An increase of both shallow and intraslab intermediate-depth seismicity has been observed days to years before some great subduction earthquakes, as before Tohoku-oki (Mw 9.0, 2011), Maule (Mw 8.8, 2010) or Iquique (Mw 8.2, 2014) earthquakes (Bouchon et al., 2016, Jara et al,. 2017). These observations suggest that a link exists between these deep and shallow foreshocks, but it is still poorly understood and not characterized in a systematic manner. Some studies have attempted to address this lack of systematic characterization by using a statistical approach (Delbridge et al., 2017).
The aim of this study is to systematically and statistically identify and characterize the potential correlations between deep and shallow seismicity. We want to assess whether or not such interactions exist. If they exist, we plan to characterize when and where they occur, at what frequency, their characteristic duration, and with what spatial pattern.
For this purpose, we develop a statistical method to assess the relevance of deep-shallow interactions, that allows to identify statistically significant correlations between deep and shallow seismicity. We focused on the seismicity of the Japan trench subduction zone during the decade prior to the Tohoku-oki earthquake, because deep-shallow interactions were identified there, and because we can test the events picked by our method against the correlations highlighted in published papers (Bouchon et al., 2016). The correlation values between the deep and shallow events from the Japan Meteorological Agency catalog are calculated on various different sliding-windows with durations from month to week. These correlation values are then compared to the ones obtained using synthetic series of shallow events that meet the spectral properties of the real series, and the significance of the correlation is calculated.
Some windows show a strong correlation. The dependence of our results to different parameters, such as the completeness magnitude, the length of the window, the lag, the smoothing etc… are evaluated. The spatio-temporal analysis of the seismicity on maps for these windows is also explored. While the results are still preliminary, we believe that this method has the potential to systematically and quantitatively assess the current presumptions on the link between deep and shallow seismicity, that would lead to a better understanding of the mechanisms leading to megathrust earthquakes.
Bouchon, M., Marsan, D., Durand, V., Campillo, M., Perfettini, H., Madariaga, R., & Gardonio, B. (2016). Potential slab deformation and plunge prior to the Tohoku, Iquique and Maule earthquakes. Nature Geoscience, 9(5), 380.
Delbridge, B. G., Kita, S., Uchida, N., Johnson, C. W., Matsuzawa, T., & Bürgmann, R. (2017). Temporal variation of intermediate‐depth earthquakes around the time of the M9. 0 Tohoku‐oki earthquake. Geophysical Research Letters, 44(8), 3580-3590.
Jara, J., Socquet, A., Marsan, D., & Bouchon, M. (2017). Long-Term Interactions Between Intermediate Depth and Shallow Seismicity in North Chile Subduction Zone. Geophysical Research Letters, 44(18), 9283-9292.