SM2.2

A new high-precision ground- or platform-rotation sensor called the Quartz Rotation Sensor (QRS) has been developed and tested. The QRS is a mechanical angular accelerometer that senses rotational torque with an inherently digital, load-sensitive resonant quartz crystal. It is a portable broadband sensor with a noise floor measured to be ∼45 pico-radian/root (Hz) near 1 Hz, and a resonant period of ~10 s. The noise floor of the sensor near 0.1 Hz is more than two orders of magnitude lower than other similarly sized instruments enabling a dramatic improvement in ability to measure rotational teleseismic signals and tilt contamination in horizontal seismometers. We will present details of the sensor and measurements of rotational components of teleseismic waves recorded with the sensor at a vault. The QRS is useful for rotational seismology and for improving low-frequency seismic isolation in demanding applications such as the Laser Interferometer Gravitational-Wave Observatories.

How to cite: Venkateswara, K., Paros, J., Bodin, P., Wilcock, W., and Tobin, H. J.: Quartz Rotation Sensor, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-13511, https://doi.org/10.5194/egusphere-egu22-13511, 2022.

Over the last decade, the interest of rotational ground movements has become significant in the field of seismological research, especially in seismic engineering. Being able to reliably detect and record rotational motions is a key point in rotational seismology to better understand the origin of earthquakes and in particular to relate them to the geological context. The area of rotational seismology includes seismology, earthquake engineering, seismotectonic, geodesy as well as gravitation waves. Generally, in classical approach, seismic events are monitored by underground and surface seismic stations based on translational vibration sensors (seismometers, geophones, accelerometers). However, a full description of wave motion requires information about both displacements along the three perpendicular axes X, Y, and Z as well the rotation around these axes. The lack of a possibility of complete wave motion measurements results mainly due to technical difficulties in providing the appropriate sensors meeting all technical requirements of rotational seismology.

In this paper we present the laboratory analysis and field records of the fibre-optic seismograph (FOS) that utilizes the Sagnac effect based on a minimum optical configuration designed for a huge fibre-optic gyroscope with special attention to angular motion detection. Presented FOS utilizes a closed-loop configuration, which is based on the compensatory phase measurement method as well as specific electronic system. The experimental results showed that described FOS is characterized by a wide measuring range, it detects signals with amplitudes ranging from several dozen nrad/s up to even few rad/s and frequencies from 0.01 Hz to 100 Hz. The determined angle random walk was equal to 3∙10⁻⁸ rad/s and bias instability was equal to 2∙10⁻⁸ rad/s. Moreover,  besides the laboratory verification of FOS’s proper operation, the field observation results are also presented. Aforementioned device is constantly registering rotational motions in the seismological observatory located in the basement of the Książ Castle near Wałbrzych, Poland. We present the rotational events induced by the exploitation of the copper ore deposit in this area as well as long-term measurements, showing results confirming positive detection of small differences in Earth’s rotation rate – mainly diurnal and semi-diurnal. The presented data give broad view of the potential FOS’s application in the area of rotational seismology, including seismic monitoring in observatories, buildings, mines, chimneys and even on glaciers and in their vicinity.

How to cite: Jaroszewicz, L., Kurzych, A., and Dudek, M.: A highly sensitive instrument for direct and long-term observations of seismic and natural-mode rotational movements, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-9352, https://doi.org/10.5194/egusphere-egu22-9352, 2022.

A gradiometer array was deployed as part of the wavefields community experiment conducted by IRIS in the summer of 2016 near Enid, Oklahoma, USA. The gradiometer consisted of 7 levels of concentric square rings with each ring being four times the area of the immediate smaller ring; the largest ring spanned an 800X800 km² area. Each ring was made up of 16 three-component, 4.5 Hz nodal instruments. In a bid to appraise the effectiveness of the gradiometer in characterizing seismic waves, we computed seismic wave attributes in the form of apparent slowness and signal azimuth from gradiometer records of a magnitude 4.2 event that occurred during the wavefields experiment and compared these attributes with those computed from a coincidental, 3-km aperture phased array by means of a new array analysis method based on the continuous wavelet transform (CWT). Just as in gradiometry, the phased array technique provides wave attributes for all time points, which allows a point-for-point comparison of the gradiometry attributes with those for the phased array method. Prior to analysis, we extracted body wave phases from the gradiometer and phased array data by means of scale-time gating in the CWT space. This step was necessary to reduce the effect of seismic phase interference that can negatively impact gradiometry results. Gradiometry analysis of the vertical component data revealed a P wave horizontal phase velocity of 6.17+-0.04 km/s, which only deviates by 0.03 km/s from the phase array result obtained over an identical time window. The corresponding azimuth computed using gradiometry is 2.2 degrees off the great circle path between the event’s epicenter and the gradiometer center. If the smallest gradiometer ring is labelled 1 and the rest progressively labelled based on their sizes up to 7, this optimal result was obtained using the gradiometer subarray that combines rings 1,3 and 5. Thus, the gradiometer with its relative portability may be preferred over a traditional phased array deployment in some geophysical campaigns.  Using CWT thresholding techniques finds those areas of the wavelet transform plane that contain high SNR for useful processing using beam forming or gradiometry.

How to cite: Bolarinwa, O. and Langston, C.: Wave Gradiometry and Continuous Wavelet Transform Thresholding, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-6577, https://doi.org/10.5194/egusphere-egu22-6577, 2022.

Observations of ground motion in the near-field of induced earthquakes are important to assess ground shaking limits and design pumping protocols for geothermal stimulation projects, in particular near densely popluated urban areas where such zero-emission geo-energy systems can supply heat and electricity close to the consumer. Diverse seismic networks around the 2018 and 2020 geothermal stimulations in the Otaniemi district in the Espoo/Helsinki area, southern Finland, recorded the ground motions of 6-km-deep induced events at epicentral distances in the 2 to 20 km range. Key features of the seismic networks are seismic arrays consisting of 3 to 25 three-component 4.5 Hz geophones recording at 400 Hz, with interstation distances in the 50 m range. From the array seismograms of translational motion it is possible to compute rotational motion for some 200 events with local magnitudes between 0 and 1.8. The data allow the rare assessment of ground motion patterns at small distances in the cratonic low-attenuation environment of the Fennoscandian shield. Here wo focus on a systematic evaluation of the scaling relations between array-derived peak ground rotation rate (PRR) and peak ground acceleration (PGA) that have been shown to be linearly related. Array-derived motion around all three axes is computed using the ObsPy community tool implementation of Spudich and Fletcher’s seismogeodetic approach. The array and subarray size controls the frequency range for which the rotational motion can be reliably estimated, hence we focus on the robustness and accuracy of the obtained PRR values. We explore the array shape dependent frequency range by a combined analysis of the quality of the PRR estimates, the quality of the linear relationship between PRR and PGA, and the wavelength-to-array-size ratio. The target frequency range is 2 – 15 Hz. We further test if the bandlimited PRR-PGA scaling differs from PGA-scaling obtained from the full bandwidth records. For narrow-band signals the proportionality factor or slope of the PRR-PGA scaling is the local slowness, which opens intriguing opportunities to probe the local velocity structure. From our data we can analyze the scaling relations and therefore consistency between the nine different component pairs of PRR and PGA motion. These results based on ratios of single peak values in a 2 s long seismogram—the S minus P time is about 1 s—are compared to local phase speed estimates from a previous analysis based on optimizing translational acceleration and vertical rotation of the full S-waveform. Data from the many small arrays are used to explore the attenuation of PRR with distance from the source. The deployment of broadband rotational sensors and DAS systems for wavefield gradiometry analyses is anticipated to become more common in future networks; this study contributes to bridging the waiting time by providing low-tech observations of band-limited array-derived rotational motion estimates from induced seismicity for seismic engineering studies.

How to cite: Hillers, G., Sadeghi-Bagherabadi, A., Vuorinen, T. A. T., and Taylor, G.: Array-derived peak ground rotation rate vs. peak ground acceleration: scaling relations from seismicity induced by the Espoo/Helsinki geothermal stimulation, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-11072, https://doi.org/10.5194/egusphere-egu22-11072, 2022.

The quantitative low/high noise models (L/HNM) for translational ground motions (e.g., Petersen 1993) based on many observations of acceleration power-spectral densities has been extremely successful for the evaluation of site quality, as well as the development of seismic sensors for passive experiments on Earth. No such L/HNM exists for rotational ground motions, primarily because 1) there are close to no direct sensors that measure below the Earth’s smallest rotational motions (large ring laser are currently the most sensitive instruments), and 2) small-scale seismic arrays can be used to derive rotational motions, but are limited in frequency range. A (even approximate) rotational L/HNM would be useful in particular for the development of new rotation sensors considering the numerous possible applications of 6 degree-of-freedom observations in terrestrial and planetary seismology as well as ocean bottom observations. As the terrestrial low-noise motion is primarily dominated by surface waves, the well-known connection between plane surface waves and rotational motions can be used to estimate rotational motions from classic seismometer records using local velocity information. We propose a methodology to derive a rotational L/HNM and support the model by ring laser and seismic array observations.

How to cite: Igel, H., Brotzer, A., Stutzmann, E., Montagner, J.-P., Lin, C.-J., Hadziioannou, C., Wassermann, J., and Schreiber, U.: A Low Noise Model for Rotational Ground Motions, EGU General Assembly 2022, Vienna, Austria, 23–27 May 2022, EGU22-3112, https://doi.org/10.5194/egusphere-egu22-3112, 2022.