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

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.