ALBERT

Description: We present observations of cross coupled spheroidal modes in the Earth's free oscillation spectrum recorded by the vertical component G-ring laser (Geodetical Station Wettzell) of the 2011 M w 9.0 Tohoku-Oki earthquake. In an attempt to determine which are the mechanisms responsible for spheroidal energy in a vertical axes rotational spectra, we first rule out instrumental effects as well as the effect of local heterogeneity. Secondly, we carry out simulations of an ideal rotational sensor taking into account the effects of the Earth's rotation, its hydrostatic ellipticity and structural heterogeneity, which results in a good fit to the data. Simulations considering each effect separately are performed in order to evaluate the sensitivity of rotational motions to various phenomena compared to traditional translation measurements.

Description: Microzonation, the estimation of (shear) wave velocity profiles of the upper few 100 m in dense 2D profiles, is one of the key methods for understanding the variation in seismic damage caused by ground-shaking events and thus for mitigating the risk of damage in the future. In this article, we present a novel method for estimating the Love-wave phase velocity dispersion using ambient noise recordings. We use the vertical component of rotational motions inherently present in ambient noise and the established relation to simultaneous recordings of transverse acceleration, in which the phase velocity of a plane SH (or Love)-type wave acts as a proportionality factor. We demonstrate that the developed inversion technique shows comparable results to more classical, array-based methods. Furthermore, we demonstrate that if portable weak-motion rotational motion sensors are available and the installation of a seismic network or array is not possible, a single point, multicomponent approach for estimating the dominant direction of the incident wavefield and the local velocity structure will be feasible with similar performance compared to more classical techniques.

Description: Dynamic tilts (rotational motion around horizontal axes) change the projection of local gravity onto the horizontal components of seismometers. This causes sensitivity of these components to tilt, especially at low frequencies. We analyse the consequences of this effect onto moment tensor inversion for very long period (vlp) events in the near field of active volcanoes on the basis of synthetic examples using the station distribution of a real deployed seismic network and the topography of Mt. Merapi volcano (Java, Indonesia). The examples show that for periods in the vlp range of 10–30 s tilt can have a strong effect on the moment tensor inversion, although its effect on the horizontal seismograms is significant only for few stations. We show that tilts can be accurately computed using the spectral element method and include them in the Green's functions. The (simulated) tilts might be largely influenced by strain–tilt coupling (stc). However, due to the frequency dependence of the tilt contribution to the horizontal seismograms, only the largest tilt signals affect the source inversion in the vlp frequency range. As these are less sensitive to stc than the weaker signals, the effect of stc can likely be neglected in this application. In the converse argument, this is not necessarily true for longer periods, where the horizontal seismograms are dominated by the tilt signal and rotational sensors would be necessary to account for it. As these are not yet commercially available, this study underlines the necessity for the development of such instruments.

Description: We report in this paper an original analysis of microseismic events (MSEs) induced by an excavation operation in the clay environment of the Mont Terri underground rock laboratory. In order to identify the MSEs with confidence, we develop a restrictive but efficient multistep method for filtering the recorded events. We deduce the spatial distribution and processes associated with the excavation-induced damage from the spatial location and focal mechanisms of the MSEs. We observe an asymmetric geometry of the excavation damaged zone around the excavated gallery, without notable microseismic activity in the sandy facies sidewall, in contrast with the shaly facies sidewall where a first burst of events is recorded, followed by two smaller bursts: one locates ahead of the excavation front and is associated with a dominant double-couple component, suggesting bedding plane reworking, that is, shear fracture mode, and the MSEs of the other cluster inside the shaly sidewall of the gallery, with a dominant compensated linear vector dipole component, suggesting extensive cracking. We identify and discuss four major factors that seem to control the MSEs source mechanisms: lithology, geometry of the geological features, gallery orientation and direction of the main compressive stress.

Description: The unique instrument setting at the Piñon Flat Observatory in California is used to simultaneously measure 10 out of the 12 components, completely describing the seismic-wave field. We compare the direct measurements of rotation and strain for the 13 September 2015 M w  6.7 Gulf of California earthquake with array-derived observations using this configuration for the first time. In general, we find a very good fit between the observations of the two measurements with cross-correlation coefficients up to 0.99. These promising results indicate that the direct and array-derived measurements of rotation and strain are consistent. For the array-based measurement, we derived a relation to estimate the frequency range within which the array-derived observations provide reliable results. This relation depends on the phase velocity of the study area and the calibration error, as well as on the size of the array.

Description: We introduce a new event database for rotational seismology. The rotational seismology website (see Data and Resources ) grants access to 17,000+ processed global earthquakes starting from 2007. For each event, it offers waveform and processed plots for the seismometer station at Wettzell and its vertical-component ring laser (G-Ring), as well as extensive metrics (e.g., peak amplitudes, signal-to-noise ratios). Tutorials and illustrated processing guidelines are available and ready to be applied to other data sets. The database strives to promote the use of joint rotational and translational ground-motion data demonstrating their potential for characterizing seismic wavefields.

Description: We report the first ground rotation observations on the seafloor from an experiment we carried out in the North Sea close to the island of Heligoland. A slightly modified commercial fiber optic gyroscope was mounted on an ocean-bottom seismometer (OBS) platform together with an intermediate-period seismometer. The system was lowered to the seafloor for 4 days. To investigate a potential tilt contamination of horizontal translational recordings, we calculate the coherence between the corresponding motion components (rotations around x axis and translations along y axis, and vice versa). We find very high correlations in the 5–13 s period interval, in which the correlation coefficient reaches 0.94 over 8.5 hrs. This clearly indicates that horizontal translational components are severely contaminated by rotations. We find that these rotational motions are caused by seafloor currents or deformation of the seafloor rather than by seismic waves. The ground rotation observations allow correcting for the cross-coupling effect, thereby decreasing the power spectral density up to 11 dB at 10 s period on horizontal OBS components. We discuss general requirements for broadband rotation sensors for OBS applications as well as for possible further applications.

Description: 〈span〉〈div〉Summary〈/div〉The broadband surface wave magnitude equation assigns magnitudes based on source-receiver distance and peak surface wave amplitude. It is standard practice to use the vertical component of peak ground velocity to determine magnitude, such that only contributions from the vertical motion of Rayleigh waves are present in the surface wave train. With the advent of rotational ground motion observations from instruments such as ring laser gyroscopes, it is possible to measure rotational ground motions about three orthogonal axes. For surface waves, observations of rotations about the vertical axis are theoretically only sensitive to the transverse nature of Love waves, unaffected by either component of Rayleigh waves. We use this concept to separate and study the amplitude information of surface waves independently. With a large database of recorded seismic waveforms for co-located translations and rotations, collected in Wettzell, Germany, we empirically define magnitude scale attenuation constants as a method for quantifying amplitude decay. Through this differential analysis, we determine a necessity for separate surface wave magnitude equations through measurements of translations and rotations. Synthetic seismograms were concurrently produced using an open-source spectral-element wave propagation code, for comparisons against observations. Though synthetically derived amplitude decays agree for translations, they do not accurately predict the decay found for rotations. Synthetics also overpredict amplitudes for both rotations and translations. Results from observations imply that rotation amplitudes decay faster over distance with respect to velocity amplitudes, and that the current surface wave magnitude equation is insufficient for predicting observed translation and rotation amplitudes. We attribute variations in amplitude decay characteristics to the different effects of attenuation on Love and Rayleigh waves, with potential influence from local velocity structure, and scattering effects. The lack of agreement in synthetics is attributed to the insufficiency of synthetic attenuation and velocity structure to replicate the effects seen in observations.〈/span〉