The Size and Rupture of the Great 2013 Deep-Focus Earthquake Beneath the Sea of Okhotsk: Constraints from GPS

The Sea of Okhotsk 24-May-2013 Mw 8.3 earthquake, which ruptured the lower end of the Kuril-Kamchatka Benioff zone, is the largest deep-focus event ever observed with seismometers and the first such event observed with GPS. According to the global centroid moment solution (GCMT), the centroid depth is 607 km. We captured coseismic static offsets on continuous GPS stations of the networks: the Kuril-Kamchatka GPS Array, Sakhalin Island, the far-western Aleutians, and the northern coast of the Sea of Okhotsk, totally 14 sites. The peak GPS offsets are 15 mm horizontally and 12 mm vertically. We did not detect afterslip for the period of 57 days following the earthquake. Coseismic slip was estimated from observed offsets by least-squares inversion using a spherical, elastically layered earth model (PREM). We tested several rectangular rupture models with strike, dip, and rake of the GCMT solution for both nodal planes. The mean slip on the fault was treated as an unknown. In all inversions, predicted surface displacements fit the data well at all GPS sites, with the reduced chi-square 0.9 assuming the standard deviation 1.5 mm for horizontal GPS offsets. The product DA, where D is the mean fault slip, and A is the area of the fault, varies by less than 1% regardless of the chosen fault dimensions for down-dip widths and along-strike lengths < 200 km and regardless of the chosen fault plane. In other words, because the rupture of this earthquake is so deep, the surface deformation does not constrain the distribution of coseismic slip in space; however it constrains well the product DA. To estimate the geodetic earthquake moment GDA from DA, where G is the shear modulus, we assume G to be that of PREM at the depth 607 km. The moment for the best fit DA is 3.47 x 10^21 N m, which is 15% smaller than from GCMT, and the magnitude Mw is 8.29 compared with 8.34 from GCMT. We explore possible explanations for the difference including the local structure of the slab.