Inversion of 3-D crustal P-wave velocity structure in Ningxia and its neighborhood by using direct, reflected and refracted waves

Rayleigh wave group velocity distribution in Ningxia

“…The lateral velocity variation at shallow depth is correlated well with surface geology and relief, which is consistent with our tomographic results. At deep depth, results from Jin et al (1999), Li et al (2002), and Zhang et al (2009) found low-velocity layers in the middle-to-lower crust beneath the Yinchuan basin that are interpreted as partial melting in this area; however, they are not observed in our study. Since in this study the longest period we measured for the group velocity is 22 s, which corresponds to the resolving depth around 25-5 km, in order to detect the middle-to-lower crustal structures, we have to rely on group velocity measurements from longer periods.…”

“…Tomographic studies using earthquake data from Jin et al (1999) and deep seismic sounding profiles (Li et al, 2002;The Committee Compiled Earth Science Profiles, SSB, 1992) all showed that lateral velocity variations in the Ningxia area are evident, and the Yinchuan basin is featured with low velocities at a depth down to at least 7.5 km, and high velocities are revealed beneath the mountainous region. The Yinchuan basin is a trough-like Cenozoic graben-basin located in the west edge of Ordos massif, it has very thick sediment layers, and the relatively high velocity beneath the Helan Mountain and southern mountainous region is probably due to their shallow bedrock depths.…”

“…We adopted isotropic layered models that are composed of 50 layers with 1-km thickness of each layer overlying a halfspace. The S-wave velocity in each layer is referred to deep seismic sounding profiles and tomography results (Zhao et al, 2007;Li et al, 2002;Jin et al, 1999; The Committee Compiled Earth Science Profiles, SSB, 1992), and P-wave velocity and density are computed from S-wave velocity based on empirical formulas. Since the maximum period we measured for group …”