Ground-motion Prediction Equations For The Vertical Component From Subduction Zone Earthquakes In Japan

Ground Motion Prediction Equations(GMPEs)is an important part in probabilistic seismic –hazard Analysis and determining the seismic loads and resistance levels in structural design.In the past,the study of GMPEs is mainly concentrated on the horizontal component.However,in recent years,the earthquake observation shows that the strong vertical ground motions will cause as serious damage on the building structure as horizontal.The study of vertical ground motion is seriously lagging behind.Due to the large difference between the attenuation characteristics of vertical and horizontal ground motion,it is unreasonable to use the simple relationship to describe the ratio of vertical and horizontal response spectrum in many design codes.It is imminent to establish the GMPEs for vertical ground motions.The subduction zone has complex geological conditions,not only the common crustal earthquakes,but also other types of earthquakes such as subduction slab and subduction interface.Strong earthquakes observation station in Japan are densely,and a large number of strong motion records have been obtained in recent years.It is reasonably practicable to use the abundant strong motion records in the subduction zone of Japan to establish a high quality GMPE for vertical ground motions.In this paper,4710 records of 136 subduction slab earthquakes,5647 records of the 89 shallow crustal earthquakes,and 1988 records of 47 subduction slab earthquakes,were selected,which were obtained from Ki K-net and K-NET since 1968 to 2012.GMPEs for vertical component were established form these records.Two models of for different earthquakes categories was presented:(1)the subduction slab model,(2)the shallow crustal and the upper mantle model.We referred and followed the previous study Zhao2016 models of horizontal component for subduction zone,contrasted and selected three reasonable seismic catalogues to determine the Focal location information,and used the Slab1.0 subduction geometric model.Earthquakes were typically divided into four categories with different tectonic locations.According to the characteristics of ground motion of subduction zone,the influence variables of each effect were discussed and compared respectively.The significance test and the AIC rule were used to determine the applicability of the model parameters.Regarding source effect,we used a bilinear magnitude scaling rate hinged at a moment magnitude(M_W)of 7.1 to describe the magnitude effect.The fault mechanism and the depth terms were considered as source parameters.Regarding path effect,the method of separating the geometric attenuation rate and the anelastic attenuation rate were adopted.The influence factors of the near-field or far-field path effect and the special properties of the deep rock in the active volcanic area were took into account.The correction parameter terms to describe these effects were used.Regarding site effect,we computed site period from shear-wave velocity profile and the engineering bedrock depth.Four site classes(SCs)based on site period were used and an independent constant term was selected for each site class.In this paper,the random effect method was used to model regression.The two sets of records were selected to determine the model coefficients.The first set contained as many large earthquakes as possible to determine the large earthquakes and the second set excluded the records with inferred site information to get more accurate model coefficients.The magnitude of the records was chosen as M_W≥4.9,and the Magnitude-dependent distance truncation was used for all records to avoid possible effects from untriggered stations and 300 km was chosen as the maximum distance.Finally,a prediction model was established for calculating the acceleration response spectrum at 5% damping ratio over 37 spectral periods(including PGA)in the range of 0 ~ 5.0s.We proposed a smoothing method to deal with the regression parameters of the attenuation model.The method was based on the order of the sensitivity of parameter and the process of considering the random residuals.The model parameters are smoothed in a certain order.Finally the smooth model parameters and predictive response spectrum was obtained,and the divided model standard deviation was given.Depending on above result and the comparison with several groups of GMPEs,we drew the conclusions as follow:1.The vertical attenuation characteristics of the three categories of earthquakes are quite different concerning the model parameters and the response spectrum prediction values.The random residuals and standard deviation of the two groups attenuation models are also different.These results indicate that it is reasonable to separate the three categories of earthquakes and establish the attenuation relationship using the two groups of attenuation models.2.Compared with the horizontal-component model of Zhao2016,there are many differences between the vertical and horizontal ground motions in source effect,path effect and site effect.The standard deviation of vertical-component model from the site effect is significantly smaller than the horizontal-component model in the long periods.Horizontal-to-vertical(H?V)spectrum ratio is a complex function affected by multiple parameters.The H?V spectrum ratio is further reduced by the conditions of near field,large earthquake,soft soil and others.H?V spectrum ratio in the short period of 0.04 s ~ 0.06 s gets the minimum,in this condition the vertical ground motion is quite obvious.3.Compared with the SBSA16 model,more comprehensive parameters such as depth term,near field and far field path attenuation rate terms were taken into account in the model of this paper.