Estimation Of Source Parameters And Site Response Using Strong Motion Records From Lusahn Earthquake

Lushan Earthquake is another destructive earthquake in Lushan county, Sichuanprovince, China mainland on Apirl20,2013following the Wenchuan Earthquake.Many aftershocks have occurred after the main shock, and more than1000groupaccelegrams have been collected by the National Strong Motion Observation NetworkSystem in the Lushan Earthquake sequences. Based on the generalized inversion ofthe S-wave Fourier amplitude spectra of179recordings with the hypocenter from25to70km in30earthquakes with magnitude from3.5to5.4triggered by15fixed andmobile stations and selecting one aftershock as the reference event, the effects ofsource, site and propagation path on strong motion have been divided.(1) The source displacement spectra obtained from inversion were analyzed toestimate various source parameters using grid search method, including seismicmoment, moment magnitude, corner frequency, stress drop, source radium, seismicenergy and apparent stress for each event. It is shown that the mean value of M0fc3isequal to1.7298×1023dyne·cm·s-3,and the average stress strop is equal proximately to3.14Mpa, accordingly, lower than the moderate aftershocks in Wenchuan earthquake.Stress drop is not related to the magnitude and the focal depth, while, the stress dropis much higher on the edge of asperity and barrier on the fault plane. The averageratio of S-wave energy and seismic moment is3.01×10-12, and the stress drop andapparent stress is positive correlation. By fitting the moment magnitude and thesurface wave magnitude, we find the moment magnitude is lower for the smallaftershocks, but higher for the moderate aftershocks.(2) In the frequency range from0.5to20Hz, the frequency-dependent qualityfactor Q(f) in this region is found to be31.867f1.0375, lower than the other regions inSichuan, which is consistent with the result obtained by the code normalizationmethod. Quality factor in this paper is stable and we confirmed the uncertaintyresulted from the shear wave velocity for the quality factor can not been considered.(3) Comparing the site response inverted by this paper with results obtained byH/V spectra ratio method and one-dimension method, we find consistent peakfrequency and different peak amplitude, the other two method underestimate the siteresponse. The topography have the major effect on the site response for thecomplicated topography area, while the VS30locates in the secondary position. Wealso estimated the Kappa value and the sediment depth using recordings, and theeffects on site response has been confirmed. Nonlinear site responses have been revealed in the aftershock and main-shock larger strong motion recordings, comparedwith the site response from the smaller strong motion recording, site response becomehigher for the aftershock, while, consistent in the low frequency and lower in the highfrequency for the main-shock.This paper will be useful for the simulation of strong ground motion, groundmotion prediction equations, seismic hazard analysis, seismic zonation maps and soon.