The recent 2011 M_w 9.0 Tohoku, Japan, and the 2004 M_w 9.15 Sumatra–Andaman superquakes have humbled many in earthquake research. Neither region was thought capable of earthquakes exceeding M_w∼8.4. Appealing proposed relationships to predict the size of earthquakes in subduction zones, such as that between earthquake magnitude and parameters such as lower plate age and convergence rate (Ruff and Kanamori, 1980) and plate coupling based on anchored slabs (Scholz and Campos, 1995), at least have many exceptions and may not be valid. Both earthquakes occurred where the subducting plate edge was quite...

Although much work has been done on active tectonics of eastern Tibet, little is known about the Longriba fault zone and its role in strain partitioning. Whether its two sub‐parallel strands (Longriqu and Maoergai faults) can rupture simultaneously in a large earthquake remains unknown. We conducted trenching combined with the interpretation of satellite imagery, field investigations, topographic surveys, and radiocarbon and Optically Stimulated Luminescence (OSL) dating to reconstruct paleoseismic history, and we used displaced terrace risers to estimate geological slip rates. Our results demonstrate that the Longriba fault zone is predominantly right‐lateral with a small southeast‐verging thrust component. Four surface‐rupturing events occurred on the Longriqu fault at 5080 ± 90, 11,100 ± 380, 13,000 ± 260, and 17,830 ± 530 cal yr B.P. Together with our previous trenches on the Maoergai fault, we found that the last event probably ruptured both the Longriqu and Maoergai faults. Prior to the last event, the two strands of the Longriba fault zone experienced alternating earthquakes. The fault zone has a high potential for an earthquake larger than Mw 7. The slip rate of the Longriba fault zone decreases from ~7.5 mm/yr in latest Pleistocene to ~2.1 mm/yr in the Holocene, probably related to a slowing down of the eastern motion of the Tibetan Plateau. The comparison with slip rates at the Longmen Shan fault zone suggests that the Longriba fault zone has an equally important role in strain partitioning in eastern Tibet. This study is helpful to seismic hazard assessment and an understanding of deformation mechanism in eastern Tibet.

The 1933 M 7.5 Diexi earthquake is another catastrophic event with the loss of over 10,000 lives in eastern Tibet comparable to the 2008 M_w 7.9 Wenchuan earthquake. Because of its unknown surface rupture, the seismogenic tectonics of the 1933 earthquake remains controversial. We collected unpublished reports, literatures and old photos associated with the 1933 earthquake and conducted field investigations based on high-resolution Google Earth imagery. Combined with paleoseismological analysis, radiocarbon dating and relocated earthquakes, our results demonstrate that the source of the 1933 earthquake is the northwest-trending Songpinggou fault. This quake produced a >30 km long normal-faulting surface rupture with the coseismic offset of 0.9–1.7 m. Its moment magnitude (M_w) is ∼6.8. The Songpinggou fault undergoes an average vertical slip rate of ∼0.25 mm/yr and has a recurrence interval of ∼6700 yrs of large earthquakes. The normal-faulting surface rupture of this quake is probably the reactivation of the Mesozoic Jiaochang tectonic belt in gravitational adjustment of eastern Tibet. Besides the major boundary faults, minor structures within continental blocks may take a role in strain partitioning of eastern Tibet and have the potential of producing large earthquake. This study contributes to a full understanding of seismotectonics of large earthquakes and strain partitioning in eastern Tibet.