Multiple point source solution
The key rupture characteristics of the Maduo earthquake are further confirmed at the intermediate frequency bands, with a multiple point source (MPS) inversion method [Q Shi et al. , 2018] using high-rate GPS waveforms, regional broadband waveforms, and teleseismic body waves (Fig.S9) (Supplement text-3 ). An MPS solution usually uses 2-10 pure dislocation point sources to approximate the complex fault geometry and the rupture process of a large earthquake ([e.g. Duputel et al. , 2012]). Compared with a finite fault model, a smaller number of MPS parameters can resolve the most robust earthquake rupture features at 0.2 Hz and lower frequencies for the Maduo earthquake. In this frequency range, we find that 6-point sources (M1-M6, table S3, Fig.S10-13) are needed to adequately approximate the first order geometry and rupture complexity of the earthquake.
Our preferred MPS solution is presented as beach balls in map view (Fig.2) and depth profiles (Fig.3), with M1-2 and M3-6 located to the west and east of the epicenter, respectively. Uncertainty distributions of the solution (Fig.S10-11) show that the location, starting time, length and shape of the source time function, and focal mechanism parameters are well resolved, mostly owing to the constraints of nearby high-rate GPS observations. Fig.S12 presents the decomposition of synthetics from each sub-events in fitting the closest high-rate GPS stations, which demonstrates the sensitivity and variation of the contributions from different stations. The centroid depths of M3 (7.8 km) and M4 (7.8 km) correspond to deep ruptures on fault segments S4-6, while M1, M2, M5, and M6 (3-6 km) indicate relatively shallow ruptures. This is highly consistent with the FFM slip distribution (Fig.3). The summation of source time functions from the MPS solution also highly resembles the FFM moment-rate function, except for several high-frequency features, which is expected, as we use lower-frequency waves in the MPS inversion. Interestingly, M5 and M6 are located on the northern and southern branches of the fault bifurcation, respectively. The strike of E-W oriented fault plane of M5 is ~10 degrees counter-clockwise to the strike of M6 (Fig. S11), roughly consistent with the strike difference between the two fault branches. The source durations of M5 and M6 are shorter than those of the other sub-events (Fig.S10b), as also shown in the waveform decomposition in fitting the closest high-rate GPS data, where M5 and M6 show sharper pulse in fitting the later portion of E-W component at HSHX station (Fig.S12). This shorter source time function feature is in line with more high-frequency energy radiation from the bifurcation rupture as revealed by the FFM model and the following back-projection analysis.