Delayed triggering of early aftershocks by multiple surface waves circling the earth
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It is well known that direct surface waves of large earthquakes are capable of triggering shallow earthquakes and deep tremor at long-range distances. Recent studies have shown that multiple surface waves circling the earth could also remotely trigger microearthquakes. However, it is still not clear whether multiple surface waves returning back to the main shock epicenters could also trigger/modulate aftershock behavior. Here we conduct a study to search for evidence of such triggering by systematically examining aftershock patterns of earthquakes with magnitude ≥ 8 since 1990 that produce observable surface waves circling the globe repeatedly. We specifically examine the 2011 M9 Tohoku-Oki event using a composite catalog of JMA, HiNet and newly detected events obtained by waveform cross correlation. We compute the magnitude of completeness for each sequence, and stack all the sequences together to compute the seismicity and moment rates by sliding data windows. The sequences are also shuffled randomly and these rates are compared to the actual data as well as synthetic aftershock sequences to estimate the statistical significance of the results. Our results suggest that there is some moderate increase of early aftershock activity after a few hours when the surface waves return to the epicentral region. However, we could not completely rule out the possibility that such an increase is purely due to random fluctuations of aftershocks or caused by missing aftershocks in the first few hours after the mainshock.