【摘要】 利用青藏高原东缘1999—2013年间多期GPS水平速率观测数据,基于多面函数拟合,计算球面坐标系下区域不同时期的面应变和最大剪应变,分析地应变的时空演化特征,结合不同时期发生的中强以上地震(MS>6.0),研究期间大震分布与地应变时空演化特征的关系,主要结论如下:(1)青藏高原东缘面应变分布与地块有一定的对应关系,面应变的差异会在块体边界和内部形成不同的断层闭锁形式,与地震发生位置和震源机制有一定的关联;(2)区域最大剪应变的高值区对应于构造活动性较强的断裂带,这些断裂带鲜有地震发生;低值区对应于活动性较弱的断裂带,在区域地壳运动剧烈的背景下,在这些活动性相对较弱的断层上易形成应变能积累,因而会发生地震。区域绝大多数地震都发生在最大剪应变的低值区。更多还原

【Abstract】 A study on the evolution of crustal strain in the eastern Tibetan Plateau not only provides a perspective regarding regional crustal strain characteristics but is also very useful for analyzing the relationship between earthquakes and strain rates.Here,we first calculate the crustal strain in spherical coordinates around the eastern Tibetan Plateau in different periods based on a multi-surface function using regional GPS velocities for the periods 2001 to 2004,2004 to 2007,and 1999 to 2013.Then,we study the characteristics of the evolution of surface strain and maximum shear strain in this area,analyzing its relationship with the large earthquakes(>MS6.0)that occurred during each period.It was determined that during the years 2001 to 2004,the eastern Tibetan Plateau generally suffered surface compression.The value of the surface compression of the southern part of the Longmenshan sub-block was lower than the northern part.The Sichuan-Yunnan rhombus-shaped block generally suffered surface compression,but the value of the Baoshan sub-block’s surface compression was lower than the other three sub-blocks.From the distribution and focal mechanisms of the earthquakes(>MS6.0),we found that the three strike-slip earthquakes between the Baoshan and Dianzhong sub-blocks might be a response to the considerable difference in surface compression between the two sub-blocks.The normal-slip earthquake in the Yajiang sub-block might be a response to the difference of the N-S orientation in the surface compression of the sub-block.During the years 2004 to 2007,the emergence of two areas of strong surface compression formed a "strong-weak-strong"pattern of compression along the Longmenshan Fault,which could have triggered the May 12,2008,Wenchuan MS8.0earthquake.The surface strain in the Sichuan-Yunnan rhombus-shaped block changed dramatically from2004 to 2007;this may have had some impact on the occurrence of earthquakes after 2008.From the surface strain rate of 1999 to 2013,we found the evidence of strain responses to the Wenchuan earthquake,manifested as the uniform surface compression around the Xianshuihe faults.Generally,the distribution of surface strain rate agrees well with the scope of the sub-blocks,but the distribution of maximum shear strain rate agrees with the fault systems among the sub-blocks.The areas of high values of maximum shear strain rate correspond to the fault zones of strong tectonic activity,where it is difficult for the strain accumulation to grow and no large earthquakes occur.From the distribution of maximum shear strain rate during 2001 to 2004,we found that most earthquakes were located around the area of low values.This means that lower the maximum shear strain rate,greater the strain accumulation,and higher the risk of large earthquakes.During2004 to 2007,the low-value area of maximum shear strain rate was located around the Longmenshan Fault,as was the situation during 2001 to 2004.This means that the Longmenshan Fault is not active and it can accumulate considerable strain energy over a very long period,which is an important requirement for the occurrence of the MS8.0 Wenchuan earthquake.更多还原