The shallow crustal S-velocity structure of the Longmenshan fault zone using ambient noise tomography of a seismic dense array

• 作者: Dandan Li Gaochun Wang Ruihua Lin Kai Deng Xiaobo Tian
• 来源期刊: 地震学报（英文版）
• 年，卷（期）: 2019,（5）
• 页码范围: 197-206
• 页数: 10页
• 语种: 英文
• DOI: 10.29382/eqs-2019-0197-02

摘要

The Longmenshan fault zone(LMSF),characterized by complex structures and strong seismicity,is located at the junction between the eastern margin of the Tibetan Plateau and the north-western Sichuan basin.Since the Wenchuan earthquake on May 12,2008,abundant studies of the formation mechanism of earthquakes along the LMSF 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 empirical Green's functions(EGFs)between different station-pairs and extracted 776 Rayleigh-wave dispersion curves between 2 and 7 s.And then,we used the direct-inversion method to obtain the fine shallow crustal S-wave velocity structure within 6 km depth in the middle section of the Longmenshan fault zone and nearby areas.Our results show that the sedimentary layer(＞5 km)exists in the northwest margin of Sichuan Basin with a low S-wave velocity(～1.5-2.5 km/s)which is much thicker than that beneath the Longmenshan fault zone and the Songpan-Garzê block.The high-velocity structures with clear boundaries below the middle of Longmenshan fault zone(～2-4 km)and the Songpan-Garzê block(～4.5-6 km)probably reveal the NW-SE distribution patterns of both the Pengguan complex and the high-density belt hidden in the northwest of the Pengguan complex.And the obviously high-velocity anomalies observed at the depth of ～ 1-2 km in the southeastern margin of the Songpan-Garzê block can be considered as the Laojungou granites.Our results provide a high-resolution shallow velocity structure for detailed studies of the Longmenshan fault zone.