## 2 Materials and Methods
### Double Beamforming Tomography  
In the frequency domain, the double beamforming process measures the Rayleigh wave phase velocity between 0.3 s and 0.8 s periods. We adopted 0.35 km as the width of the beam and ensured that there were sufficient numbers of stations under the selected width for obtaining stable results. The array was divided into several beams and one beam was used as the source beam. After the signal was transmitted through the medium, it was received by the other beams, which acted as receiver beams. The interstation distance between every source-receiver pair was required to be longer than one wavelength of the Rayleigh waves in order to satisfy the far-field criterion [16]. The less strict criterion in this study was used to retain a sufficient number of cross-correlation functions for the double beamforming analysis [17; 18]. For stacking the fundamental Rayleigh waves, period-dependent velocity thresholds were adopted for cutting and tapering the cross-correlation functions. The period-dependent slowness thresholds were 0.2-5.0 s/km between 0.3 s  
Figure 2: Vertical to vertical component cross correlation from the source station to the receiver stations for each section. The first section is north of the Meishan Fault (MS01\(-\)MS17), and the next section (MS18\(-\)MS29) is inside CCU. The third (MS30\(-\)MS45) and fourth (MS45\(-\)MS60) sections are south of CCU.  
and 0.8 s (Figure 2). After cutting and tapering, the waveforms were normalized by their maximum amplitudes, transferred to the frequency domain, and stacked and shifted according to the slowness. The beam-stacked cross-correlation functions were then converted back to the time domain and the amplitudes of the envelopes were measured. The grid search technique was used in the source and receiver beams to obtain the best slowness. If the searched slowness closely represents the slowness of the structure at the source side or receiver side, the waveforms will stack constructively, and the stacked waveform will have the largest envelope amplitude. Taking Stations MS22 and MS34 as an example, the maximum amplitude of the envelope of the stacked waveforms with respect to the source beam slowness and receiver slowness was obtained by applying a 2D grid search of the slowness (Figure 3). After repeatedly measuring the slowness with different pairs of source and receiver beams, the mean slowness and its standard deviation were calculated to give final slowness and uncertainty. Figure 4 shows the histograms of repeated measurements of the slowness at Station MS22 for the following three periods: 0.3 s, 0.5 s, and 0.7 s. In the 0.3 s and 0.5 s periods, the statistical results of the slowness show a more centralized distribution resulting in lower uncertainty. For the 0.7 s period, the distribution shows more scatter than for the shorter periods. The mean slowness and the uncertainty at each beam location were used in building the Rayleigh wave phase slowness cross-sections of the entire array for the 0.3 s, 0.5 s, and 0.7 s periods (Figure 5). Significant variations in the gradient at certain locations may be related to variations in the geological structure.