• Rotational Seismology

During the transit of seismic waves the earth and the ground is not only translating but it also rotates. Traditionally seismologists could only measure translations along three cardinal axes but earthquakes also generates tilt motions which rotate the ground. This fact was predicted by the linear elasticity theory but it took more than thirty years of technological progress in the instrumentation to achieve the sensitivity needed to record this tiny but extremely important ground motion. The most reliable instruments to cap- ture rotational motions are optical gyroscopes. The large ring lasers (RLG) provide top sensitivity and are able to detect any M7+ earthquake. The new portable fiber optic gyroscopes (FOG) specifically designed for seismology, at the expense of a lower sensitivity, provide the great advantage of portability, which is a mandatory requirement for in-field measurements. We have, since at least ten years, consistent four components (three translations and one vertical rotation rate) observations from the G-ring in Wettzell, Germany. These observations permitted to establish the importance of colocated rotational and translational measurements for the study of earthquakes and ocean generated noise. Still at the beginning of this Ph.D. project there was need of confirming and expanding the observations to different sites, possibly in a different structural context and exploring higher amplitude signals and closer epicentral distances with a large RLG. Broadband six component (6C) measurements from a portable rotational sensor of local earthquakes were missing too. These open questions are faced in this work whose chapters are constituted by scientific publications in chronological order. From 2015 I contributed to the experimental activity, construction and data analysis of a new RLG located in gran Sasso underground laboratories named Gingerino. The first three chapters regard the measurements of the Gingerino RLG instrument inside the Gran Sasso, in a deep underground environment. In the first chapter I report the detection and the analysis of the first underground rotational signals from a tele-seismic event. The characterization of the instrument as well as an analysis of the noise of the installation site can be found in chapter two. The analysis of the data from the 2016 central Italy seismic sequence is presented in chapter three. In this chapter we report a large dataset of events that a are studied with new methods based on the wavelet decomposition of the signals. The last chapter shows the results of the first field campaign with 6C observations (three rotational and three translational degrees of freedom) during the aftershocks of the MW 6.1 Norcia earthquake of 2016. This configuration can be alternative to a seismometer array; this is an undeniable logistic advantage for future applications in extreme environments as well as in planetary seismology. By the time of writing this abstract we have three large ring laser gyroscope operative in the world: G-wettzell, Gingerino and the new ROMY, a four components RLG that allows the reconstruction of the ground rotation vector with a record sensitivity. We expect then very soon advances in this research field thanks to the recent developments in instrumentation and processing techniques, some of them are already present in this work.