Broadscale postseismic gravity change following the 2011 Tohoku-Oki earthquake and implication for deformation by viscoelastic relaxation and afterslip

A linear viscoelasticity for decadal to centennial time scale mantle deformation

The extended Burgers material (EBM) model provides a linear viscoelastic theory for interpreting a variety of rock deformation phenomena in geophysics, playing an increasingly important role in parameterizing laboratory data, providing seismic wave velocity and attenuation interpretations, and in analyses of solid planetary tidal dispersion and quality factor Q. At the heart of the EBM approach is the assumption of a distribution of relaxation spectra tied to rock grain boundary and interior granular mobility. Furthermore, the model incorporates an asymptotic long-term limiting behavior that is Maxwellian. Here we use the extensively developed linear theory of viscoelasticity to isolate those parameters of EBM that apply to both post-seismic relaxation processes involving flow of olivine rich upper mantle material and to studies of tides, where periods of forcing range from 12 h to 18.6 years. The isolated EBM parameters should also apply to theoretical and geodetic studies of glacial isostatic adjustment, especially when the initiation of continuous cryospheric surface unloading dates to the 20th or 21st century. Using analytical Laplace transformed solutions of Boussinesq's half-space load problem, we show that the effects of EBM transient rheology may have substantial influence on geodetic interpretations of unloading induced crustal motions even on time scales that are sub-decadal.

The State of Remote Sensing Capabilities of Cascading Hazards over High Mountain Asia

Cascading hazard processes refer to a primary trigger such as heavy rainfall, seismic activity, or snow melt, followed by a chain or web of consequences that can cause subsequent hazards influenced by a complex array of preconditions and vulnerabilities. These interact in multiple ways and can have tremendous impacts on populations proximate to or downstream of these initial triggers. High Mountain Asia (HMA) is extremely vulnerable to cascading hazard processes given the tectonic, geomorphologic, and climatic setting of the region, particularly as it relates to glacial lakes. Given the limitations of in situ surveys in steep and often inaccessible terrain, remote sensing data are a valuable resource for better understanding and quantifying these processes. The present work provides a survey of cascading hazard processes impacting HMA and how these can be characterized using remote sensing sources. We discuss how remote sensing products can be used to address these process chains, citing several examples of cascading hazard scenarios across HMA. This work also provides a perspective on the current gaps and challenges, community needs, and view forward towards improved characterization of evolving hazards and risk across HMA.

Global sea level change signatures observed by GRACE satellite gravimetry

Ice mass loss on land results in sea level rise, but its rate varies regionally due to gravitational self-attraction effects. Observing regional sea level rates by ocean mass change using the Gravity Recovery and Climate Experiment (GRACE) gravity solutions is difficult due to GRACE’s spatial resolution (~a few hundred km) and other limitations. Here we estimate regional sea level mass change using GRACE data (without contributions from temperature and salinity variations) by addressing these limitations: restoring spatially spread and attenuated signals in post-processed GRACE data; constraining ocean mass distribution to conform to the changing geoid; and judging specific corrections applied to GRACE data including a new geocenter estimate. The estimated global sea level mass trend for 2003–2014 is 2.14 ± 0.12 mm/yr. Regional trends differ considerably among ocean basins, ranging from −0.5 mm/yr in the Arctic to about 2.4 mm/yr in the Indian and South Atlantic Oceans.

Transient rheology of the Sumatran mantle wedge revealed by a decade of great earthquakes

Understanding the rheological properties of the upper mantle is essential to develop a consistent model of mantle dynamics and plate tectonics. However, the spatial distribution and temporal evolution of these properties remain unclear. Here, we infer the rheological properties of the asthenosphere across multiple great megathrust earthquakes between 2004 and 2014 along the Sumatran subduction zone, taking advantage of decade-long continuous GPS and tide-gauge measurements. We observe transient mantle wedge flow following these earthquakes, and infer the temporal evolution of the effective viscosity. We show that the evolution of stress and strain rate following these earthquakes is better matched by a bi-viscous than by a power-law rheology model, and we estimate laterally heterogeneous transient and background viscosities on the order of ~10¹⁷ and ~10¹⁹ Pa s, respectively. Our results constitute a preliminary rheological model to explain stress evolution within earthquake cycles and the development of seismic hazard in the region.

The rheology of the upper mantle is key to understanding how plate tectonics may evolve. Here, using GPS and tide-gauge measurements along the Sumatran subduction zone, the authors’ show that a bi-viscous rheology model is needed to explain the stress and strain evolution of the upper mantle following earthquakes.

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

Large earthquakes often trigger viscoelastic adjustment for years to decades depending on the rheological properties and the nature and spatial extent of coseismic stress. The 2006 M_w8.3 thrust and 2007 M_w8.1 normal fault earthquakes of the central Kuril Islands resulted in significant postseismic gravity change in GRACE but without a discernible coseismic gravity change. The gravity increase of ~4 µGal, observed consistently from various GRACE solutions around the epicentral area during 2007-2015, is interpreted as resulting from gradual seafloor uplift by ~6 cm produced by postseismic relaxation. The GRACE data are best fit with a model of 25-35 km for the elastic thickness and ~10¹⁸ Pa s for the Maxwell viscosity of the asthenosphere. The large measurable postseismic gravity change (greater than coseismic change) emphasizes the importance of viscoelastic relaxation in understanding tectonic deformation and fault-locking scenarios in the Kuril subduction zone.