# Travel-Time Inversion Method of Converted Shear Waves Using RayInvr Algorithm
## 2 Data and Methodology  
### Data Acquisition and Processing  
A wide-angle seismic profile (NS5) was conducted by the [PERSON] of the South China Sea Institute of Oceanology in May-July 2014. The profile NS5 is 317 km long and is an NW-SE trending line crossing the Dongsha Rise (Figure 1). OBSs are deployed at an interval of \(\sim\)15-20 km along the profile with 12 OBS instruments (comprising three orthogonal geophones and one hydrophone) that record usable data. The seismic source consisted of four BOLT air-guns arrayed with a total volume of 6000 in 3 (\(\sim\)98.3 L),  
Figure 1: Topography and bathymetry map of north-eastern South China Sea (SCS) with the location of deep seismic survey lines. The gray lines represent the deep seismic survey lines which have conducted the studies of converted S-waves, including OBS2001 [4], OBS2006-3 [13], OBS2016-2 [14]. The thick, black, solid line is the profile NS5 in this study. Red dots represent ocean bottom seismometers (OBSs). Gray areas represent the distribution of the Pearl River Mouth Basin and Taixinan Basin. The yellow curve with triangles denotes Manila Trench. CSD: Chaoshan Depression.  
which were operated at a pressure of 2000 psi (13.79 MPa). A total of 965 shots were fired along profile with a shooting interval time of 120 s. The recorded OBS data are of high quality, and most instruments have recorded clear air-gun signals. The processing of OBS data includes data format conversion, correction of OBS position and clock drift, and thereby four components of seismic profile are plotted. We have identified five P-wave phases on the vertical component profile: refracted/reflected phases in the Cenozoic sediment layers (Ps2/Psp), refracted phases in the Mesozoic sediment layers (Ps3), refracted phases in the upper and lower crust (Pg), refracted phases in the upper mantle (Pn), and reflected phases at the crust-mantle boundary (PmP). Based on the seismic phases described above, we set seven layers in the initial velocity model (water, Cenozoic sediment, Mesozoic sediment, upper crust, lower crust, high-velocity lower crust, and upper mantle), corresponding to velocities of 1.5 km/s, 1.7-3.5 km/s, 3.5-5.5 km/s, 5.5-6.5 km/s, 6.5-7.0 km/s, 7.0-7.5 km/s, and 7.8-8.2 km/s. The Cenozoic sediment layer was further divided into two layers, each corresponding to a high-amplitude reflection in single-channel reflection seismic data [19] and OBS data (Figure 2). Furthermore, we performed forward P-wave velocity modelling using the RayInvr [17]. We adjusted the boundary nodes and velocity values by using a manual trial-and-error method to make the modelled, theoretical travel times consistent with observed times, based on ray tracing and travel-time fitting. The final model (Figure 3 and Figure S1) had the root-mean-square travel-time residuals (RMS) of 81 ms, and normalised chi-squared value (\(\chi\)2) of 1.468. The uncertainty of the Moho depth and P-wave velocity of the high-velocity lower crust were determined to be \(-\)0.2-0.2 km and \(-\)0.2-0.2 km/s (Figure S2), respectively. The converted S-waves phases are primarily concentrated in the two horizontal components. We rotated two horizontal components into radial and transverse components to make the S-wave phases clearer. The polarisation angle (for details see Table S1) is calculated using the energy scanning method [20]. Then, the S-wave modelling is conducted using the S-wave phases recorded on the radial component.