Short Duration Events on OBS recordings in the Northwestern Sub-basin of the South China Sea
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摘要: 本文对2019年10月—2020年5月在南海西北次海盆布放的宽频带海底地震仪的数据进行了分析,利用长短时窗均值算法在单个OBS台站拾取了多达12万个短时事件。根据信号特征以及可能的成因机制,可将事件分为连续性短时事件(C-SDE)和随机短时事件(R-SDE)。C-SDE的主要特征是在一段时间内连续且有规律发生的短时事件;包含多个事件组,事件组间隔约100—300 s,每个事件组包括3—10个持续约1 s的单波;其主频介于20—44 Hz之间,不同台站信号主频的不同可能是因信号源到台站距离不同而衰减的结果;C-SDE能量较强,在间距约105 km的三个OBS台站的三个分量上均有记录。R-SDE的主要特征是随机发生,持续时间约0.3—2 s;主频集中于8—25 Hz,在频谱上常呈现双波峰;由于其事件能量较小,很难在多个台站上拾取到同一个事件。本文探讨了C-SDE和R-SDE的信号源可能的产生机制,推断该时间段内南海西北次海盆存在着活跃的C-SDE信号源,可能来自人类活动,如气枪放炮或者低频水声通信;R-SDE则来源于离台站很近的源,很可能是海底沉积层中的气体逃逸所产生的破裂过程所导致,表明南海西北次海盆海底沉积层中存在气体泄露。Abstract: Based on the analysis of the data on broadband OBSs deployed in the northwestern sub-basin of the South China Sea (SCS) from October 2019 to May 2020, we detected more than 120 000 Short Duration Events (SDEs) at one station by using STA/LTA algorithm. These SDEs are classified as consecutive short duration events (C-SDEs) and random short duration events (R-SDEs) according to their characteristics and generation mechanism. C-SDEs occur over several hours or days consecutively and regularly. The interval between the two Group Events (GEs) is about 100-300s and contains 3−10 single-wave (SW) with a duration of about 1s. The dominant frequencies of C-SDEs range from 20 Hz to 44 Hz, likely depending on the distance from the sources. C-SDE has strong energy and record on three OBS stations spaced about 105 km apart. The R-SDEs have the dominant frequency ranging from 8 Hz to 25 Hz with double frequency peaks on the spectrum and a duration of 0.3−2 s. Due to the limited energy of the R-SDEs, it is not easy to detect the same event on multiple stations. We present the possible generation mechanisms for C-SDEs and R-SDEs, respectively. We infer there are active C-SDE sources in the northwestern sub-basin of the SCS. They may result from human activities, such as air gun firing or low-frequency hydroacoustic communication signals. The R-SDE sources appear close to the OBS stations, likely generated by the ruptures due to gas escape from seafloor sediments. R-SDEs on OBS recordings indicate persistent gas leakages from the sedimentary layer in the northwestern sub-basin of the SCS.
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Key words:
- OBS /
- short duration events /
- STA/LTA /
- gas escape /
- Whale Song
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图 1 典型SDE波形及幅度频谱
(a—b)马尔马拉海2011年布放的OBS(OBS-01台站)记录的海底沉积层中气泡逃逸产生的SDE的三分量波形和幅度频谱( Tsang Hin Sun et al,2019); (c—d)东北太平洋布放的OBS台站记录的2012-11-03 19:00:00的鲸鱼叫声产生的SDE的三分量波形和幅度频谱(数据来源:IRIS ,网络号:X9 ,台站号:BS080) ( Kuna,Nábělek,2021)。
Figure 1. Waveforms and amplitude spectra of typical SDEs。
(a,b) Three-component waveforms and amplitude spectra of SDEs from bubble escape in seafloor sediment recorded by OBS (OBS-01 station) in the Sea of Marmara (Tsang Hin Sun et al,2019); (c,d) Three-component waveforms and amplitude spectra of SDEs produced by whale calls on OBS stations (IRIS data. Network:X9,Station:BS080,Begin time:2012-11-03 21:16:52) in the Northeast Pacific.) (Kuna ,Nábělek,2021).
图 3 STA/LTA方法拾取SDE示意
(a)单个SDE波形序列;(b)连续SDE波形序列;(c)单个SDE的STA/LTA的特征曲线;(d)连续SDE的STA/LTA的特征曲线 。粉色区域为噪音数据,浅绿色区域为SDE信号数据。
Figure 3. Example of SDEs detected by the STA/LTA method
(a,b). The waveform of a single SDE and consecutive SDEs. The pink box is noise data and the light green box is the SDE signal data. (c,d). Character curves of a single SDE and the consecutive SDEs in detections using STA/LTA method.
图 4 SDE数量随时间的变化
(a) 2019年10月—2020年5月间,K02,K03,K08数量变化(日统计)的时序图; (b) 2020年5月3日—10日间,SDE在K02,K03,K08台站拾取的数量变化(小时统计)时序图; (c) 2020年1月3日—10日间,SDE在K02,K03,K08台站拾取的数量变化(小时统计)时序图
Figure 4. Temporal distribution of the SDE number
(a). Temporal distribution of the SDE number in OBS stations (K02,K03,K08) from October 2019 to May 2020;(b). Temporal distribution of the SDE number in OBS stations (K02,K03,K08) from May 3 to 10, 2020;(c). Temporal distribution of the SDE number in OBS stations (K02,K03,K08) from January 3 to 10,2020.
图 6 C-SDE波形图
(a)相距约52 km的K02,K03,K08台站上垂直分量的C-SDE信号,R1,R2,R3,R4分别表示R-SDE, E1表示地震事件;(b) 相距约52 km的K02,K03,K08台站上垂直分量的单一GE波形;(c) K08台站上三个分量(E,N,Z)的波形
Figure 6. Waveform of C-SDEs
(a) The vertical components of C-SDE signals on OBS stations (K02, K03, K08) about 52 km apart,R1,R2,R3,R4 are the R-SDEs,and E1 is an earthquake event.(b) The GEs waveforms of vertical components on OBS stations (K02,K03,K08).(b) The waveform of the three components (E,N,Z) on K08 station.
图 9 不同时间尺度下各种机制导致的C-SDE的波形对比。(a,b)OBS台站上的鲸鱼叫声记录(数据来源:IRIS data。网络号:X9,台站号:BS080,时间:2012-11-03 19:00:00) (Kuna ,Nábělek, 2021)、(c,d)短周期OBS的人工震源(气枪)信号和(e,f)本文研究
Figure 9. Comparison of C-SDEs caused by various mechanisms on different time scales. (a,b) Whale songs recorded by OBS station (IRIS data records at OBS stations Network:X9, Station:BS080, Begin time:2012-11-03 19:00:00) (Kuna ,Nábělek, 2021),(c,d) Artificial sources (air guns) signal recorded by short-period OBS and this paper studied(e,f).
表 1 宽频带海底地震仪实验信息
Table 1. Experimental information of broadband OBS
台站编号 经度/°E 纬度/°N 深度/m 投放日期
年-月-日回收日期
年-月-日K08 115.499 17.799 −3 780.01 2019-10-20 2020-05-15 K03 115.999 17.799 −3 835.82 2019-10-20 2020-05-15 K02 116.499 17.799 −3 896.38 2019-10-21 2020-05-14 表 2 STA/LTA方法拾取SDE参数设置表
Table 2. The parameter set of SDE pickup with the STA/LTA method
参数名称 STA 时间 LTA 时间 开始触发值 结束触发值 持续时间 信噪比 设置值 0.1(s) 50 (s) 8 6 0.05—2 (s) 3 表 3 SDE拾取结果
Table 3. The pick-up quantity of SDEs on each station
Station HHE HHN HHZ Total K02 28020 85721 40010 153751 K03 41549 122051 18829 182429 K08 77693 83942 93849 255484 表 4 2020年5月3—9日,C-SDE信号发生及持续时间
Table 4. Occurrence and duration of C-SDE signal from May 3 to 9,2020
序号 日期
年-月-日开始时间
时:分:秒结束时间
时:分:秒持续时间
时:分:秒1 2020年5月3日 0:54:15 7:43:10 6:48:55 2 2020年5月3日 16:30:35 1:12:50(+1天) 8:42:15 3 2020年5月5日 0:33:30 7:55:30 7:22:00 4 2020年5月6日 3:58:25 13:50:25 9:52:00 5 2020年5月8日 3:28:40 8:24:10 4:55:30 6 2020年5月9日 1:30:12 8:38:55 7:08:43 7 2020年5月9日 18:42:07 22:20:10 3:38:03 -
Allen, R. V. Automatic earthquake recognition and timing from single traces[J]. Bull. Seismol. Soc. Am. 1978, 68, 1521–1532. Ardhuin, F. , Gualtieri, L. , & Stutzmann, E. (2015). How ocean waves rock the Earth: Two mechanisms explain microseisms with periods 3 to 300 s[J]. Geophysical Research Letters, 42, 765–772.https: //doi.org/10.1002/2014GL062782 Baer, M. , Kradolfer, U. An automatic phase picker for local and teleseismic events[J]. Bull. Seismol. Soc. Am. 1987, 77, 1437–1445. Batsi, E. , Tsang-Hin-Sun, E. , Klingelhoefer, F. , Bayrakci, G. , Chang, E. T. Y. , Lin, J. , Dellong, D. , Monteil, C. , & Géli, L. Nonseismic signals in the ocean: Indicators of deep sea and seafloor processes on ocean-bottom seismometer data[J]. Geochem. , Geophys. , Geosyst. 2019, 20, 3882–3900. https://doi.org/10.1029/2019GC008349 Bayrakci, G. , Scalabrin, C. , Dupré, S. , Leblond, I. , Tary, J. B. , Lanteri, N. , Augustin, J. M. , Berger, L. , Cros, E. , Ogor, A. , Tsabaris, C. , Lescanne, M. , Géli, L. Acoustic monitoring of gas emissions from the seafloor. Part II: A case study from the Sea of Marmara[J]. Mar. Geophys. Res. 2014, 35, 211–229.https: //doi.org/10.1007/s11001-014-9227-7 Berndt, C. , Feseker, T. , Treude, T. , Krastel, S. , Liebetrau, V. , Niemann, H. , Steinle, L. (2014). Temporal constraints on hydrate-controlled methane seepage off Svalbard[J]. Science, 343, 284–287.https: //doi.org/10.1126/science.1246298 Beyreuther M. , Barsch R. , Krischer L. , Megies T. , Behr Y. , Wassermann J. , ObsPy: A Python Toolbox for Seismology[J]. Seismological Research Letters 2010, 81 (3): 530–533. doi: https: //doi.org/10.1785/gssrl.81.3.530 Brodie, D. C. , Dunn, R. A. Low frequency baleen whale calls detected on ocean-bottom seismometers in the Lau basin, southwest Pacific Ocean[J]. J. Acoust. Soc. Am. . 2014, 137, 53–62.https: //doi.org/10.1121/1.4904556 Buskirk, R. E. , Frohlich, C. , Latham, G. V. , Lawton, J. , Chen, A. T. Evidence that biological activity affects ocean bottom seismograph recordings[J]. Mar. Geophys. Res, 1981, 5(2), 189–205. Dréo, R. , Bouffaut, L. , Leroy, E. , Barruol, G. , Samaran, F. Baleen Whale distribution and seasonal occurrence Revealed by An Ocean Bottom Seismometer Network in The Western Indian Ocean[J]. Deep Sea Research Part II: Topical Studies in Oceanography. 2019, vol. 161, pp. 132-144. Dunn, R. A. , Hernandez, O. Tracking blue whales in the eastern tropical Pacific with an ocean-bottom seismometer and hydrophone array[J]. J. Acoust. Soc. Am. . 2009, 126(3), 1084–1094. Earle, P. S. , Shearer, P. M. Characterization of global seismograms using an automatic-picking algorithm[J]. Bull. Seismol. Soc. Am. 1994, 84, 366–376. Embriaco, D. , Marinaro, G. , Frugoni, F. , Giovanetti, G. , Monna, S. , Etiope, G. , Gasperini, L. , Polonia, A. , del Bianco, F. , Çağatay, M. N. , Ulgen, U. B. , Favali, P. Monitoring of gas and seismic energy release by multiparametric benthic observatory along the North Anatolian Fault in the Sea of Marmara (NW Turkey) [J]. 2014, Geophysical. J. Int. , 196(2), 850–866.https: //doi.org/10.1093/gji/ggt436 Franek, P. , Plaza. F, A. , Mienert, J. , Buenz, S. , Ferré, B. , Hubbard, A. Microseismicity linked to gas migration and leakage on the Western Svalbard Shelf. Geochem[J]. Geophys. , Geosyst. 2017, 18, 4623–4645.https: //doi.org/10.1002/2017GC007107 Hilmo, R. , Wilcock, W. S. D. Physical sources of high‐frequency seismic noise on Cascadia Initiative ocean bottom seismometers[J]. Geochem. , Geophys. , Geosyst. 2020, 21, e2020GC009085.https: //doi.org/ 10.1029/2020GC009085 Kuna, V. M, N´abˇelek, J. L. Seismic crustal imaging using fin whale songs[J]. Science, 2021, 371, 731–735.Https: //doi.org/10.1126/science.abf3962 Lepore, S. , and Grad, M. Analysis of the primary and secondary microseisms in the wavefield of the ambient noise recorded in northern Poland[J]. Acta Geophys. 66, 915–929 (2018).https: //doi.org/10.1007/s11600-018-0194-2 Liu, C. , Hua, Q. , Pei, Y. , Yang, T. , Xia, S. , & Xue, M. , et al. (2014). Passive-source ocean bottom seismograph(obs) array experiment in south china sea and data quality analyses[J]. Chin. Sci. Bull. 59(33): 4524–4535DOI 10.1007/s11434-014-0369-4 刘晨光,华清峰,裴彦良,杨挺,夏少红,薛梅,黎伯孟,霍达,刘芳,黄海波. 2014. 南海海底天然地震台阵观测实验及其数据质量分析[J]. 科学通报(16),:11. Liu D,Yang T,Le B M,Wu Y C,Wang Y Z,Huang X F,Du H R,Wang J,Chen Y S. 2022a. Seismometer-detached broadband ocean bottom seismograph (OBS):development,test,and data quality analysis[J]. Chinese J Geophys,65(7):2560–2572 (in Chinese). doi: 10.6038/cjg2022P0441 刘丹,杨挺,黎伯孟,吴越楚,王宜志,黄信锋,杜浩然,王建,陈永顺. 2022. 分体式宽频带海底地震仪的研制、测试和数据质量分析[J]. 地球物理学报,65(7):2560–2572. doi: 10.6038/cjg2022P0441 Marcon, Y. , Kelley, D. , Thornton, B. , Manalang, D. , Bohrmann, G. Variability of natural methane bubble release at Southern Hydrate Ridge[J]. Geochem. , Geophys. , Geosyst. 2021, 22, e2021GC009894.https: //doi. org/10.1029/2021GC009894 McDonald, M. A. , Hildebrand, J. A. , Webb, S. C. Blue and fin whales observed on a seafloor array in the Northeast Pacific[J]. J. Acoust. Soc. Am. 1995, 98(2), 712–721. Ostrovsky, A. A. (1989). The estimation of ocean bottom seismographs’ coupling characteristics by means of microshock recordings[J]. Mar. Geophys. Res. 11, 119–127, doi: 10.1007/BF00285663. Pereira, A. , Romagosa, M. , Corela, C. , Silva, M. A. , Matias, L. Source Levels of 20 Hz Fin Whale Notes Measured as Sound Pressure and Particle Velocity from Ocean-Bottom Seismometers in the North Atlantic[J]. J. Mar. Sci. Eng. 2021, 9, 646.https: //doi.org/10.3390/ jmse9060646 Person, R. , Favali, P. , Ruhl, H. A. , Beranzoli, L. Rolin, J. -F. , Waldmann, C. , Thomsen, L (2015). From ESONET multidisciplinary scientific community to EMSO novel European research infrastructure for ocean observation[J]. In P. Favali, L. Baranzoli, & A. De Santis (Eds. ), Seafloor observatories: A new vision of the Earth from the Abyss (pp. 531–563). Berlin, Germany: Springer.https: //doi.org/10.1007/978-3-642-11374-1 Soule D. C. , Wilcock, W. S. , Fin whale tracks recorded by a seismic network on the Juan de Fuca Ridge, Northeast Pacific Ocean[J]. J. Acoust. Soc. Am. 2013, 133, 1751–1761. Sultan, N. , Marsset, B. , Ker, S. , Marsset, T. , Voisset, M. , Vernant, A. M. , Bayon, G. , Cauquil, E. , Adamy, J. , Colliat, L. , Drapeau, D. Hydrate dissolution as a potential mechanism for pockmark formation in the Niger delta[J]. J. Geophys. Res. 2010. 115. B08101, 1-33.https: //doi.org/10.1029/2010JB007453 Sultan, N. , Riboulot, V. , Ker, S. , Marsset, B. , Géli, L. , Tary, J. B. , … Grimaud, S. (2011). Dynamics of fault-fluid-hydrate system around a shale-cored anticline in deepwater Nigeria[J]. Journal of Geophysical Research, 116, B12110.https: //doi.org/10.1029/2011JB008218 Tsang-Hin-Sun, E. , Batsi, E. , Klingelhoefer, F. , Géli, L. Spatial and temporal dynamics of gas-related processes in the Sea of Marmara monitored with ocean bottom seismometers[J]. Geophys. J. Int. 2019, 216(3), https: //doi.org/10.1093/gji/ggy535 Pontoise B. , Hello, Y. Monochromatic infra-sound waves recorded offshore Ecuador: Possible evidence ofmethane release[J]. Terra Nova. 2002, 14(6), 425–435.https: //doi.org/10.1046/j.1365-3121.2002.00437.x Tary J. B. , , Geli, L. , , Guennou C. , , Henry, P, , Sultan, N. , , Çağatay N. , , Vidal, V. Microevents produced by gas migration and expulsion at the seabed: A study based on sea bottom recordings from the Sea of Marmara[J]. Geophysical. J. Int. 2012, 190(2), 993–1007.https: //doi.org/10.1111/j.1365-246X.2012.05533.x Trnkoczy A. Understanding and parameter setting of STA/LTA trigger algorithm. In New Manual of Seismological Observatory Practice 2[G]. (NMSOP-2). 2012, IS 8.1, 20 pp. Ugalde A. , , Gaite, B. , , Ruiz, M. , Villaseñor, A. , , & Ranero, C. R. Seismicity and noise recorded by passive seismic monitoring of drilling operations offshore the eastern Canary Islands[J]. Seismol. Res. Lett. . 2019, 90. 1565-1576.https: //doi.org/10.1785/0220180353 Wang Y Z,Yang T,Wu Y C,Liu D,Huang X F,Wang J,Zhong W X,Shou H T,Zhou Y,Chen Y S. 2022. A new broad-band ocean bottom seismograph and characteristics of the seismic ambient noise on the South China Sea seafloor based on its recordings[J]. Geophys J Int,230(1):684–695. doi: 10.1093/gji/ggac092 Weirathmueller, M. J. , Wilcock, W. S. , Soule, D. C. Source levels of fin whale 20 Hz pulses measured in the Northeast Pacific Ocean[J]. J. Acoust. Soc. Am. 2013, 133, 741–749. Wilcock, W. S. D, , Hilmo, R. S. A method for tracking blue whales (Balaenoptera musculus) with a widely spaced network of ocean bottom seismometers[J]. PLoS ONE. 2021, 16(12): e0260273.https: //doi.org/10.1371/journal. pone.0260273 -