THE CRUSTAL DENSITY STRUCTURE UNDERNEATH LONGMENSHAN FAULT ZONE

doi:10.3969/j.issn.0253-4967.2012.01.004

Abstract

Abstract:

It is of importance to study the crustal density structure of Longmenshan Fault zone for understanding the seismicity of this area. Using commercial software of Geosoft, gravity inversion for 6 lines of Bouguer gravity anomaly data was carried out under the constraint of deep seismic sounding (DSS) data obtained in the last two decades. The depth and distribution of sediments, crustal thickness and Conrad discontinuity of Longmenshan tectonic zone and its adjacent area(100°～105°E, 28°～33°N)are obtained. The inverted results indicate that the crustal density structure of the Longmenshan Fault zone is obviously different between the two sides. The sedimentary layer in western plateau is thinner and base rocks outcrop almost in most parts; while the thickness of sedimentary layer in eastern Sichuan Basin is much larger, in most parts, it is larger than 6km. The Conrad and Moho discontinuities are relatively smooth in both sides of the fault and the depths of them increase gradually from the eastern 24km and 42km to 35km and 67km under Tibetan Plateau, respectively. The whole fault of Longmenshan displays as a SN-direction gravity gradient zone, with each of its interfaces in the crust under the faults being offset. The displacements of the Conrad and Moho discontinuities are 6～7km and 3～5km, respectively. The sharp change in crustal thickness is maybe one of the causes of strong earthquakes in this area.

Key words: Wenchuan earthquake, Longmenshan Fault zone, crustal density structure, gravity inversion, Moho discontinuity

摘要：

研究龙门山及邻区地壳密度结构对于认识该地区地震活动性具有重要意义。根据龙门山及邻区(100°～105°E,28°～33°N)的布格重力异常资料,选取了跨越龙门山断裂带的6条重力测线,在深地震测深资料约束下,使用Geosoft软件分别反演出了龙门山地区地下的沉积层、康拉德界面和莫霍面的深度分布。研究结果表明:龙门山断裂带两侧的地壳结构明显不同,西面高原地区沉积层较薄,大部分为基岩出露;而东边盆地沉积层明显较厚,多在6km以上。莫霍面和康拉德面在两侧均相对平缓,康拉德面从东部的大约24km增加到青藏高原山区的35km左右;莫霍面深度从东部盆地的大约42km增加到西部青藏高原的67km左右。龙门山断裂带整体表现为一条近SN向的陡变重力梯度带,并在其地壳内各界面均发生错断,莫霍面和康拉德面错断距离分别达6～7km和3～5km。该区地壳的这种陡变和不均匀性是导致地震活动性强烈的主要原因之一。

关键词: 汶川地震, 龙门山断裂带, 重力反演, 地壳密度结构, 莫霍面

CLC Number:

P315.2

TANG Xin-gong, YOU Shuang-shuang, HU Wen-bao, YAN Liang-jun. THE CRUSTAL DENSITY STRUCTURE UNDERNEATH LONGMENSHAN FAULT ZONE[J]. SEISMOLOGY AND GEOLOGY, 2012, 34(1): 28-38.

唐新功, 尤双双, 胡文宝, 严良俊. 龙门山断裂带地壳密度结构[J]. 地震地质, 2012, 34(1): 28-38.

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