Number 157 (Story #2), December 23, 1993 by Phillip F. Schewe and Ben Stein|
GEOELECTRIC SIGNALS: DO THEY PRECEDE EARTHQUAKES? Speaking at the recent meeting of the American Geophysical Union, Anthony Fraser Smith of Stanford (415-723-3687) reviewed his data from four years ago which showed that local measurements of Earth's magnetic field fluctuated much more vigorously than usual in the days and hours before the 7.1-magnitude Loma Prieta earthquake. Many scientists hesitate to infer any correction between the signals and the quake, particularly on the basis of only one such data set. To study the matter further, Fraser Smith has set up several detectors around California near faults. Simon Klemperer, also of Stanford (415-723-7344), attempts to model Fraser Smith's signals by suggesting that in the buildup to a quake, a flexing fault system might squeeze pockets of water together (which are sparse at these depths--18 km), altering the electrical conductivity of the fault, which in turn can act like an antenna to modulate the measured magnetic field at the surface. Other types of geoelectric signals possibly related to quakes were reported at the AGU meeting. Seiya Uyeda of Tokai University in Japan and Texas A&M cited data linking four recent earthquakes in Japan with anomalies in the static voltage differences between various measurement stations. Jean Chu of MIT presented a small portion of an extensive Chinese study (over 20 years) of earthquakes and possible precursors in the form of changes in the conductivity of the Earth.