壳幔物质流变的底辟作用孕育大地震——卫星重力资料的证据

根据2-180阶与2-360阶的数字全球模型恢复的卫星重力场数据,主要源于地下18~36 km深处的密度变化,多数大地震的震源也正是位于该层段。 因此卫星重力场资料对于地震研究有很大的意义。笔者详细分析了中国汶川、墨脱、仲巴、伽师和台湾等地的卫星重力资料(这些地区在近几十年来均发生过6级以上的大地震),惊奇地发现,在这些地区,卫星重力场的样式具有极大的相似性,它们表现在:(1)卫星重力场的正负异常值成对出现;(2)震中通常位于重力场峰值间的陡梯度带;(3)由低阶到高阶的重力场正异常的峰值增大而宽度变窄;(4)正、负峰值间的距离随所处的大地构造背景而变; (5)不同应力场壳幔物质底辟的样式不同,从地质上看,高密度壳幔物质的流变底辟作用,可以合理地解释这种独特的样式变化。文中提出高密度生长指数的概念,用以度量密度由深部到浅部的增加。岩石圈断裂带、深大断裂带及多层次滑脱层是壳幔高密度塑性物质底辟活动的优势通道。壳幔物质的非均质性反映了大陆板块构造动力学过程的复杂性。

Major earthquakes by rheological diapirism of crust mantle material—Evidence from satellite gravity data

Abstract: Satellite gravity field data recovered by digital global models with 2180 to 2360 order are mainly originated from the density variation in the depth of 1836 km, at which epicenters of most major structure earthquakes were located. Therefore, satellite gravity data can make great contribution to earthquake research. The author has made a detailed analysis of the satellite gravity data over Wenchuan, Motuo, Zhongba, Jiashi and Taiwan Areas in China, where earthquakes more than magnitude of 6 took place in recent several decades. It is surprising that the gravity field patterns over these areas are extremely similar, which is characterized by the following features: (1)Paired positive and negative anomaly. (2)Epicenters usually were located between the pairs where the gradient of gravity field was the steepest. (3)The value of positive peak increased and its width decreased at higher order field. (4)The distance between positive and negative peaks varies in different tectonic background. (5)Different styles of crust mantle material diapirism in different stress fields. Geologically, rheological diapirism of high density crust mantle material could be a reasonable explanation to this unique pattern. The proposed high density growth index is a measurement of increase in density from deep to shallow. The passageways of diapirism for high density crust mantle plastic material may be formed by the crust faults or giant fault zones and detached strata. The heterogeneity of crust mantle material reflects the complexity of dynamic processes in continental plate tectonics.