1999年集集地震前后台湾地区地应变率场的分布及其动力学成因

DISTRIBUTION OF STRAIN RATES IN TAIWAN BEFORE AND AFTER 1999 CHI-CHI EARTHQUAKE AND THEIR GEODYNAMIC MECHANISMS

利用集集地震震前(1990—1995年)及震后(2003—2005年)的GPS观测资料,对台湾及附近地区的地应变率场进行了计算。计算结果显示,集集地震前后,台湾地区的应变率场分布格局基本一致,最大主压应变率的最大值位于台湾东部的海岸山脉地区,方向与台湾岛倾斜相交;主压应变由东向西迅速衰减,在中央山脉地区主压应变率小于主张应变率,呈现拉张的应变状态;在台湾西部平原主压应变率很小。同时,计算的主应变率方位与台湾中强地震震源机制解中的P、T轴方向基本一致。最大剪应变率的高值区分布在海岸山脉地区,该地区面膨胀为负,其数值在整个台湾地区为最大;但沿着中央山脉的南北两侧面膨胀率为正值。这些计算结果说明海岸山脉地区是台湾与吕宋弧碰撞的中心地带,碰撞后物质分别向东北和西南两个方向逃逸。集集地震4年后的2003—2005年,最大剪应变率在主震及余震区比震前明显增大,这可能是地震后震源区介质物性变化造成的;同时也说明在这些地区介质还没有完全恢复到主震前的状态。

The destructive Chi-Chi earthquake occurred in 1999,which had a full impact on the stress state in Taiwan.The strain rates based on the GPS data before and after the 1999 Chi-Chi earthquake in the period of 1990-1995 and of 2003-2005 were estimated respectively.The computed results show that the pattern of strain rate distribution in Taiwan is not changed much before and after the Chi-Chi earthquake.The highest value of the maximum principle strain rates is located on the Coastal Range,which orientated obliquely to the trend of Taiwan.Deceasing rapidly from east to west,the values of principle compressive strain rates are less than those of extensive strain rates on the Central Range,behaving on tensile strain state there.And principle compressive strain rates are very small on the western Coastal Plain.At the same time,the orientations of principle strain rates are agreement with those of P-and T-axis in focal mechanisms in Taiwan.The high value of the maximum shear strain rates is located on the Coastal Range where the surface dilatation rates are negative,and the absolute value is highest in Taiwan.However surface dilatation rates are positive on both the northern and southern sides of the Central Range.The computed results demonstrate that the Coastal Range is the central collision zone between Taiwan and Luzon arc,and material escapes toward two directions of NE and SW,respectively.In the period of 2003-2005,after 4 years since the Chi-Chi earthquake,the maximum shear strain rates become much larger than those in the period of time before the mainshock which may be resulted from the changes of the physical properties of the medium in focal regions,also suggesting that the medium did not resume to the state before the Chi-Chi earthquake,yet.