Fine three-dimensional P-wave velocity structure beneath the capital region and deep environment for the nucleation of strong earthquakes

Abstract

A detailed 3-D P-wave velocity model of the crust and uppermost mantle under the capital region is determined 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 region. 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 lithological features. In the North China Basin, the depression and uplift areas are imaged as slow and fast velocity belts, respectively, 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 anomalies in our tomographic images, while the Quaternary intermountain 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 earthquake (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.North China is a platform with old tectonic basement, but it is a very active region from a seismotectonic point of view. The present study region is the northern portion of North China, which is located in the intersection of the Yanshan and the Taihangshan uplift regions (Fig. 1). Generally speaking, seismicity is less active in old platforms of the world. However, many strong earthquakes with magnitudes greater than M 7.0 have occurred in this region. Among them, the 1966 Xingtai earthquake (M 7.2) and the 1976 Tangshan earthquake (M 7.8) caused tremendous casualty and losses. Why do so strong earthquakes occur in this old platform? What are the tectonic backgrounds in the epicentral areas of these strong earthquakes? These questions have drawn broad attention. Some researchers have investigated the 3-D seismic velocity structure of the crust and upper mantle under this region using arrival times recorded by the previous seismic network [2][3][4][5][6] , and some deep seismic soundings (DSS) were also conducted to investigate the crustal structure in this region [...

Tomographic imaging of P wave velocity structure beneath the region around Beijing

“…Because of the political and economic importance for the capital area, many scholars investigated the 3-D seismic velocity structure models in this region by using P wave arrival times (Jin et al, 1980;Liu et al, 1986Liu et al, , 1989Zhu et al, 1990;Sun and Liu, 1995;Ding, 1999;Yu et al, 2003;Huang and Zhao, 2005;Lei et al, 2008). With the increasing seismic observatories and the improvement of the coverage of the seismic stations, especially installation of the seismic array in North China, the collected seismic dataset supplies more complete database.…”