Characteristics and impact of environmental shaking in the Taipei metropolitan area

Examining continuous seismic data recorded by a dense broadband seismic network throughout Taipei shows for the first time, the nature of seismic noise in this highly populated metropolitan area. Using 140 broadband stations in a 50 km × 69 km area, three different recurring, strong noise signals characterized by dominant frequencies of 2–20 Hz, 0.25–1 Hz, and < 0.2 Hz are explored. At frequencies of 2–20 Hz, the seismic noise exhibits daily and weekly variations, and a quiescence during the Chinese New Year holidays. The largest amplitude occurred at a station located only 400 m from a traffic-roundabout, one of the busiest intersections in Taipei, suggesting a possible correlation between large amplitude and traffic flow. The median daily amplitude for the < 0.2 Hz and 0.2–1.0 Hz frequency bands is mostly synchronized with high similarity between stations, indicating that the sources are persistent oceanic or atmospheric perturbations across a large area. The daily amplitude for the > 2 Hz band, however, is low, indicating a local source that changes on shorter length scales. Human activities responsible for the 2–40 Hz energy in the city, we discovered, are able to produce amplitudes approximately 2 to 1500 times larger than natural sources. Using the building array deployed in TAIPEI 101, the tallest building in Taiwan, we found the small but repetitive ground vibration induced by traffic has considerable effect on the vibration behavior of the high-rise building. This finding urges further investigation not only on the dynamic and continuous interaction between vehicles, roads, and buildings, but also the role of soft sediment on such interaction.

Joint Inversion of Rayleigh and Love Dispersion Curves Extracted from Ambient Seismic Noise Based on Secular Function

Traditional multi-mode dispersion curve inversion requires correct mode discrimination. However, when the stratum contains complex structures such as low-speed soft interlayer or high-speed hard interlayer, the dispersion curve may show phenomena such as “mode kissing” and “mode jumping”, which can easily cause mode misjudgment and lead to erroneous inversion results. Based on the “secular function”, this paper constructs a new type of objective function applied to the inversion of dispersion curve. This objective function does not require prior mode discrimination, which effectively solves the “mode misjudgment” problem of multi-mode dispersion curve inversion. The joint inversion of Rayleigh and Love dispersion curves extracted from ambient seismic noise is used to improve the constraint of the inversion and avoid the inversion falling into a local minimum in the case of a large-scale search of parameters. Finally, a numerical simulation was performed to verify the feasibility of the new inversion method.

Spectra of seismic noise at selected Indian stations

India Meteorological Department (IMD) is operating five digital seismograph systems at New Delhi (NDI),.Shillong (SHL), Pune (POO), Kodaikanal (KDK) and Dhamlsala (DHM) since last few years. The details pertaining to instrumental characteristics and software for data retrieval and processing are presented in this paper, Through PC based algorithms. noise pectra are computed and interpreted for these five stations. It is found that the maximum peak occurs at about 6Hz for Pune. Shillong and Kodaikanal while at New Delhi and Dharmsala, it is noted at about 2 Hz. The spectral peak at Shillong as deduced from the SRO system shifts to about I Hz which is in agreement with a similar observation reported at Gauribidanur seismic array.

SEISMICITY of the NORTHERN CAUCASUS in 2015

Seismic monitoring in the region in 2015 was carried out by a seismic network consisting of 59 stations. Digital equipment was installed at all stations in the second half of the year. The network capability was assessed by the level of seismic noise at the stations: in most of the region, the network provided registration of an earthquake from КR7.0, in the central (including the Greater Sochi region) and eastern parts of the region – КR6.0, and in some local zones with КR5.5. 2,276 earthquakes were registered, 17 earthquakes were felt in the settlements of the Caucasus. The maximum intensity VII at MSK-64 (SSI-17) scale was noted from the earthquake in the territory of Azerbaijan. The earthquake on November 3, which occurred on the platform territory within the Stavropol arch, felt IV at MSK-64. The strongest earthquakes were recorded in the Terek-Caspian and Kura troughs and in the eastern part of the Greater Caucasus. The seismicity of the North Caucasus in 2015 in accordance with the seismicity scale "SOUS-09" was set as the "background average" for the observation period from 1962 to 2015.

Daily monitoring of scattered light noise due to microseismic variability at the Virgo interferometer

Data acquired by the Virgo interferometer during the second part of the O3 scientific run, referred to as O3b, were analysed with the aim of characterising the onset and time evolution of scattered light noise in connection with the variability of microseismic noise in the environment surrounding the detector. The adaptive algorithm used, called pytvfemd, is suitable for the analysis of time series which are both nonlinear and nonstationary. It allowed to obtain the first oscillatory mode of the differential arm motion degree of freedom of the detector during days affected by scattered light noise. The mode’s envelope i.e., its instantaneous amplitude, is then correlated with the motion of the West end bench, a known source of scattered light during O3. The relative velocity between the West end test mass and the West end optical bench is used as a predictor of scattered light noise. Higher values of correlation are obtained in periods of higher seismic noise in the microseismic frequency band. This is also confirmed by the signal-to-noise ratio (SNR) of scattered light glitches from GravitySpy for the January-March 2020 period. Obtained results suggest that the adopted methodology is suited for scattered light noise characterisation and monitoring in gravitational wave interferometers.

Application of an improved particle filter for random seismic noise suppression

Random noise is inevitable during seismic prospecting. Seismic signals, which are variable in time and space, are damaged by conventional random noise suppression methods, and this limits the accuracy in seismic data imaging. In this paper, an improved particle filtering strategy based on the firefly algorithm is proposed to suppress seismic noise. To address particle degradation problems during the particle filter resampling process, this method introduces a firefly algorithm that moves the particles distributed at the tail of the probability to the high-likelihood area, thereby improving the particle quality and performance of the algorithm. Finally, this method allows the particles to carry adequate seismic information, thereby enhancing the accuracy of the estimation. Synthetic and field experiments indicate that this method can effectively suppress random seismic noise.