Working Towards Including Rotational Ground Motions for Regional Long Period Full-Moment Tensor Inversion

Date: 4/25/2019

Time: 04:45 PM

Room: Cascade I

There is a potential benefit from adding rotational motions to moment tensor (MT) inversions because they provide information about the wavefields spatial gradients on the free surface that cannot be directly obtained using only three-component (3‑C) particle displacements (e.g., Donner et al., 2016; Li and van der Ban, 2017). Rotational data may provide additional constraints in resolving M_xzand M_yzfor shallow seismic events which are poorly resolved due to the free-surface vanishing traction and provide an additional radiation pattern in cases of sparse station coverage. Seismology is also long awaiting a broadband rotational seismometer and fiber-optic gyroscope technology appears promising. We computed 3‑C particle and 3‑C rotational displacements synthetics using f‑k reflectivity method from a 1‑D velocity model. Rotational Greens functions are formed by making spatial gradients from displacement synthetics. To compare data with synthetics, we use array particle displacements to derive rotational motions. We examined 4 earthquakes recorded from the small aperture Piñon Flats Observatory Array in California and Golay gradiometer array deployed during the IRIS Community Wavefield Demonstration Experiment in Oklahoma. We verified the observed array derived rotational motions by comparing them with translational motions to estimate phase velocity and back-azimuth. We obtained similar phase velocity and back-azimuth results using seismic gradiometry. Well-constrained MT solutions were estimated for each earthquake using long-period regional waves and then the MT solutions were used to model the observed array derived long-period rotational ground motions. We obtained good fit to the data with the rotational synthetics. Future work will include statistical tests to show when rotational motions best improve MT inversion. Prepared by LLNL under Contract DE-AC52-07NA27344.

Presenting Author: Gene A. Ichinose

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