Sensitivity of prompt elasto-gravitational signals to 3-D Earth structure

SUMMARY Mass redistribution during earthquake rupture, along with subsequent wave propagation, perturbs Earth’s gravity field, generating the so-called ‘prompt elasto-gravitational signals’ (PEGS) and ‘prompt gravity-strain signals’ (PGSS). These signals are detectable on accelerometers and gradiometers before P-wave arrivals, and therefore offer the potential for early-warning systems and rapid assessment of event magnitude and tsunamigenic risk. Despite their significance, numerical modelling of PEGS and PGSS has been restricted to 1-D Earth models, assuming negligible effects of 3-D heterogeneities. In this study, we utilise a spectral-infinite-element method to compute these PEGS and PGSS for both 1-D and 3-D Earth models. Kernels of sensitivity to model heterogeneity are investigated using the adjoint method, which is re-formulated to write the self-gravitating adjoint equations directly in terms of density and elastic perturbations only. We find that PEGS and PGSS display the greatest sensitivity to model perturbations below the source and receiver; however, sensitivity is generally weak, validating previous assumptions that 1-D models are sufficient.