黄土覆盖山地对岷县、漳县M_s6.6地震余震动力响应的影响

Influence of Aftershocks of the Minxian-Zhangxian MS6.6 Earthquake in a Loess-rich, Hilly Region

摘要:

2013年7月22日岷县、漳县MS6.6地震发生后,我们立刻在极震区架设三台K2强震动记录仪器,以了解地震对黄土覆盖山地的影响.截至8月11日共获取地震记录172个（516条）,ML≥3.0的地震48个,最大震级ML4.4,最大加速度值65.9 gal.我们从永星台阵的强余震观测资料中,选取震级较大的余震记录进行分析,经过初步处理,分别读取各个台站的地表最大加速度值,由于观测地区的黄土覆盖层较薄,土质松软,永星村台、永星小学台这两个台的加速度记录,不能与大竜村台的记录直接比较,我们将观测记录进行傅里叶分析,统计其优势频率,展示部分典型记录的傅里叶谱发现,黄土对高频成分的吸收作用不可忽视,最大加速度值随震中距的增大而衰减迅速.在近场情况下,地表最大加速度对震中距十分敏感,所以无法直接对比不同地形对于地表加速度的影响,分析结果表明：大竜村台优势频率为5.2 Hz;永星村台优势频率为4.1 Hz;永星小学台优势频率为5.3 Hz,注意到位于山脚下的大竜村测点为基岩,加速度记录的优势频率自然较高,而位于山顶的永星村是黄土地基,但是此地的加速度记录的优势频率明显高于山腰记录,几乎与基岩台基的优势频率相当.宏观调查也表明：低频率、高烈度对于房屋的破坏力更大.ML3.8地震观测表明,地表加速度值随震中距的增大而迅速衰减,距离最近的大竜村台（基岩台址）获取的加速度记录北南向最大,另外两个分向的加速度记录也大于较远的两个流动台的记录.基岩台北南向比垂直向要大,和这次地震的震源机制（逆冲兼走滑）相关.地形影响依然存在,本次观测台阵中,位于山顶的永星村观测记录的傅里叶谱,明显高于山腰的记录,地表最大加速度值也稍高于山腰观测,由于地形相差不够大,规律性也不太明显.本次观测结果和以往在孤立山峰的观测情况略有不同,一是记录地震的震级偏小,而观测距离又偏近,震中距的影响可能超过地形因素的影响程度,所以其原因更加复杂,有待进一步研究.

Abstract:

Minxian-Zhangxian M6.6 earthquake occurred on July 22,2013 in Dingxi,Gansu province.Shortly after the earthquake,we placed eight digital flow strong motion observation recording instruments in the surrounding area which were produced by Kinemetrics Company in the USA.We set up three K2 strong motion recording instruments in Yongxing village,Yongxing primary school and Dalong village.A large number of seismic records that can be used to better understand the effects of earthquakes on the loess layer were obtained.Up to August 11,172 （516） seismic records were acquired,including 48 earthquakes of magnitude ML ≥ 3.The maximum magnitude was ML4.4,and the maximum acceleration value was 65.9 gal.From the observed data of strong aftershocks from the Yongxing array,we selected records of large magnitude aftershocks for analysis.The Minxian ML 3.8 earthquake record from August 8th was the largest,and we focused on those results.The results showed that the maximum acceleration value decayed rapidly with increasing distance from the epicenter,and the largest acceleration value was from Dalong village station,which was the nearest station from the epicenter.Because of the presence of a thin and soft loess layer in the observation area,the acceleration records from the Yongxing village station and the Yongxing primary school station could not be directly compared with those from Dalong village,but we conducted a further spectrum analysis on each record,and the effect of absorption by loess on the high-frequency components was significant.To further explore the impact of topography on the observed values,we further expanded the scope of the data filtering,and used the seismic data of four ML ≥2.5 earthquakes as the basis for measuring the peak ground acceleration value of each station.We conducted Fourier analysis on the records,obtained the dominant frequencies,and illustrated the Fourier spectrum of some typical records.The results show that the dominant frequency of Dalong village is 5.2 Hz,that of Yongxing village is 4.1 Hz and that of Yongxing primary school is 5.3 Hz.Noted that Dalong village was in foothills and its observation point was bedrock,the dominant frequency of acceleration was higher.The survey also showed that low frequency and high intensity events had greater destructive power for buildings.The seismic observation of ML3.8 earthquake indicated that the acceleration records of NS direction of bedrock point are larger than vertical direction,which is related to the focal mechanism of the earthquake.The observation results are slightly different than the previous observation in isolated peaks,because the magnitudes of earthquakes are small,and the observation distance is near,the influence of epicentral distance may exceed the influence of topographic factors,which is too complex to be further studied.