HR: 08:30h
AN: S41D-03    [Abstracts]
TI: Re-affirming the Magnetic Precursor to the 1989 Loma Prieta, CA, Earthquake Using Magnetic Field Data Collected in the US in 1989 and 1990
AU: * Culp, D
EM: dculp@purdue.edu
AU: Klemperer, S
EM: sklemp@stanford.edu
AF: Department of Geophysics, Stanford University, Stanford, CA 94305-2215, United States
AU: Glen, J
EM: jglen@usgs.gov
AF: USGS Menlo Park, 345 Middlefield Road, Menlo Park, CA 94025, United States
AU: McPhee, D K
EM: dmcphee@usgs.gov
AF: USGS Menlo Park, 345 Middlefield Road, Menlo Park, CA 94025, United States
AB: Electromagnetic (EM) signals are one of the most commonly claimed precursors to earthquakes. The most cited of these proposed EM events is the report of ultra-low frequency (ULF) EM anomalies prior to the 10/17/1989 magnitude 7.1 Loma Prieta earthquake (Fraser-Smith et al., 1990, GRL 17, 1465) recorded at Corralitos (7 km from the epicenter). Although some critical aspects of this ULF-EM signal (such as the increase of amplitude at lower frequency) are entirely consistent with an origin for the signal at the earthquake hypocenter, some scientists question the validity of the data simply because only a single instrument recorded the signal. Recently, W.H. Campbell (2005, Eos Trans. AGU, 86(18), Jt. Assem. Suppl., Abstract GP23A-01; and unpublished manuscript) has claimed that the existence of a ULF precursor signal to the Loma Prieta earthquake "is very likely false". Campbell argues that the reported "precursor" was not of local origin but was recorded throughout the western US and was therefore a solar-terrestrial disturbance; and that the extreme amplitudes reported by Fraser-Smith must have been due to a change of gain of the Corralitos magnetometer "by a factor of 5 to 10, on (or about) 4 to 5 October", i.e. an instrumental malfunction. J.N. Thomas et al. (2007, Abstract 4036, XXIV General Assembly IUGG, Perugia; and unpublished manuscript) repeat and amplify this claim. Because of the importance of the Loma Prieta ULF-EM signal to earthquake-prediction studies, we have critically examined the Campbell and the Thomas et al. claims about the veracity of the signal. Campbell made a visual comparison/correlation of the magnetic signals recorded at Corralitos with those recorded at U.S.G.S. magnetic observatories at Fresno, Boulder and Tucson, but only for the first 18 days of October 1989. Thomas et al. made numerical correlations of data from Fresno and Kakioka, Japan, with the Corralitos data-set, but only from September 18 through October 27, 1989. We have duplicated much of the Campbell and the Thomas et al. methodologies, but we are extending our comparison over two years of observations including time periods significantly before and after the Loma Prieta earthquake. Even a simple visual comparison between very long time series from Fresno and Corralitos shows that the single instrumental gain change postulated by Campbell and by Thomas et al. does not suffice to explain the data. Unless one postulates multiple fluctuations of instrumental response, with time-varying gain, the simplest explanation of the Corralitos magnetic field anomalies is that some form of ULF precursor was recorded prior to the Loma Prieta earthquake.
DE: 1500 GEOMAGNETISM AND PALEOMAGNETISM
DE: 1513 Geomagnetic excursions
DE: 7200 SEISMOLOGY
DE: 7223 Earthquake interaction, forecasting, and prediction (1217, 1242)
SC: Seismology [S]
MN: 2007 Fall Meeting