Seismic Corner Frequency Inversion And Its Influencing Factors In Southwest China

Source parameters are quantitative descriptions of earthquake source characteristics,and their accurate inversion is important for understanding the causes of earthquakes and for subsequent earthquake warning.As two important seismic source parameters,the inflection frequency and seismic moment directly affect the source spectrum model,so their accurate inversion and discussion of the correspondence between them is an important task in seismic research.Due to the influence of the signal-to-noise ratio of seismic waves,propagation path effects,differences in source mechanisms and source processes,the results of the traditional spectral ratio method of inversion of the corner frequencies are inaccurate and discrete.There are some problems in establishing the correspondence between the corner frequency and the seismic moment,and there are relatively few explanations of the intrinsic mechanisms affecting the correspondence between the two,and there is a lack of analysis of the factors in the rupture of the seismic source.In this paper,we introduce the Bootstrap resampling method,the Kolmogorov-Smirnov test and the screening method of the spectral trend test to evaluate and filter the accuracy of the inversion results of the spectral ratio method and improve the accuracy of the inversion of the inflection frequency by the spectral ratio method.This method is applied to the inversion of seismic source parameters such as the inflection frequency,seismic radiant energy and energy-moment ratio of earthquakes in southwest China,and the reliability of the spectral ratio method and the three screening methods is verified.The results of the inversion of the inflection frequency and seismic moments are fitted to obtain the correspondence between the inflection frequency and seismic moments in southwest China.Finally,the spectral boundary integral method is used to construct a fault rupture model,and the effect of different fault rupture parameters on the correspondence between the inflection frequency and seismic moment is analysed by calculating the far-field displacement and the inflection frequency from the source rupture.The main conclusions are obtained as follows:（1）the correspondence between the inflection frequency and seismic moment in southwest China is f_c∝M₀^{-1/3.205±0.25}.This result is not consistent with fc∝M₀^-1/3 under self-similarity conditions.the study in terms of seismic radiant energy finds that the seismic energy moment ratio in southwest China increases with increasing earthquake magnitude.This indicates that large earthquakes are more efficient radiators of seismic energy compared to small magnitude earthquakes.（2）By calculating the inflection frequency values for different aspect ratios of faults with the same seismic moment and fault area,it is found that the inflection frequency values gradually decrease as the aspect ratio of the fault increases.The seismic inflection frequency value decreases from 0.3 to 0.24 as the aspect ratio increases from 1:1 to 6:1.（3）By studying the variation of the calculated inflection frequency for different earthquakes with a constant seismic moment,due only to the change in fault rupture velocity.It is found that the magnitude of the rupture velocity will affect the high-frequency as well as the low-frequency components of the source spectrum,resulting in a change in the calculated values of the seismic corner frequencies.An increase in rupture velocity leads to a high peak but short duration in the seismic source time function.The corresponding far-field displacement results are richer in low-frequency components and less in high-frequency components,leading to increased values of the corner frequencies.（4）By considering three different rupture initiation locations and calculating the corner frequency values for different seismic moments in the magnitude range of 4.0-6.0,the effect of different rupture initiation locations on the correspondence between the two is investigated.It is therefore inferred that the different rupture initiation locations may be one of the reasons for the self-similar deviation in the correspondence between rupture frequency and seismic moment in southwest China.