希腊克里特MW6.0地震后的应力方向变化与强余震发生

Stress direction change and strong aftershock occurrence after the M S6.0 earthquake in Crete,Greece

地震的震源机制是地壳应力变化的指示器,而地壳应力变化与强震的发生直接相关。前人研究了地震震源机制变化在视应力较高的走滑型大震前的应力变化过程,而未见到震源机制变化对视应力较低的正断型大震发生的指示作用的研究。文章以2021年希腊克里特M W6.0正断型地震序列为例,通过计算地震序列震源机制解与区域应力场方向之间最小空间旋转角的变化,揭示应力变化与强震发生的关系。为保证震源机制解的准确性,采用多家机构确定的震源机制得到中心震源机制作为该地震的震源机制,而后采用该地震序列精确的震源机制求解当地应力场,最后计算地震震源机制与主震震源机制及与所估计的地壳应力场方向的空间旋转角随时间的变化,探索强震发生与应力场变化的关系。结果表明:在主震发生的短期内,余震震源机制与该区域应力场方向的空间旋转角较大,与其后小震级的弱地震活动对应;随后余震震源机制与应力场方向的空间旋转角减小,对应后面发生的3次M W>5.0的强余震,在此之后的长时间内余震震源机制和应力场方向的空间旋转角再次增大,对应的余震震级及频度皆明显下降。文章以2021年希腊克里特M W6.0地震序列为例,发现视应力较低的正断型地震前也存在应力方向集中现象,为探索地震应力前兆提供了范例。

The focal mechanism of earthquakes is an indicator of the variation of crustal stress,which is directly related to the occurrence of strong earthquakes.To date,no previous study has investigated the indication of focal mechanism change to the occurrence of normal-fault earthquakes with low apparent stress.This paper takes the 2021 M W6.0 earthquake sequence in Crete,Greece,as an example to determine the relationship between stress variation and the occurrence of strong earthquakes by calculating the minimum spatial rotation angle between the earthquake sequence's focal mechanisms and the direction of the regional stress field.The focal mechanisms determined by multiple agencies were used to obtain the central focal mechanism as the focal mechanism of the earthquake,thus ensuring the accuracy of the focal mechanism.Then,the local stress field was determined by the earthquake sequence's accurate focal mechanism.Finally,the spatial rotation angle of the crustal stress field was calculated,and the relationship between the occurrence of strong earthquakes and the variation of the stress field was explored.Results indicate that the spatial rotation angle between the focal mechanism of aftershocks and the direction of the regional stress field is large in the short term after the mainshock,thereby corresponding to the weak seismicity of subsequent earthquakes with small magnitudes.Consequently,the spatial rotation angle decreases,corresponding to the occurrence of three strong aftershocks(M W>5.0).After that,the spatial rotation angle increases again for a long time,and the magnitude and frequency of the corresponding aftershocks decrease.Using the same 2021 M W 6.0 earthquake sequence as an example,the results reveal that the stress direction concentration also exists before the normal-fault earthquakes with low apparent stress.Therefore,this study provides an example for the exploration of earthquake stress precursors.