新疆伽师强震群区上部地壳细结构的高分辨折射地震探测研究

FINE UPPER CRUSTAL STRUCTURE OF JIASHI STRONG EARTHQUAKE SWARM REGION IN XINJIANG INFERRED FROM HIGH RESOLUTION SEISMIC REFRACTION PROFILE DATA

2004年4月在伽师强震群区完成了一条高分辨折射地震探测剖面．我们采用有限差分反演和哈格多恩原理折射波前成像方法进行了处理，得到了该地区上部地壳细结构图象．结果显示上部地壳结构总体上横向比较均匀，纵向分层明显．该地区地壳顶部11km以上可分为4层：顶部约400m以上为浅表盖层，P波速度约1．65～1．8km／s，该层为较松散的风化层；第二层为纵向强梯度层，P波速度在1．8～4．5km／s之间，纵向梯度约1．2km·s^-1／km，其厚度在2．96～3．0km之间，其底界面几乎水平；第三层的厚度有较大变化，其埋深从东北向西南逐渐变深，西南端层厚约6．5km，东北端减薄为5．5km．该层的上部和下部又表现为不同的速度结构特征，其上部速度较均匀，平均P波速度约4．8km／s，下部相对于第二层来说为一弱梯度层，梯度约0．35km·s^-1／km，P波速度在5．1～6．25km／s之间．该层的底界为结晶基底，其结晶基底西南深东北浅，形成一个向天山方向上翘的斜坡，似乎显示出坚硬的塔里木块体在插入天山下时受阻的上部地壳结构特征；第四层比较均匀，速度约为6．3km／s．在4km深度左右有一横向速度异常变化，推断可能与隐伏的麦盖提断裂及下苏洪一麦盖提断裂有关，但未见这些断裂延伸至地表及穿过基底的结构特征．伽师强震群的发震构造至少应位于11km深度以下的中下地壳之中．

In April of 2004, a high resolution seismic refraction profile was carried out in Jiashi strong earthquake swarm area, Xinjiang, China. The data were processed by means of both finite difference inversion and refractor wavefront imaging technique in terms of Hagedoorn principle and the fine upper crustal structure was determined. The results show that the upper crustal structure is relatively well-distributed laterally and obviously by layers vertically. There are about four layers above 11.0 km depth in the studied area. The top layer above 400 m is the shallow cover with comparatively loose weathered layer and its P wave velocity is about 1.65-1.8 km/s; the second layer is a vertically strong gradient layer where the P wave velocity ranges from 1.8 km/s to 4.5 km/s and the vertical P wave velocity gradient is about 1.2 km·s^-1/kin, the thickness of which is 2.96-3.0 km, and its bottom boundary is almost horizontal; the third layer thickness, which is about 6.5 km at SW edge of the studied region and decreases to 5.5 km at NE edge, has comparatively large variation and the buried depth increases gradually from NE to SW. In the third layer, the upper and lower part have different structure features. The P wave velocity of upper part is comparatively even and the average P wave velocity is about 4.8 km/s. Raltive to the second layer, the lower part of the third layer is a weak P wave velocity gradient layer, the gradient of which is about 0.35 km· s^-1/kin and P velocity is about 5. 1-6. 25 km/s. The bottom boundary of the third layer is crystalline basement which is deeper in SW and shallower in NE and forms the slope toward Tianshan Mountain, which seemly shows that the upper crust had been resisted while the hard Tarim block inserting into Tianshan Mountains the forth layer is relatively even and its P wave velocity is about 6.3 km/s. There are a lateral velocity variation at the depth of about 4.0 kin, suggesting that it has something to do with the hidden Meigaiti fault and Meigaiti-Xiasuhong fault but there are no structure features about these faults stretching to the surface and passing through the crystalline basement. The seismogenic tectonic of Jiashi strong earthquake swarm at least lies in middle or lower crust beneath 11.0 km depth.