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Exploring the uncertainty range of co-seismic stress drop estimations of large earthquakes using finite fault inversions

Mareike N. Adams, Cedric Twardzik, & Chen Ji

Under Review September 6, 2016, SCEC Contribution #7062

A new finite fault inversion strategy is developed to explore the uncertainty range for the energy based average co-seismic stress drop ((Δτ_E ) ̅) 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 (Δτ_E ) ̅ 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 (Δτ_E ) ̅, which allows one to define the range of (Δτ_E ) ̅ 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 (Δτ_E ) ̅ (5-10 MPa) for this earthquake is successfully constrained. However, the same dataset exhibits no sensitivity to its upper bound of (Δτ_E ) ̅ 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 (Δτ_E ) ̅ cannot be constrained with them. This has consequences for the earthquake energy budget. Failing to constrain the upper bound of (Δτ_E ) ̅ leads to the conclusions that 1) the seismic radiation efficiency determined from the inverted model might be significantly overestimated; 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 (Δτ_E ) ̅ can be used as an energy-based smoothing scheme during finite fault inversions.

Adams, M. N., Twardzik, C., & Ji, C. (2016). Exploring the uncertainty range of co-seismic stress drop estimations of large earthquakes using finite fault inversions. Geophysical Journal international, (under review).