基于形变观测分析中国台湾东部花东纵谷断层运动特征

Analysis of movement characteristics of the Longitudinal Valley Fault in eastern Taiwan,China based on GPS observations

花东纵谷断层是中国台湾动力作用和地壳运动变形最强烈的断层之一,其断层运动特征和强震危险程度一直备受学者的关注。文中分别以同震地表位移、1992—1999年震间形变数据为约束,反演2003年成功M_W6.8地震同震位错分布和花东纵谷断层震间运动特征。结果表明:花东纵谷断层北段处于强闭锁状态(闭锁率高达0.9),闭锁深度深(约27km);南段闭锁程度较弱(闭锁率约0.5),闭锁深度较浅(约12km);中段闭锁程度与闭锁深度介于南北段之间。另一方面,2003年成功MW6.8地震微观震中位于震间无震滑移区与闭锁区的过渡带附近。依据同震位错、震间断层运动反演结果,以及历史强震破裂分布特征,分析认为,花东纵谷断层南北段运动方式存在差异性,北段主要以强震形式运动,南段以蠕滑和地震两种形式运动。自1951年花莲—台东M_L7.3地震序列后,花东纵谷断层南段、中段和北段至2016年所累积的矩能量分别等价MW 6.4、M_W7.0、MW_7.4地震;若发生级联破裂,整个断层至2016年所累积的矩能量等价M_W7.5地震。

The Longitudinal Valley Fault is one of the faults with the most active dynamic action and crustal movement deformation in Taiwan, China. Its fault movement characteristic and strong seismic risk have been concerned by scholars. Using constraints from coseismic surface displacement and interseismic deformation observations between 1992 and 1999, this paper inverted the coseismic slip distribution of the 2003 Chengkung Earthquake （MW 6.8） and the interseismic slip characteristic of the Longitudinal Valley Fault. The results show that the northern Longitudinal Valley Fault was almost fully locked （locking fraction up to 0.9） to the depth of 27 km; the southern fault was partially locked （locking fraction ~0.5） with shallower locking depth （~12 km）; and the locking fraction and locking depth in the central fault was between north and south. The epicenter of the 2003 Chengkung earthquake was near the transition zone between the creeping zone and the locked zone. Based on the coseismic slip distribution and interseismic slip characteristic results, and combined with the characteristics of historical earthquake fault rupture distribution, one can see that there was a difference in the form of fault movement between southern and northern Longitudinal Valley fault： the northern is mainly in the form of seismic slip, and the southern the form of seismic and aseismic slips. Since the 1951 HualianTaitung ML 7.3 earthquake sequence, the southern, central and northern faults have become capable of releasing the stored strain in a future MW 6.4, 7.0 and 7.4 earthquakes, respectively. In case of a cascading rupture, however, the accumulated strain under the entire Longitudinal Valley Fault can engender a MW 7.5 earthquake.