Rotational Ground Motions Research Articles

Seismic Anisotropy From 6C Ground Motions of Ambient Seismic Noise

AbstractWe propose a new approach capable of measuring local seismic anisotropy from 6C (three‐component translation and three‐component rotation) amplitude observations of ambient seismic noise data. Our recent theory demonstrates that the amplitude ratio of 6C cross‐correlation functions (CCFs) enables retrieving the local phase velocity. This differs from conventional velocity extraction methods based on the travel time. Its local sensitivity kernel beneath the 6C seismometer allows us to study anisotropy from azimuth‐dependent CCFs, avoiding path effects. Such point measurements have great potential in planetary exploration, ocean bottom observations, or volcanology. We apply this approach to a small seismic array at Pion Flat Observatory (PFO) in southern California, array‐deriving retrieves rotational ground motions from microseismic noise data. The stress‐induced anisotropy is well resolved and compatible with other tomography results, providing constraints on the origin of depth‐dependent seismic anisotropy.

Fiber-optic gyroscope for rotational seismic ground motion monitoring of the Campi Flegrei volcanic area

The real-time monitoring of densely populated areas with high seismic and volcanic risk is of crucial importance for the safety of people and infrastructures. When an earthquake occurs, the Earth surface experiences both translational and rotational motions. The latter are usually not monitored, but their measurement and characterization are essential for a full description of the ground motion. Here we present preliminary observational data of a high-sensitivity rotational sensor based on a 2-km-long fiber-optic Sagnac gyroscope, presently under construction in the middle of the Campi Flegrei Volcanic Area (Pozzuoli, Italy). We have evaluated its performance by analyzing data continuously recorded during an acquisition campaign of five months. The experimental setup was composed of a digital nine-component seismic station equipped with both a rotational sensor and conventional seismic sensors (seismometers, accelerometers, and tiltmeters). During this experiment we detected seismic noise and ground rotations wavefield induced by small to medium local earthquakes (M D <3). The prototype gyroscope shows a very promising sensitivity in the range of 5×10−7−8×10−9rad/s/Hz over the frequency bandwidth 5 mHz–50 Hz. Future upgrades and perspectives are discussed.

Characteristics of torsional ground motions obtained for Turkey and their appraisal against Eurocode 8

In current engineering practice, the torsional effects of ground motions are only considered implicitly through “accidental eccentricity coefficients". In seismic design codes, these coefficients account for not only the unintended (or uncertain) asymmetric distributions of structural mass and stiffness in the plan but also the torsional components of ground motions. The torsional effects may develop due to wave passage effects or the incoherency of translational ground motions along the foundation systems. While it is possible to obtain torsional components of ground motions directly from seismic records with modern instruments, such sensors are uncommon. Therefore, several numerical procedures have been proposed in the literature for estimating torsional components of ground motions from translational measurements. In this study, the Single Station Procedure (SSP)—based on the deconstruction of translational ground motions into body waves and the subsequent reconstruction of rotational ground motions—is employed to obtain torsional ground motions for seven different seismic events in Turkey. Additionally, the corresponding accidental eccentricity coefficients that can cause the same response modification with torsional ground motions are determined. First, a conventional procedure is utilized to obtain the corresponding accidental eccentricity coefficients. Various drawbacks of the conventional procedure, such as negative eccentricity coefficients, are addressed. Then, a relatively obscure definition of accidental eccentricity is revived to propose a novel procedure for determining accidental eccentricity coefficients. It is observed that the proposed procedure yields higher values for the accidental eccentricity coefficients than the conventional procedure. It also observed that the torsional spectrum provided in Eurocode 8 provisions requires higher eccentricity values than those implied by the recorded ground motions from Turkey.

Characterizing the Background Noise Level of Rotational Ground Motions on Earth

Abstract The development of high-sensitive ground-motion instrumentation for Earth and planetary exploration is governed by so-called low-noise models, which characterize the minimum level of physical ground motions, observed across a very broad frequency range (0.1 mHz–100 Hz). For decades, broadband instruments for seismic translational ground-motion sensing allowed for observations down to the Earth’s low-noise model. Knowing the lowermost noise level distribution across frequencies enabled not only to infer characteristics of Earth such as the ocean microseismic noise (microseisms) and seismic hum, but also to develop highly successful ambient seismic noise analysis techniques in seismology. Such a low-noise model currently does not exist for rotational ground motions. In the absence of a substantial observational database, we propose a preliminary rotational low-noise model (RLNM) for transverse rotations based on two main wavefield assumptions: the frequency range under investigation is dominated by surface-wave energy, and the employed phase velocity models for surface waves are representative. These assumptions hold, in particular, for a period range of about 2–50 s and lose validity towards long periods when constituents produced by atmospheric pressure dominate. Because noise levels of vertical and horizontal accelerations differ, we expect also different noise levels for transverse and vertical rotations. However, at this moment, we propose a common model for both types of rotations based on the transverse RLNM. We test our RLNM against available direct observations provided by two large-scale ring lasers (G-ring and ROMY) and array-derived rotations (Piñon Flats Observatory array, Gräfenberg array, and ROMY array). We propose this RLNM to be useful as guidance for the development of high-performance rotation instrumentation for seismic applications in a range of 2–50 s. Achieving broadband sensitivity below such a RLNM remains a challenging task, but one that has to be achieved.

Analyze the Difference Between Rotational and Translational Motions Produced by High-speed Train

The development of fiber optic gyroscope technology makes it possible to directly measure the rotational ground-motion. The joint observation of the three-component rotational seismometer and the traditional three-component translational seismometer is a trend of future. In this paper, the translational and rotational signals generated during the high-speed trainway are studied, and the three-component translational and rotational seismometers are fixed together to realize the joint recording of the high-speed trainway signal. Comparing the translational and rotational three-component data, we find that only one component waveforms and the spectrum have certain consistency. However, the difference between them indicates that the rotation and translation signals generated by the shallow surface wave signal have different frequency bands.

Modified Frequency-Bessel Transform Method for Dispersion Imaging of Surface Wave from Six-Component Ambient Noise Recordings

Surface wave analysis methods based on ambient noise have attracted considerable attention within the geophysical and civil engineering communities, particularly in light of the limitations associated with traditional seismic surveys in densely populated urban areas. Conventionally, translational seismic sensors have been employed for the observation of ambient noise. Nevertheless, recent advancements in rotational sensor technology have facilitated the direct measurement of ground motion induced by seismic waves in a rotational manner. Through the integration of these rotational sensors with traditional inertial seismometers, a novel six-component seismic sensor has been developed. This innovative sensor enables the local observation of six degrees of freedom pertaining to ground motion, encompassing three orthogonal components of translational motion and three orthogonal components of rotational motion. In order to demonstrate the seismological significance of rotational ground-motion data, we applied the modified frequency-Bessel transform method (MFJ) to six-component ambient noise recordings for surface wave analysis. To underscore the practical utility of the MFJ method, a real-world case study is presented, illustrating its effectiveness in quantifying Rayleigh and Love wave dispersion from six-component ambient noise recordings.

Study on the strong earthquake failure mechanism of space grid structure considering rotational ground motion

Space grid structures are increasingly being utilized in public buildings due to their exceptional structural performance. Given that these structures are commonly located in areas with high pedestrian traffic and significant investments, damages incurred during an earthquake could result in significant economic losses and casualties. To explore the rotational seismic response of space grid structures, this study employs the frequency domain method to compute the rotational seismic motion. Using the Lushan Middle School Comprehensive Hall's structural form as a reference, a finite element model of the space grid structure is established. Three types of seismic motions, namely three-way translational, three-way rotational, and six-way translational and rotational, are input into the structural model to investigate strong seismic translation and rotation coupling effects. The impact and features of rotational seismic motion on space grid structures are explored by studying the influence and characteristics of rotational seismic motion. Under varying seismic motion inputs, the failure mechanisms of typical space grid structures' translational and rotational components are also determined.

Efficient wave type fingerprinting and filtering by six-component polarization analysis

SUMMARYWe present a technique to automatically classify the wave type of seismic phases that are recorded on a single six-component recording station (measuring both three components of translational and rotational ground motion) at the Earth’s surface. We make use of the fact that each wave type leaves a unique ’fingerprint’ in the six-component motion of the sensor (i.e. the motion is unique for each wave type). This fingerprint can be extracted by performing an eigenanalysis of the data covariance matrix, similar to conventional three-component polarization analysis. To assign a wave type to the fingerprint extracted from the data, we compare it to analytically derived six-component polarization models that are valid for pure-state plane wave arrivals. For efficient classification, we make use of the supervised machine learning method of support vector machines that is trained using data-independent, analytically derived six-component polarization models. This enables the rapid classification of seismic phases in a fully automated fashion, even for large data volumes, such as encountered in land-seismic exploration or ambient noise seismology. Once the wave-type is known, additional wave parameters (velocity, directionality and ellipticity) can be directly extracted from the six-component polarization states without the need to resort to expensive optimization algorithms. We illustrate the benefits of our approach on various real and synthetic data examples for applications such as automated phase picking, aliased ground-roll suppression in land-seismic exploration and the rapid close-to real-time extraction of surface wave dispersion curves from single-station recordings of ambient noise. Additionally, we argue that an initial step of wave type classification is necessary in order to successfully apply the common technique of extracting phase velocities from combined measurements of rotational and translational motion.

Seismic Structure-Soil-Structure Interaction between a pair of buildings with consideration of rotational ground motions effects

During an earthquake, buildings are analysed dynamically without any consideration of their neighbouring structures and usually this also neglects the influence of surficial rotational ground motion. Population growth and increased land costs have increased the spatial density of buildings in urban areas. This closer proximity of buildings to one another leads to more complex interactions between buildings and the surficial seismic ground motion field that includes both horizontal and rotational components. To address these issues, this paper explores the Structure-Soil-Structure Interaction (SSSI) between a pair of buildings, including the surficial horizontal and rotational ground motions. A reduced-order parametric model of this complex system is proposed. Ground accelerogram data recorded from the 6-DOF sensor SINAPS@ Kefalonia (Greece) post-seismic experiment and the SMART-1 array in Taiwan were employed. This extensive parametric study analyses over six million system/ground motion cases. Results suggest that the rotational ground motion can affect the seismic response when the SSSI is considered, producing an additional amplification of acceleration responses of certain structures. The impact of the rotational ground motion depends principally on the fundamental natural period of the building, and the SSSI depends mainly on the frequency ratio (here Ωb) between the buildings.

Validation of Rayleigh Wave Theoretical Formulation with Single-Station Rotational Records of Mine Tremors in Lower Silesian Copper Basin

The classical Rayleigh surface rotational wave in terms of its theoretical notation and, resulting from this, properties associated with the induced seismic phenomena in mines are presented. This kind of seismic wave was analysed in-depth from the point of view of the parameters governing the form of its mathematical notation based on the similarity to the records obtained during the induced seismicity in near-field 6-DoF monitoring. Furthermore, conducted field measurements made it possible to relate the amount of the emitted seismic energy to the expected highest amplitude of rotational vibrations in the entire field of their impact on the rock mass. As a result, this made it possible to impose the completely defined R wave to the numerical models of given objects; the safety level, when subjected to the dynamic load induced by the rotational wave, would be an objective of the performed analyses. The conducted preliminary analyses were prepared for a plane strain state, for which the values of seismic rotations were evaluated concerning the energy and the distance of the seismic event’s source. As a result of the performed simulations, it was found that the results of the calculations matched with a satisfying degree with the field seismic measurements of the rotational ground motion induced by propagating the seismic wave. Such a verified analytical description of the theoretical formulas can be the basis for the implementation of R-wave characteristics into seismic codes and numerical analyses of object stability in the Lower Silesian Copper Basin region.

Translational and rotational ground motion simulations in homogeneous reduced micropolar half-space

The present article focusses on determining the response of reduced micropolar half-space subjected to specific loading conditions. Analytical expressions for translational and rotational displacements are derived for the half-space subjected to impulses using the method of potentials. The derived analytical expressions are numerically evaluated to determine the response of reduced micropolar half-space subjected to buried impulses acting in both horizontal and vertical directions. The derived Green’s functions are further used to obtain the surface translations and rotations for the half-space subjected to earthquake force. Parametric analysis is carried to understand the effect of additional material parameters of the reduced micropolar medium on the surface ground motions. Finally, ground motions are simulated for 2012 Wutai, Taiwan, earthquake by modelling the medium as a homogeneous reduced micropolar half-space consisting of rock material properties.

Regional Moment Tensor Inversion Using Rotational Observations

AbstractAbstractThere are benefits from the addition of rotational motions to 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 demonstrated this by simulating curl and displacement wavefields for a regional distance station. Thus, one 6‐C station (3‐Component translational + 3‐Component rotational) gathers the same information on radiation pattern as two 3‐C stations at 90° azimuth from one another along the focal plane axis, which is sometimes difficult to obtain when restricted to surface sensors. We added rotational Green's functions to a regional MT inversion scheme (long‐period, time‐domain, and linear inversion) by computing spatial gradients from f‐κ reflectivity synthetics. For rotational data, we used Array Derived Rotations (ADR) from Piñon Flats Observatory Array and Golay array deployed during IRIS Community Wavefield Demonstration Experiment. The hope is to ultimately use compact and field deployable broadband rotational seismometers instead of ADRs. Rotational motions were predicted from well‐constrained deviatoric MT solutions of nine earthquakes recorded by the arrays and other seismic networks. We formed three station sparse datasets with (3‐C and 6‐C) and without rotational ground motions (3‐C only) for eight earthquakes testing the benefits of including rotational waveforms in MT inversions when they are equally weighted with translational waveforms. Adding rotational motions improved the double‐couple components and reduced compensated‐linear‐vector‐dipole and isotropic components in full‐MT solutions using sparse datasets with poor coverage.

ROMY: a multicomponent ring laser for geodesy and geophysics

SUMMARY Single-component ring lasers have provided high-resolution observations of Earth’s rotation rate as well as local earthquake- or otherwise-induced rotational ground motions. Here, we present the design, construction and operational aspects of ROMY, a four-component, tetrahedral-shaped ring laser installed at the Geophysical Observatory Fürstenfeldbruck near Munich, Germany. Four equilateral, triangular-shaped ring lasers with 12 m side length provide rotational motions that can be combined to construct the complete vector of Earth’s rotation from a point measurement with very high resolution. Combined with a classic broad-band seismometer, we obtain the most accurate 6 degree-of-freedom ground motion measurement system to date, enabling local and teleseismic observations as well as the analysis of ocean-generated Love and Rayleigh waves. The specific design and construction details are discussed as are the resulting consequences for permanent observations. We present seismic observations of local, regional and global earthquakes as well as seasonal variations of ocean-generated rotation noise. The current resolution of polar motion is discussed and strategies how to further improve long-term stability of the multicomponent ring laser system are presented.

Seismological Processing of Six Degree-of-Freedom Ground-Motion Data.

Recent progress in rotational sensor technology has made it possible to directly measure rotational ground-motion induced by seismic waves. When combined with conventional inertial seismometer recordings, the new sensors allow one to locally observe six degrees of freedom (6DOF) of ground-motion, composed of three orthogonal components of translational motion and three orthogonal components of rotational motion. The applications of such 6DOF measurements are manifold—ranging from wavefield characterization, separation, and reconstruction to the reduction of non-uniqueness in seismic inverse problems—and have the potential to revolutionize the way seismic data are acquired and processed. However, the seismological community has yet to embrace rotational ground-motion as a new observable. The aim of this paper is to give a high-level introduction into the field of 6DOF seismology using illustrative examples and to summarize recent progress made in this relatively young field. It is intended for readers with a general background in seismology. In order to illustrate the seismological value of rotational ground-motion data, we provide the first-ever 6DOF processing example of a teleseismic earthquake recorded on a multicomponent ring laser observatory and demonstrate how wave parameters (phase velocity, propagation direction, and ellipticity angle) and wave types of multiple phases can be automatically estimated using single-station 6DOF processing tools. Python codes to reproduce this processing example are provided in an accompanying Jupyter notebook.

Prediction of rotational ground motion for mining-induced seismicity – Case study from Upper Silesian Coal Basin, Poland

For decades, coal mining in the Upper Silesian Coal Basin (USCB) in Poland has been inducing annually more than 150 seismic events with local magnitude M > 2. Two sets of sensors were installed in the USCB in Poland to measure angular velocities and translational accelerations from seismic events induced by underground coal mining. In the period from December 2015 to November 2016, 503 seismic events were recorded with local magnitude M between 1.1 and 3.5. Analysis of measurement data for induced seismicity confirmed a strong linear correlation between peak ground acceleration of shear S-waves, PGA and peak ground motion of angular velocity, PRV. The observed correlation can be a result of previously observed coupling between the angular instantaneous velocity vector and the respective displacement vector at the ground surface. Three types of empirical equations for induced seismicity in Upper Silesia in Poland were developed to predict the peak rotational ground motion of the vertical (torsion), horizontal (rocking), and instantaneous 3D-xyz components. The prediction values of horizontal peak angular velocity ground motion indicate that for strong mining-induced seismic events in the USCB with a magnitude of M = 3.5, we can expect that the mean values of PRVH will reach 7.5 mrad/s in the epicentral area (r = 500 m) and with magnitude M = 4 respectively will reach 19.7 mrad/s. Such values of ground rotation may substantially contribute to the overall seismic response of civil infrastructure, particularly for tall buildings. The study of angular velocity ground motion is important not only from the cognitive point of view, but also from practical significance for assessing the seismic impact on very high buildings, chimneys and stability of slopes.

Seismic moment tensors from synthetic rotational and translational ground motion: Green’s functions in 1-D versus 3-D

SUMMARY Seismic moment tensors are an important tool and input variable for many studies in the geosciences. The theory behind the determination of moment tensors is well established. They are routinely and (semi-) automatically calculated on a global scale. However, on regional and local scales, there are still several difficulties hampering the reliable retrieval of the full seismic moment tensor. In an earlier study, we showed that the waveform inversion for seismic moment tensors can benefit significantly when incorporating rotational ground motion in addition to the commonly used translational ground motion. In this study, we test, what is the best processing strategy with respect to the resolvability of the seismic moment tensor components: inverting three-component data with Green’s functions (GFs) based on a 3-D structural model, six-component data with GFs based on a 1-D model, or unleashing the full force of six-component data and GFs based on a 3-D model? As a reference case, we use the inversion based on three-component data and 1-D structure, which has been the most common practice in waveform inversion for moment tensors so far. Building on the same Bayesian approach as in our previous study, we invert synthetic waveforms for two test cases from the Korean Peninsula: one is the 2013 nuclear test of the Democratic People’s Republic of Korea and the other is an Mw 5.4 tectonic event of 2016 in the Republic of Korea using waveform data recorded on stations in Korea, China and Japan. For the Korean Peninsula, a very detailed 3-D velocity model is available. We show that for the tectonic event both, the 3-D structural model and the rotational ground motion, contribute strongly to the improved resolution of the seismic moment tensor. The higher the frequencies used for inversion, the higher is the influence of rotational ground motions. This is an important effect to consider when inverting waveforms from smaller magnitude events. The explosive source benefits more from the 3-D structural model than from the rotational ground motion. Nevertheless, the rotational ground motion can help to better constraint the isotropic part of the source in the higher frequency range.

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

ABSTRACTThe additional observation of three components of rotational ground motions has benefits for tilt-seismometer coupling (e.g., ocean-bottom seismometry and volcano seismology), local site characterization, wavefield separation, source inversion, glacial and planetary seismology, as well as the monitoring of structural health. Field applications have been mostly hampered by the lack of portable sensors with appropriate broadband operation range and weak-motion sensitivity. Here, we present field observations of the first commercial 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 array-derived rotation measurements and a navigation-type gyro. We present observations of the 2018 Mw 5.4 Hualien earthquake and the 2016 central Italy earthquake sequence. Processing collocated rotation and classical translation records shows the potential in retrieving wave propagation direction and local structural velocity from point measurements comparable to small-scale arrays of seismic stations. We consider the availability of a portable, broadband, high sensitivity, and low self-noise rotation sensor to be a milestone in seismic instrumentation. Complete and accurate ground-motion observations (assuming a rigid base plate) are possible in the near, local, or regional field, opening up a wide range of seismological applications.

Seismic Damage Assessment of RC Buildings Subjected to the Rotational Ground Motion Records Considering Soil-Structure Interaction

The significance of the seismic rotational components have been overlooked in the seismic evaluation of structural behavior. As researchers have measured seismic components more accurately using sensitive rotational velocity sensor, it was observed that the magnitude of rotational components is considerable and could not be neglected. Hence, some parts of seismic damage or failure of structures cannot be exclusively attributed to the translational components. In this regard, this paper used seven accelerograms in which rotational components were measured by advanced sensors. The considered RC buildings which designed as per intermediate moment-resisting frame system were analyzed using OpenSees in nonlinear dynamic domain. In the numerical modeling, lumped plasticity model was used to simulate the behavior of RC component members considering the rotational motions and soil-structure interaction as main parameters. The results of numerous nonlinear time history analyses showed that the contribution of rotational components to the seismic behavior of RC frames is considerable and should be included in the seismic design codes.

Authors’ reply to discussion on “A parametric study on the effect of rotational ground motions on building structural responses” by F. Vicencio and N. A. Alexander [Soil Dyn Earthq Eng 118 (2019) 191–206

Authors’ reply to discussion on “A parametric study on the effect of rotational ground motions on building structural responses” by F. Vicencio and N. A. Alexander [Soil Dyn Earthq Eng 118 (2019) 191–206

Love wave amplitude decay from rotational ground motions

Love wave amplitude decay from rotational ground motions