Strong ground motion characteristics of MS6.2 Jishishan earthquake,Gansu Province

Duan Xueliang; Xu Jiahang; Liu Yanqiong; Bian Zhu; Han Yi; He Jiacong; Peng Cheng; Zhang Long; Zhou Zhenghua

doi:10.11939/jass.20240020

Abstract：According to the China Earthquake Networks Center,a MS6.2 earthquake occurred in Jishishan County,Gansu Province,at 23:59 on December 18,2023,with an epicenter at(35.70°N,102.79°E)and a focal depth of 10 km.Studies indicate that the causative fault of this earthquake is a buried branch fault of the southern margin fault of the Laqi Ridge,active in the Late Pleistocene,with a strike of N311°W,a dip of N54°E,and a length of approximately 13.9 km.As of 8:00 on December 22,2023,the earthquake had resulted in 148 fatalities and three missing persons.According to seismic damage investigations,the Jishishan earthquake in Gansu Province exhibited the phenomenon of"moderate earthquake,severe disaster".To analyze this phenomenon in the event,this paper examines the spatial distribution characte-ristics,rupture directivity effect,hanging wall and footwall effect,response spectrum characte-ristics,and attenuation of peak ground acceleration(PGA),aiming to provide references for site selection in post-earthquake reconstruction and seismic design of engineering structures. In the Jishishan MS6.2 earthquake,a wealth of high-quality strong motion acceleration records were captured by China's National Strong Motion Observation Network and National Earthquake Early Warning Network,including 305 sets of three-component strong ground motion acceleration records(116 from bedrock stations and 189 from soil stations)captured by the Early Warning Network and 37 sets of three-component strong ground motion acceleration records captured by soil stations of the Strong Motion Observation Network. Firstly,the original acceleration time histories were processed.Due to factors such as instrument installation tilt and temperature effects,these original acceleration time histories exhibited zero drift.Therefore,zero offset correction was performed on the original accelera-tion time histories.After zero offset correction,acceleration time histories without zero drift for each station were obtained,totaling 342 sets.After excluding data from stations where the three-component PGAs are less than 10 cm/s2,170 sets remained.Using these data,spatial dis-tribution maps of three-component PGAs were drawn,and the spatial distribution characte-ristics of peak acceleration were reflected by comparing and analyzing them with the trend of the causative fault. Next,the directional effect of the Jishishan MS6.2 earthquake was investigated.Two sta-tions located in the forward and backward rupture zones of the causative fault,with the same site conditions and similar distances to the epicenter,were selected.The duration,peak values,and response spectra of the acceleration and velocity time histories of the two stations were com-pared and analyzed to reflect the influence of the fault rupture directional effect on the spatial distribution of ground motion.For the analysis of the hanging wall and footwall effect,two sta-tions located on the hanging wall and footwall,with the same site conditions and similar dis-tances to the epicenter,were selected.The duration,peak values,and response spectra of the acceleration time histories of the two stations were compared and analyzed to examine the influ-ence of the hanging wall and footwall effect on the spatial distribution of ground motion. Then,two nearby stations:one founded on soil and the other on bedrock were selected to compare their acceleration response spectra.By analyzing the spectral values and the length of the platform section of the acceleration time histories,the impact of site conditions on ground motion was elucidated.Finally,the ground motion of stations near the epicenter was analyzed.Stations within 20 km of the epicenter were selected,and the acceleration zoning of each sta-tion was obtained by consulting the China Ground Motion Parameter Zoning Map(GB 18306-2015).The design response spectrum parameters of the site where each station was located were obtained according to the Code for Seismic Design of Buildings(GB 50011-2010).A comparison diagram of the acceleration response spectra and design spectra of near-field stations was drawn,and the actual ground motion intensity of the near-fault area was analyzed by comparing the spectral values and the length of the platform section. The results show that:①Stations with any component PGAs greater than 10 cm/s2 are dis-tributed in the northwest direction,consistent with the distribution of the causative fault,indic-ating that the distribution of the causative fault significantly controls the spatial distribution of PGAs.②The rupture directional effect significantly influences the distribution characteristics of PGAs and PGVs near the fault.The PGVs recorded by stations located in front of the rupture are greater than those recorded by stations behind the rupture for all three components,espe-cially in the EW and NS components,reaching up to 3-4 times higher.③The hanging wall and footwall effect significantly affects the distribution of PGAs;the three-component accelera-tion response spectra recorded by hanging wall stations are greater than those recorded by foot-wall stations in the high-frequency range,indicating more pronounced high-frequency and short-period characteristics.④In the horizontal direction,different frequency bands exhibit different amplification effects on acceleration for rock and soil sites:rock sites exhibit signific-ant amplification in the high-frequency range,whereas soil sites exhibit significant amplifica-tion in the lower-frequency range.⑤The acceleration spectra observed at the epicenter signifi-cantly exceed the levels of rare and extremely rare earthquakes specified in current seismic design codes.This indicates that the actual seismic forces experienced by buildings may sur-pass their structural capacity to resist damage or prevent collapse,explaining the phenomenon of"moderate earthquake,severe disaster"in this event.