11:00 AM - 1:00 PM
[STT38-P04] Observation of rotational ground motions with a three-component rotational seismometer in the Tohoku region, Japan
Keywords:rotational ground motion, observation, Tohoku region
Rotational ground motions are sometimes measured in conjunction with translational ground motions. These kinds of measurements are called 6C measurements. They have several advantages over traditional 3C measurements, including improved S-wave detection, the single-station estimation of Love-wave phase velocity, and others (Schmelzbach et al., 2018).
Recently, our group had a chance to borrow a portable 3C broadband rotational seismometer, iXblue BlueSeis3A (e.g., Bernauer et al., 2018). This is an interferometric fiber-optic gyroscope that can measure angular rotation rates independently of translational motions based on the Sagnac effect. We conducted a 6C measurement at station TU.SHC in Miyagi prefecture using the BlueSeis3A and an STS-2 broadband seismometer from September 22 to November 5 in 2021. We confirmed that the BlueSeis3A measured DC components consistent with the Earth's rotation rate at the station's latitude. During the observation period, we detected eight local to regional earthquakes with magnitudes of 3.2 to 6.0. The largest M6.0 event took place in Central Chiba Prefecture on October 7 and is about 280km away from the station. The rotation rate on the NS component is about 1microrad/sec at the maximum, and its Fourier spectrum is predominant at around 1Hz. On the other hand, the smallest M3.2 event occurred in the middle of Fukushima Prefecture on October 3 and is just about 20km away from the station. However, the rotation rate is as large as about 5microrad/sec on the NS component. Its Fourier spectrum is large at higher frequencies than 10Hz.
Regarding the amplitude of rotation rates, if we assume a transversely polarizing plane wave, we can convert a rotation rate around the vertical axis dΩ/dt to translational acceleration on the transverse component d2u/dt2 by the following relation dΩ/dt = - d2u/dt2 / 2c where c is a phase velocity. Assuming c of 500m/s, we can roughly estimate an acceleration amplitude in m/s2 by multiplying the rotation rate by just a factor of 1000. So some ground motion prediction equation of acceleration amplitudes (e.g., Shi and Midorikawa, 1999) helps understand the decay of rotation rates with distance. From the results of the detected eight events, a threshold of event detection is about 0.5 microrad/sec, which corresponds to 0.05gal in acceleration according to the above simple conversion. If we need to perform some waveform analysis, an amplitude of a few or several times larger than this threshold would be necessary.
According to the datasheet of the BlueSeis3A, the self-noise is typically 20nrad/sec/Sqrt(Hz) over a wide frequency range from 0.01 to 50Hz. Because this value is slightly higher than the new high-noise model (Peterson, 1993) in the secondary microseism band, microseisms cannot be observed. The self-noise level is almost similar to the K-NET 95 type accelerometer (e.g., Fujiwara et al., 2007) from about 0.2 to 30 Hz. At lower frequencies, the noise level of the BlueSeis3A is much smaller. This comparison probably helps Japanese seismologists quickly understand the noise characteristics of the BlueSeis3A.
Acknowledgments:
We borrowed a BlueSeis3A rotational seismometer from Oceanwings Corp., Japan. We appreciate technical and logistical support from that company. We also thank iXblue for making available programs for logging and deramping data. We used the JMA unified earthquake catalog.
Recently, our group had a chance to borrow a portable 3C broadband rotational seismometer, iXblue BlueSeis3A (e.g., Bernauer et al., 2018). This is an interferometric fiber-optic gyroscope that can measure angular rotation rates independently of translational motions based on the Sagnac effect. We conducted a 6C measurement at station TU.SHC in Miyagi prefecture using the BlueSeis3A and an STS-2 broadband seismometer from September 22 to November 5 in 2021. We confirmed that the BlueSeis3A measured DC components consistent with the Earth's rotation rate at the station's latitude. During the observation period, we detected eight local to regional earthquakes with magnitudes of 3.2 to 6.0. The largest M6.0 event took place in Central Chiba Prefecture on October 7 and is about 280km away from the station. The rotation rate on the NS component is about 1microrad/sec at the maximum, and its Fourier spectrum is predominant at around 1Hz. On the other hand, the smallest M3.2 event occurred in the middle of Fukushima Prefecture on October 3 and is just about 20km away from the station. However, the rotation rate is as large as about 5microrad/sec on the NS component. Its Fourier spectrum is large at higher frequencies than 10Hz.
Regarding the amplitude of rotation rates, if we assume a transversely polarizing plane wave, we can convert a rotation rate around the vertical axis dΩ/dt to translational acceleration on the transverse component d2u/dt2 by the following relation dΩ/dt = - d2u/dt2 / 2c where c is a phase velocity. Assuming c of 500m/s, we can roughly estimate an acceleration amplitude in m/s2 by multiplying the rotation rate by just a factor of 1000. So some ground motion prediction equation of acceleration amplitudes (e.g., Shi and Midorikawa, 1999) helps understand the decay of rotation rates with distance. From the results of the detected eight events, a threshold of event detection is about 0.5 microrad/sec, which corresponds to 0.05gal in acceleration according to the above simple conversion. If we need to perform some waveform analysis, an amplitude of a few or several times larger than this threshold would be necessary.
According to the datasheet of the BlueSeis3A, the self-noise is typically 20nrad/sec/Sqrt(Hz) over a wide frequency range from 0.01 to 50Hz. Because this value is slightly higher than the new high-noise model (Peterson, 1993) in the secondary microseism band, microseisms cannot be observed. The self-noise level is almost similar to the K-NET 95 type accelerometer (e.g., Fujiwara et al., 2007) from about 0.2 to 30 Hz. At lower frequencies, the noise level of the BlueSeis3A is much smaller. This comparison probably helps Japanese seismologists quickly understand the noise characteristics of the BlueSeis3A.
Acknowledgments:
We borrowed a BlueSeis3A rotational seismometer from Oceanwings Corp., Japan. We appreciate technical and logistical support from that company. We also thank iXblue for making available programs for logging and deramping data. We used the JMA unified earthquake catalog.