Towards Adjoint Finite Source Inversion: Application to the 2011 M9 Tohoku Earthquake

The recent 2011 M9 Tohoku, Japan, earthquake was recorded by thousands of sensors at near-fault distance, including broad band, strong motion and continuous GPS sensors. This event provides a unique opportunity to image the earthquake rupture process with high resolution. In order to enable the exploitation of the immense dataset available, orders of magnitude larger than in previous earthquakes, we are developing a scalable source inversion procedure based on time-reversal adjoint inversion. We adopt the linear least squares formulation of the source inversion problem, whose basic unknown is the spatio-temporal distribution of slip rate. We formulate an iterative conjugate gradient procedure to minimize the L2 norm of ground velocity residuals between data and synthetics. Each iteration involves one time-reversal (adjoint) and one forward simulation. Exploiting the time-reversal symmetry and the reciprocity principle of elastodynamics, the adjoint is computed by a wave propagation simulation in which time-reversed seismogram residuals are imposed as point forces at the stations simulated. The resulting fault tractions on a locked fault are the adjoint fields, related to the gradient of the misfit function with respect to the model. The simulations are performed with a recent extension of the SPECFEM3D spectral element code to dynamic and kinematic finite sources on unstructured meshes (Galvez et al, session S24 of this meeting). The non-planar geometry of the megathrust fault is accounted for in the spectral element mesh (generated with CUBIT). The subsurface structure is incorporated, on a coarse scale, using regional 3D velocity models, e.g. from the Japan Seismic Hazard Information Station (J-SHIS) website. We will report on the results of our initial efforts, focused on exploiting the continuous 1 Hz GPS signals recorded in Japan to understand the low frequency aspects of the rupture process of the 2011 Tohoku earthquake.