Figure 2. (a) Stacking profile of the single event receiver function with a Gaussian factor of 5.0. (b) Stacking profile of all the teleseismic events receiver function with a Gaussian factor of 5.0. (c) CCP imaging of single event receiver function. (d) CCP imaging of all the teleseismic events receiver function. The CCP imaging results shown with a Gaussian factor of 2.0. The thin green line at 35 km depth represents the Moho depth under the iasp91 model (Kennett & Engdahl, 1991). The thin green dashed lines and pink circles are the theoretical arrival times and observed arrival times of the Pms phase, respectively, which are calculated by the iasp91 velocity model using the 1D time-to-depth conversion method.
For comparison, the receiver functions with high signal to noise ratio from a single Mw 7.3 event at epicentral distance of 35.2° recorded on 24 June 2019 at 02:53:39 UTC is presented in our results (Fig 2 a; Figs S2 a, c). At the same time, the CCP stacking method proposed by Yuan et al. (1997) was adopted to obtain a migrated image along the ray path (Figs 2 c, d and S2 ).
Combined with a tomographic model, we can also discuss the background S-wave velocity structure with a 0.5° × 0.5° horizontal resolution and 0.2 km vertical resolution over the depth range of 0~180 km (Zhou et al., 2012). Because this tomographic model was calculated from the ambient noise inversion of group and phase velocities, it has a relatively good vertical resolution (Fig S4). In addition, the H–k method was adopted for our research (Zhu & Kanamori, 2000), and the arrival times of the Pms, PmpPms and PmpSms phases were automatically picked after the corresponding moveout correction (Fig 3 a) (Shen et al., 2017).
3 Results
In our results, our focus is on the aligning profiles of receiver functions (Fig 2 and Fig S2). The direct P-wave arrivals are obvious in our results, showing a maximum delay of 0.2 s beneath 23.4~22.5°N, which may be caused by the thick Quaternary sediment at the Sanshui Basin (Fig S1). A secondary positive phase occurs at nearly 3~5 s, which represents the Pms converted wave. The overall trend of Pms converted wave arrivals arriving earlier indicates the slight uplift of the Moho to the southeast. The average arrival times of the Pms, PmpPms and PmpSms phases are 3.8 s, 12.2 s and 14.9 s, respectively, while the corresponding values calculated by the iasp91 velocity model are 4.4 s, 10.9 s, and 13.5 s. The average depth calculated by the H–k method is 30.9 km, while the Moho is deepest (approximately 37 km) beneath 23°N and shallowest ( approximately 26 km) beneath 22.5°N. The H–k results show that the average Vp/Vs is approximately 1.78, which is almost the same as the global average (Christensen, 1996). Importantly, Vp/Vs exceeds 1.8 beneath 22.8–23.2°N area, where the maximum Vp/Vs is approximately 1.9 (Fig 3 b).