MAGNETIC FIELD PRECURSORS TO THE 1989 M7.1 LOMA PRIETA EARTHQUAKE, CA

Two magnetic-field precursors were recorded prior to the 1989 Loma Prieta, CA earthquake (Fraser-Smith et al., 1990). The most likely mechanism for these precursors is electrokinetics (Fenoglio et al., 1995), which is the flow of ionized groundwater caused by small ruptures along the fault prior to the main earthquake (Byerlee, 1993). Temporal correlations have been established between these precursors and two small foreshock earthquakes. Earthquake scaling relationships predict earthquake-rupture sizes for these two foreshocks consistent with sizes predicted by the electrokinetic model (Hanks, 1977 and Scholz, 2002). Magnitudes of the magnetic fields by modeling the characteristics are compatible with the observed magnetic-field strengths for the two precursors. While these results support a seismic origin for the magnetic-field anomalies, further research of data from other earthquakes should be observed before a strong causal relationship between foreshocks and magnetic-field precursors can be established.