- 2018-05-11 09:28
Yunpeng Zhang1, Baoshan Wang1, Tao Xu2, Weitao Wang1, Yihe Xu1
1. Key Laboratory of Seismic Observation and Geophysical Imaging, Institute of Geophysics, China Earthquake Administration, Beijing, 100081, China
2. State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, 100029, China
Key Words: Southern segment of Tan-Lu fault; Airgun source; Body-wave tomography
Objectives: The Tan-Lu fault is a complex deep fault that has obvious sub-instability areas. However, the velocity structure is poorly imaged due to lack of natural earthquakes. Recently, with some distinct features as highly repeatable and environmentally friendly, airgun array was adopted as active source for imaging the high resolution local crustal structure. So to explore the high-resolution crustal structures in the southern segment of the Tan-Lu fault, we carried out an experiment by firing portable airgun source along the Yangtze River in Anhui Province during Oct. 10th-20th, 2015 (referred to as Anhui Airgun Experiment hereafter).
Methods: For the Anhui Airgun Experiment, an airgun array composed of four individual guns with total volume 8000 in3 mounted on R/V Yanping II was used as the seismic source. 4845 shots were fired along the 300 km segment of the Yangtze River from Ma’anshan to Anqing, among them 2973 shots were fired at 20 fixed sites and 1872 shots were fired in the walkway. The receiving system was stations within 300km of the Yangtze River, including 109 broadband permanent stations and more than 2000 portable short period seismometers. We obtained the high-resolution crust structure in the southern segment of Tan-Lu fault using TOMOG3D (Zhao et al., 1992; 1994) by 4118 P-wave and 1906 S-wave arrives which manually picked from the stacked waveforms.
Results: The 3-D inverted velocity structure is overall consistent with the geological settings. Imaging result in the Middle-Lower Yangtze River Metallogenic Belt presents a strong deformation in the upper crust with higher velocity in eastern side. Low Vp, Vs and Vp/Vs value correspond to the volcanic basin like Lujiang-Zongyang and Ningwu basin, while ore deposits have high Vp and Vp/Vs in the upper crust. Low-velocity anomaly zone beneath the Changjiang fracture zone corresponds to junction zone of Tongling, Anqing and Luzong ore deposits. The velocity structures in the upper-middle crust and upper mantle suggest that the Tan-Lu fault might be a deep fracture cutting down to the Moho discontinuity.
Conclusion: The Anhui Airgun Experiment is the first experiment by firing portable airgun source, and the inverted velocity structure confirms that it can make body-wave tomography and has high resolution in shallow crust.
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