ULF magnetic field measurements near the epicenter of the Ms 7.1 Loma Prieta earthquake

Abstract

Anomalous measurements of the ultra-low frequency (ULF) magnetic field fluctuations prior to the magnitude 7.1 Loma Prieta earthquake of October 17, 1989, have been studied. For the past few years we have been monitoring fluctuations of the magnetic field of the Earth in the ULF range at Corralitos, California; our instruments were located 7 km from the epicenter of the earthquake. We have observed four anomalies in our data which may turn out to be precursors to the earthquake. First we observe narrowband noise fluctuations centered at 0.1 Hz and estimated to have a bandwidth of 0.00143–0.00167 Hz, i.e. a Q value of 60–70. The narrowband fluctuations appear to have a maximum equivalent amplitude of over 1400 pT/√Hz which is roughly 31 dB higher than the typical quiet average amplitude background levels. These fluctuations begin around September 12 and last until October 5. Next we observe the appearance of additive wideband noise fluctuations beginning around October 5 and continuing until the occurrence of the earthquake. These wideband fluctuations, which cover almost the entire 0.01–10 Hz frequency range of the system, have an average amplitude that is approximately 19 dB larger than typical levels in the 0.01–0.02 Hz band. Thirdly, we observe an atypical decrease in noise levels in the 0.2–5 Hz band throughout the day prior to the earthquake. The fourth anomaly is a jump in the power of magnetic field fluctuations, mostly in the 0.01–0.5 Hz band, in the three hours preceding the earthquake. This activity reached its highest level in the lowest 0.01–0.02 Hz band, and had a magnitude of roughly 60000 pT/√Hz, which is about 40 dB larger than typical background noise levels in the band. Our anomalous measurements do not appear to be the result of any magnetic field fluctuations generated in the upper atmosphere or to movement of our sensor caused by shocks preceding the quake. In describing these anomalous magnetic field fluctuations, both electrokinetic and piezoelectric theories of the generation of magnetic field fluctuations prior to earthquakes suggest wideband fluctuations. In our measurements, we have observed both wideband and narrowband fluctuations, so that our narrowband measurements do not seem to be easily explicable by the above theories. However, the wideband fluctuations could turn out to be attributable to the above mechanisms, since at the monitored frequencies, the skin depths of electromagnetic waves are comparable with the distances from our sensors to the earthquake focus, and therefore our measurements would not be unexpected.