Fault geometry and finite fault inversion
We use the geodetic observations and the relocated aftershocks
[W Wang et al. , 2021] to define the geometry of rectangular
fault segments that will be used in the finite fault model (FFM)
inversion. Ten fault segments are needed to mimic the first-order strike
variations due to bending and bifurcation, as shown in the geodetic
observation (Fig.1a and Fig.2a). These ten segments are sub-vertical
faults, as aftershocks are distributed quite close to the surface
rupture trace (Fig.2b), and the E-W deformations across the fault are
nearly symmetric (Fig.1b). Aftershocks are located mostly to the north
of surface rupture at fault segments 1-6 while aftershocks are
distributed primarily to the south of surface rupture at fault segments
7-10. Based on this relative location between seismicity and surface
rupture trace, we set the dipping direction of fault segments 1-6 and
7-10 towards north and south, respectively (Fig.2). Note, that the
seismic stations used in earthquake relocation study are distributed
quite uniformly in the source region [W Wang et al. , 2021].
Therefore, we can rule out the systematic location bias between the
seismicity to the west and east of the epicenter. Geodetic-only
inversions can then be conducted to determine the dip angle of each
fault segment (Table S2).
Based on this fault geometry, we jointly invert geodetic data, nearby
high-rate Global Positional System (GPS) waveforms, regional broadband
waveforms and teleseismic body waves using a finite fault inversion
method [Ji et al. , 2002] to recover the kinematic rupture
history and slip distribution on the fault segments (Supplement
text-2 ). We divide the rectangular fault segments into 3 km × 2 km
patches, and invert for the slip amplitude, slip direction, rupture time
and rise time on each patch. A Laplacian smoothing algorithm is applied
for the slip distribution during the inversion. High-quality nearfield
geodetic and high-rate GPS observations greatly suppress the trade-offs
between the parameters in the inversion.
Our preferred FFM is presented in Fig.2b in a map view and in Fig.3 as a
depth view along the strike. This model produces excellent fits to both
seismic waveforms (Fig.S3-5) and static surficial deformations
(Fig.S6-8), suggesting a reliable model resolution. Most of the slips
are distributed at the depth shallower than 8 km except for S5 and S6,
where slips are as deep as 15 km. Interestingly, the distribution of the
aftershocks also shows a gap at S5 and S6 (Fig.2b and Fig.3d),
indicating the rupture released most of the stress accumulated in the
entire seismogenic zone, which has a depth of ~15 km. On
the other segments, where the seismogenic zone is only partially
ruptured, aftershocks are much denser, and distributed in the depth
range of 5-15 km, showing a clear complementary feature with the
coseismic slip distribution (Fig.3d), similar to that observed for other
strike-slip earthquakes (e.g., [Wei et al. , 2011]). The
aftershocks were most likely triggered by down-dip post seismic slip and
Coulomb stress change imposed by coseismic slip. Therefore, slip deficit
happens both at shallow [Jin and Fialko , 2021] and at greater
depths, that is, at the upper and lower bounds of the seismogenic zone.
The equivalent moment tensor of the FFM shows relatively small
non-double-couple component (Fig.3b), which is highly similar to that
from multiple point source solution (see next section). Our equivalent
moment tensors are more similar to the GCMT solution, including the Mw,
but much more different from the W-phase solution, in which the
east-west oriented fault plane has a shallower dip angle (67°) and quite
strong normal faulting component in rake (-40°). It appears that a
reliable FFM of Maduo earthquake provides an independent verification to
the global moment tensor solutions.
There are substantial slip distributions on the two branches of the
bifurcated fault, where the moment magnitude of the northern branch
(S7-8, Mw6.8) is slightly larger than that on the southern branch
(S9-10, Mw6.7). The sizable and comparable moment distribution on the
two branches is a key feature that allows robust resolution on their
ruptures. The waveform observations on high-rate GPS station HSHX
exhibit a clear frequency-dependent feature that the rupture from the
bifurcated branches (S7-10) produced more high-frequency seismic waves
than that from earlier ruptures (S1-6) (Fig.4). HSHX station is located
at almost the same distances away from S4-10 (Fig.2b), therefore the
geometric spreading induced attenuations at HSHX are similar for
ruptures on these fault segments. If we assume pure strike-slip motion
uniformly distributed on these fault segments, we would expect stronger
seismic waves from S4-6 as the azimuth of HSHX is closer to the
strongest SH-wave radiation direction of S4-6. Indeed, in the
displacement waveforms, which are dominated by low-frequency energy,
seismic waves excited from S1-S6 show larger amplitudes than those from
S7-10, as shown in the synthetic waveform decompositions (Fig.4).
However, in velocity waveforms, in which higher-frequency features are
presented, the waveform amplitudes from S7-S10 are larger than those
from S1-S6. This suggests that S7-10 radiated more high-frequency waves
than those on the other fault segments. Note that the displacement
waveforms are dominated by the periods of 10 - 20 s, while the velocity
waveforms show stronger energy at 5 - 10 s. We did not fit the N-S
velocity waveform as good as the E-W component for the rupture from
S7-10 (highlighted by circles in Fig.4). This portion of the waveform
shows more high frequency energy in the N-S component than the E-W
component. This larger misfit to the higher frequency waveform is likely
because our FFM inversion is dominated by relatively low frequency
energy in the observations.