2018年M_W4.3河北永清地震的震源机制与破裂特征

Source mechanism and rupture characteristics of the 2018 M_W4.3 Yongqing earthquake

采用区域宽频带地震波数据,反演确定了2018年2月12日河北永清地震的地震矩张量解和破裂过程.震源机制结果显示,这次地震的矩心深度约为16 km,发生在走向北东、倾向南西的断层上,断层错动以左旋走滑为主、兼具少量正断层分量.破裂模型显示,地震的最大滑动量约为1.3 cm,破裂尺度约2 km,二者皆明显高于根据现有地震定标律估计得到的结果.断层最大应力降和平均应力降分别约为0.6和0.3 MPa,明显低于板内地震应力降平均水平.分析认为,这一较低的应力降水平与缺少余震的"孤立型"地震序列对应,反映了较弱的断层强度和破碎的震源区介质,可能是华北部分地区地震的共有特征.

The 12 February 2018 M_W4.3 Yongqing earthquake,is the largest shock around the Beijing area since the 2006 Wen＇an MS5.1 earthquake.Similar to the 2006 Wen＇an earthquake and many other earthquakes in this region,very few aftershocks were recorded after the Yongqing mainshock.It raises a question whether the isolated earthquake sequence is related or not to the source characteristics.To answer this question,investigations of the source mechanism and rupture process are necessary.We estimated the moment tensor solution and rupture process by inverting seismograms of 38 local broadband stations within the epicentral distances of 200 km.The duration and shape of the source time function were considered in the moment tensor inversion.Through a grid search,we found a centroid depth of about 16 km of the earthquake.The scalar seismic moment is 3.1×1015 Nm,equivalent to amoment magnitude M_W4.3.From the source time functions,the rupture duration is about 3.5-4.0 s.The mechanism solution suggested a strike slip event,with a minor normal-faulting component.Combined with the geological background,the NE striking and SE dipping nodal plane was identified as the causative fault.The rupture model was estimated on the NE striking fault plane by using a linear technique.From the static slip distribution,the peak slip is about 1.3 cm,and the rupture dimension is approximately 2 km.Most slips are concentrated around the major slip asperity.The source time function suggests a source duration of about 3.5 s,and the peak moment rate appearing at 1 s,which are well consistent with those from the moment tensor inversion.The earthquake ruptures expanded from the hypocenter in a circular dislocation model at 0-2 s.A few weak slips appeared at shallow depth after 2 s,which might be caused by errors in observation and Green＇s functions and were less reliable.Both the peak slip and the rupture dimension of the 2018 Yongxing earthquake are higher than the values of an M_W4.3 earthquake estimated from existing earthquake scaling law.They may provide additional constraints for further correction of the parameter-magnitude scaling laws.The maximum and average stress drops are about 0.6 and 0.3 MPa,respectively,remarkably lower than 10 MPa of a typical intraplate earthquake.The relatively low stress drop indicates that the fault strength is weak.Seismic tomography in the Huabei area has found that the crust is fractured,which may lead to weak faults there.The fractured crust and weak fault can block the stress transformation to some extent,and are likely to cause isolated earthquake sequence.