Emergence, moment change and disappearance of repeating earthquakes following the 2011 Tohoku earthquake

Abstract:

Some repeating earthquake sequences show systematically increased seismic moments after the 2004 M6.0 Parkfield (Chen et al., 2010) and the 2011 M9.0 Tohoku earthquake (Tohoku EQ; Uchida et al., 2015) in the areas with large afterslip. These can be explained by the slip behavior changes in conditionally stable regions due to high loading rates.

We examined temporal changes in interplate repeater activities following the Tohoku EQ in a small area off Iwate, Japan (~39.75°N, 142.35°E) where large afterslip was estimated to have been occurring (e.g., Sun & Wang, 2015). We performed hypocenter relocations by the double-difference method (Waldhauser & Ellsworth, 2000) using arrival-time differences estimated from waveform cross-spectra.

Before the Tohoku EQ, M2.5-2.9 events (Group A) repeatedly occurred at almost the same location in the area with quasi-periodic recurrence intervals (9-12 months). There were no M≥2.0 events within 5km from the Group A.

After the Tohoku EQ, three significant changes were observed.

(1) At the location of the Group A, larger earthquakes (M≥3.0) than before started to repeat with much shorter recurrence intervals, similar to the previous report (Uchida et al., 2015).

(2) Two repeating sequences emerged to the ~600m NNW (Group B; M3.2-3.9) and ~700m NNE (Group C; M2.2-4.4) of the Group A, where there had been no events before the Tohoku EQ. This can be interpreted as the aseismic-to-seismic transitions in conditionally stable patches due to the fast loading rate.

(3) Magnitudes tended to become smaller as time passed in each sequence. The Group C-events disappeared after the M2.2 event in January 2012. This suggests the seismic-slip areas gradually shrunk over time due to the decay of the afterslip.

These observations show that temporal changes in the loading rate play an important role in the diversity of interplate earthquakes as well as the spatial heterogeneity of frictional properties along the plate boundary.