Studies On Seismic Characteristics And Dynamic Mechanisms Of Great Earthquakes

The great earthquakes(M≥8.0) play significant roles on behalf of the devastating destructive in disasters so its prediction is the primary object in the study of tectonic earthquakes. Frequently great shocks have caused severe seismic hazard since2004. Study on seismic activity, especially on the great earthquake prediction, has become a focus in the research field of seismology.While the preparation and occurrence of great earthquakes requires specific geological and tectonic environment, including geological settings, tectonic environment, as well as geophysical conditions inside the Earth. Therefore, in this paper we try to numerically simulate some issues of the deep and shallow seismogenic environments based on the statistical studies of the characteristics of great earthquake activities.In this paper, the great earthquakes were studied in two aspects of seismcity and numerical simulation.①In the aspect of seismic activity, firstly, we quantitatively calculated the seismicity periods of the globe and the main tectonic zones. Next, the spatial characteristics of the worldwide great earthquakes were studied. Then, the similarities and differences of seismic activity prior to the great intraplate and interplate shocks were researched. Finally, we analysis the statistical features of the great earthquake sequences.②In the aspect of numerical simulation, we firstly studied on the2008Wenchuan M8.0earthquake from the aspects of regional stress and strain field, the geodynamic mechanics of high-angle reverse fault slip, the influence from the Zipingpu reservoir, and the Coulomb stress triggering from the strong earthquakes in the boundary zones of the Bayan Har block since1900. Next, we made analysis of the regional stress field characteristics and explored the impact of deep focus earthquake on great shallow earthquakes. Finally, we explain why the great shallow shocks with thrusting-slip rupture occurred in the area where the dip-angle of the Benioff zone is rather small.The results of the study on seismic activity show that①There are two principal periodic components,45.5-year and32.0-year, in the global seismicity. For the Circum-Pacific seismic belt, the principal period is45.5-year. The principal periodic components are30.9-year and47.5-year in the seismicity of the low-latitude circum-earth zone.②Most of the great shallow shocks are the type of thrusting-slip rupture, which mainly occurred in the subduction plate boundary zones. In the source region the dip-angle of the subdution plate is small and the movement direction of the subduction plate is approximately vertical with the trench. The great shocks with thrusting-slip rupture occurred in the contact area of the two plates, while the great normal-fault events occurred in the oceanic crust.③Two types of gap usually appear before the most of the great intraplate and interplate shocks. The doughnut-shape pattern with large scale often appear around the source region prior to great intraplate shocks, which is the long-term background anomaly. The spatial distribution of seismic activity shows the heterogeneous features and the seismicity of earthquake swarms is significant. While in the long term prior to the great interplate earthquakes, the notable anomaly is the seismic strengthening or the obvious quiescence of strong shocks. In the short term the frequency of earthquake swarms increase and some seismic patterns are formed. While in the middle-short term prior to the great shallow interpolates events, the strengthening of deep and intermediate-depth earthquake activity in the subduction zone, showing an increased frequency, magnitude and depth, is the special anomaly.④Most of the global great shallow shocks since1976are the type of thrusting-slip rupture and most of the sequences are the type of main shock-aftershock. The types of foreshock-mainshock-aftershock and multiple mainshock sequences show the feature of thrusting-slip rupture. Statistical records show that17earthquakes with Mw≥8.5have occurred since1900globally. Five of these were preceded by a foreshock of M≥7.0, accounting for29.4%. This is significantly higher than the proportion of foreshocks identified before moderate or strong earthquakes.The results of numerical simulation research show that①Under strong subduction of Indian Plate and the horizontal drag of the lower crust, the horizontal southeast-ward movement of the Bayan Har block was counterworked from the Sich basin and caused the differential uplift of the western Sichuan plateau relative to Sichuan basin. It provides an essential geodynamic condition for the M8.0Wenchuan earthquake on high dip-angle reverse fault. The influence of stress triggering on the Wenchuan shock from the earthquakes with Ms≥7.0occurred in the boundary zones of the Bayan Har block from1900is very weak. Earthquake preparation depends on the seismogenic fault's own energy accumulation in the background stress field, and stress triggering between earthquakes is only external cause. Earthquake intensity and distance between shocks are the main factors affecting the Coulomb stress triggering. In addition, we explain why the southern section of the Longmenshan fault did not rupture based on the research of stress triggering. The seismic source of the Wenchuan earthquake is in the field where the static Coulomb changes are positive when the Zipingpu reservoir storage, as well as are negative when the reservoir sluice. But because of the remoteness from the reservoir, the magnitude of about0.0021MPa is too small. So the reservoir has no significant influence on the Wenchuan earthquake.②Results based on the Coulomb stress calculation show that the fault easily generates thrusting-slip rupture when the dip-angle is about30°, yet the possibility that the thrusting rupture of the fault is small when the dip-angle is larger then60°or less than10°.③The lithosphere horizontal movement differences in the hierarchy(or the differential horizontal movement of the Mantle and the lithosphere) has great influence on topography.