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 February 28, 2001 at a depth of 50-55 km within the subducting Juan de Fuca plate. The fault parameters are estimated from moment tensor solutions, and by comparing the surface deformation from GPS data and from satellite interferometry with predictions from elastic deformation models. The deformation model calculates the coseismic surface deformations caused by an earthquake on a tensile rectangular fault in a uniform half space. Continuous GPS stations within 200 km of the epicentre resolved horizontal displacements as great as 9 mm (210°) and vertical displacement as great as 13 mm of subsidence near the epicentre. The near-vertical displacements were also determined from a differential interferogram created from synthetic aperture radar data from RADARSAT, the only satellite data available for the event. A maximum vertical subsidence of approximately 20 mm is observed 30 km west of the epicentre. 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° or a strike of 360° and a dip of 70°. The trade-off between fault area and rupture displacement is not resolved by our data but a good fit is found with an area of 230 km2 and an along-strike length of 23 km and downdip width of 10 km. If the event nucleated within the crust it occurred at a depth of 51 km with a scalar moment of 2.0*10^19 Nm and, with the above area, a rupture displacement of 1.4 m, whereas if the event nucleated within the mantle it occurred at a depth of 54 km with a scalar moment of 1.8*10^19 Nm and rupture displacement of 1.5 m.