Low‐frequency magnetic field measurements near the epicenter of the M_s 7.1 Loma Prieta Earthquake

Abstract

We report the results of measurements of low frequency magnetic noise by two independent monitoring systems prior to the occurrence of the MS 7.1 Loma Prieta earthquake of 17 October 1989. Our measurements cover 25 narrow frequency bands in the more than six‐decade frequency range 0.01 Hz–32 kHz, with a time resolution varying from a half hour in the ULF range (0.01–10 Hz) to one second in the ELF/VLF range (10 Hz–32 kHz). The ULF system is located near Corralitos, about 7 km from the epicenter. The ELF/VLF system is located on the Stanford campus, about 52 km from the epicenter. Analysis of the ELF/VLF data has revealed no precursor activity that we can identify at this time. However, the ULF data have some distinctive and anomalous features. First, a narrow‐band signal appeared in the range 0.05–0.2. Hz around September 12 and persisted until the appearance of the second anomalous feature, which consisted of a substantial increase in the noise background starting on 5 October and covering almost the entire frequency range of the ULF system. Third, there was an anomalous dip in the noise background in the range 0.2–5 Hz, starting one day ahead of the earthquake. Finally, and perhaps most compelling, there was an increase to an exceptionally high level of activity in the range 0.01–0.5 Hz starting approximately three hours before the earthquake. There do not appear to have been any magnetic field fluctuations originating in the upper atmosphere that can account for this increase. Further, while our systems are sensitive to motion, seismic measurements indicate that there were no significant shocks preceding the quake. Thus, the various anomalous features in our data, and in particular the large‐amplitude increase in activity starting three hours before the quake, may have been magnetic precursors.

Correlation Between Time Change in Modulus of Short-Period Geomagnetic Variation and Seismicity in Taiwan

“…The theory of stress-induced changes in rock magnetization is generally in agreement with laboratory studies (Stacey 1964;Nagata 1970;Stacey and Johnston 1972). Anomalous electromagnetic precursors to earthquake have been reported in many papers (e.g., Beamish 1982;Rikitake 1987a, b;Fraser-Smith et al 1990;Yoshino et al 1993;Oike and Murakami 1993;Fujinawa and Takahashi 1994;Pulinets et al 1998;Rikitake 1997;Liu et al 2000;Zeng et al 2001;Nagao et al 2002;Zeng et al 2002;Yen et al 2004). Research on the relation between time change of conductivity and seismic activity is an important subject in earthquake prediction.…”

Investigation of ULF magnetic anomaly during Izu earthquake swarm and Miyakejima volcano eruption at summer 2000, Japan

“…Different long-term observatories are already in operation in Japan (Hayakawa, et al, 1996;Uyeda et al, 2000), Russia (Ismaguilov et al, 2001), Greece (Varotsos et al, 1996), U.S. (Fraser-Smith, et al, 1990) …”

Some aspects of man-made contamination on ULF measurements

“…These measurements, however, may be interesting also for other aspects of geophysical sciences: for example, geomagnetic field variations in the frequency range of micropulsations provide useful information on the structure of the Earth's crust (Meloni et al, 1985;Pilipenko and Fedorov, 1993;Vellante, 1997;. In addition, local ULF emissions are considered a promising candidate for short-term earthquake predictions (Fraser-Smith et al, 1990Molchanov et al, 1992;Hayakawa et al, 1996;Nagao et al, 2002;Gotoh et al, 2002). Lastly, special care has been dedicated to the possible influence of natural and artificial…”