3-D isotropic and anisotropic shallow crustal structure on Pingtan Island, Fujian, southeastern coast of China
Graphical abstract
Introduction
Pingtan Island (also known as Haitan Island) (Fig. 1) is located on the southeastern coast of China. It is the fifth largest island in China and the largest island in Fujian Province. More than 400,000 people live on the island. In 2009, the government of Fujian Province established the Pingtan experimental zone. Since then, Pingtan Island has gradually become the core area for the development of the Straits Economic Zone (Guo et al., 2019). This Island is located in a seismically active region (Fig. 1a). Therefore, it is necessary to uncover the subsurface structure of the Pingtan Island which may play an important role in evaluating the potential earthquake risks in the future based on numerical simulation of strong ground motion.
Pingtan Island is distributed between two large NE-oriented fault zones (Changle-Zhao'an fault zone and Binhai fault zone), at the northeastern end of the Pingtan-Dongshan metamorphic belt. This area has complex geological conditions and well-developed faults. The shape of the island is an irregular ellipse with the long axis oriented in the NNE direction. This orientation is basically consistent with the direction of the regional tectonic structure (Chen et al., 2016). The southeastern Fujian coast was a bulge before the Mesozoic (Zhu et al., 2008). As the Yanshanian movement began, large-scale volcanic eruptions and intrusions together with strong fault activity occurred. The Changle-Zhao'an fault zone (Lin, 1999), Binhai fault zone (Zhu et al., 2008) and the main faults on the Pingtan Island (Chen, 1994) all formed during this period. Large-scale volcanic eruptions and intrusions occurred along the fault strikes (Zhao, 1987), which determined the tectonic structure of Pingtan Island. Therefore, the deformation of the subsurface Pingtan Island is effected largely by the regional tectonic structure. The anisotropic velocity structure can reflect the directional arrangement of fractures and mineral lattices under stress and strain, as well as the variations of the deformation within the crust and mantle. Studies on anisotropy generally use following methods: anisotropic tomographic inversion of traveltime data (Eberhart-Phillips and Henderson, 2004; Eberhart-Phillips and Reyners, 2009; Wang and Zhao, 2013), shear wave splitting (Zhang et al., 2007; Liu et al., 2015; Chang et al., 2018), receiver functions (Levin and Park, 1997; Ozacar and Zandt, 2004; Liu and Niu, 2012; Sun et al., 2012), and surface wave tomography (Yao et al., 2010; Wang and Tape, 2014).
Common methods for detecting shallow crustal structures include body wave traveltime data tomographic inversion (Schuler et al., 2015), ambient seismic noise tomography (Lin et al., 2013; Fang et al., 2015; Li et al., 2016), receiver functions (Leahy et al., 2012), joint inversion (Zhang et al., 2014), etc. Recently, Active source exploration has been widely used. Active sources are not dependent on the occurrence and distribution of earthquakes, and the location and excitation time are accurate. Common active sources include shots (Fuis et al., 1995; Teves-Costa et al., 1996; Li et al., 1997; Catchings et al., 2002), vibrator control systems (Al-Ali, 2006), CASS (Controlled Accurately Seismic Source) (Yang et al., 2011; Liu et al., 2016), airgun source, etc.
An airgun generates seismic waves by the sudden release of high-pressure air underwater. It is environmentally protective and has high repeatability (Chen et al., 2007, Chen et al., 2017). These are great advantages for obtaining a high-resolution velocity structure of the island. With the continuous improvement in theory and technology, airguns have been widely used. Examples are the LARSE (Los Angeles Region Seismic Experiment), a cooperative study of the crustal structure (Brocher et al., 1995; Okaya et al., 1995; Godfrey et al., 2002); the SIGHT (South Island GeopHysical invesTigation), a multidisciplinary investigation of continental deformation at convergent plate boundaries (Okaya et al., 2002; Stern et al., 2002); the onshore-offshore seismic experiment in the northeastern South China Sea (Qiu et al., 2003; Zhao, 2004); and the Anhui Airgun Experiment in the Yangtze River (Hu et al., 2016; Zhang et al., 2016; She et al., 2018; Tian et al., 2018).
Previous studies on southeastern coast of China are generally about the structures of large areas (Zhang et al., 2018; Zhang, 2019), while the deformation of the subsurface Pingtan Island is unclear. In this paper, we performed isotropic and anisotropic inversions with body wave traveltime data from marine large-volume airgun sources, and obtained the 3-D high-resolution anisotropic velocity model of Pingtan Island. We analyzed the relationship between the inversion results and the regional structural features, especially for the fault structures. Then, we compared the anisotropic structure at shallow crust with that of deep crust and upper mantle, and further discussed the causes of azimuthal anisotropy and deformation mechanisms from the shallow crust to the upper mantle of the southeastern coast of China.
Section snippets
Data
The body wave traveltime data were obtained from the dense array deployed for a large-volume airgun shot experiment organized by the Fujian Earthquake Agency in 2018. The scientific research ship “Yanping No.2” excited 387 airgun signals (Fig. 2, blue dots) along two nearly parallel lines. The average gun spacing of each line is approximately 250 m. The airgun source consists of six 1500 LL bolt airgun array with a total volume of 12000 in3. The dense array consists of 112 portable seismographs
Checkerboard resolution test
To test the model resolution under the current data distribution and inversion grid, we conduct a checkerboard resolution test. The checkerboard model is built by adding ±5% velocity anomalies to adjacent grid points based on the 1-D velocity model. Then, we calculate the theoretical P-wave and S-wave traveltimes under this model using the real distributions of stations and shots. The theoretical traveltime is used to perform isotropic inversion, and the recovery of the model reflects the
Checkerboard resolution test
Similar to the 3-D isotropic inversion process, we also perform a checkerboard resolution test to evaluate the spatial resolution. The checkerboard model is built by adding the perpendicular azimuthal anisotropy direction to adjacent grid points on the 1-D velocity model. The anisotropy parameters of the adjacent grid points are A = B=3 × 10−3 (s/km) and A = B=−3 × 10−3 (s/km). According to formula (2-2), the anisotropy directions of adjacent grid points are 22.5° and 112.5°. The azimuthal
Discussion
We obtained 3-D shallow crustal isotropic and azimuthally anisotropic velocity models of Pingtan Island through the inversion of large-volume airgun body wave traveltime data. The results can help us better understand the regional tectonic structure as well as the anisotropic characteristics and deformation mechanisms.
Conclusions
We obtained the 3-D isotropic and anisotropic shallow crustal velocity structure on Pingtan Island and the surrounding areas through the tomographic inversion of body wave traveltime data from a large-volume airgun source. Our results lead to the following conclusions.
(1) The range of velocity anomalies decreases with depth, and the velocity structure corresponds well with the landforms. The sedimentary plains in the midwestern area have a higher velocity, while the eroded hills in the southern
Data availability
The 3-D anisotropic model ‘PingtanVpVsAzimAniso’ has been uploaded with this research article. For traveltime data, please contact the co-author Huiteng Cai ([email protected]) from Fujian Earthquake Agency.
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work is supported by National Natural Science Foundation of China [grant number 41790464, 41790461] and the Seismic Science and Technology Spark Program CEA (XH19023Y). We appreciate Dr. Zhouchuan Huang, an anonymous reviewer, and the editor for their constructive comments for our manuscript paper.
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