唐山4.8级地震前后视应力变化研究

Variations in Apparent Stress Prior to and after theTangshan MS4.8 Earthquake

摘要:

2012年5月28日唐山发生MS4.8地震，之后宝坻发生MS4.0和MS3.5地震，这三次地震是近十年来首都圈地区较显著的事件。本文利用首都圈数字化地震台网2011年1月至2013年6月唐山地区ML≥2.5地震的宽频带数字地震波资料计算地震视应力。首先采用傅里叶变换获得地震观测谱，扣除仪器响应后进行非弹性衰减和几何扩散校正，几何扩散采用Gail提出的震中距分段模型，用随频率变化的Q值来表示非弹性衰减；然后在Brune圆盘模型下利用遗传算法求出一组fC（拐角频率）和Ω0（零频极限），使得理论震源谱和校正的观测谱残差最小；通过零频极限求得地震矩M0，通过有限仪器带宽补偿并加上积分已校正的观测谱求得辐射能量ES；最后即可求出视应力。计算结果表明：MS4.8地震视应力表现出震前趋势上升，震时达到峰值，震后趋势下降的变化特点，可以较清晰地反映出区域应力场的演化过程，故可作为地震趋势判断的依据之一；MS4.8地震前后地震视应力与震级表现出一定的正相关，随着震级增大视应力也增大，但MS4.8地震前视应力值总体上大于震后视应力。

Abstract:

On May 28，2012，an earthquake of MS4.8 occurred in Tangshan，and was followed by earthquakes of MS4.0 and MS3.5 in Baodi.Data from this series of earthquakes （Tangshan MS4.8，and the Baodi MS4.0 and MS3.5 earthquake series，from January 2011 to June 2013），recorded by the Digital Seismic Network in Tianjin，were used to calculate the apparent stress in this region.First，the multi window FFT was used to obtain the observed spectrum，and then after extracting the instrument response it was corrected for geometry spreading and attenuation.For geometry spreading correction，Gail’s three section geometry spreading model was used.The frequency-dependent Q value was used for inelastic attenuation correction.A genetic algorithm was applied to determine the pair of fC and Ω0 of Brune’s ω2 model，which yields the least deviation between the theoretical ω2 model and the corrected spectral.The seismic moment （M0） was calculated from Ω0，and ES was obtained by integrating the corrected spectral with limited bandwidth compensation.Apparent stress was calculated from M0 and ES.Furthermore，the time distribution of apparent stress and the average value of apparent stress are presented.The results are as follows：First，a high apparent stress appeared before the MS4.8 earthquake and its peak value appeared at the mainshock；then，the apparent stress decreased after the MS4.8 earthquake.The size of the apparent stress accurately reflects the state of the stress field in the hypocenter region.Therefore，it can be used as the basis for determining the seismic trend.Second，the apparent stress before and after the MS4.8 earthquake is generally positively correlated.The apparent stress increases with magnitude，but，on the whole，the apparent stress value is greater before the MS4.8 earthquake than after.