摘要
A detailed 3-D P-wave velocity model of the crust and uppermost mantle under the capital region is de- termined with a spatial resolution of 25 km in the horizontal direction and 4—17 km in depth. We used 48750 precise P-wave arrival time data from 2973 events of local crustal earthquakes, controlled seismic explosions and quarry blasts. These events were recorded by 123 seismic stations. The data are analyzed by using a 3-D seismic tomography method. Our tomographic model provides new information on the geological structure and complex seismotectonics of this re- gion. Different patterns of velocity structures show up in the North China Basin, the Taihangshan and the Yanshan Mountainous areas. The velocity images of the upper crust reflect well the surface geological, topographic and lithologi- cal features. In the North China Basin, the depression and uplift areas are imaged as slow and fast velocity belts, re- spectively, which are oriented in NE-SW direction. The trend of velocity anomalies is the same as that of major structure and tectonics. Paleozoic strata and Pre-Cambrian basement rocks outcrop widely in the Taihangshan and Yanshan uplift areas, which exhibit strong and broad high-velocity anoma- lies in our tomographic images, while the Quaternary inter- mountain basins show up as small low-velocity anomalies. Most of large earthquakes, such as the 1976 Tangshan earthquake (M 7.8) and the 1679 Sanhe earthquake (M 8.0), generally occurred in high-velocity areas in the upper to middle crust. However, in the lower crust to the uppermost mantle under the source zones of the large earthquakes, low-velocity and high-conductivity anomalies exist, which are considered to be associated with fluids, just like the 1995 Kobe earthquake (M 7.2) and the 2001 Indian Bhuj earth- quake (M 7.8). The fluids in the lower crust may cause the weakening of the seismogenic layer in the upper and middle crust and thus contribute to the initiation of the large crustal earthquakes.
A detailed 3-D P-wave velocity model of the crust and uppermost mantle underthe capital region is determined with a spatial resolution of 25 km in the horizontal direction and4-17 km in depth. We used 48750 precise P-wave arrival time data from 2973 events of local crustalearthquakes, controlled seismic explosions and quarry blasts. These events were recorded by 123seismic stations. The data are analyzed by using a 3-D seismic tomography method. Our tomographicmodel provides new information on the geological structure and complex seismotectonics of thisregion. Different patterns of velocity structures show up in the North China Basin, the Taihangshanand the Yanshan Mountainous areas. The velocity images of the upper crust reflect well the surfacegeological, topographic and lithologi-cal features. In the North China Basin, the depression anduplift areas are imaged as slow and fast velocity belts, respectively, which are oriented in NE-SWdirection. The trend of velocity anomalies is the same as that of major structure and tectonics.Paleozoic strata and Pre-Cambrian basement rocks outcrop widely in the Taihangshan and Yanshanuplift areas, which exhibit strong and broad high-velocity anomalies in our tomographic images,while the Quaternary inter-mountain basins show up as small low-velocity anomalies. Most of largeearthquakes, such as the 1976 Tangshan earthquake (M 7.8) and the 1679 Sanhe earthquake (M 8.0),generally occurred in high-velocity areas in the upper to middle crust. However, in the lower crustto the uppermost mantle under the source zones of the large earthquakes, low-velocity andhigh-conductivity anomalies exist, which are considered to be associated with fluids, just like the1995 Kobe earthquake (M 7.2) and the 2001 Indian Bhuj earthquake (M 7.8). The fluids in the lowercrust may cause the weakening of the seismogenic layer in the upper and middle crust and thuscontribute to the initiation of the large crustal earthquakes.
基金
This work was partially supported by the Chinese Earthquake Study Foundation(Grant No.103076)
the Basic Research from the Ministry of Science and Technology(Grant No.2002CCD01700)
the Japan Society for the Promotion of Science(Grant Nos.11440134,12002006).
关键词
三维P波
速度结构
成核现象
地震
流动性
地质特征
capital region
seismic tomography
3-D P-wave velocity structure
nucleationof strong earthquakes
low-velocity layer
fluids