Exploring the uncertainty range of coseismic stress drop estimations of large earthquakes using finite fault inversions

Abstract

Anewfinite fault inversion strategy is developed to explore the uncertainty range for the energy based average coseismic stress drop (ΔτE) of large earthquakes. For a given earthquake, we conduct a modified finite fault inversion to find a solution that not only matches seismic and geodetic data but also has a ΔτE matching a specified value. We do the inversions for a wide range of stress drops. These results produce a trade-off curve between the misfit to the observations and ΔτE, which allows one to define the range of ΔτE that will produce an acceptable misfit. The study of the 2014 Rat Islands Mw 7.9 earthquake reveals an unexpected result: when using only teleseismic waveforms as data, the lower bound of ΔτE (5-10 MPa) for this earthquake is successfully constrained. However, the same data set exhibits no sensitivity to its upper bound of ΔτE because there is limited resolution to the fine scale roughness of fault slip. Given that the spatial resolution of all seismic or geodetic data is limited, we can speculate that the upper bound of ΔτE cannot be constrained with them. This has consequences for the earthquake energy budget. Failing to constrain the upper bound of ΔτE leads to the conclusions that (1) the seismic radiation efficiency determined from the inverted model might be significantly overestimated and (2) the upper bound of the average fracture energy EG cannot be constrained by seismic or geodetic data. Thus, caution must be taken when investigating the characteristics of large earthquakes using the energy budget approach. Finally, searching for the lower bound of ΔτE can be used as an energy-based smoothing scheme during finite fault inversions.