Correlation between time change in modulus of short-period geomagnetic variation and seismicity in Taiwan

In this study, geomagnetic data of the Lunping observatory from 1993 to 2000 are utilized for computing the amplitude variation of short-period geomagnetic total intensity data, using the complex demodulation method (CD method). In order to compare these time changes with seismicity, earthquakes that occurred within 150 km of Lunping, with magnitude M_L greater than 3.0, are located. The total sum of those earthquakes, summed month by month, is correlated with the modulus. After removing seasonal effect, our results show that the modulus of periods 24, 12, and 8 hr reveals a notable change that seems to be related to the total sum of events within the whole study period. One possible precursor is found 6 months prior to the 1999 high seismicity. The modulus for the periods 24, 12, and 8 hr increased gradually from the beginning of 1999 to August 1999. After earthquake occurrence the modulus decreased again to a normal level. We propose that this notable increase might be related to a preparation process for this strong earthquake.