Séminaire ISTerre

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Complex Earthquake Triggering in the Ibaraki Region by the 2011 Mw9.0 Tokoku-Oki Earthquake

mardi 14 septembre 2021 - 14h00

Asaf Inbal - Tel Aviv University
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The importance of aftershocks in redistributing crustal stresses has been known for several decades. However, a growing body of evidence also highlights the role of transient aseismic slip for triggering of large earthquakes. These slip transients may spontaneously nucleate around the site of the future rupture, or may be triggered remotely. Triggered aseismic off-fault transients generated on frictionally heterogeneous faults can cause damaging earthquakes. Only in a few cases, however, the off-fault deformation field is captured with sufficient spatio-temporal resolution, allowing one to elucidate the complex interactions leading to a large, destructive remote aftershock. One such case is the March to April 2011 seismic sequence in Ibaraki prefecture, Japan, which occurred shortly after the Mw9.0 Tohoku-Oki mainshock. This sequence lasted for about a month, and calumniated with the 11 April 2011 Mw6.7 Iwaki-Fukushima earthquake, which ruptured a pair of shallow sub-parallel inland normal faults that were unfavorably oriented with respect to Tohoku-Oki co-seismic strain regime. To constrain the processes responsible for rupture under seemingly unfavorable conditions, we analyze the GPS-derived strain field in Ibaraki during the month preceding the Mw6.7. We find that most of the local deformation in this time period is accommodated aseismically, and that the ratio of the aseismic-to-seismic moment release is largest in the two sub-regions neighboring the Mw6.7 source area. That pre-Mw6.7 strain distribution is only partially compatible with slip along the shallow normal faults in Ibaraki, but is consistent with motion along a deep-seated splay fault that branches from the subduction interface beneath Ibaraki. Our modeling suggests that, to account for the strain accumulated between March 11 and April 11, 2011, the shallow portion of the splay fault must have slipped aseismically by about 40 cm. Thus, the Tohoku mainshock initiated a chain-reaction in Ibaraki, whereby triggered slip on the splay fault induced stresses on shallow normal faults, whose activation eventually led to the Mw6.7 Iwaki-Fukushima rupture.

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Equipe organisatrice : Cycle sismique et déformations transitoires

Amphithéâtre Killian, Maison des Géosciences, 38400 Saint Martin d'Hères