Electricomagnetic Coseismic signal associated with aftershock of Wenchuan Ms 8.0 earthquake

The disastrous earthquake with magnitude Ms 8.0 had occurred in Wenchuan, Sichuan province at 14:28, May 12, 2008. Two sets MT system V5-2000 ware set up at Hanwang (HW) and Chouziba (CB) on May 22 and May 25 respectively in Wudu,Gansu province where is about 100 km away from Qingchang epicenter of aftershock zone. They are 3 km away from each other. The data recording with 24 Hz sampling rate had been done continuously except changing battery at 8:00 to 8:30 for 22 days and recorded many aftershocks events. 19 events were recorded at both measurement sites with magnitude bigger than Ms 4.0 in distance shorter than 120 km away from the observation sites during 22 days, including the biggest one Ms 6.4 in Qingchuan at 16:21, May 25. A seismometer with sampling rate 200 Hz was set up at Hanwang which is 2.5 km away from HW and 5.5 km from CB in order to compared electromagnetic wave variation. Both seismometer and MT system adopt GPS time service. It is easy to synchronize for both data. The coseismic signal obtained on all electric and magnetic field channels at both HW and CB site for most of events, but few events did not very significant on electric channels. We analysesed 120 seconds sampled data during event occurrence. The time series of both seismic wave and MT records show that co-seismic signals exist on all components of the magnetic and electric fields of two different sites, but that they begin with the arrival of seismic waves and not at the origin time of the earthquake. The most easily understood signals on all our sensors are always the co-seismic responses. These results are consistent with those of predecessor. It is very clear that co-seismic coseismic signals are not produced at the earthquake source, but rather are due to local effects of passing seismic waves. The visible co-seismic signal records the motion of the sensors in the earth’s magnetic field and also an electro-seismic component due to charge-separation induced by passage of the seismic waves through saturated or partially saturated near-surface strata adjacent to the sensors.