Seismo-geomagnetic anomalies and M ≥ 5.0 earthquakes observed in Taiwan during 1988-2001

In this paper, a relationship between M ≥ 5.0 earthquakes and diurnal variations of the total geomagnetic field recorded at eight magnetometers in Taiwan during 1988-2001 are examined. One magnetometer station was setup in a seismic quiet area as a reference, while the others were located in areas of high seismicity or crustal activity observing earthquake effects. We compute the distribution of diurnal range ratios between the reference and each observation station for the entire thirteen years as a background and compare it with the monitored distributions during five different time periods before and after an M ≥ 5.0 earthquake occurring within a distance of 50 km from the observation station. Three specific earthquakes with different magnitudes, including the M = 7.3 Chi-Chi earthquake show that the monitored distributions one month before and during the month of the earthquakes significantly depart from the associated background. It is found that changes of underground conductivities and currents around the forthcoming epicenter significantly affect the near-by geomagnetic field on the ground during the earthquake preparation period. The statistical results demonstrate that the monitored distributions of geomagnetic anomalies are highly related to the focal mechanism.