Six Degree-of-Freedom Broadband Ground-Motion Observations with Portable Sensors: Validation, Local Earthquakes, and Signal Processing

Abstract

The additional observation of three components of rotational ground motions has benefitsfor tilt-seismometer coupling (e.g., ocean-bottom seismometry and volcano seismology),local site characterization, wavefield separation, source inversion, glacial and planetaryseismology, as well as the monitoring of structural health. Field applications have beenmostly hampered by the lack of portable sensors with appropriate broadband operationrange and weak-motion sensitivity. Here, we present field observations of the firstcommercial portable broadband rotation sensor specifically designed for seismology.The sensor is a three-component fiber-optic gyro strictly sensitive to ground rotation only.The sensor field performance and records are validated by comparing it with both arrayderivedrotation measurements and a navigation-type gyro. We present observations ofthe 2018 Mw 5.4 Hualien earthquake and the 2016 central Italy earthquake sequence.Processing collocated rotation and classical translation records shows the potential inretrieving wave propagation direction and local structural velocity from point measurementscomparable to small-scale arrays of seismic stations. We consider the availabilityof a portable, broadband, high sensitivity, and low self-noise rotation sensor to be a milestonein seismic instrumentation. Complete and accurate ground-motion observations(assuming a rigid base plate) are possible in the near, local, or regional field, openingup a wide range of seismological applications.