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.