We image the rupture process of the 2021 Mw 7.4 Maduo, Tibet earthquake using slowness-enhanced back-projection and joint finite fault inversion, which combines teleseismic broadband body waves, long-period (166-333 s) seismic waves, and 3D ground displacements from radar satellites. The results reveal a left-lateral strike-slip rupture, propagating bilaterally on a 160-km-long north-dipping sub-vertical fault system that bifurcates near its east end. About 80% of the total seismic moment occurs on the asperities shallower than 10 km, with a peak slip of 5.7 m. To simultaneously match the observed long-period seismic waves and static displacements, notable deep slip is required, despite a tradeoff with the rigidity of the shallow crust. This coseismic deep slip within the ductile middle crust could result from strain localization and dynamic weakening. Local crustal structure and synthetic long-period Earth response for Tibet earthquakes thus deserve further investigation. The WNW branch ruptures ~75 km at ~2.7 km/s, while the ESE branch ruptures ~85 km at ~3 km/s, though super-shear rupture propagation possibly occurs during the ESE propagation from 12 s to 20 s. Synthetic back-projection tests confirm overall sub-shear rupture speeds and reveal a previously undocumented limitation caused by the signal interference between two bilateral branches. The stress analysis on the forks of the fault demonstrates that the pre-compression inclination, rupture speed, and branching angle could explain the branching behavior on the eastern fork.
An Mw 7.8 earthquake occurred on the East Anatolian Fault (EAF) and the secondary Narlı Fault (NF) on Feb 6, 2023, closely followed by an Mw 7.5 event on the Sürgü Fault 9 hours later. We analyze the distant and local seismic data, high-rate GPS recordings, and radar satellite images by Slowness Enhanced Back-Projection and joint Finite Fault Inversion for the Mw 7.8 event to resolve its rupture process. The rupture first initiates and propagates on the NF. After reaching the junction with the EAF, it propagates bilaterally on the EAF, extending 120 km to the northeast at 3.05 km/s and 200 km to the southwest at 3.11 km/s. The southwest speed is further verified by local seismic recordings and the absence of Mach surface wave characteristics. Compared with the EAF, the NF features denser seismic activity in recent decades, suggesting that it was more favorable for rupture nucleation. The EAF segments where the largest coseismic slip occurred have been relatively quiescent since the late 1800s. But the coseismic slip is much larger than the slip deficit accumulated during this period, which could be attributed to an ~900-year supercycle. The EAF geometry is similar to other active fault systems, such as the San Andreas Fault (SAF) and San Jacinto Fault (SJF). Considering high slip rates, resemblant supercycle mode, and the lack of large earthquakes on the southern SAF and SJF since 1857, an M8 earthquake could potentially occur there if most moment accumulation is released at once.
On 04 March 2021, a Mw 7.3 earthquake occurred approximately 182 km to the northeast of the city of Gisborne, at the southern end of the Kermadec Trench, where the Hikurangi Plateau underthrusts beneath the Australian plate. The Mw 7.3 event is followed by two larger Mw 7.4 and Mw 8.1 quakes ∼4 hr later ∼800 km to the north (Okuwaki et al., 2021). The thickness of Hikurangi Plateau is approximately 12-15 km (the average thickness of Pacific oceanic crust is 8 km), close to the critical thickness (17 km) where buoyancy switches from driving to resisting subduction (Collot & Davy, 1998). Due to the thickening of the oceanic plate and the increasing obliquity from north to south, the convergence rate decreases from 6.3 near 30°S to 4.9 cm/yr at 37°S (de Mets et al., 1990). The subduction stops around 44°S, where the Chatham Rise indicates the transition from oceanic to continental crust of the Pacific plate. The obliquity in the Hikurangi trench causes a slip-partitioning motion with the trench-parallel motion accommodated by strike-slip faulting and the rotation of the eastern North Island arc (Wallace & Beavan, 2010;Wallace et al., 2004).There have been several Mw 6.5+ historical normal-faulting earthquakes but few large megathrust earthquakes around the source region of the 2021 Mw 7.3 earthquake. The only two recorded large interplate thrust earthquakes along the Hikurangi subduction zone are the 1947 Mw 7.1 Poverty Bay earthquake and the 1947 Mw 6.9-7.1 Tokomaru Bay earthquakes. Both of them are tsunamigenic with long source duration (∼40 s) and slow rupture speeds (∼1 km/s). They produced disproportionately large tsunamis for their magnitudes (Doser & Webb, 2003). The lack of large megathrust events in this region can be explained by the small interseismic plate coupling coefficient of ∼0.6 at seismogenic depth from 0 to 10 km inferred from GPS studies (Wallace & Beavan, 2010;Wallace et al., 2004). The plate convergence seems to be accommodated by repeating aseismic slow slip events,
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