山西原平4.2级地震前的重力场异常特征

Characteristics of Gravitational Field Anomalies prior tothe Yuanping M4.2 Earthquake in Shanxi

摘要:

利用晋蒙重力测网2007—2009年的流动重力资料，研究了原平4.2级地震前的重力场变化特征。结果表明，以4.2级地震震中为界，流动重力呈现出东西向的负条带异常，而南部、北部均上升（正异常）的特征，震中附近的大部分测点点值在震前均出现上升—下降—恢复的变化特征，地震发生在重力恢复阶段。在远离震中区的内蒙重力测网并没有出现上述重力场变化特征。

关键词:原平4.2地震; 晋蒙地区; 重力场变化; 孕震体膨胀

Abstract:

On March 28，2009，an earthquake of M4.2 occurred in Yuanping in Shanxi Province that affected people’s residences 200 km the epicenter area.The epicenter was at middle part of Yiding Basin，which is a horseshoe-shaped extensional basin，and the edge faults of the basin include the northern Wutaishan，northern Xizhoushan，eastern Yizhongshan，and southern Henshan faults.In this study，we collect the gravity data of the Portable Gravity Observation Network from 100 observed points distributed in the Shaxi-Inner Mongolia region during 2007—2009.The values of gravity field variation with time are calculated in terms of six months and one year at every observed point，and values at every network point are obtained by interpolation with kriging and trend extrapolation methods in that region. Analysis of gravity variation characteristics prior to the M4.2 Yuanping earthquake shows that the gravity field data in the epicenter area exhibited negative banded anomalies in the E-W direction before the M4.2 earthquake and those positive anomalies appeared both north and south of the epicenter zone.The observed gravity field values show characteristics of ascent-descent-restitution at most observation stations near the epicenter.The Yuanping earthquake occurred during the restitution period.The varied characteristics of the gravity field described above did not appear in gravity observation points in the Inner Mongolia region far from the Yuanping epicenter zone.The Yuanping epicenter is located in a negative anomaly zone of the Bouguer gravity gradient and in a negative gradient anomaly zone of variation of Bouguer gravity，which corresponds with the northern Wutaishan Fault.This phenomenon implies that a corresponding relationship existed between gravity changes and tectonics in the study area Due to media inhomogeneity of the earth，earthquakes cause different areas to exhibit varied values of media density，which is generally more obvious near the faults.It is widely accepted that variation in the gravity field can be deduced from changes in media density and can be observed at the surface.Seismogenic body impending is a basic physical process of seismogenic theory.During the process of seismogenic body impending，media density in the focal zone is reduced；therefore，the gravity field value would decrease at the Yuanping epicenter.This reduction in media density may have caused the gravity field values observed at Yuanping epicenter prior to the earthquake occurrence.In addition, the epicenter is generally near the largest gravity change zone between ascending and descending variations.Moreover，the epicenter does not coincide with the largest position change of gravity field variation.These results support that the shape of the Bouguer gravity field gradient varied with time before the earthquake occurrence according to the seismogenic body impending theory.

Key words:Yuanping M4.2 earthquake; Shanxi-Inner Mongolia region; gravity field anomaly; seismogenic body impending