Regional moment tensor inversion using rotational observations

There are potential benefits from the addition of 3-C rotational motions to traditional 3-C translational displacements for moment tensor (MT) inversions. The rotational radiation pattern is orthogonal to the shear radiation pattern, thus incorporating rotations is equivalent to gaining another observation point on the focal sphere. We demonstrate this by simulating the curl and displacement wavefields in a half-space for a regional distance station. We also demonstrate the effect of velocity gradients beneath a station on rotational motions compared to displacements. Further investigation is needed to study how this affects the MT inversion. We added rotational Greens functions to regional long-period MT inversion computing spatial gradients from f- reflectivity synthetics. We used array derived rotational motions from the Piñon Flats Observatory Array in California and Golay arrays deployed during the IRIS Community Wavefield Demonstration Experiment in Oklahoma. Well-constrained MT solutions were estimated for three earthquakes using long-period regional waves with and without rotational ground motions as test cases. Prepared by LLNL under Contract DE-AC52-07NA27344. This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344. LLNL release number LLNL-ABS-781439. Index Terms: 7215 Earthquake source observations, 7290 Computational seismology Plain-Language Summary Scientists have traditionally used seismometers to record earthquake generated translational ground motions using three-dimensional axes typically oriented in a vertical, north-south and east-west directions in other words a “Cartesian coordinate system”. Recent advancements in seismometer development for recording rotational or twisting ground motions about the same three-axes provide additional information, which, in addition to translational seismometers can help resolve the radiation patterns of quakes. In cases where physical access is limited, the sparse distribution of seismometers caused by, for example, ocean coasts, islands, Lunar, and Mars surfaces can prevent the complete observation of the quake radiation pattern. The combination of these two types of seismometers at a single point are useful to infer faulting mechanisms of quakes or other seismic source types than using just one seismometer. Submitted to American Geophysical Union Fall Meeting, San Francisco, CA 9-13 December 2019. S032 – Rotation and Strain in Seismology – Applications, Instrumentation and Theory S21G-0589