This project made progress on two important tasks. First, GPS time series covering the postseismic period from a large megathrust event (2006-2007 Kuril earthquake doublet) were extended to 10 years length. Therefore, this is one of leading datasets of observed postseismic deformation following a megathrust seismic source in the era of modern space geodesy. Second, advanced mechanical models were used to match the GPS time series, resulting in improved understanding of mantle rheology and the nature of locking at a subduction zone.

The Kuril subduction zone is one of the most seismically active regions on the Earth. It was the last major subduction zone totally unexplored by the methods of space geodesy. In 2006-2007, a doublet of great earthquakes in the Kuril arc partly ruptured one of the most conspicuous gaps in subduction-zone seismic activity (the 15 November 2006 M_W 8.3 thrust event and the 13 January 2007 M_W 8.1 extensional event). The Kuril GPS Array was installed several months before the 2006-2007 earthquakes in collaboration between Russian and US scientists. Observations from the Array have documented coseismic and postseismic surface deformation following the great Kuril earthquakes, in their near and far fields for 10 years. The study area is one of only a handful to capture a coherent postseismic signal with continuous GPS. For a decade, the near field stations have moved trenchward, towards the seismic source at a speed of several tens of millimeters per year initially and an order of magnitude slower currently.

We assume that viscoelastic relaxation is the dominant signal in GPS displacements (compared to afterslip) after about a year of the postseismic time interval. Our modeling of postseismic deformation has explored realistic three-dimensional subduction structures accounting for the dipping rigid slab and for a low-viscosity mantle wedge above it (using the program RELAX of S. Barbot). We tested linear (Maxwell) and nonlinear (power-law) rheologies for the asthenosphere, The data are best fit by the Maxwell asthenospheric viscosity 1 × 10¹⁸ Pa s for the interval 2007.5–2016.5. An open and intriguing question in connection with postseismic data following the 2006-2007 Kuril earthquakes is whether the long-term asthenospheric viscosity is much higher, on the order of ~1 × 10¹⁹ Pa s in the Kuril subduction zone, as deduced from postseismic deformation observed several decades after the 1960 Chile and 1964 Alaska M_W ~9 earthquakes.

A nonlinear power-law rheology predicts the growth of apparent viscosity of the asthenosphere with time. From laboratory experiments with olivine, two alternative power-law mechanisms are possible: dislocation creep (stress power-law exponent n = 3.4-4.5) or diffusion creep (n = 0.9-1.5). Our numerical tests spanned the expected range of n, as well as a range of values of the initial apparent viscosity. The data of the Kuril displacement field are best fit by diffusion creep with n = 1.2 although the fit is not as good as for the Maxwell model.

The project relates to a region subject to disastrous earthquakes, and better understanding of the earthquake cycle and mantle rheology is important for understanding seismic hazards here and in other similar settings. This work is a positive step for international relations because it has fostered cooperation between scientists of the Russian Academy of Sciences and of US universities. The data provided by the project are available for use by the broad scientific community.

In connection with this project, we observed by GPS the May 2013 deep focus earthquake in the Sea of Okhotsk. This event was the largest ever recorded (magnitude 8.3), and it occurred at 611 km (about 380 miles) depth. To our surprise, despite its great depth the earthquake caused clear and significant permanent deformation of the Earth’s surface. The surface displacements can be explained well by a simple dislocation model consistent with the Global CMT earthquake source. This event demonstrated that some of the motions we observe on the surface are not due only to the shallow tectonic processes that we have usually considered. The impact of deep earthquakes may have implications for the long-term uplift or subsidence at certain subduction zones, and for the limits on the stability of tectonic plates, as the cumulative uplift and subsidence rates from the history of deep and intermediate depth (>100 km) earthquakes from 1960 is comparable to long-term geological uplift and subsidence rates in some subduction zones.

Last Modified: 09/01/2016
Modified by: Mikhail Kogan