Ultra-low frequency electromagnetic measurements associated with the 1998 Mw 5.1 San Juan Bautista, California earthquake and implications for mechanisms of electromagnetic earthquake precursors

Abstract Ultra-low frequency (ULF: 0.01–10 Hz) magnetic field anomalies prior to M ≥6.0 earthquakes have been reported in various regions of the world. Here, we test whether smaller magnitude earthquakes have associated ULF anomalies by using continuous measurements made ∼9 km above and ∼2 km NE of the epicenter of the M w 5.1 8/12/98 San Juan Bautista, CA earthquake. Half-hour spectral averages of the magnetic field data in nine different ULF frequency bands show no long-term (days to weeks) magnetic field anomaly prior to the earthquake. In addition, magnetic and electric field polarization shows no anomalous behavior clearly associated with seismic activity. An ∼0.02 nT increase in activity for 2 h prior to the earthquake is close to the background noise levels precluding identification of a precursor. We present scaling calculations to show if precursory ULF anomalies are related to the size of the earthquake. The observed ∼0.02 nT increase is the approximately expected magnetic anomaly prior to the San Juan Bautista (SBJ) earthquake, based on the anomaly observed prior to the 1989 Loma Prieta (LP) earthquake. Our reexamination of previously proposed dilatant-conductive and piezomagnetic mechanisms for ULF precursors shows that these effects are consistent with the observations prior to the SJB earthquake based on the observations (i.e., lack of a strong precursor). In contrast, previously proposed electrokinetic and magnetohydrodynamic (MHD) mechanisms do not appear to be appropriate for this particular earthquake. This analysis further supports the hypothesis that precursory ULF signals are related to the size of the earthquake.