An energy-based smoothing constraint and the uncertainty range of co-seismic stress drop of large earthquakes

Mareike N. Adams, Jinglai Hao, Cedric Twardzik, & Chen Ji

Submitted August 25, 2016, SCEC Contribution #6921, 2016 SCEC Annual Meeting Poster #240

The large uncertainties of some basic earthquake source parameters, such as average stress drop (Cotton et al., 2013), are largely caused by over-simplifications of the source (e.g., Madariaga, 1979). In contrast, the results of finite fault source inversions suffer from the uncertainties caused by over-parameterization. Hundreds of free parameters are often used to represent spatiotemporal rupture history, despite the fact that not all of them are well constrained by the available observations. The uncertainties of individual sub-fault parameters are often difficult to access because they are not only caused by the limited data but also the regularizations applied to stabilize the inversions. We develop a new finite fault inversion strategy to explore the uncertainty range for the energy based average stress drop (Δτ_E ) of large earthquakes. For a given earthquake, a series of modified finite fault inversions are conducted to search for the solution that not only best fits seismic and geodetic data but also has a Δτ_E matching a given value. It results in a trade-off curve between the misfit to the observations and Δτ_E , allowing the range of Δτ_E constrained by the given geophysical dataset to be robustly defined. Our initial study of the 2014 Mw7.9 Rat Islands earthquake using teleseismic data revealed that only the lower bound of Δτ_E could be constrained with far field seismic data (Adams et al., 2016). To investigate whether such a conclusion also holds for near field data, the slip distribution and Δτ_E of the 2015 Mw7.9 Gorkha, Nepal earthquake is studied using near-source geodetic data. Our result reveals a similar pattern that the misfit gradually improves when the target Δτ_E increases from 0.5 to 8 MPa; but becomes nearly constant from 10 MPa to 50 MPa. Hence, only the lower bound (~8-10 MPa) of Δτ_E can be constrained, even with such a comprehensive geodetic dataset. As Δτ_E is proportional to the available seismic energy, our results imply that only the lower bound of the available energy can be constrained. We notice that the Laplacian of fault slip monotonously increases with Δτ_E and propose to use Δτ_E as a new regularization tool to smooth fault slip during the source inversion. The inverted result can be viewed as the solution with minimum available seismic energy for one earthquake.

Adams, M. N., Hao, J., Twardzik, C., & Ji, C. (2016, 08). An energy-based smoothing constraint and the uncertainty range of co-seismic stress drop of large earthquakes . Poster Presentation at 2016 SCEC Annual Meeting.

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