Robustness and singularity of pre-seismic signals in GRACE gravity solutions: application to the 2011 Tohoku Mw9.0 Earthquake

Documenting the preparation phase of giant earthquakes and retrieving pre-seismic signals of upcoming events is a crucial challenge. Over the short term, various deformation transients have been detected before large subduction events, emphasizing in particular the role of the slab pull in driving plate motions (e.g., Bouchon et al., 2016 ; Bedford et al., 2020). Among them, we previously evidenced an anomalous gravity gradient signal during the months before the March 2011 Tohoku earthquake, likely originating from the solid Earth (Panet et al., 2018). We showed that it could reflect a broad deformation of the subducted slab prior to the event, generating the giant earthquake as the deformation propagated from depth to surface.

Taking the example of the 2011 Tohoku earthquake, we conduct here a systematic and global retrospective analysis of time series of GRACE-reconstructed gravity gradients truncated in February 2011. Our aim is to test whether the gravity gradient variations preceeding the earthquake can be identified as singular and potentially originating from the solid Earth in an automated way and without knowledge on the upcoming event. First, we enhance the angular resolution of the gravity gradients in order to extract signals closely aligned with a chosen plate boundary orientation. Along this preferred orientation, we extract fast temporal variations at the sub-annual timescales of geodetic, gravitational and seismic signals reported before the event in previous studies. To evaluate the significance of the obtained gravity gradient anomalies, we design a method to extract coherent signals between different GRACE gravity field models and assess their sensitivity to the dealiasing ocean model. We present and discuss the results of these analyses applied to different sets of GRACE gravity models: the GRGS03, GRGS04 and CSR06 solutions, as well as their respective ocean dealiasing models. Beyond the case of the Tohoku earthquake, our approach can be applied to the systematic monitoring of the Pacific subduction belt, to detect gravity variations potentially linked with sudden changes in slab motions in-depth these plate boundaries.

 

References

Bedford, J. R., et al. (2020). Months-long thousand-kilometre-scale wobbling before great subduction earthquakes, Nature 580, 628-635.

Bouchon, M., et al.  (2016). Potential slab deformation and plunge prior to the Tohoku, Iquique and Maule earthquakes, Nature Geoscience 9, 380-383.

Panet, I., et al. (2018). Migrating pattern of deformation prior to the Tohoku-Oki earthquake revealed by GRACE data, Nature Geoscience 11, 367-373.