The 1200-km long Kuril arc is the last subduction zone never previously explored by space geodetic methods. In 2006, we installed the continuous GPS network (CGPS) over the whole arc augmented with several survey-mode stations (SGPS). In 2006-2007, two great earthquakes near the central Kurils occurred several months after we installed the network: Mw 8.3, 15 Nov 2006 underthrusting event, and Mw 8.1, 13 Jan 2007 tensional outer-rise event. Although the earthquakes have prevented us from estimating reliable interseismic surface velocities for most of the Kuril arc, we got the chance to examine great earthquakes and their transient response in the region that was a seismic gap for a century.
Two SGPS stations nearest to the hypocenters captured the largest observed offsets of about 0.6 m reflecting the superposed effect of both events. These offsets are mostly attributed to the Nov 2006 event. More distant stations captured coseismic offsets caused by each event ranging from several mm to ~60 mm. Significant transient signals associated with the rapid postseismic afterslip or with the relaxation in the viscous mantle were noticed for the Nov 2006 event but not for the Jan 2007 event. To ensure correct estimation of the Nov 2006 coseismic offsets, we modeled postseismic transients by the logarithmic approximation in agreement with the rate-strengthening friction.
For both events, we inverted observed GPS coseismic offsets to evaluate the size and rake of the coseismic slip, with the rupture approximated by six or twelve cells for the Nov 2006 event and by one or two cells for the Jan 2007 event. We used damped least squares for inversion, with the damping factor varied so that the chi-square of observed minus predicted offsets did not exceed the unity by more than 20%. In forward solutions required by the iterative procedure, we used the method of Pollitz with the PREM layered model of the spherical Earth. For the Nov 2006 earthquake, we constrained the rupture dimensions and geometry by distribution of aftershocks, shallow seismicity, and plate tectonics considerations, but not by the global CMT solution. For the Jan 2007 earthquake, we constrained the rupture by distribution of aftershocks and by the global CMT solution.
Regardless of the degree of smoothing, inversion for the Nov 2006 earthquake shows the following stable characteristics of the slip distribution: (1) the largest slip peaks in the southern region of the rupture near the epicenter; (2) the slip magnitude is near zero at the northwestern side of the rupture nearest to the island arc; (3) the rake is closer to the value predicted by the Pacific - North American plate convergence than to the value of 90° predicted by the trench-normal thrust; (4) the geodetic moment exceeds the global CMT moment by at least 20%. The last characteristic implies that a significant part of the coseismic slip occurred aseismically. Inversion for the Jan 2007 earthquake shows that the geodetic moment agrees with the global CMT moment within the data uncertainties.
For the period of a year since the Nov 2006 event, GPS time series demonstrate that the Kuril stations are moving to SE, opposite to the direction of interseismic motion. Our ongoing observations will allow us to determine whether the observed drift reflects the postseismic afterslip or the viscous relaxation in the mantle.