Abstract: In this paper, a seismomagnetic phenomenon due to piezomagnetic effect is numerical calculated to help understanding the magnetic anomaly associated with earthquakes. The theory of piezomagnetic effect is introduced and the piezomagnetic field resulting from strike slip fault and dip slip fault are calculated. The pattern of the piezomagnetic field is geometrical symmetry for simple slip model and depends on the strike direction of the fault. The total intensity of the geomagnetic field changes is influenced by the burial depth of slip. According to the piezomagnetic field resulting from two slip models of the 2008 Wenchuan M_W7.9 earthquake, larger magnetic field changes occur on slip plane and decays quickly with the distance to epicenter in the bilateral of slip plane. The piezomagnetic field is reduced to about 1 nT or less in area which locates tens kilometres away from epicenter. Based on the observed data of Chengdu geomagnetic observatory before and after the earthquake, a geomagnetic field change of 0.45 nT which is above the background noise level is obtained. The piezomagnetic effect calculated from slip model according to the inversion of geodetic surveying data is consistent with observed data which infers that suitable piezomagnetic model can explain static coseismic geomagnetic changes.

Key words: Seismomagnetic anomaly, Piezomagnetic effect, Numerical calculation, the 2008 Wenchuan M_W7.9 earthquake