Moment Tensor Inversion And Rupture Process Of Earthquakes With Obvious NDC Components

Accurate seismic moment tensor and focal depth and other source parameters are the important basis for assessing earthquake disasters,analyzing active structures,and studying volcanic processes.The greater the magnitude of an earthquake,the more serious the damage will often be.Studying its source characteristics and rupture process will help to further analyze the causes of earthquakes and explore the potentials of destructive earthquakes.The typical tectonic earthquake mechanism often conforms to the double-couple source model,but some earthquakes often have non-double-couple components,which cannot be represented by a simple plane fault model.The complexity of non-double-couple earthquakes,such as nuclear explosions,mine collapses,and volcanos,as well as the complexity of ruptures such as complex fault geometries and non-planar ruptures,have important indications for understanding the cause and mechanism of earthquakes.Compared with large earthquakes(Mw>7.0),there arc more moderate-strong earthquakes(5.5<Mw<7.0),and sometimes the damage is serious as well.Some moderately strong earthquakes have obvious non-double-couple components,which are different from typical tectonic earthquakes.Such source processes may also contain some degrees of complexity.Determining the causes of non-double-couple earthquakes are also difficult in seismic source research.The proportion of non-double-couple earthquakes is about 10%of moderate-strong earthquakes(Mw>5.5),and traditional methods are often not effective in distinguishing their source composition.With the deployment of digital broadband seismic stations and submarine seismometers,more and more seismic source parameter studies are carried out by means of a combination of multiple data and multiple methods.Thousands of broadband digital seismographs around the world usually have good azimuth and epicentral distance coverage at the teleseismic epicentral distance(30-90°),and the seismic body wave signals have a high signal-to-noise ratio and are often used to invert for the source parameters.For moderate-strong earthquakes,the layout of seismic stations in some areas is still relatively sparse.Using only local or teleseismic stations cannot accurately determine the moment tensor and focal depth of the earthquake.At that time,it is necessary to perform joint inversion to improve the accuracy.This paper firstly uses the joint inversion of local and teleseismic data to determine the full moment tensors of moderate-strong earthquakes.For moderate-strong earthquakes,such as the Jiuzhaigou earthquake in Sichuan in 2017,the Kaohsiung earthquake in Taiwan in 2010,and the Mw6.6 earthquake in Pamirs in 2016,the corresponding local and teleseismic joint inversion of source parameter solutions were tested.It is found that the focal depth and moment tensor of an earthquake cannot be effectively measured for only local or teleseismic waveforms and the deviation is about 5°.The use of joint inversion can effectively improve the reliability of the determination.When the weights of the local and teleseismic waveforms are different,the moment tensor changes more obviously,and the focal depth also changes significantly.The overall results are more reliable when the error weight of the local and teleseismic waveforms is equal.The rupture process of a large earthquake is more complicated,and it is often not explained by a simple point source.At this time,some earthquakes with obvious non-double-couple components may have multiple seismic sub-even ts.The rupture process of an earthquake with multiple sub-events can be studied using multi-point source simulation or finite fault inversion.According to the initial point source parameter solution,it can be preliminarily judged whether the non-double-couple component of the earthquake is obvious,and then according to the multi-point source inversion,the number of point sources of the rupture,the location,and the depth of the seismic processes,etc.,are obtained.Finally,based on the result of the multi-point source,the finite faults inversion of multiple fault planes determines the slip process of the earthquake.This paper studies the rupture process of the 2009 North Andaman Mw7.5 earthquake and discusses the cause of the non-double-couple component of the earthquake.The North Andaman earthquake has obvious non-double-couple components(>60%).Through multi-point source inversion,this earthquake has four sub-events,two more obvious strike-slip events in the shallow part and two more obvious normal event in the relatively deep part.Finite fault inversion shows that the strike-slip of the earthquake is mainly in the shallow part of the subduction interface,which is different from most subduction zone earthquakes that are thrust earthquakes.In addition,the deep slip may be closely related to the subduction interface,and the shallow slip and the turning effect of the subduction boundary jointly affect the occurrence of subsequent normal fault events.