Characteristics And Extraction Of Typical Regional Seismic Phases

Seismic waveforms provide key information for the exploration of the earth's internal structure.Among them,the first arrival wave Pn at regional distanceand the surface wave with large amplitude have the characteristics of easy identification and high signal-to-noise ratio.Both of them contain rich information of the earth and have been applied widely.However,the information of Pn wave is not yet investigated sufficient.The surface wave imaging is mainly based on dispersion and amplitude between source-station and station-station,and the spatial coverage of ray path in earthquake area needs to be improved.In this study,we investigate more information from Pn waveforms to constrain velocity structure of crust-mantle transition zone.Rayleigh wave between seismic events is also studied and the reliability of results are discussed.In addition,we analyze the advantages and disadvantages of single-seismic phase and multi-seismic phase,and investigate the role of multi-seismic phase(crustal multiples)in the study of velocity structure and source parameters.In this study,we frst explore an effect of velocity-gradient structures in the CMTZ through synthetic Pn waveforms,and demonstrate that the shape of Pn changes from step-like to pulse-like when the CMTZ becomes from sharp to a gradient velocity structure,respectively.We then use Pn waveforms of the regional earthquakes(the July 2012 Mw 4.8 earthquake in east China and the September 2016 Mw 5.1 earthquake in South Korea)to examine the CMTZ structures to the west and east of the Yellow Sea and thus to constrain tectonic afnity between the eastern China and southern Korean Peninsula.The Pn waveforms show that the Moho in southern Yangtze Craton is sharp whereas there may be a gradient structure in CMTZ with a 6-10 km thickness in northern Yangtze Craton.The CMTZ in South Korea is also sharp,similar to the southern Yangtze Craton.Observed difference in CMTZ structures suggest that Sulu Orogenic Belt may extend along the north of the Gyeonggi Massif in Korea.Eastern margin of Tibetan plateau shows complicated deformation and active seismicity,providing important condition for the study of inter-source interferometry.Taking this area as an example,the Rayleigh wave empirical Green's function between seismic events is extracted,and the accuracy of the results are discussed.Synthetic tests show that when using stationary phase approximation,the azimuth angle between the station and the line of two events should not exceed 20 degrees.The observation that Rayleigh wave dispersion curves are divided into two branches with different speed may due to the error of event time and horizontal position of the earthquakes.Reliable Rayleigh wave empirical Green's function of inter-sources also depends on appropriate focal mechanism.Rayleigh wave from thrust earthquakes are relatively stable while a proper azimuth should be selected to avoid interference of strong S wave for strike-slip earthquakes.In addition,the comparison of cross-correlation functions in different azimuth shows that the instability of empirical Green's function between earthquakes may caused by the complex three-dimensional structure in study area.This study indicates that we can extract reliable inter-sources Rayleigh wave empirical Green functions based on seismic interferometry under suitable focal mechanism,station distribution,accurate event time and horizontal position of earthquakes in areas with good one-dimensional structures.For areas with complex three-dimensional structures,it is difficult to obtain an accurate result.Single-seismic phases play an important role in specific research issues according to their clear charicteristics and form the foundation for the study of multi-seismic phases.Appropriate combinations of seismic phases improve the spatial coverage of seismic rays which provide more constraints on source parameter and structure.The complex sedimentary structure and sparse station distribution in the Songyuan area increase the difficulty of determining the depth of moderately strong earthquakes.In this paper,we try to constrain the shallow crustal velocity structure and focal depth in this area by multiple seismic phase in crust.The preliminary results show that the focal depth of Qianguo earthquakes in the Songyuan area are distributed in the sedimentary layer or under the bottom of the sedimentary layer.Further researchs are preferred to take the complicated sedimentary structure in basin,variation of earthquake mechanism and crustal attenuation factor into consideration.In this study,the regional typical seismic phase Pn,Rayleigh wave and shallow crust multiple waves are analysed based on observed data and synthetics.We investigate more information from Pn waveforms to constrain the velocity structure of crust-mantle transition zone.The study of Rayleigh wave between earthquakes shows advantages and limitations of inter-sources interferometry which help us understand the method better.The analysis of multi-seismic phases provide a further idea for the study of earthquake and Earth's internal structure.