Viscoelastic relaxation following subduction earthquakes and its effects on afterslip determination

Abstract

Afterslip is commonly thought to be the controlling process in postseismic deformation immediately following a great megathrust earthquake and is usually inferred from geodetic observations using purely elastic models. However, observed motion reversal of the near-trench area right after the 2011 M w 9 Tohoku-oki earthquake demonstrates the dominance of viscoelastic relaxation of coseismically induced stresses. To understand the importance of incorporating viscoelasticity in afterslip determination, we employ biviscous Burgers mantle rheology and use finite element models to explore how viscoelastic relaxation in short-term postseismic deformation is controlled by various geometrical and rheological factors. Our results indicate that immediately after large megathrust earthquakes (M w > 8.0), viscoelastic deformation should always cause opposing motion of inland and trench areas and subsidence around landward termination of the rupture, although the rate of such postseismic motion depends on local conditions such as the age and hence thickness of the slab and transient mantle viscosity values. While elastic models may be adequate for afterslip estimation for earthquakes of M w < 7.5, neglect of viscoelasticity for larger events leads to overestimate of afterslip downdip of the rupture and underestimate of afterslip at shallower depths. Reassessing shallow afterslip following the 2005 M w 8.7 Nias earthquake using 2-D viscoelastic models suggests that the actual afterslip may be greater than that estimated using an elastic model by more than 50%. Similarly, interpreting trenchward motion of some seafloor GPS sites following the Tohoku-oki earthquake using a viscoelastic model suggests large shallow afterslip outside of the main rupture area.

Importance of rheological heterogeneity for interpreting viscoelastic relaxation caused by the 2011 Tohoku-Oki earthquake

“…For example, Ozawa et al (2012) argued that afterslip was the dominant mechanism; Diao et al (2014) and Han et al (2014) examined the combined effects of afterslip and viscoelastic relaxation; Hu et al (2014) explored the effect of poroelastic rebound; and several authors showed that viscoelastic relaxation plays an important role even in short-term deformation (Sun et al 2014;Sun and Wang 2015;Hu et al 2016a). The most important results from these studies are those obtained using seafloor geodetic observation (SGO) (Watanabe et al 2014;Tomita et al 2015).…”

Interpretation of Offshore Crustal Movements Following the 2011 Tohoku-Oki Earthquake by the Combined Effect of Afterslip and Viscoelastic Stress Relaxation

“…In addition, to obtain unbiased results of the inversion analysis, we need to use (1) proper plate interface geometry, (2) proper crustmantle rheological structure, and also (3) a consistent coseismic slip distribution, which causes coseismic stress changes in the asthenosphere. Diao et al (2014) and Yamagiwa et al (2015) as well as the present study performed simultaneous inversion for coseismic and postseismic slip distributions with a standard crust-mantle rheological structure model to satisfy the second and third requirements, but Sun and Wang (2015) and Hu et al (2016) did not. In our common understanding, as mentioned above, afterslip is considered to occur in a deep brittle-ductile transition zone so as to release the stress concentrations caused at the tip of main rupture.…”

“…For example, the result of inversion analysis by Perfettini and Avouac (2014) shows a dominant zone of shallow afterslip extending over the main rupture area near the trench. Sun and Wang (2015) have suggested the existence of large shallow afterslip outside of the main rupture area. Yamagiwa Vol.…”

“…Given the coseismic slip distribution by Iinuma et al (2012) and the afterslip distribution by Ozawa et al (2012), they tried to tune the viscosity parameters but could not explain the postseismic seafloor movements at two GPS/Acoustic stations off Iwate and Fukushima. Sun and Wang (2015) resolved this problem by introducing the shallow afterslip areas off Iwate and Fukushima. Using a finite element plate-subduction model, Suito (2017) examined individually the contributions of viscoelastic relaxation in the mantle wedge and the oceanic mantle to the postseismic crustal deformation.…”

Simple Crack Models Explain Deformation Induced by Subduction Zone Megathrust Earthquakes

“…Several recent great subduction zone earthquakes, such as the 2010 M w 8.8 Maule earthquake in Chile and the 2011 M w 9.0 Tohoku earthquake in Japan, have greatly sharpened our view of various aspects of earthquake physics, including foreshock activity and its influence on mainshock nucleation, depth-dependent earthquake source properties, and structural and rheological control on coseismic and postseismic slip (Ando and Imanishi, 2011;Meng et al, 2011;Lay et al, 2012;Moreno et al, 2012;Sun and Wang, 2015). In particular, changes in seismicity patterns have shed light on the spatiotemporal evolution of the stress state around subducting plate boundaries.…”