Post-seismic slip of the 2011 Tohoku-Oki earthquake from GPS observations: implications for depth-dependent properties of subduction megathrusts

Abstract

S U M M A R YHere we inverted the GPS data to infer the coseismic slip of the Tohoku-Oki earthquake and the time-dependent afterslip distribution in the 4 months following the main shock. The Tohoku-Oki earthquake showed an unexpected magnitude and a characteristic depth-dependent differentiation of seismic energy radiation. In this context the estimation and comparison of the distribution of the fault portions that slip coseismically and post-seismically contribute to a better understanding of the variation of frictional characteristics of the plate interface. The inferred coseismic slip extends in a relatively compact region located updip from the hypocentre and reaches its highest value (about 60 m) near the trench. Afterslip occurs mostly outside the coseismic rupture and is distributed in two main modal centres. It reaches its largest values in an area located downdip of the coseismic slip and extends to a depth of 80 km. In the depth range between 30 and 50 km afterslip overlaps the portion of the fault that experienced historical moderate earthquakes, high-frequency seismic radiation and thrust-type aftershocks. The behaviour of this area can be explained by a rheologically heterogeneous region made of a ductile fault matrix interspersed with compact brittle asperities. On the contrary, the region beneath 50-60 km depth is probably characterized by a fully velocity strengthening behaviour. Southern afterslip, located off-Chiba Prefecture, is probably related to the M w 7.9 Ibaraki-Oki aftershock. The northward extension of the afterslip stops at a latitude of about 40• N, just south of the off-Aomori region. This may be related to three large events occurred in this area during the last century and the consequent strong coupling or complete depletion of the accumulated strain that characterize this region.

Dynamics of a fault model with two mechanically different regions

“…The moment rate function associated with this event was dominated by one single hump (Wei et al 2012), making it appropriate to ascribe the seismic slip to a large unstable region on the fault plane; what is more, this earthquake was followed by a prolonged afterslip episode (Ozawa et al 2011). Observations show that seismic slip concentrated in a compact area at shallow depth, while afterslip occurred on a similar area downslip (Lay et al 2012;Silverii et al 2014). A part of postseismic deformation was certainly due to viscoelastic relaxation in the asthenosphere (Sun et al 2014;Yamagiwa et al 2015), and an attempt will be made to discriminate between the two mechanisms.…”

Ground uplift related to permeability enhancement following the 2011 Tohoku earthquake in the Kanto Plain, Japan

“…The uplift in the eastern part is mostly greater than that in the west. This uplift pattern is well known and is explained by the numerical modeling of viscoelastic relaxation and afterslip (Ozawa et al 2012;Sun et al 2014;Diao et al 2014;Silverii et al 2014;Yamagiwa et al 2015).…”

“…Four possibilities have been proposed for the mechanism of post-seismic displacement: (1) afterslip of an earthquake fault, (2) viscoelastic relaxation, (3) fault-zone dilatancy recovery (displacement from post-seismic recovery from fault-zone contraction), and (4) poroelastic rebound (displacement owing to groundwater level change) (e.g., Fielding et al 2009;Wang et al 2012;Ozawa et al 2012;Sun et al 2014;Diao et al 2014;Silverii et al 2014;Hu et al 2014;Yamagiwa et al 2015). For the 2011 Tohoku earthquake, previous studies have explained island-scale post-seismic deformation using a contribution of these effects.…”

“…For the 2011 Tohoku earthquake, previous studies have explained island-scale post-seismic deformation using a contribution of these effects. Ozawa et al (2011) and Silverii et al (2014) modeled island-scale deformation during several months (seven and four months, respectively) after the earthquake, as afterslip of a fault. Diao et al (2014) modeled post-seismic deformation of GEONET data up to 560 days after the earthquake by considering afterslip and viscoelastic relaxation.…”

“…Vertical uplift occurred in the fore-arc region, which is relatively close to the epicenter, whereas subsidence resulted in the back-arc region. The observed land displacement was modeled by viscoelastic rebound and afterslip on the mainshock fault (Ozawa et al 2012;Sun et al 2014;Diao et al 2014;Silverii et al 2014;Yamagiwa et al 2015). Although these crustal displacements on the scale of an island arc have been discussed, local surface displacement has rarely been investigated.…”