Fig. 1 of Kawanishi et al.[25]
Summary
Many small earthquakes, called aftershocks, ensue. Additional large earthquakes typically do not occur, despite the fact that the large static stress near the edges of the fault is expected to trigger further large earthquakes at these locations. We propose that the mainshock rupture was arrested within areas characterised by substantial stress relaxation prior to the main earthquake This stress relaxation along fault edges could explain why mainshocks are rarely followed by further large earthquakes. Shear stress increases along the edges and extensions of the fault where the slip of the earthquake did not occur[1]. If the strength is assumed to be uniform on the fault and its extension, it is clear that the stress after the mainshock becomes greater than the strength near the edges of its rupture area and beyond. The question of whether or not a subsequent major earthquake will occur along the edges and extensions of a mainshock fault is compounded by the questions of why rupture propagation of a large earthquake stops at its edge and how the size of the earthquake is determined[14,15]. The red circle indicates the mainshock hypocentre determined by JMA. c Cross-section perpendicular to the fault strike. d Magnitude−frequency distributions of the aftershocks focal mechanisms of which were determined (black dots) and manually picked (grey circles). e Large scale map showing the aftershock region modified from Fig. 1 of Kawanishi et al.[25]
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