Fault Parameters of the Nisqually Earthquake Determined from Moment Tensor Solutions and the Surface Deformation from GPS and InSAR

The magnitude 6.8 Nisqually earthquake occurred on 28 February 2001 at a depth of 50-60 km within the subducting Juan de Fuca plate. The fault parameters are estimated from moment tensor solutions and by comparing the surface displacements from Global Positioning System (GPS) data and from satellite interferometry, with predictions from elastic deformation models. The simple deformation model calculates the coseismic surface displacements caused by an earthquake on an extensional rectangular fault in a uniform half-space. Continuous GPS stations within 200 km of the epicenter resolved horizontal displacements as great as 9 mm (210°) and vertical displacements as great as 13 mm of subsidence near the epicenter. The near-vertical displacements were also determined from a differential interferogram created from synthetic aperture radar (InSAR) data from RADARSAT, the only satellite data available for the event. A maximum vertical subsidence of approximately 20 mm is observed 30 km east of the epicenter. The GPS, InSAR, and moment tensor solutions provide a consistent solution for the rupture parameters of the Nisqually earthquake. The fault has a strike of 180° and a dip of 20° and is centered at 47.10° N and 122.67° W (4 km east and 6 km south of the epicenter) or has a strike of 360° and a dip of 70° and is centered at 47.10° N and 122.69° W (2 km east and 6 km south). The trade-off between fault area and rupture displacement is not resolved by our data, but a good fit is found with the main rupture having an area of 230 km^2 and an along-strike length of 23 km and downdip width of 10 km. The rupture area is centered at a depth of 51 km with a scalar moment of 2.0 x 10^19 N m and, with the given area, an average rupture displacement of 1.4 m. To refine the interpretation of the GPS and InSAR data, a 3D heterogeneous numerical model was generated having realistic shear moduli structure, a 3D model of the subducting slab, and a spherical Earth. The results are similar to those from the half-space model, but there is significant refinement with a deeper rupture center at 60 km.