Researchers reporting in AGU’s JGR Solid Earth identified changes in the magnetic field near intermediate-large earthquakes in California in the days before the earthquakes happened, providing evidence that there is a physical change that can be observed in the days before an earthquake.
- Magnetic field changes as earthquake precursors have been the subject of numerous studies and some controversy
- The statistical signal presented in the study is of modest size, which means that researchers still can’t directly provide a prediction that can be used to alert the public
- This study provides evidence that there is a physical change that can be observed in the days before an earthquake, but further scientific study is needed to understand this process
The authors present an analysis of ground-based magnetic time-series measurements before 19 earthquakes ≥M4.5 in California drawing from over 330 000 site-days of measurement spanning a decade.
To perform a fair existential test for electromagnetic antecedents they applied a pre-specified statistical analysis with two key ideas.
First, the researchers combined signals from nearby (≤40 km) sites via spectral cross-power, and then looked for large spikes in the frequency domain (0.016–25 Hz).
“In this statistical case-control study, we used the machine learning concept of rigorously separated train and test sets of earthquakes which were generated via a rule-based query of the USGS earthquake catalog.
“Before each declustered earthquake, we constructed one period 24–72 hr before (the “precursor” or “p-period”) and a series of seven equally-sized preceding periods (“quiescent” or “q-periods”).
“We distilled the data in each period to a frequency-dependent feature—the 98th percentile of spectral cross power.”
They trained a model based on Linear Discriminant Analysis and applied the discriminator to the test set revealing a modest effect in the days leading up to an earthquake.
While the observed effect size is not directly useful for earthquake prediction (long a scientific goal), it suggests a relationship that should be further investigated for a physical link.
This study should enable further principled examinations of geomagnetic precursor signals and serve as a model for a sound statistical analysis process in this very challenging domain for data-driven science.
“We hope that these observations may ultimately contribute constructively to the literature on understanding processes that lead up to earthquakes.”
“Case-Control Study on a Decade of Ground-Based Magnetometers in California Reveals Modest Signal 24–72 hr Prior to Earthquakes” – AGU JGR Solid Earth – https://doi.org/10.1029/2022JB024109 – OPEN ACCESS
Featured image: Map of site locations and earthquakes considered in this study. Credit: AGU JGR Solid Earth
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