芦山Ms7.0地震余震期间大地电磁视电阻率变化

摘要

The present paper analyzes the relationship between the apparent resistivity change and major aftershock activities based on magnetotelluric meas-urements data from May 3 to 24,2013.A group of self-power and cross-power spectra are obtained with the robust method and coherence limit based on char-acteristics of magnetotelluric method.Then the spectra causing apparent resis-tivity and phase curve unsmooth are removed according to their shape,and the high-quality data are obtained after removing disturbed data.The results show that the apparent resistivity tended to increase gradually with maximum change by 27％ during the measurement period but not in a monotonous manner.The yx polarized apparent resistivity ρayx;reached its maximum after nearly every main aftershock cluster,but among each clusters the minimum resistivity appeared.Finally the analyses on earthquake generating mechanisms indicate that the earthquake resistivity effect is due to the change in pore fluid in micro fractures,therefore it is speculated that the increase trend of the resistivity during the period of Lushan aftershocks is caused by stress recovery and porosi-ty recovery after the Lushan earthquake.%以2013年4月20日芦山Ms7.0地震震后5月3-24日距主震震中约7 km处所观测到的大地电磁数据为基础,分析地下电阻率变化与较大余震(Ms＞3.0)活动的关系.为确保用于地震异常分析的观测数据的可靠性,基于大地电磁方法的特点,使用稳定估算(robust)技术和相干度因子约束等方法得到各频点一定数量的自功率谱和互功率谱,再根据视电阻率和相位曲线的分布形态,剔除引起视电阻率和相位曲线突变的谱值之后得到高质量的视电阻率和相位曲线.此外,根据曲线形态和误差分布来判断数据质量,剔除可能受到干扰影响的数据,以获得能够真实地反映地下电阻率变化的视电阻率分析结果.结果显示:观测期内的yx极化模式的视电阻率值总体呈非单调的增大趋势,最大增幅达27％;yx极化模式的视电阻率值几乎在每个相对集中的较大余震丛集后均会出现高值,而较大余震丛集期间对应的则是视电阻率低值.从物理机制上分析地震的电阻率效应源于微裂隙内孔隙流体的变化,故推测芦山地震余震期间电阻率的增加趋势是由于震后应变释放区应力恢复和孔隙度恢复所致.