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The Shallow Crustal S-Velocity Structure of the Longmenshan Fault Zone Using Ambient Noise Tomography of a Seismic Dense Array*

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The Shallow Crustal S-Velocity Structure of the Longmenshan Fault Zone Using Ambient Noise Tomography of a Seismic Dense Array* Earthq Sci (2019)32: 197–206 197 doi: 10.29382/eqs-2019-0197-02 The shallow crustal S-velocity structure of the Longmenshan fault zone using ambient noise tomography of a seismic dense array* Dandan Li1,2 Gaochun Wang1,2 Ruihua Lin3 Kai Deng4,* Xiaobo Tian1,5,* 1 State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing 100029, China 2 University of Chinese Academy of Sciences, Beijing 100049, China 3 Sinosteel Tianjin Geological Academy LTD, Tianjin 300181, China 4 College of Geophysics, Chengdu University of Technology, Chengdu 610059, China 5 CAS Center for Excellence in Tibetan Plateau Earth Sciences, Beijing 100101, China Abstract The Longmenshan fault zone (LMSF), shallow velocity structure for detailed studies of the characterized by complex structures and strong seismicity, is Longmenshan fault zone. located at the junction between the eastern margin of the Tibetan Plateau and the north-western Sichuan basin. Since Keywords: Longmenshan fault zone; ambient noise tomography; the Wenchuan earthquake on May 12, 2008, abundant studies S-wave velocity structure; short-period dense seis- of the formation mechanism of earthquakes along the LMSF mic arrays were performed. In this study, a short-period dense seismic array deployed across the LMSF was applied by ambient noise tomography. Fifty-two 3-D seismic instruments were used for data acquisition for 26 days. We calculated the 1 Introduction empirical Green's functions (EGFs) between different station- pairs and extracted 776 Rayleigh-wave dispersion curves Located at the junction of the Sichuan foreland basin between 2 and 7 s. And then, we used the direct-inversion and the Songpan-Garzê block, the Longmenshan fault zone method to obtain the fine shallow crustal S-wave velocity structure within 6 km depth in the middle section of the is formed by the strong eastward compression from the Longmenshan fault zone and nearby areas. Our results show Tibetan Plateau and the blocking of Sichuan Basin that the sedimentary layer (>5 km) exists in the northwest (Figure 1) (Burchfiel et al., 1995, 2008; Jia et al., 2006; margin of Sichuan Basin with a low S-wave velocity Hubbard and Shaw, 2009). Characterized by complex (~1.5−2.5 km/s) which is much thicker than that beneath the structures and strong seismicity, the huge Longmenshan Longmenshan fault zone and the Songpan-Garzê block. The fault zone is composed of three major faults (F1: high-velocity structures with clear boundaries below the Wenchuan-Maoxian Fault; F2: Yingxiu-Beichuan Fault; middle of Longmenshan fault zone (~2−4 km) and the F3: Hanwang-Anxian Fault) (Figure 1). Since the 21st cen- Songpan-Garzê block (~4.5−6 km) probably reveal the NW- tury, two strong earthquakes have occurred on the Long- SE distribution patterns of both the Pengguan complex and menshan fault zone, causing terrible casualities and eco- the high-density belt hidden in the northwest of the Pengguan nomic losses. The Wenchuan earthquake (M 8.0) in 2008 complex. And the obviously high-velocity anomalies S observed at the depth of ~1−2 km in the southeastern margin was caused mainly by thrust motion with stike-slip along of the Songpan-Garzê block can be considered as the F1 and F2 (Wang et al., 2008). And occurring at the F3 on Laojungou granites. Our results provide a high-resolution the southwest end of Longmenshan fault zone, the Lushan earthquake (MW6.7) in 2013 was a thrust event (Lin et al., * Received 16 January 2020; accepted in revised form 3 April 2020; 2013). Therefore, it is urgent to utilize various geophysical published 16 May 2020. methods to study the underground structure of the * Corresponding author. e-mail: [email protected], txb@mail. Longmenshan fault zone. iggcas.ac.cn © The Seismological Society of China and Institute of Geophysics, In recent years, a series of work was carried out to China Earthquake Administration 2019 study the internal crustal structures of the Longmenshan 198 Earthq Sci (2019)32: 197–206 101°E 102° 103° 104° 105° 33°N Hongyuan Seismic station Thrust fault epicenter Songpan-Garzê MJF terrane LRBF zone HYF L0 32° Beichuan MEKF 6 MYLF LJG Wenchuan 4 XLB L0 PG 2 Wenchuan 31° Elevation (km) Danba earthquake LMSF zone 0 Chengdu Qaidam F 2 Lushan F1 F earthquake 3 LQSF 30° Tibet Sichuan Basin SB Figure 1 Tectonic setting and distribution of seismic stations in the Longmenshan fault zone. LQSF: Longquanshan fault zone; IMSF zone: Longmenshan fault zone; F1: Wenchuan-Maoxian fault; F2: Yingxiu-Beichuan fault; F3: Hanwang-Anxian fault; MEKF: Barkam fault; HYF: Huya fault; LRBF zone: Longriba fault zone; MJF: Minjiang fault; LJG: Laojungou granite (Mesozoic granite); XLB: Xuelongbao complex (Neoproterozoic metamorphic complex); PG: Pengguan complex (Neoproterozoic metamorphic complex); SB: Sichuan Basin; Red dashed line L0: A profile line; Blue triangle: Seismic station. The red square is study area fault zone and the adjacent areas, including seismic travel the Longmenshan fault zone near the Songpan-Garzê area time tomography (Lei et al., 2009; Wu et al., 2009; Li et is characterized by low and medium wave velocity while al., 2011; Deng et al., 2014), S-wave velocity structure high velocity below the Sichuan Basin, and the weak zone inversion (Li et al., 2009a; Liu et al., 2014), deep seismic thickening towards the Sichuan Basin in the deep crust was reflection profile (Li et al., 2009b; Guo et al., 2013), wide- interpreted as a channel flow (Li et al., 2009a; Liu et al., angle reflection/refraction seismic profile (Jia et al., 2014; 2014). It’s worth noting that most of these studies Zhang et al., 2017), gravity and magnetic modeling (Tian concentrated on large-scale regional problems. et al., 2017; Xue et al., 2017), electrical detection (Zhao et Due to the rich information of the underground media al., 2012; Wang et al., 2014b), drilling exploration (Wang contained in the seismic ambient noise, researchers have et al., 2016), etc. The results show that seismic wave proposed seismic ambient noise imaging technique for 3D velocity in the shallow layer of the Sichuan Basin is velocity structure imaging (Claerbout, 1968; Michel and significantly lower than those in the Longmenshan fault Anne, 2003; Roux et al., 2005; Shapiro et al., 2005; Yao et zone and Songpan-Garzê block (Jia et al., 2014; Liu et al., al., 2006). This imaging method is not limited by seismic 2018). And the thrust strongly uplifted the upper crust and sources with the advantages of low requirements, high crystalline basement under the central fault system of efficiency and high imaging resolution. Nowadays, it has Longmenshan (Guo et al., 2013; Jia et al., 2014; Zhang et been widely applied to revealing the fine velocity structure al., 2017). The seismic wave velocity structure also at the junction of Songpan-Garzê block and Sichuan Basin, revealed that the high-velocity anomalous zone in the which plays an important role in studying the mechanism upper and middle crust of Longmenshan blocked the low- of earthquakes in Longmenshan fault zone (Li et al., 2010; velocity material come from the interior of the plateau, Chen et al., 2015). However, the previous researches also which leads to strain accumulation and controls the focused more on the deep crust and mantle velocity direction of earthquake occurrence and rupture propaga- structures, but less on the shallow fine velocity structure tion (Lei et al., 2009; Wu et al., 2009; Li et al., 2011; Deng with high imaging spatial resolution. In recent years, short et al., 2014; Li et al., 2019). The middle and lower crust of period ambient noise tomography (~1 s, station spacing Earthq Sci (2019)32: 197–206 199 less than 10 km) has been proposed with the improvement We calculated the cross-correlation functions (CFs) of of seismic instruments and gradually applied to high- the every hour's background noise data of different station resolution shallow velocity structure imaging, which pairs and superposed the CFs. The periods of CFs are proved that ambient noise tomography can be developed 0.5 to 10 s, and the length is 100 s. The cross-correlation from large-scale research (hundreds of kilometers) to calculation results are shown in Figure 2. small-scale research (hundreds of meters) (Lin et al., 2010; 150 Hannemann et al., 2014; Wang et al., 2018). The purpose of our study is to obtain the fine velocity structure of the middle section in the Longmenshan fault zone and the adjacent areas. The research steps include background noise data pre-processing, cross-correlation calculation, phase velocity dispersion curves extraction of 100 Rayleigh wave, and S-wave velocity structure inversion (Bensen et al., 2007). In the inversion process, the direct inversion method of surface-wave dispersion was used to minimize the influence of complex terrain (Fang et al., Distance (km) 2015). 50 2 Data and methods 2.1 Station distribution and data collection In our study, the data were recorded by an array of 52 0 0 50 100 EPS-2 instruments deployed across Longmenshan, with a −100 −50 t (s) runtime of up to 26 days (November 1 to November 26, 2017). The sampling frequency of the EPS-2 instrument is Figure 2 0.5−10 s interstation cross-correlation functions 100 Hz, and the frequency band width is 5 s−200 Hz. The 3) The extraction of Rayleigh-wave phase velocity array is almost perpendicular to the Longmenshan Range dispersions and extends from the Songpan-Garzê block to the edge of The empirical Green's functions (EGFs) of the medium the Sichuan Basin (Figure 1). The distance between between stations can be calculated from the first-order stations is about 2 km, and the total length of the array is time derivative of the CFs.
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