Study of the early postseismic phase of the Tohoku-Oki earthquake

Supervisor : Hugo Perfettini (hugo.perfettini@ird.fr) ; Co-supervisor : Mathilde Radiguet (mathilde.radiguet@ujf-grenoble.fr)

Title : Study of the early postseismic phase of the Tohoku-Oki earthquake

Supervisor : Hugo Perfettini (hugo.perfettini ird.fr) ; Co-supervisor : Mathilde Radiguet (mathilde.radiguet ujf-grenoble.fr)

Key Words : Tohoku-Oki earthquake ; coseismic slip ; postseismic slip ; early afterslip ; inversion ; link between coseismic rupture, afterslip and aftershocks

The purpose of this master project is to analyze the early (up to several days after the mainshock) postseismic phase of the Tohoku-Oki earthquake (2011, Mw9.0) and in particular the relationship between coseismic rupture and early afterslip, but also the way afterslip might drive the early aftershocks which are the most numerous.

The Tohoku-Oki earthquake is the largest earthquake ever recorded with seismologic, geodetic and sea-floor measurements data. All the data shows that the large tsunami induced by the mainshock is consistent with the largest coseismic slip being located near the trench.
The location of afterslip (the aseismic slip that occurs on the fault after the earthquake) that followed coseismic rupture is still debated. Ozawa et al. (2012) found a large region of afterslip bellow the coast but no shallow afterslip. Perfettini and Avouac (2014) evidenced two region of afterslip : one bellow the coast, consistent with the study of Ozawa et al. (2012) and a second shallow region, located near the region, and showing the largest amount of afterslip. Recently, Sun and Wang (2015) suggested that the observed post-seismic deformation could be explained by a combination of viscoelastic stress relaxation and shallow afterslip. Anyhow, the largest amount of coseismic slip being located near the trench, a significant amount of afterslip should have been released there.

One way to reconcile the published studies is to assume that a large amount of shallow afterslip has been released in the first hours to days following the mainshock. To address this issue, we will analyze the early postseismic phase considering GPS data at a 30s sampling rate recently acquired by ISTerre (GEONET Japanese GPS network). The GPS time series of 400 GPS stations will be inverted to get the early afterslip on the fault using the PCAIM inversion software (http://www.tectonics.caltech.edu/resources/pcaim/). The evolution of afterslip will be compared with the evolution of aftershocks in order to test the idea that aftershocks are driven by afterslip.
During the master project, a coseismic model will be derived and compared to the afterslip model in order to understand how the stable regions of the fault (evidenced by the early afterslip) could have influenced seismic rupture.

A solid background in Mathematics, Physics and Computation are required as it is a theoretical master project. This master project aims at being continued during a PhD for which half of the scholarship is already guaranteed by the ANR Atypic SSE that started in October 2017. During the PhD, the pre-seismic phase of the Tohoku-Oki earthquake will be studied, and all the results obtained by studying the pre-, co- and post-seismic phase will be modelled using the 3D seismic cycle model CYCLE3D considering realistic fault geometries and elastic properties.

REFERENCES
• Ozawa, S., Nishimura, T., Munekane, H., Suito, H., Kobayashi, T., Tobita, M., et al. (2012). Preceding, coseismic, and postseismic slips of the 2011 Tohoku earthquake, Japan. Journal of Geo- physical Research, 117, B07404. doi:10.1029/2011JB009120.
• Perfettini, H., & Avouac, J. P. (2014). The seismic cycle in the area of the 2011 Mw9.0 Tohoku-Oki earthquake. Journal of Geo- physical Research : Solid Earth, 119, 4469–4515.
• Sun, T., & Wang, K. (2015). Viscoelastic relaxation following subduction earthquakes and its effects on afterslip determination. Journal of Geophysical Research : Solid Earth, 120, 1329–1344.