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Early Postseismic Phase of the 2011 Tohoku-Oki Megathrust Earthquake: Observations by High-Rate Gps Solutions and Deformation Mechanisms Involved

Description: 

Post-seismic deformation involves several mechanisms (afterslip, poroelastic rebound, viscoelastic relaxation), acting at different spatial and temporal scales. The early post-seismic phase, from a few minutes to a few hours after the mainshock, is interesting because it marks the transition between rapid-coseismic slip and subsequent slow slip and aftershocks. Since it is characterized by high deformation rates, analyzing the deformation in this early period can bring strong constraints on the postseismic mechanisms. Yet, the early postseismic deformation has been little studied, because high-rate time series of the surface deformation in the vicinity of the rupture zone are not always available and time series are noisier that daily positioning.

Here, we focus on the early postseismic of the 2011 Mw 9 Tohoku-Oki earthquake. A detailed analysis of the temporal evolution of the deformation, using high-rate GNSS time series over one month after the earthquake, is used to identify the underlying mechanisms at play at short time scale.

Our result show that the temporal evolution of the early postseismic signal can be mostly explained by an Omori-like Transient Brittle Creep mechanism with a p-value around 0.75. The first 40 hours of signal are key to characterize this behavior. The spatial analysis reveals that the mainafterslip zone locates downdip from the main rupture. A secondary afterslip zone, close to the Ibaraki-Oki aftershock, has a distinct temporal evolution with a p value around 1.

To interpret these p-values, we develop a simple numerical model that qualitatively reproduced p values lower than one. We show that performing a creep test in a progressive damage model including thermal activation allow to qualitatively reproduce the temporal evolution observed for the early postseismic signal of Tohoku.

Finally, we compare the temporal evolution with the aftershock activity, and we also extend the GNSS analysis to longer time scales (9 years). Over longer time scales, we combine the transient brittle creep mechanism with viscoelastic relaxation to explain the surface displacements.

Session: From Earthquakes to Plate Boundaries: Insights Into Fault Behavior Spanning Seconds to Millennia [Poster]

Type: Poster

Date: 4/20/2023

Presentation Time: 08:00 AM (local time)

Presenting Author: Mathilde Radiguet

Student Presenter: No

Invited Presentation: 

Authors