1927年古浪8级大震及其周边地区三维地壳P波速度结构特征

本文联合利用甘肃及周边测震台网记录的古浪及周边地区4592次地震的P波绝对到时资料和相对到时资料，采用双差地震层析成像方法反演了古浪震源区高分辨率的三维P波速度精细结构.结果显示，浅部P波速度分布与地表地质之间具有很好的对应关系.皇城—双塔断裂带在6 km以上深度表现为高速异常带，而在6~15 km逐渐转换为明显的低速特征，之后再次转换为高速体.震区下部在10~20 km深度有一个尺度约200 km²的低速异常体，地震发生时破裂首先在该低速体发生，与主震空间位置非常吻合.主震区的岩石结构主要由奥陶纪变质砂岩、石英岩和加里东期的花岗岩等坚硬岩体组成.这种坚硬岩体对应的P波速度结构为高速体，有利于能量积累.武威盆地在20 km以上深度表现为明显的低速异常，在25 km深度之下，整体显示为高速体，表现出稳定块体的特征.表明武威盆地中下地壳和上地幔顶部已插入到冷龙岭隆起带之下.震区小震重新定位发现，皇城—双塔断裂带东、西两段表现出不同的力学运动性质，西段以逆冲运动为主，地震主要发生在断裂的下盘.而东段地震却主要发生在上盘，断层活动以局部拉张为主.我们还首次发现在皇城—双塔断裂带的中段与主破裂呈垂直方向存在有在主震发生时新产生的一条共轭断层，基于小震的断层面参数反演显示该断裂是一高倾角运动性质以右旋为主兼具正断的断裂.

3D P-wave crustal velocity structure of the 1927 Gulang M8 earthquake and its adjacent area

3D P-wave velocity structure in crust and uppermost mantle in and around Gulang earthquake source region was studied by double-difference tomography with the direct use of both absolute arrival times and the more accurate differential travel times. These data come from the earthquakes catalog records of totally 4592 events recorded by 125 stations from the digital seismic network in Gansu and its adjacent seismic network during the years of 2000 to 2015. The result shows that shallow P-wave velocity structure has a good correlation with surface geology. Huangcheng-Shuangta fault zone is imaged as a high P-wave velocity region in 0~6 km depth, while it is imaged as a low P-wave velocity region in 6~15 km depth. It is converted into a high-velocity anomaly beneath 15 km again. There is a significant low-velocity anomaly with about 200 km² size where the first rupture generated as the great earthquake occurred beneath the source region with a 10~20 km depth. It corresponds closely to the location of mainshock. The mainshock areas consists of metamorphic sandstones, quartzite and granite.These kinds of rock correspond to obvious high-velocity anomalies that are beneficial to the accumulation of energy. The Wuwei basin is characterized by its obvious low velocity anomaly above the depth of 20 km and high-velocity anomalies under the depth of 25 km as a whole, which can reflect the stability of the block. Its front extends to Lenglongling uplift belt and reaches to the west of Wuwei-Gulang fault zone in the lower crust and upper mantle with depth increasing. It is showed that the eastern segment and western segment have different mechanics property. The motion of the western segment is a thrust fault pattern, and earthquakes occurred in the footwall. Nevertheless, the motion of the eastern segment is a partial tension style, and earthquakes mainly occurred in the hanging wall of the fault. Besides, we found a conjugate fault that located at the central of Huangcheng-Shuangta fault zone and is perpendicular to the main rupture. It is right-lateral strike-slip faulting with minor tension component with high angle, that we obtained from inversion of fault parameter based on the distribution of small earthquakes.