Estimation of Source Spectra, Attenuation, and Site Responses from Strong-Motion Data Recorded in the 2019 Changning Earthquake Sequence

Abstract

ABSTRACTThis study used 306 accelerograms recorded at 22 strong-motion stations to investigate the source parameters, quality factor (Q), and site effects of S-wave Fourier acceleration amplitude spectrum (FAS) of the 2019 MS 6.0 Changning earthquake sequence in China with surface-wave magnitudes (MS) of 4.1–6.0. The generalized inversion technique (GIT) was adopted. The inverted stress drop of the mainshock was 1.15 MPa, and those of the aftershocks varied from 0.11 to 1.04 MPa with an average value of 0.43 MPa. The MS of these earthquakes were larger than Mw with an average magnitude difference of 0.22. The inverted Q values increase rapidly with frequencies at 0.5–4.0 Hz from 62 to 2920 and become less dependent at 4.0–25.0 Hz. Such a phenomenon indicates that the propagation path attenuation mechanism transited to intrinsic at high frequencies. A bilinear Q(f) model for which Q(f)=237.6f1.27 (Q<1280) and Q=1280 at higher frequencies was obtained. The high-frequency attenuation model of the study area was κ=0.0420+0.0001262R. The inverted site responses of the 22 stations were compared with those calculated using the horizontal-to-vertical spectral ratio (HVSR) method. In general, the amplification curves of most stations obtained with the GIT were similar to those of HVSR, and the amplification levels were relatively higher. Contrarily, obvious discrepancies were observed between the results estimated from the two methods at several stations. Such effects were attributed to the limitation that the majority of the stations were distributed along the boundary of the basin and mountainous areas, and the inverted Q values were representative of the specific area rather than the pure basin and mountainous areas. Finally, a nonlinear soil site effect was observed at 51GXT in earthquakes with peak ground acceleration greater than 300 cm/s2. The nonlinearity obviously aggravated the site amplification at 1.0–5.0 Hz.