#### Transcript Modeling_GPS_Velocities - GeoWeb

Introduction to the modelling of GPS results • GPS provides • Surface crustal velocities in a global reference frame, or with respect to a block, realized through a set of stations (globk ‘plate’ command) • Time dependent deformation (mainly CGPS & time series from glred) What can be asked of geodetic results ? Secular velocities • Kinematic boundary conditions around deformation zones • Deformation regime across deforming zones and its relation to global plate motion – input for dynamic models • slip rates along major faults* • locking depth, spatial distribution of coupling* * constraints on seismic hazard What can be asked of geodetic results ? Time dependent deformation • co-seismic displacement: location, slip distribution, moment • post-seismic deformation: afterslip, visco-elastic processes • detection and quantification of slow slip events Modelling velocities for tectonics In actively deforming zones, the velocities are the sum of several contributions: global plate motion – estimated & removed during Globk analysis to obtained velocities with respect to stable plate interiors • • long-term tectonic motion – modeled using either blocks or distributed deformation • because most faults are locked during the period separating two earthquakes (inter-seismic phase), they induce surface elastic deformation Rigid blocks approach Assumptions • deformation is localized along major faults • small internal deformation (<< deformation across major boundaries) • blocks defined by closed boundaries Methods solve for rigid rotation rates (Euler poles) Exemple: Nazca/South America convergence Villegas, 2009 Refinement: elastic blocks approach Same assumptions as for the blocks But, interseismic elastic deformation accounted for Results slip rates along major faults locking depth, coupling coefficient large faults like subduction interface: spatial distribution of coupling Figure from Z.-K. Shen, in Stein and Wysession, 2006 Strain rate • We now consider the local change of velocity • the velocity gradient tensor is defined by the derivative of the velocity components wrt to the coordinates • It can be divided into: • A local rotation • A strain rate tensor (symetric) Example of strain rate analysis using polygons Aktug et al., 2009 Strain rate analysis using a regular grid Kreemer et al., 2003 Thin viscous sheet approach Assumptions • deformation at scale (>>100 km) is driven by the balance of stress distribution induced by boundary conditions and stress arising from crustal thickness lateral variations • Lithosphere modeled as fluid • thin sheet approximation: all quantities are averaged over lithosphere thickness Example: large scale velocity field in Asia Vergnolle et al., 2006 Using SHELLS, http://peterbird.name Example: large scale velocity field in Asia Vergnolle et al., 2006 Using SHELLS, http://peterbird.name Elastic Block Models as a Tool for GPS Analysis • Account for surface deformation from fully or partially locked faults • Provide secular constraints in estimating timedependent motion • Create a kinematically consistent model for largescale motions There are (at least) two well-developed, documented software packages freely available: DEFNODE / TDEFNODE Rob McCaffrey, Portland State University (formerly at RPI) http://web.pdx.edu/~mccaf/www/defnode/ BLOCKS Brendon Meade, Harvard University (formerly at MIT) http://summit.fas.harvard.edu/~meade/meade/Software.html Region is divided into ‘blocks’, contiguous areas that are thought to rotate rigidly. The relative long-term slip vectors on the faults are determined from rotation poles. Each block rotates about a pole. Back-slip is applied at each fault to get surface velocities due to locking. Velocities due to fault locking are added to rotations to get full velocity field. The rotating blocks are separated by dipping faults. Courtesy Rob McCaffrey Okada model applied at boundaries Meade et al., [2002] Model velocities same for any path integral Program Flow for DEFNODE Data GPS velocities InSAR line-of-site rates Uplift rates Tilt rates Slip vectors Transform azimuths Spreading rates Fault slip rates Strain rates Parameters Block rotations Reference frame Fault locking Uniform strain rates Output Text files GMT mappable files Uncertainties (linearized) Solution Grid search Downhill simplex McCaffrey [1995; 2007] Example from the eastern Mediterranean Seismicity and earthquake focal mechanisms provide a first-cut for block boundaries GPS velocities refine the boundaries GMT representation of DEFNODE output GPS velocities observed modeled block motion Residuals Large-scale rotation with subduction lockingExample superimposed from the western Mediterranean : GPS velocities from 10 years of CGPS and SGPS measurements Note rigid rotation of Africa with respect to Iberia and independent motion of the Rif (Morocco) and Betic (Spain) mountains Koulali et al. (2011) Large-scale rotation with subduction locking superimposed Velocity residuals from a 3-block model Error elliipses are 70% confidence Example from Cascadia: Large-scale rotation with subduction locking superimposed Example from Cascadia: Large-scale rotation with subduction locking superimposed Surface velocities from subduction alone Deep red is fully locked; deep blue freely slipping GPS velocities from continuous (red) and surveymode (blue) sites. Insert shows depth of the subducting slap and fault nodes used in the inversion. Triangles are volcanoes. McCaffrey et al. [2007] A block model can be used in non-steady state settings to separate kinemtics from transients Example: Spatially propagating slow slip events (SSEs) in Cascadia Time series data from PANGA Model showing rotating blocks, subduction locking, and rates of uniform strain