Influence of anelastic surface layers on postseismic thrust fault deformation

We present the results of a systematic modeling study of postseismic deformation following blind thrust earthquakes. The results include qualitative and quantitative predictions' of the surface movement caused by relaxation in viscoelastic near-surface layers. Finite element forward models are used in conjunction with elastic dislocation inversions to characterize the postseismic deformation. A viscoelastic surface layer overlying a blind thrust fault in an elastic basement shows characteristic signatures of postseismic surface movement. Simple equivalent elastic dislocations located in the hanging wall wedge are found to provide an effective proxy for near-surface postseismic relaxation in two-dimensional numerical simulations. A model survey of a range of fault dip angles and layer geometries shows the time evolution and geometry of the proxy fault to be simply related to fault dip and sediment thickness. The results are of significance in the interpretation of postseismic Global Positioning System (CPS) strain data from the 1994 Northridge, California, earthquake and other similar events in regions characterized by poorly consolidated or otherwise anelastic layers overlying the brittle seismogenic zone.