Abstract
We infer the postseismic deformation mechanisms following the 2011 Tohoku megathrust earthquake via an analysis of onshore geodetic observations. We focus on the temporal decay characteristics of postseismic deformation using continuous time-series data at time scales that span many orders of magnitude by means of high-rate GNSS data. Our analysis indicates Omori-like power-law decay of the horizontal ground displacement rates, with p-value (0.69) that is significantly smaller than that of the aftershock occurrence (~ 1). This slow decay implies a (non-Maxwellian) viscoelastic relaxation mechanism other than afterslip since immediately after the mainshock, which is inferred using only onshore geodetic data. Spatial distribution of the Omori parameters implies that the postseismic deformation will continue over 100 years in a down-dip area of the northern part of the mainshock fault. The decay characteristics of vertical displacement rates are also almost Omori-like, but data deviation from the fitting line several 100 days after the mainshock might reflect the change of the dominant mechanism of the postseismic deformation. This multi-time scale geodetic approach will provide important constraints for future viscoelastic models of Earth’s interior.
Highlights
Postseismic deformation following large earthquakes has been quantitatively investigated using geodetic observations (Segall 2010)
A recent continental earthquake study (Ingleby and Wright 2017) demonstrated that afterslip is the dominant postseismic deformation mechanism. They focused on nearfault geodetic observations at time scales that spanned over five orders of magnitude, and found that the postseismic velocities were inversely proportional to time since the earthquake (following the Omori law for aftershocks (Omori 1894), with a p-value of ~ 1), with logarithmic increase in postseismic displacement over time
We found that the p-value is significantly smaller than that for continental earthquakes (Ingleby and Wright 2017), which indicates the slow decay of postseismic deformation following the 2011 Tohoku Earthquake
Summary
Postseismic deformation following large earthquakes has been quantitatively investigated using geodetic observations (Segall 2010). A recent continental earthquake study (Ingleby and Wright 2017) demonstrated that afterslip (or localized power-law creep in a shear zone) is the dominant postseismic deformation mechanism. They focused on nearfault geodetic observations at time scales that spanned over five orders of magnitude, and found that the postseismic velocities were inversely proportional to time since the earthquake (following the Omori law for aftershocks (Omori 1894), with a p-value of ~ 1), with logarithmic increase in postseismic displacement over time. For the 2011 Tohoku earthquake, Ozawa et al (2012) suggested a good correlation between the aftershock number and the moment of the afterslip based on daily data during 7 months after the mainshock (but they did not propose a linear relation of them)
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