| Parkfield segment of San Andreas fault is located in southern California,USA.It is a small segment of many segments of San Andreas fault,with a total length of about40 km,which is consistent with the general trend of San Andreas fault from northwest to southeast.Since the beginning of the record,there have been six earthquakes of magnitude 6 in Parkfield,namely,the Parkfield earthquakes in 1857,1881,1901,1922,1934 and 1966,which occurred once every 22 years on average.Park segment fault has active and regular seismic activity,and it is the fault with the most instruments installed in the world at present,so it attracts researchers’ attention and becomes the best place to study fault activity.In this paper,GNSS and InSAR observation data are used to study the crustal deformation characteristics of the Parkfield segment of the SAN Andres fault.Through the study of fault zone,we can further understand the lithosphere composition,crustal activity and deformation mechanism of the earth,which will lay a foundation for the study of earthquake prediction in the future and provide reference for the earthquake risk assessment.The main research contents of this paper include the following aspects:(1)In view of the problem that the existing researches on small-scale fracture are not sufficient and the details are easy to be ignored,this paper directly uses the processing strategy of small-scale short baseline length change to explore the relative motion on both sides of the fracture.Based on the global Navigation Satellite System(GNSS)observation data from 2001 to 2004,five baseline time series in the region were calculated.The time series results of baseline length change show that the Parkfield Mw0.6 earthquake has an effect on the baseline length,and the closer the baseline direction is to the fracture direction,the more sensitive the baseline length change is to earthquake.The crustal deformation in the study area is characterized by E-W stretching and NE-SW compression,which is consistent with the dextral shear in the EW direction of the Parkfield fault.(2)In order to improve the reliability and adapt to the small-scale characteristics of the Parkfield segment of the SAN Andreas fault,the strain tensor is calculated by dividing polygonal deformation elements based on the projected plane baseline,and the dynamic evolution process of deformation in the fault zone is studied.Based on GNSS observation data,a polygonal mesh method is proposed to calculate the surface strain parameters,namely,maximum shear strain,maximum surface expansion,first shear strain and second shear strain.The results of plane strain time series show that the strain parameters have abnormal trend of acceleration,locking and reverse acceleration before the earthquake,and the trend of periodic change is consistent with elastic rebound theory and concave-convex distribution.The significant change of strain parameters also shows that the dextran shear tectonic dynamic changes caused by the fracture in the Parkfield Mw0.6 earthquake.(3)In order to better reflect the crustal deformation characteristics in the fault zone,GNSS station velocity was used to analyze the relative movement on both sides of the fault,and the whole study area was used as a section to carry out slip velocity inversion.The results show that the strike-slip rate of the fault obtained by inversion is 31.22±4.6mm/a,and the deformation transformation zone is between 2km on both sides of the fault,which conforms to the characteristics of fault creep motion and is in good agreement with the results in other literatures.Therefore,the Parkfield section of the SAN Andres fault presents a trend of dextral sliding.(4)Based on InSAR data from 2019 to 2021,p S-INSAR technology was used to extract the LOS-direction deformation rate of the fault zone,and then the slip rate inversion was carried out.The fault slip rate obtained by inversion was 27.35±3.2mm/a,which was verified by comparison with GNSS results.The two results have good consistency.According to InSAR and GNSS results,the Parkfield section of the SAN Andreas fault presents a trend of dextran creep,with small accumulation of strain energy and continuous dislocation and slow sliding in the stress process.The seismic risk is relatively low,but it should still be considered that in the context of the Peristaltic section of the SAN Andreas fault,The possibility of an earthquake in the Parkfield section.There are 22 figures,5 tables and 96 references. |
