On 27 March 1964, a monster earthquake struck beneath Alaska’s Prince William Sound. A 9.2 on the Richter scale, the tremor was the most powerful recorded earthquake in U.S. history. Five months later, a letter published in Nature reported that a magnetometer in the city of Kodiak had recorded disturbances in the Earth’s magnetic field just before the quake struck. In the 50-plus years since, several more studies have shown magnetic changes—that they labeled anomalous—preceding quakes.
Such anomalies have provided scientists and the lay public with the tantalizing notion that earthquakes might come with warnings and might even be predictable. Unfortunately, strong evidence that these anomalies are directly related to impending quakes has never been found. Conversely, recent research has shown that the majority of magnetic changes recorded before earthquakes to date were related to malfunctioning equipment, problems with analyses, or normal magnetic fluctuations that arise as the Earth’s magnetic field clashes with solar wind. Still, the search for earthquake precursors persists.
Here Masci and Thomas look at two of the most recent reports suggesting that precursory signals may lie in ultralow-frequency (ULF) magnetic data—electromagnetic waves with frequencies between 0.001 and 10 Hz. ULF signals are generated by interactions between the Sun and the Earth’s magnetic field at the magnetosphere, some artificial noises, and processes beneath the Earth’s surface. However, any changes in ULF signals attributed to earthquakes thus far are suspect, according to the authors, as they may be explained by disturbances in the geomagnetic field in response to Sun-Earth interactions. Further, any valid mechanism by which fault zones would generate precursory electromagnetic perturbations remains to be seen.
First, the authors refute the claim made by J. L. Currie and C. L. Waters (J. Geophys. Res. Space Physics, 2014, doi:10.1002/2013JA019530) that the magnetic polarization ratio—a relationship between components of the geomagnetic field—is a valid measure in the search for precursor signals. According to the authors, the polarization ratio cannot be considered as a promising parameter to identify possible earthquake precursors in ULF magnetic field data. They also contend that the superposed epoch analysis—a statistical analysis that spotlights weak signals—fails to show preseismic and coseismic ULF magnetic signals before the Japanese earthquakes studied by P. Han et al. (J. Geophys. Res. Space Physics, 2014, doi:10.1002/2014JA019789) and, as such, cannot be used to predict future quakes.
The search for precursory signals in ULF magnetic data has been thoroughly pursued for decades with little progress toward practical results, the authors conclude. (Journal of Geophysical Research: Space Physics, doi:10.1002/2015JA021336, 2015)
—Kate Wheeling, Freelance Writer
Citation: Wheeling, K. (2016), Are earthquakes predictable?, Eos, 97, doi:10.1029/2016EO046955. Published on 1 March 2016.