Study On The Characteristics Of Dynamic Triggering Of Small Earthquakes Activity In The Source Area Before Strong Earthquakes

Seismic stress triggering problem is one of the more current concerns in seismology,which mainly includes static stress triggering and dynamic stress triggering.Static stress triggering refers to the stress changes associated with an earthquake that can trigger or slow down the occurrence of earthquakes in the surrounding area;dynamic stress triggering refers to the seismic activity triggered by the propagation of seismic waves excited by the rapid rupture of a large earthquake to a distant location.The most direct and compelling evidence of dynamic stress triggering lies in the triggering effect of large earthquakes on far-field earthquakes.The dynamic stress triggering mechanism has not been fully understood yet,and relatively few studies on dynamic stress triggering were reported at home and abroad until the 1992 Landers 7.3 magnitude earthquake in California,USA,which became the first classical earthquake case to study dynamic Coulomb stress triggering,and since then dynamic stress triggering studies have been gradually carried out.Although dynamic triggering studies have been carried out for a long time,they only analyzed the change of small earthquake frequency in the long-distance region after the earthquake,and there is no conclusive analysis on the risk of future strong earthquakes in the region.In-depth study of the relationship between distant strong earthquakes and regional small-earthquake frequency changes can not only improve the dynamic stress triggering theory,but also provide important support for the development of future strong-earthquake hazard analysis.Based on the observation data from digital seismic stations in the North-South seismic zone of mainland China,this paper investigates the abnormal activity characteristics of the frequency of small earthquakes in the source area and the neighboring areas,based on the nine earthquakes of magnitude 6.5 or higher that occurred in the North-South seismic zone(N20°-40°,E93°-110°)since 2008.From the frequency of small earthquake activity in the neighboring areas of these nine earthquakes’ generation structures alone,no significant anomalous activity of seismic activity was found after the occurrence of a single distant earthquake of magnitude 7or higher.This paper adopts the idea of "superimposing" the effects of global magnitude 7 or higher remote earthquakes by zeroing in on the occurrence time of global magnitude 7 or higher earthquakes,and then superimposing the small earthquake catalogs of the selected mainshock source areas and neighboring areas.The small-earthquake frequency ratio was close to 1 for a long time before the mainshock,and its fluctuation was not large;however,the small-earthquake frequency ratio increased significantly 1 to 2 years before the mainshock;after the small-earthquake frequency ratio reached its peak,the small-earthquake frequency ratio decreased significantly until the mainshock occurred.The peak frequency ratio of small earthquakes does not appear at the same time in the cases with anomalous small earthquakes,which may be related to the magnitude of the mainshock and the regional tectonic background.The remaining reasons for the absence of small-earthquake frequency ratio anomalies are also discussed in this paper,which may be due to limitations in local seismic monitoring capabilities,incomplete records or missing data,or may be due to different gestation mechanisms of strong earthquakes,different tectonic environments or triggering factors,etc.,which make the anomalies difficult to identify.The "superposition" method can effectively highlight the anomalous activity characteristics of remote dynamically triggered small earthquakes in the source area and adjacent areas before the occurrence of strong earthquakes,by selecting different parameters for the calculation,mainly including lower magnitude limits and time scales,to exclude the possibility of anomalous "coincidental" corresponding earthquakes.The anomalies small-earthquake frequency ratio anomalies of each earthquake case are systematically summarized,and it is found that the magnitude of small-earthquake frequency ratio anomalies is related to different parameter choices,and the lower limit of small-earthquake magnitude and time scale of different earthquake cases are different.The results of the spatial scan analysis of the study area further verify the reliability of the remotely dynamically triggered small-earthquake activity anomalies in the source area and neighboring areas before strong earthquakes.On the basis of summarizing the small-earthquake frequency ratio anomalies in different earthquake cases,the possible mechanisms leading to the anomalous characteristics of small-earthquake frequency ratios are discussed,and the possible application prospects of dynamically triggered small-earthquake activities by remote in the future strong-earthquake hazard analysis are also foreseen.