ALBERT

Description: The Chile fore-arc at 37°S-47°S represents the co-seismic deformation zone of the 1960 M w 9.5 Valdivia earthquake. Here we report on the paleomagnetism of 43 Oligocene-Pleistocene volcanic sites from the fore-arc sliver between 38°S and 42°S. Sites were gathered west of the 1000 km long Liquiñe-Ofqui dextral fault zone (LOFZ) that represents the eastern fore-arc sliver boundary. Nineteen reliable sites reveal that the fore-arc is characterized by counterclockwise (CCW) rotations of variable magnitude, except at 40°S-41°S, where ultra-fast (〉50°/Myr) clockwise (CW) rotations occur within a 30 km wide zone adjacent to the LOFZ. CCW rotation variability (even at close sites) and rapidity (〉10°/Myr) suggest that the observed block rotation pattern is related to NW-SE seismically active sinistral faults crosscutting the whole fore-arc. According to previously published data, CW rotations up to 170° also occur east of the LOFZ, and have been related to ongoing LOFZ shear. We suggest that the occurrence and width of the eastern fore-arc sliver undergoing CW rotations is a function of plate coupling along the subduction zone interface. Zones of high coupling enhance stress normal to the LOFZ, induce high LOFZ strength, and yield a wide deformation zone characterized by CW rotations. Conversely, low coupling imply a weak LOFZ, a lack of CW rotations, and a fore-arc entirely dominated by CCW rotations related to sinistral fault kinematics. Our locking inferences are in good agreement with those recently derived by GPS analysis, and indicate that seismotectonic segment coupling has remained virtually unchanged during the last 5 Ma.

Description: [1]  We developed a three-dimensional scheme to invert geoid anomalies aiming to map density variations in the mantle. Using an ellipsoidal-Earth approximation, the model space is represented by tesseroids. To assess the quality of the density models, the resolution and covariance matrices were computed. From a synthetic geoid anomaly caused by a plume tail with Gaussian noise added, the inversion code was able to recover a plausible solution about the density contrast and geometry when it is compared to the synthetic model. To test the inversion algorithm in a natural case study, geoid anomalies from the Yellowstone Province (YP) were inverted. From the EGM2008 geopotential model expanded up to degree 2160, lower crust and mantle-related negative geoid anomalies with amplitude of approximately 70 m were obtained after removing long-wavelength components (〉 5400 km) and crustal effects. We estimated three density models for the YP. The first model, the EDM-1, uses a starting model with density contrast equal to zero. The other two models, the EDM-2 and EDM-3 use an initial density derived from two S-velocity models for the western United States, the DNA10-S by Obrebsky et al . (2011) and the NWUS11-S by James et al . (2011). In these three models, a lower and an upper bound for the density solution was also imposed as a priori information. Regardless of the initial constraints, the inversion of the residual geoid indicates that the lower crust and the upper mantle of the YP have a predominantly negative density contrast (~ -50 kg/m 3 ) relative to the surrounding mantle. This solution reveals that the density contrast extends at least to 660 km depth. Regional correlation analysis between the EDM-1 and NWUS11-S indicates an anti-correlation (coefficient of ~ -0.7) at 400 km depth. Our study suggests that the mantle density derived from the inversion of geoid could be integrated with seismic velocity models to image mantle anomalous features beyond the depth limit of investigation achieved combining gravity and seismic tomography.

Description: The postseismic deformation captured with continuous Global Positioning System (cGPS) monitoring following many recent mega-thrust events has been shown to be a signal composed of two dominant processes: afterslip on the plate interface and viscoelastic relaxation of the continental and oceanic mantles in response to the coseismic stress perturbation. Following the south-central Chile 2010 Maule Mw 8.8 earthquake, the time series from the regional cGPS network show a distinct curvature in the pathway of the horizontal motion that is not easily fit by a stationary decaying pattern of afterslip in combination with viscoelastic relaxation. Here we show that with realistic assumptions about the long-term decay of the afterslip signal, the postseismic signal can be decomposed into three first-order contributing processes: plate interface re-locking, plate interface afterslip, and mantle viscoelastic relaxation. From our analyses we conclude that the plate interface recovers its interseismic locking state rapidly (model space ranges between an instant recovery and a period of 1 year); a finding that supports laboratory experimental evidence as well as some recent studies of aftershocks and postseismic surface deformation. Furthermore, re-locking is the main cause of the curvature in the cGPS signal, and this study presents a plausible range of geodetic re-locking rates following a megathrust earthquake.