从地质特征分析成都主城区在地震中的安全性——以汶川大地震为例

Safety discussion on Chengdu city in earthquake according to geological features:Take Wenchuan earthquake as an example

汶川M_S 8.0大地震发生后,关于成都市主城区安全性的讨论一度十分激烈。本文从成都市及周边的基本地质构造和沉积特征入手,讨论成都市主城区在地震中的安全性问题。经过分析认为:川西坳陷刚性稳定的岩石圈结构决定了成都平原不会发生破坏性的大地震;成都平原沉积盖层厚度大、质量佳、分布适宜决定了龙门山发生的地震不会对成都主城区造成巨大破坏。因此,面对天然地震,成都市主城区是安全的。

The earthquake safety of the Chengdu city has been a focus issue since the Wenchuan big earthquake. This present paper will talk about the safety of Chengdu urban area in an earthquake based on the basic geological structures and sedimentary features in Chengdu and its neighborhood area. Devastating earthquakes usually occur at active huge fault belts, and are of repetitive. The lithosphere structure of western Sichuan depression （ Chengdu plain） is rigid and stable. Being situated at Chengdu plain, Chengdu city is on a stable tectonic unit. There is no large fault under the city. There is no record about devastating earthquake in Chengdu plain in history. Thus, there is no any possibility for a devastating earthquake to occur at Chengdu city. Secondly,Chengdu plain has a thick, good and well-distributed sedimentary cover, which determines that any earthquake occurs in Longmenshan can not make fatal damage to Chengdu urban area. The strata under Chengdu plain are composed up of four layers ： 1 ） The Pre-Sinian basement,whose surface is buried 11 kilometers and is the deepest in Sichuan Basin; 2）Upper Sinian-Middle Triassic marine carbonates with the thickness of 4 000 m to 5 000 m ; 3 ） Uupper Triassic-Cretaceous continental clastic rocks with the thickest thickness of 5 000 m to 6 000 m in Sichuan Basin. This strata texture is ＂the protection god＂ warding the Chengdu city off being ruined by Wenchuan earthquake. Because the rigidity of the strata becomes smaller and smaller from the bottom to the top, their ability absorbing the seismic energy resulted by earthquakes is bigger and bigger. This makes the seismic waves from Longmen Mountain earthquakes very weak with no destroy energy when they propagated to Chengdu city. Therefore,Chengdu city is safe to the earthquakes which happen in Longmen Mountains. There, faced with natural earthquake,Chengdu urban area are safe.