Taiwan region is one of the most tectonically active area of China. The regional tectonic activity includes subduction between Philippian Sea plate (PSP) and Eura-Asian plate in tow different directions, and mountain building. The Eastern Taiwan range experienced rapid change from the building of Coastal Mountains (CoR) to subduction in 100 km distances. The Meilun tableland (part of Hualien city) is at the center of this transition. This regional also has high seismic risk. The 1951 Eastern Taiwan earthquake sequence is the most serious earthquake disaster in the Taiwan history. The geological investigation indicates an 67 years of occurrence interval for characteristic earthquakes on the Meilun Fault. On Feb 6th 2018 ( 67 years after the 1951 event), an strong earthquake shocked the Hualien region. The moment magnitude of this event is 6.4, which caused 300 injuries and 17 fatalities.

　　Han Yue and Li Zhao’s groups performed study on this event and published an paper in Earth and Planetary Science Letters (EPSL). This study investigated the fault structure, rupture processes, ground deformation pattern and discussed the associated tectonic mechanisms. Based on earthquake relocation results, 3D ground displacement field reconstruction, multiple point source inversion and finite fault model inversion, we resolved a cascaded rupture process of this event, which involves sequential rupture on three faults. The rupture d started on an off-shore fault, jumped on to the Meilun Fault under the Hualien Tableland and finally ruptured to the Long-Valley fault (LV). In comparison with the velocity model off the Hualian City, we found the off-shore fault is also an boundary between low-velocity volcaniclastic rock (CoR) and high velocity volcanic rocks (CeR), thus we infer the off-shore fault is an inter-plate fault. The Meilun tableland is a wedge structure cut from the surrounding structures. “Dynamic slip partitioning” occurred during this event involving reversed and strike-slip slips on the off-shore fault and Meilun fault, respectively (figure 1). Slip partitioning commonly occurs in oblique subduction zones, which shows reversed slips and strike-slips on slab interface and volcanic arcs, respectively. Though slip partitioning of subduction zones rarely occur dynamically, near the Meilun Tableland different slips on two faults occurred in an cascaded rupture process: reversed slips on off-shore faults and strike-slips on Meilun faults.

　　The movements of Meilun tableland reflects a special tectonic motion occurred between mountain building and subduction. The 2018 Hualian earthquake presents different senses of motion on two side-walls of the Meilun tableland. However, during the 1951 event, significant reversed slips ( ~1m) occurred on the Meilun fault. Hence, the 2018 Hualien earthquake is not a simple repeating of 1951 slips and motion of Meilun tableland is more like an “teeterboard”. The exchanging reversed slips on two sidewalls of the Meilun tableland is the uplifting mechanism of the Meilun Tableland over a geological history. Thus the 2018 Hualien earthquake reflects an special sense of motion occurred on the transition between mountain building and subductions.

Figure 1, Meilun tableland locates between the costal range (CoR) and the subduction zone. The 2018 Hualian earthquake involves slips on two side-walls of Meilun Tableland.

Lo Y C, Yue H, Sun J, et al. The 2018 Mw6. 4 Hualien earthquake: Dynamic slip partitioning reveals the spatial transition from mountain building to subduction[J]. Earth and Planetary Science Letters, 2019, 524: 115729. https://doi.org/10.1016/j.epsl.2019.115729