Research On Seismic Intensity Assessment Method Based On Aftershocks And Ground Motion Prediction Equations

Seismic intensity must be considered in earthquake disaster prevention and mitigation.Reasonable estimates of intensity distributions and ground motion values from large historical earthquakes are useful references for seismic hazard fortification and historical disaster research,among other purposes;making full use of postearthquake seismic station records for rapid intensity assessment is critical for effective emergency response.Currently,intensity decay relations based on epicenter distance are widely used in the technical systems of the earthquake industry in China,among which the elliptical intensity decay relation is the most commonly used.Although this empirical model could rapidly provide results for assessing the extent and degree of damage in seismogenic zones,it suffers drawbacks when applied to earthquakes with a large magnitude.The estimation of the extent of each intensity zone is excessively regular and is difficult to reflect the effects of a large earthquake surface rupture zone;providing fine ground motion distribution data is difficult,and the accuracy of human casualties and disaster damage assessed based on the results will be limited.The method of estimating the seismic intensity of historical earthquakes is relatively simple.Currently,the assessment technology for disaster information services during the earthquake emergency phase lags behind the country’s progress in station network construction and data accumulation.In response to the abovementioned research and application status,this study introduces a ground motion prediction equation based on fault distance and source distance into the historical earthquake intensity assessment.We propose and implement a method and process to use the geological survey data or aftershock data of large historical earthquakes to project the seismic intensity,which can be used to estimate the intensity distribution of historical earthquakes more accurately and rapidly.Inspired by this study,the feasibility of using early aftershock sequences to assess seismic intensity during the earthquake emergency phase is discussed,and a method based on aftershock data within 2 h post-earthquake is developed.Conditions for using the method are summarized by applying it to multiple earthquake cases,and its robustness and time efficiency are discussed.The proposed method is enhanced by combining the results of a previous study to resolve its shortcomings of application to moderate-to-strong earthquakes.The intensity distribution of moderate-to-severe earthquakes can be evaluated with higher precision by performing a buffer zone analysis on the fitted curve of the spatial distribution of aftershock sequences.The following are the main conclusions of this study:(1)In the estimation of maximum intensity values of large historical earthquakes,elliptical intensity decay relations can provide a reference for the determination of maximum intensity values,but they cannot provide a more detailed ground motion or intensity distribution data of the region and are easily limited in use by magnitude and region;additionally,the estimation results of different elliptical intensity decay relations for a same earthquake case may considerably vary.Ground motion prediction equations based on the shortest fault distance or equivalent hypocentral distance could be used to estimate the intensity distribution of large historical earthquakes,producing relatively fine-grained ground motion distribution grid data.Generally,the ground motion prediction equation based on the shortest fault distance is more suited for the intensity estimation of large historical earthquakes in China.When a historical earthquake has already been thoroughly researched and investigated,the seismic intensity can be calculated by intercepting the corresponding length of trace on the fault closest to the epicenter on the basis of the rupture described in the literature.Without sufficient seismic data,the corresponding traces can be intercepted using the empirical formula of surface rupture,and the seismic intensity can be assessed using the default mode of the symmetric rupture centered on the epicenter.Furthermore,historically calibrated and reliable strong aftershock data can be used to estimate the intensity distribution of large historical earthquakes.(2)The spatial distribution of the aftershock sequence in the immediate aftermath of an earthquake represents basic information,such as the length and direction of the mainshock rupture.Within 2 h of an earthquake,the aftershock sequence can be used to assess the seismic intensity and provide a more accurate estimate of the intensity distribution.The curve fitted to the geographic coordinates of the early aftershock sequence using robust locally weighted regression(r Lowess)can more accurately represent the length and direction of the surface rupture in the case of a simple and welldefined fault system in the seismogenic region.When the fault system is complicated,the method can also accurately assess the overall rupture trend and scale.The r Lowess curves are always slightly longer than the actual surface rupture,indicating that aftershocks that occurred at some distance from the fault shortly end after the earthquake.Aftershocks frequently exacerbate the damage in the disaster area,resulting in more serious casualties and property losses.The intensity distribution,as determined by the results of the spatial distribution analysis of early aftershock sequences,can reveal areas with serious casualties and economic losses,allowing for the rational allocation of rescue forces.The proposed method of assessing seismic intensity based on early aftershocks possesses a higher potential for use in real-time or near-real-time intensity distribution assessment.(3)The incorporation of geographical information system(GIS)spatial analysis into the early aftershock data preprocessing stage effectively addresses the shortcomings of the proposed method in assessing the intensity of moderate-to-strong earthquakes.As a nonparametric method,RLowess is well suited to analyzing events such as aftershocks caused by complex geophysical processes,for which no reliable theoretical model is currently available.Ground motion or intensity distributions estimated using earthquake intensities based on early aftershock assessment or improved methods thereof can be applied to test the accuracy of source rupture processes traced by geophysical means(for example,back-projection methods).We believe that this study expands the use of GIS spatial analysis in disaster information services,and the intensity assessment results can open up novel avenues for determining the location of seismogenic faults,studying earthquake-radiated energy,and simulating the relationship between seismogenic faults and aftershocks.