In the month prior to the 1989 Loma Prieta earthquake measurements of the earth 's magnetic field at ultra-low frequencies by a magnetometer in Corralitos , California , just 7 km from the epicenter of the impending earthquake , started showing anomalous increases in amplitude . Just three hours before the quake the measurements soared to about thirty times greater than normal , with amplitudes tapering off after the quake . Such amplitudes had not been seen in two years of operation , nor in a similar instrument located 54 km away . To many people such apparent locality in time and space suggested an association with the earthquake
1989 Loma Prieta earthquake
1989 Loma Prieta earthquake
Seismo-electromagnetics