EPP

The hourly data of the vertical Z and the horizontal H components of 37 ground–based DC–ULF geomagnetic stations are examined during 20 April–12 May 2008. On 9 May 2008, three days before the Wenchuan M_S 8.0 shock, anomalies — a double low-point and a decreased amplitude — are registered on the curves of the Z component at 25 stations in a large-scale area surrounding the Wenchuan epicentral area. The H component shows none of the double low-point phenomenon but does exhibit a reduced magnitude at the same time. The geomagnetic index Kp is also examined and indicates that the anomalies appear at a solar quiet period. The appearing time shift (T_zs) between the first low-point on May 9 and the minimum point occurring time of May 1–5, 2008 is also checked. The results show that T_zs is on the order of 1–2 hours earlier or later than usual and there is a 2–6 hours’ gap between these two low-points. However, there is still a transition area which includes the epicenter where T_zs = 0. Variation amplitude examined on vertical Z increases as the distance from the epicenter decreases. An Earth–air–ionosphere model has been employed to investigate a possible mechanism of this phenomenon and positive results have been unexpectedly attained. All these above-related results tend to prove that the variations of the Z and H on May 9, 2008 during the solar quiet period are probably associated with the forthcoming Wenchuan M_S 8.0 earthquake.

Twenty-seven FHDZ-M15 combined geomagnetic observation systems (each of which is equipped with a fluxgate magnetometer and a proton magnetometer) had been installed in the China geomagnetic network before the 2008 Wenchuan earthquake, during which coseismic disturbances were recorded by 26 fluxgate magnetometer observatories. The geomagnetic disturbances have similar spatial and temporal patterns to seismic waves, except for various delays. Six proton magnetometer observatories recorded coseismic disturbances with very small amplitudes. In addition, fluxgate magnetometers registered large-amplitude disturbances that are likely to have included responses to seismic waves. However, two problems remain unresolved. First, why do these geomagnetic disturbances always arrive later than P waves? Second, why do the geomagnetic disturbances have spatial and temporal directivity similar to the main rupture direction of the earthquake? Solving these two problems may be crucial to find the mechanism responsible for generating these geomagnetic anomalies.