Research On The Crustal Structure Of Cascadia Sea Area By Receiver Function Method

The Cascadia sea area,which is located at the northeastern part of the Pacific,includes many active tectonic units,such as mid-ocean-ridge,fracture zone,oceanic plate,and the subduction zone.As a typical intermediate spreading center,the Juan de Fuca Ridge and the plate generated from it have unique characteristics in global oceanic lithosphere evolutions.In addition,the possible earthquakes in the Cascadia subduction zone pose a serious risk to the densely populated area in western North America.Therefore,studying the lithospheric structure in this sea area has geological and societal significances.This paper uses the passive-source seismic data recorded by the Ocean Bottom Seismometer(OBS)array in the Cascadia sea area,which was supported by the US“Cascadia Initiative”program and employs the receiver function method to investigate the crustal structure beneath this region.Due to the fact that OBS cannot achieve perfect coupling with the seabed and various types of noise caused by ocean activities,the quality of OBS data is relatively low,particularly for the horizontal components.In order to obtain valid receiver function,it is critical to improve the quality of OBS waveform by applying a few preprocessing techniques.First,we verified the horizontal orientation of the OBS.The conventional method determining the orientation of OBS is to utilize the polarization of Rayleigh waves.We use a different approach,which is based on the relative changes of Rayleigh wave and Love wave amplitudes on the radial and tangential components,respectively,to revalidate the horizontal orientation for each OBS.Secondly,the back-azimuth dependent,negative polarities of the P signature on receiver functions at some OBSs indicate that the seismometers may be tilted.We corrected the tilts of seismometers.In addition,we improved the signal to noise ratio of low frequency contents by removing the compliance noise on waveforms.The quality of receiver functions had been noticeably improved after those procedures.We calculated receiver functions at 15 OBSs along two profiles traversing the Juan de Fuca plate.214 earthquakes with magnitude greater than 5.5 and epicenter distance in the range of 30°to 90°were selected.By superimposing the receiver functions at each OBS,we obtained the average timing of the Moho converted phase Ps.We then applied the H-?method to reveal characteristics of crustal structure in the region.The results show that,from the Juan de Fuca Ridge to Cascadia subduction zone,the timings of Ps on the receiver functions at both profiles change systematically from 0.7 s to 1.8 s,suggesting the crustal thickness gradually increases.Consolidated sediments on the top of the crust are likely responsible for increasing crustal thickness as the plate aging.To further estimate the influences of the sedimentary layer on the receiver function in the region,we measured the time difference between the direct P wave on the vertical component and the slightly delated P_dS on the radial component due to conversion occurred at the bottom of sediment.These measurements indicate the thickness of sedimentary layer is negligible near the Juan de Fuca Ridge with the time differences close to 0.3s.Their influences on the receiver function,therefore,can be neglected;However,for OBSs near the subduction zone,the thickness of sediment layer is significant with the average time difference greater than 1.5s.Such large time differences suggest the sediment has a huge impact on the receiver functions.Based on these observations,we conclude that it is difficult to use conventional receiver function analysis to constrain the crustal thickness due to the thick sediments near the Cascadia subduction zone.