Spatiotemporal variations in the statistical parameters of the shear wave splitting from local events beneath Eastern Hokkaido

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Statistical analysis is carried out of the distribution of the fast Swave $$(\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{\phi } )$$ , polarization azimuths, time delays between the split shear waves $$(\delta \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{t} )$$ and the dominant frequencies of P waves $$(\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{f} _P )$$ from weak seismic events that hit Eastern Hokkaido during the period of 1998–2003, which also includes strong events with M > 6 and the strong Tokachi-oki earthquake (M8.0) of September 26, 2003. The analysis revealed oscillatory pattern of variations in wave parameters along Eastern Hokkaido. In the rigid crustal blocks beneath the Tokachi and Kushiro Plains, stability is observed in the fast azimuth orientation and unstable behavior of $$\delta \overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{t}$$ and $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{f} _P$$ in the interval of periods 0.5–2.5 years. In the weakened zones beneath the Hidaka Range and the Nemuro Peninsula, periodical alternation of $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{\phi }$$ along orthogonal directions is revealed. It is found that the parameters of the waves coming from the seismic events originating in different depth intervals experience anomalous changes before strong earthquakes. These anomalies reflect depth changes in the type of deformation of the medium. Active deformation is observed beneath the Hidaka Range. Dynamic changes in beneath the stations on either side of the range (ERM, MYR), their antiphase pattern, and the discrepancies in the behavior of the parameters from the events that originate in different depths indicate that these regions act as active buffers in the redistribution of stresses in the Kuril-Japan arc-arc junction zone. The onset of instability that develops before the Tokachi-oki earthquake is marked by gradual shortening of the period of oscillations in $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{\phi }$$ and transition to the north and east azimuths as well as by the decreasing of $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{f} _P$$ from the intermediate-depth events starting from middle 2002, and for deep events, starting from 2000. The acceleration of the process begins in the middle of 2003 and is recognized by a noticeable increase in and a decrease in $$\overset{\lower0.5em\hbox{$\smash{\scriptscriptstyle\frown}$}}{f} _P$$ beneath almost all stations.