CAJ | 학술논문

2008年5月12日汶川M_s8.0地震前后,距震中320 km甘肃陇南汉王地震台的地电阻率、大地电场、ELF电磁等多种观测手段捕捉到这次地震前后的异常信息.本文采用解析方法,着重探讨汶川8.0级地震前后观测到的电磁场变化特征,从电磁现象来探讨汶川M_s8.0地震发生前后,电磁场长、中、短、临期的异常特征和变化规律.自2003年以来甘东南地区及甘川交界发生4次5.0级以上地震,从空间上看,有从南北地震带中段的西北向西南方向移动的特点,强度逐渐增强,其中,文县5.0和两次岷县5.0级以上地震可以视为汶川8.0级地震的前震;从时间上分析,震前陇南汉王地震台观测到电磁场出现了长趋势变化和短临异常变化,其特征：①2003～2008年,ZD8B地电仪观测的N54°W、N85°E和N73°W测道地电阻率均显示出长趋势变化和短期变化;②视电阻率各向异性度'S''年变化率在有震年(异常年)是无震年(正常年)的两倍;③ELF电磁仪观测到了(0.5～39Hz)频率段的电场、磁场在汶川8.0级地震及余震序列中异常变化分别比正常月份大1～5个数量级,并记录到电磁同震现象;④大地电场仪(ZD9A-Ⅱ)在汶川震前17个小时也记录到高频临震变化信息.
2008년5월12일문천M_s8.0지진전후,거진중320 km감숙롱남한왕지진태적지전조솔、대지전장、ELF전자등다충관측수단포착도저차지진전후적이상신식.본문채용해석방법,착중탐토문천8.0급지진전후관측도적전자장변화특정,종전자현상래탐토문천M_s8.0지진발생전후,전자장장、중、단、림기적이상특정화변화규률.자2003년이래감동남지구급감천교계발생4차5.0급이상지진,종공간상간,유종남북지진대중단적서북향서남방향이동적특점,강도축점증강,기중,문현5.0화량차민현5.0급이상지진가이시위문천8.0급지진적전진;종시간상분석,진전롱남한왕지진태관측도전자장출현료장추세변화화단림이상변화,기특정：①2003～2008년,ZD8B지전의관측적N54°W、N85°E화N73°W측도지전조솔균현시출장추세변화화단기변화;②시전조솔각향이성도'S''년변화솔재유진년(이상년)시무진년(정상년)적량배;③ELF전자의관측도료(0.5～39Hz)빈솔단적전장、자장재문천8.0급지진급여진서렬중이상변화분별비정상월빈대1～5개수량급,병기록도전자동진현상;④대지전장의(ZD9A-Ⅱ)재문천진전17개소시야기록도고빈림진변화신식.
Before and after the 12 May M_s8.0 Wenchuan earthquake,we have observed anomalies associated with this event by several geophysical methods,such as DC resistivity,geoelectric field and electromagnetic (ELF),at the Hanwang station,Longnan,Gansu province 320 km away from the epicenter.In this paper,we use the analytical method to study the features and changing regularities on long-,medium-and short-term scales of the electromagnetic field prior to and after this quake based on these data.Since 2003,four earthquakes of M≥5 occurred in the southeastern Gansu and around the border between Gansu and Sichuan province.These events seemed to shift from the middle section of the North-south seismic zone in central China mainland toward southwest with increasing magnitude.Among them,the Wenxian M5.0 and other two M5.0 events at Minxian were likely the precursory quakes to the Wenchuan M_s8.0 in 2008.Corresponding to these quakes,the observations at the Hanwang station exhibit long-term tendency and short-time anomalous changes described as follows.During 2003～2008,the resistivity values measured by the instrument ZD8B in directions N54°W,N85°E and N73°W all showed long- and short-term variations.The annual change rates of the anisotropic degree “S′” of apparent resistivity in the years with M5.0 events (anomalous years) are two times that of the years without M5.0 quakes (normal years).The anomalies of the electric and magnetic fields on the frequency band (0.5～39 Hz) measured by the instrument ELF during the times of the Wenchuan M_s8.0 and its aftershocks are greater than those in normal times by 1～5 orders of magnitude.Meanwhile this instrument has also recorded electromagnetic coseismic signal.And 17 hours prior to the main shock,the telluric electric field instrument (ZD9A-Ⅱ) detected high-frequency anomalous information.