ALBERT

Description: Summary Northeastward subduction of the oceanic Rivera and Cocos plates in western Mexico poses a poorly understood seismic hazard to the overlying areas of the North America plate. We estimate the magnitude and distribution of interseismic locking along the northern ∼500 km of the Mexico subduction zone, with a series of elastic half-space inversions that optimize the fits to the velocities of 57 GPS stations in western Mexico. All velocities were corrected for the coseismic, afterslip and viscoelastic rebound effects of the 1995 Colima-Jalisco and 2003 Tecomán earthquakes. We explore the robustness of interseismic locking estimates to a variety of mantle Maxwell times that are required for the viscoelastic corrections, to the maximum permitted depth for locking of the subduction interface, and to the location assigned to the Rivera-Cocos-North America plate triple junction offshore from western Mexico. The best fitting locking solutions are associated with a maximum locking depth of 40 km, a triple junction location ∼50 km northwest of the Manzanillo Trough, and a mantle Maxwell time of 15 yr (viscosity of 2 × 1019 Pa·s). Checkerboard tests show that the locking distribution is best resolved at intermediate depths (10-40 km). All of our inversions define a gradual transition from strong locking (i.e. 70-100 percent) of most (70%) of the Rivera-North America subduction interface to strong but less uniform locking below the Manzanillo Trough, where oceanic lithosphere transitional between the Cocos and Rivera plate subducts, to weak to moderate locking (averaging 55 percent) of the Michoacán segment of the Cocos-North America interface. Strong locking of the ∼125-km-long trench segment offshore from Puerto Vallarta and other developed coastal areas, where our modelling indicates an average annual elastic slip-rate deficit of ∼20 mm yr−1, implies that ∼1.8 m of unrelieved plate slip has accrued since the segment last ruptured in 1932, sufficient for a M ∼ 8.0 earthquake.

Description: Summary We invert ∼25 years of campaign and continuous Global Positioning System daily positions at 62 sites in southwestern Mexico to estimate coseismic and postseismic afterslip solutions for the 1995 Mw = 8.0 Colima-Jalisco and the 2003 Mw = 7.5 Tecomán earthquakes, and the long-term velocity of each GPS site. Estimates of the viscoelastic effects of both earthquakes from a 3-D model with an elastic crust and subducting slab, and linear Maxwell viscoelastic mantle are used to correct the GPS position time series prior to our time-dependent inversions. The preferred model, which optimizes the fit to data from several years of rapid postseismic deformation after the larger 1995 earthquake, has a mantle Maxwell time of 15 years (viscosity of 2 × 1019 Pa·s), although upper mantle viscosities as low as 5 × 1018 Pa·s cannot be excluded. Our geodetic slip solutions for both earthquakes agree well with previous estimates derived from seismic data or via static coseismic offset modelling. The afterslip solutions for both earthquakes suggest that most afterslip coincided with the rupture areas or occurred farther downdip, and had cumulative moments similar to or larger than the coseismic moments. Afterslip thus appears to relieve significant stress along the Rivera plate subduction interface, including the area of the interface between a region of deep non-volcanic tremor and the shallower seismogenic zone. We compare the locations of the seismogenic zone, afterslip and tremor in our study area to those of the neighboring Guerrero and Oaxaca segments of the Mexico subduction zone. Our newly derived interseismic GPS site velocities, the first for western Mexico that are corrected for the coseismic and postseismic effects of the 1995 and 2003 earthquakes, are essential for future estimates of the interseismic subduction interface locking and hence the associated seismic hazard.