Spatial And Temporal Variation Characteristics Of B-value Before Strong Earthquakes And Their Significance For Earthquake Prediction

Earthquake is a major natural disaster faced by human beings,and effective earthquake prediction is an important guarantee to mitigate earthquake disasters.Some earthquake-prone countries such as China have carried out planned research on earthquake prediction,and although great progress has been made in related research in China and abroad over the past 50 years,earthquake prediction is a worldwide scientific problem that has not yet been solved.The reason is that the physical process of earthquake inucleation and genesis is extremely complicated,and the source of strong earthquakes is located deep underground,which is not directly observable by humans at present,so it is impossible to use the same method as numerical weather prediction for earthquake prediction today.Therefore,the exploration of earthquake prediction based on statistical methods of seismic activity is still an effective means of making earthquake prediction at present.The frequency and magnitude of earthquake is expressed quantitatively by the Gutenberg-Richter relationship in which an important parameter is the b-value.The results of previous studies show that the b-value not only portrays the quantitative relationship between different magnitude earthquakes,but also its magnitude is negatively correlated with the regional stress level;In particular,the b-value shows obvious precursor anomalies before some large earthquakes(e.g.,Tangshan earthquake,Haicheng earthquake,etc.)and is an important parameter widely used in earthquake prediction.However,the 2008 Wenchuan Mw7.9 mega-earthquake was not found to have any precursor anomalies in b-value before the earthquake.Based on the previous work,the b-value calculation strategy was modified by learning from the experience of previous b-value calculations:the western Sichuan plateau and the Sichuan basin area(surrounded by the epicenter of the Wenchuan earthquake)were divided into several circular subregions with a radius of 50 km,and the evolution of b-values over time from 1998 to 2008 before the Wenchuan earthquake was calculated in each subregion.It is found that only the subregion containing the epicenter of the Wenchuan earthquake shows the precursor anomalous features of b-value,while all other subregions do not.Although this is a retrospective test of the Wenchuan earthquake anomaly,this method of b-value time and space scanning can be used in future prediction studies of other large earthquakes.On the basis of this,the study also calculated the b-value temporal and spacial scans for the 2011 Tohoku-Oki Mw9.0 earthquake in Japan,and found that the overall b-value before the main earthquake was slightly lower compared to that of the Wenchuan earthquake.Spatial and temporal low b-value anomalies were observed before some of the larger magnitude earthquakes,and were accompanied by earthquake clusters.Before the mainshock,the b-value shows a decreasing pattern over time for about 8 months until the mainshock occurs.The spatial distribution of seismic activity and b-value for each year shows that the b-value is lower in the region where the earthquake cluster occurs,and the b-value drops to the lowest level within a decade in the region where the Mw9.0 mainshock and the largest foreshock occur,further confirming that the low b-value region corresponds to the part of the high stress state and the strong coupling of the overlying plate.Therefore,the spatial and temporal variation characteristics of this b-value can reflect the stress variation before a strong earthquake and provide an important reference for strong earthquake prediction and seismic hazard assessment.