Low frequency cultural noise

Transcription

1 GEOPHYSICAL RESEARCH LETTERS, VOL. 36, L17314, doi: /2009gl039625, 2009 Low frequency cultural noise Dong-Hoon Sheen, 1 Jin Soo Shin, 1 Tae-Seob Kang, 2 and Chang-Eob Baag 3 Received 16 June 2009; revised 4 August 2009; accepted 13 August 2009; published 9 September [1] Abnormal cultural seismic noise is observed in the frequency range of Hz. Cultural noise generated by human activities is generally observed in frequencies above 1 Hz, and is greater in the daytime than at night. The low-frequency noise presented in this paper exhibits a characteristic amplitude variation and can be easily identified from time domain seismograms in the frequency range of interest. The amplitude variation is predominantly in the vertical component, but the horizontal components also show variations. Low-frequency noise is markedly periodic, which reinforces its interpretation as cultural noise. Such noise is observed world-wide, but is limited to areas in the vicinity of railways. The amplitude variation in seismograms correlates strongly with railway timetables, and the waveform shows a wavelength shift associated with the Doppler effect, which indicates that the origin of seismic background noise in the frequency range Hz is railways. Citation: Sheen, D.-H., J. S. Shin, T.-S. Kang, and C.-E. Baag (2009), Low frequency cultural noise, Geophys. Res. Lett., 36, L17314, doi: /2009gl Introduction 1 Earthquake Research Center, Geological Research Division, Korea Institute of Geoscience and Mineral Resources, Daejeon, South Korea. 2 Department of Environmental Geosciences, Pukyong National University, Busan, South Korea. 3 School of Earth and Environmental Sciences, Seoul National University, Seoul, South Korea. Copyright 2009 by the American Geophysical Union /09/2009GL [2] The origin of seismic background noise has been extensively investigated, and although such noise is not fully understood, it can be classified according to its frequency range. Rhie and Romanowicz [2004] linked the Earth s free oscillations to atmosphere-ocean-seafloor coupling using seismic data in the period band of s. Stutzmann et al. [2000] interpreted diurnal variations in seismic noise, particularly the horizontal components in the s period band, as thermal in origin. Microseisms are the dominant seismic background noise and are strongly linked to oceanic activities [Longuet-Higgins, 1950; Hasselmann, 1963]. There are two dominant peaks in microseism signals, primary microseisms (approximately 16-s period) and secondary microseisms (approximately 8-s period). [3] Noise at frequencies above 1 Hz can be generated by a variety of human activities as well as by wind turbulence. Wind speed is known to have a strong correlation with the seismic noise [Young et al., 1996; Withers et al., 1996]. Cultural (anthropogenic) noise differs from wind-generated noise; cultural noise is generally periodic with diurnal variability, a characteristic that allows it to be isolated from wind-related noise [Ringdal and Bungum, 1977; McNamara and Buland, 2004; Sheen et al., 2009]. This paper documents the global existence of this low-frequency cultural noise and proposes that the noise is associated with railways. 2. Data [4] Broadband seismic stations are usually installed in quiet locations some distance from significant sources of cultural noise, such as roads, railroads, and machinery, in order to increase the signal-to-noise ratio. Some stations, however, are located in urban areas and record seismic noise produced by cultural activities at a broad range of frequencies. [5] Diurnal noise variations at rural and urban stations in Korea are compared using the daily power spectral density (PSD) of seismic background noise for stations BGD and SNU during 2005 and 2007 (Figure 1). Station BGD is located in a building on a small, forested island, whereas station SNU is in a tunnel on the outskirts of Seoul. Both stations are equipped with STS-2 seismometers. The PSD is evaluated using the method of McNamara and Buland [2004] with seismograms recorded at a sampling rate of 20 sps, and the mode of PSDs is plotted as a function of time of day. [6] Both stations show higher daytime than nighttime noise levels in frequencies above 1 Hz, and the urban station SNU shows higher noise levels and greater variation than does the rural station BGD. Sheen et al. [2009] showed daily variations of daytime noise level at broadband stations in South Korea and concluded that vibrations due to human activities are dominant for most of the broadband stations in South Korea in frequencies above 1 Hz. In the microseismic frequency range ( Hz), little daily variation is observed, but seasonal variations are evident [Sheen et al., 2009]. [7] Noise level variations in the frequency range of 0.01 to 0.05 Hz are usually explained by temperature variation [Stutzmann et al., 2000; McNamara and Buland, 2004] or local atmospheric pressure variability [Beauduin et al., 1996]. However, the amplitude of the vertical component of station SNU shows clear diurnal variations of up to 18 db, which is too sharp a change to be explained by daily thermal or barometric variation. Of the horizontal components, the N S component also shows a similar variation. [8] Figure 2 presents a one-day-long seismogram of the vertical component at station SNU for 4 January After removing the instrument response, converting into displacement, and band-pass filtering between Hz, abrupt amplitude changes are evident: amplitude decreases around 01:00 local time and increases around 05: Analysis [9] The origin of low-frequency noise is investigated through detailed analysis of data from station SNU. Spectro- L of5

2 Figure 1. Power spectral densities for stations BGD and SNU in Korea from 2005 to KST denotes Korea Standard Time. grams of the vertical component of seismic data from 22 December 2006 to 11 January 2007 (Figure 3) show a nighttime decrease every day except for 26 December, when a m b 6.4 earthquake occurred in Taiwan. Spectral amplitudes decrease at approximately 01:00 on weekdays and at about 00:00 (midnight) on Saturday, Sunday, and holidays. Although 25 December 2006 was a Monday, it was also the Christmas holiday, which is celebrated in Korea; on that day, the amplitude decreased around 00:00 rather than 01:00. Furthermore, 31 December was a Sunday but also the nationwide New Year s celebration; on that day, amplitude decreased at about 02:00. [10] No kind of natural non-human activity shows these kinds of periodic variations. These low-frequency amplitude variations are a function of human activities, especially railways. Seoul has an extensive metropolitan subway system. Two subway lines pass within 2 3 km of station SNU. The subway operates until 01:00 on weekdays and until 00:00 on Saturdays, Sundays, and holidays. These schedules strongly correlate with the observed amplitude variations (Figure 3). During the annual, nationwide New Year s celebration in Seoul on 31 December, the subway operates until 02:00, which clearly explains the amplitude variation found on 31 December. [11] Other stations in Korea have also recorded lowfrequency noise. The Seoul (SEO) and Incheon (INCN) stations are each located 2 to 3 km from a rail line. Both stations are equipped with STS-2 seismometers. The rail line near station INCN is at the surface, indicating that lowfrequency noise can be linked to surface railway lines as well as subways. The amplitude variations at these stations are also larger in the vertical component than in the horizontal component. [12] Seismic stations that record a broad range of frequencies are usually installed away from significant sources of cultural noise, which makes it difficult to observe lowfrequency cultural noise. Seismograms from several stations in countries other than Korea, however, have recorded the same characteristic low-frequency features. [13] Difference spectrograms for 2008 from stations in Palisades, New York (PAL), Memphis, Tennessee (MPH), and Trieste, Italy (TRI) are shown in Figure 4. PAL uses an STS-2 sensor, but stations MPH and TRI are equipped with CMG-40T and STS-1 sensors, respectively. We computed spectrograms, using the vertical component seismograms with a sampling rate of 1 sps, every 5 min for 30-min time spans. Difference spectrograms are obtained by subtracting the mode of spectrograms and averaging for selected frequencies, and are averaged over frequencies from 0.01 to 0.02 Hz for stations PAL and TRI, and from 0.02 to 0.03 Hz for station MPH. [14] Railways exist in the vicinity of all the above stations, supporting the argument that railways produce low-frequency noise. The timing of low-amplitude seismic background noise clearly coincides with times when the railways are inactive. For example, the railway near station PAL operates from about 05:00 to 02:30 the next day, which matches well with the high-noise and low-noise time intervals. Furthermore, the time shift with the switch to daylight saving time is easily identified. In the United States, clocks are adjusted in early March and early November, whereas in Italy the change takes place in late March and late October. [15] Waveforms in the time domain provide further evidence pointing to railways as the origin of the lowfrequency noise. Waveforms at a single station at similar times of day should be similar if the source mechanism and its location are identical (Figure 5). To enhance the signal, 8 days of vertical component seismograms from station PAL are stacked. To exclude interference caused by the continual passing of trains, seismograms from inactive times are used (early on Saturday and Sunday mornings). Each seismo- Figure 2. One-day-long seismogram of the vertical component at station SNU. Instrument response has been removed, and integration and a Hz bandpass filter applied. 2of5

3 Figure 3. Spectrograms of the vertical component seismogram for station SNU from 22 December 2006 to 11 January Color of the date indicates the day of the week: blue represents Saturday and red denotes Sunday or a holiday. 3of5

4 gram is corrected for the instrument response, converted into displacement, and band-pass filtered between 0.01 and 0.03 Hz. Before stacking the seismograms, the lag time is computed by cross-correlating with a seismogram for the Julian day of 97, [16] The stacked seismogram (Figure 5, bottom) shows the signal is enhanced, while background noise is reduced. The time of the signal corresponds to the passage of the first train of the day; the signal clearly originates from the train. The signal lasts for up to 20 min, which may be related to the train s length and velocity. The wavelength increases with time, which can be recognized as the result of the Doppler effect, i.e., the source is moving. The wavelength is shorter as the train approaches the station and longer as the train moves away from the station. 4. Discussion [17] The amplitude variations of seismic background noise in the frequency range of 0.01 to 0.05 Hz are observed at broadband seismic stations installed near railways. Temporal patterns in the occurrence of low-frequency noise correspond strongly with railway schedules. A wavelength shift illustrated by stacked waveforms results from the passage of a train. For these reasons, the low-frequency noise observed in several seismic stations is likely to be produced by railways. [18] Ground vibrations produced by moving trains have been extensively investigated because a moving train can produce significant ground vibration that may propagate Figure 5. Vertical component seismograms and stacked seismogram for station PAL. The number in each plot represents the Julian day in 2008, and the number in parentheses indicates the lag time in seconds, based on cross-correlation with the seismogram for the Julian day of 97, over several kilometers and cause building damage [Gutowski and Dym, 1976; Krylov and Ferguson, 1994; Ju et al., 2009]. However, most studies on vibrations caused by railways have focused on frequencies above 1 or 10 Hz [Sanford et al., 1968; Krylov and Ferguson, 1994; Ditzel et al., 2001]. The fact that railways can also generate such low-frequency vibrations could be of considerable importance. Although the observed seismic energy due to railways is rather low, it is almost 20 db above that at quiet time even at more than 2 km distance from a railway. Therefore, it is expected that in situ measurement and detailed analysis of low-frequency seismic energy from moving trains can provide valuable insights for engineering and scientific studies. Figure 4. Difference spectrograms of the vertical component seismogram for stations PAL, MPH, and TRI in Vertical gray stripes indicate times for which data are missing. [19] Acknowledgments. We used the seismic data recorded by several seismic networks, PAL of Lamont-Doherty Cooperative Seismographic Network operated by Lamont-Doherty Earth Observatory, TRI of Mediterranean Very Broadband Seismographic Network by Istituto Nazionale di Geofisica e Vulcanologia, Italy, and MPH by Earthquake Center, Saint Louis University, and the data were obtained from IRIS data management system. We thank these institutions for their efforts and services. This work 4of5

5 was funded by the Korea Meteorological Administration Research and Development Program under grant CATER References Beauduin, R., P. Lognonne, J. P. Montagner, S. Cacho, J. F. Karczewski, and M. Morand (1996), The effects of the atmospheric pressure changes on seismic signals or how to improve the quality of a station, Bull. Seismol. Soc. Am., 86, Ditzel, A., C. Herman, and G. G. Drijkoningen (2001), Seismograms of moving trains: Comparison of theory and measurements, J. Sound Vib., 248, Gutowski, T. G., and C. L. Dym (1976), Propagation of ground vibration: A review, J. Sound Vib., 49, Hasselmann, K. (1963), A statistical analysis of the generation of microseisms, Rev. Geophys., 1, Ju, S.-H., H.-T. Lin, and J.-Y. Huang (2009), Dominant frequencies of traininduced vibrations, J. Sound Vib., 319, Krylov, V., and C. Ferguson (1994), Calculation of low-frequency ground vibrations from railway trains, Appl. Acoust., 42, Longuet-Higgins, M. S. (1950), A theory of the origin of microseisms, Philos. Trans. R. Soc. London, Ser. A, 243, McNamara, D. E., and R. P. Buland (2004), Ambient noise levels in the continental United States, Bull. Seismol. Soc. Am., 94, Rhie, J., and B. Romanowicz (2004), Excitation of Earth s continuous free oscillations by atmosphere-ocean-seafloor coupling, Nature, 431, Ringdal, F., and H. Bungum (1977), Noise level variation at NORSAR and its effect on detectability, Bull. Seismol. Soc. Am., 67, Sanford, A. R., A. G. Carapetian, and L. T. Long (1968), High frequency microseisms from a known source, Bull. Seismol. Soc. Am., 58, Sheen, D.-H., J. S. Shin, and T.-S. Kang (2009), Seismic noise level variation in South Korea, Geosci. J., 13, Stutzmann, E., G. Roult, and L. Astiz (2000), GEOSCOPE station noise levels, Bull. Seismol. Soc. Am., 90, Withers, M. M., R. C. Aster, C. J. Young, and E. P. Chael (1996), Highfrequency analysis of seismic background noise as a function of wind speed and shallow depth, Bull. Seismol. Soc. Am., 86, Young, C. J., E. P. Chael, M. M. Withers, and R. C. Aster (1996), A comparison of the high-frequency (>1 hz) surface and subsurface noise environment at three sites in the United States, Bull. Seismol. Soc. Am., 86, C.-E. Baag, School of Earth and Environmental Sciences, Seoul National University, Seoul , South Korea. (baagce@snu.ac.kr) T.-S. Kang, Department of Environmental Geosciences, Pukyong National University, Busan , South Korea. (tskang@pknu.ac.kr) D.-H. Sheen and J. S. Shin, Earthquake Research Center, Geological Research Division, Korea Institute of Geoscience and Mineral Resources, 92 Gwahang-no, Yuseong-gu, Daejeon , South Korea. (dhsheen@ kigam.re.kr; jinsoo@kigam.re.kr) 5of5