Low-frequency magnetic field measurements near the epicenter of the M sub s 7. 1 Loma Prieta earthquake

Journal Article · Wed Aug 01 04:00:00 EDT 1990 · Geophysical Research Letters (American Geophysical Union); (USA)

DOI:https://doi.org/10.1029/GL017i009p01465· OSTI ID:6205765

Fraser-Smith, A C; Bernardi, A; McGill, P R; Ladd, M E; Helliwell, R A; Villard, Jr, O G ^[1]

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1. Stanford Univ., CA (USA)

The authors report the results of measurements of low frequency magnetic noise by two independent monitoring systems prior to the occurrence of the M{sub s} 7.1 Loma Prieta earthquake of 17 October 1989. Their 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 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 they 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 the systems are sensitive to motion, seismic measurements indicate that there were no significant shocks preceding the quake. Thus, the various anomalous features in the data, and in particular the large-amplitude increase in activity starting three hours before the quake, may have been magnetic precursors.

• OSTI ID: 6205765
• Journal Information: Geophysical Research Letters (American Geophysical Union); (USA), Journal Name: Geophysical Research Letters (American Geophysical Union); (USA) Vol. 17:9; ISSN GPRLA; ISSN 0094-8276
• Country of Publication: United States
• Language: English

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58 GEOSCIENCES
580000* -- Geosciences
CALIFORNIA
DATA ANALYSIS
EARTHQUAKES
ELECTRICAL SURVEYS
ELECTROMAGNETIC SURVEYS
EPICENTERS
FEDERAL REGION IX
FLUCTUATIONS
FORECASTING
FREQUENCY RANGE
GEOPHYSICAL SURVEYS
HZ RANGE
MAGNETIC FIELDS
MONITORING
NORTH AMERICA
SEISMIC EVENTS
SIGNAL-TO-NOISE RATIO
SURVEYS
TIME DEPENDENCE
USA
VARIATIONS