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1976年以来,在加利福尼亚帕克菲尔德地区沿圣安德烈斯断层上长约60km的闭锁—蠕动过渡带,布设了由7个采样同步的质子磁力仪构成的差动连接的台阵,对该地区的局部磁场进行了准确的测量。2004年9月28日的M6.0级帕克菲尔德地震的震中恰好位于这个台阵之内,此次地震在其中的5个台站的观测中产生了0.2nT～0.5nT的同震磁场变化。但是从地震发生前1个月、前1周及前数天的地磁观测数据中,都检测不到明显高于背景噪声水平的震前磁场变化(由于没有可检测出的形变、地震活动或孔隙压力变化数据,理论上也应观测不到震前磁场变化)。在地震破裂末端附近的一个台站也记录了0.01Hz～20Hz频带内的电场和磁场数据,借助于沿断层的北西延长线上距该台站约115km的另一个台站的观测数据,对来自电离层和磁层的同模式噪声进行了校正。校正后的数据中既没有发现1989年ML7.1级洛马普列塔地震前可能存在ULF频带(0.01Hz～20Hz)的异常磁噪声,也没有发现相同震级的希腊地震震前产生的电场变化。基于应变、位移和地震数据反演得到的地震一致滑动和可变滑动压磁模型,能够计算出与磁力仪台阵观测到的结果相符的磁场扰动。1993年以来,该区域内长期存在着与荷载增加一致的较高磁场变化,与之相伴的是由一个两色EDM台网和另一个较小的钻孔应变仪台网观测到的剪应变率的长期增长及表现在1992、1993和1994年各发生了一次M4.5～5.0级地震的地震活动性的增强。集成了所有这些数据建立的模型揭示了该区域在深度剖面上有增大的滑动,这也许对2004年9月28日M6.0级帕克菲尔德地震的最后发生起到了重要的作用。本次地震以及其他发生于圣安德烈斯断层系内其他地方的M5～7.3级地震的震前电场和磁场前兆的缺乏,表明了仅利用现有的这些电磁数据来对破坏性地震做出有意义的预测看来并不可能。
Since 1976, a differential-connected array of seven sample-synchronized proton magnetometers has been deployed in a latticed-peristaltic transitional zone about 60km along the San Andres Fault in Parkfield, California, The local magnetic field was accurately measured. The epicenter of the M6.0 Parker Earthquake on September 28, 2004 happened to be within this array. The earthquake produced a coseismic magnetic field change of 0.2nT ~ 0.5nT at five of these stations. However, from the geomagnetic observations 1 month before, 1 week before and 1 day before the earthquake, no pre-seismic magnetic field changes significantly above the background noise level were detected (due to the absence of detectable deformation, seismic activity or porosity Pressure changes in the data, in theory, should not be observed before the magnetic field changes). A station near the end of the earthquake rupture also recorded electric and magnetic field data in the frequency band from 0.01 Hz to 20 Hz. By means of observation data from another station about 115 km along the north west extension of the fault, The same mode noise of ionosphere and magnetosphere was corrected. In the corrected data, no abnormal magnetic noise of the ULF band (0.01 Hz ~ 20 Hz) may exist before the 1989 ML7.1 Loma Prieta earthquake and no change in the electric field before the Greek earthquake of the same magnitude was found . Based on the consistent earthquake sliding and the variable sliding piezomagnetic model retrieved from the strain, displacement and seismic data, the perturbation of the magnetic field can be calculated according to the observed results of the magnetometer array. Since 1993, there has been a long history of higher magnetic field changes consistent with increased loading in the region, accompanied by shear strain rates observed by a two-color EDM network and another smaller borehole strain gauge network Long-term growth and performance In 1992, 1993 and 1994, there was an increase in the seismicity of the M4.5-5.0 earthquake. The model that incorporates all of these data sets out to reveal increased slippage in the depth profile of the region, which may play an important role in the eventual occurrence of the M6.0 Parkfield earthquake on September 28, 2004. This earthquake and other M5-7.3 earthquakes that occurred elsewhere in the San Andreas Fault System lack of precursory electric and magnetic precursors indicate the use of these existing electromagnetic data to make destructive earthquakes Meaningful prediction does not seem possible.