Rapid postseismic relaxation after the great 2006–2007 Kuril earthquakes from GPS observations in 2007–2011

Abstract

The 2006–2007 doublet of Mw > 8 earthquakes in the Kuril subduction zone caused postseismic transient motion in the asthenosphere, which we observed on the Kuril GPS Array in 2007–2011. Here we show that the Maxwell asthenospheric viscosity that best fits the geodetic data increased by nearly an order of magnitude over the interval of 4 years, from 2 × 1017 to 1 × 1018 Pa s. These effective values of viscosity can be explained by a power law rheology for which strain rate is proportional to stress raised to a power n > 1. The apparent change in viscosity can also be caused by other factors such as coupling between afterslip and viscoelastic flow. The open and intriguing question in connection with postseismic data after the Kuril earthquake doublet is the magnitude of the long‐term asthenospheric viscosity, which shall be revealed by continued observations. An asthenosphere with viscosity of about 1 × 1019 Pa s is favored by the postseismic deformation still observed several decades after the 1960 Chile and 1964 Alaska Mw ~9 earthquakes. However, postseismic deformation associated with the 1952 southern Kamchatka Mw ~9 earthquake currently is not observed in the northern Kurils, an indication that the long‐term asthenospheric viscosity in the Kurils is lower than that in Chile and Alaska.

Postseismic gravity change after the 2006–2007 great earthquake doublet and constraints on the asthenosphere structure in the central Kuril Islands

“…Kogan et al . [, ] presented a postseismic uplift rate up to 1.2 cm/yr (KURILNET site MATC) from near‐field GPS stations along the volcanic arc, which confirmed that the postseismic deformation is caused predominantly by viscoelastic relaxation (rather than afterslip). We henceforth assume that the GRACE gravity changes are shaped primarily by viscoelastic relaxation and ignore the possible contribution of afterslip.…”

“…We will present the details of GRACE observations and viscoelastic modeling results with a range of rheological parameters (thickness of lithosphere and asthenosphere viscosity) and of coseismic moment magnitude ratios and compare with the previous Kuril GPS studies of Kogan et al . [, ].…”

Coseismic seafloor deformation in the trench region during the Mw8.8 Maule megathrust earthquake

“…This process has been explained in the context of the velocity strengthening behavior24, and has been observed mainly in the down-dip region of megathrust-earthquakes2526. At longer time scales, post-seismic deformation can be modeled as viscoelastic relaxation of the subducted slab, underlying asthenosphere and overlying asthenospheric mantle wedge232728. Viscoelastic relaxation models predict post-seismic deformation observed for several years or decades after the earthquake, including subsidence over the down-dip limit of co-seismic slip and on the rupture area and uplift seaward29.…”

Evidence of viscoelastic deformation following the 2011 Tohoku‐Oki earthquake revealed from seafloor geodetic observation

“…[] for the 2004 Sumatra earthquake and Kogan et al . [] for the 2006 Kuril earthquake. Both of their geographical settings are also along the subduction trenches, similar to that for the Tohoku‐Oki event, so they are expected to provide approximate interpretation for the present observations.…”

“…On the other hand, such postseismic phenomena after large earthquakes are in some cases discussed with another transient process, specifically, viscoelastic relaxation of stress induced by the coseismic slip in the upper mantle [e.g., Wang et al ., ]. Several researches have performed evaluation of viscoelastic responses following megathrust earthquakes for several years together with the afterslip, e.g., for the 2004 Sumatra earthquake [ Pollitz et al ., ; Hu and Wang , ] and the 2006–2007 Kuril earthquakes [ Kogan et al ., ]. However, they used only terrestrial geodetic data, which are insufficient to resolve the deformation beneath the ocean [e.g., Loveless and Meade , ].…”