A Preliminary Study On Meta-instability Stage And Instable Section Of The Fault

The meta-instability stage refers to a transition that the stress of loading evolves from the peak stress into the onset of sudent stress drop.The meta-instability stage is the final stage before the fault reaches instability,and also the conversional stage of stress state from main accumulation to dominating release.It is a very practical but challenging scientific problem to identify the meta-instability stress state of the fault and to discuss the characteristics of the unstable part.In the laboratory,the meta-instability stage of fault has been identified,and the results of this study have laid the foundation for field identification of meta-instability stage.This paper is a preliminary exploration of the state of meta-instability stress from laboratory to field,the research results of instability in laboratory are combined with the analysis of the stress state before the actual earthquake.The evolution of the pre earthquake fault is studied,and the stress state of fault and synergy phenomenon are analyzed.We explore the characteristics of meta-instability stage of fault before strong earthquake,reveal its evolution mechanism,and give decision index.Strain is a physical quantity related to tectonic stress and earthquake occurrence.Benioff strain can quantitatively describe seismicity,can be used to analyze the characteristics of strain accumulation and release.It is often used to study whether earthquake acceleration or quiescence exists before earthquakes and can be analyzed in conjunction with laboratory strain observations.First of all,we selected the west section of the southern Tianshan fault in Xinjiang as the study area,where the strong earthquake is active and the stress accumulation leve is high.Through the 2008 Wuqia 6.8 earthquake,We study the temporal and spatial evolution of seismic activity and the Benioff strain characteristics of fault retrospectively,analyz the release section and accumulation section of fault,looking for fault in meta-instability stage and determining the location of earthquake.In this paper,the research method of sub stable fracture is given,and the present stress state of the study area is discussed.Secondly,we take the wenchuan Ms8.0 earthquake as a example,and study the variation characteristics of teleseismic converted wave to time difference due to medium changes before and after earthquakes.Thus,the stress state of the medium in the source region is analyzed.The main results of this thesis are summarized below:(1)We have referred to the fault in meta-instability stage as the meta-instability fault.Based on the research results of the stress state of the fault in meta-instability stage in laboratory,we propose an method for determination of the meta-instability fault with Benioff strain accumulation supports earthquakes above magnitude 7:?Benioff strain accumulation in tectonic zones supports MS ?7 earthquake occurrence.? The fault has high Benioff strain accumulation and the strain of it began to release.? The fault is divided into accumulation and release segment.We judge whether the fault in the Meta-instability stage and search for instability section according to the degree of collaborative activity.? Determining whether the Benioff strain conforms to the accelerating seismic patterns,analyzing the time urgency of the meta,instability fault.(2)In this paper,we analyze the existing problems and controversies about the Accelerating Strain Release method,improve the method of choosing the spatial range and time scale of earthquake sequences:the space range is selected along the fault,the time scales is taken the most recent accumulation-release periods for analysis.In laboratory,with the stress loading,the stress accumulation and release characteristics of the meta-instability stage appear mainly near the fault.The selection method has some structural and physical significance according to the research results of meta-instability stage in laboratory.(3)We take the Western section of Southern Tianshan as the study area,use of meta-instability fault method study on Wuqia Ms6.8 Earthquake on October 5,2008 retrospectively.The results show the Kazkeaerte fault have these 4 conditions before the mainshock,the prediction has achieved good results.We use this method to study the stress state in the study area,found the Kalpin fault satisfies the first three conditions of meta-instability stage characteristics.We should pay attention to the seismic activity of Kalpin fault.(4)A series of PS type converted waves are produced by using the first wave of teleseismic waves passing through the velocity interfaces such as the crust and the upper mantle.It is possible to monitor the variation of the physical properties of the medium in the limited formation space above the transition zone of the seismogenic zone by measuring the time difference between the teleseismic PS converted wave and the first arrival P wave ?tps=tps-tp.We selected 2001 to 2012 as the research period,use teleseismic seismic in Sichuan YZP and JJS stations record at the south of the Hindu Kush and sumatra,then obtained the change characteristics of converted wave to time difference Atps recorded during the study period in two stations.The results show that there is a slow increasing trend of delta TPS before 2006,and delta?tps appeared obviously low value process in the period of about 2 years before the Wenchuan earthquake.The maximum decline was about 0.2?0.3 s,more than 4?5 times the measurement error.The low value has a certain degree of return about 2?3 months before the earthquake.It is shown that the teleseismic converted wave method in this paper has a good prospect in seismic monitoring and is worth further experimental study.