Some researchers have suggested that significant electromagnetic changes that can be detected prior to strong earthquakes are associated with the tectonics and subsurface structures beneath monitoring stations. Numerical simulation seems to support this phenomenon. However, to date, few field measurement data are available to address this issue. In this work, we used the shifting correlation method to analyze 2-month recordings of 20 geoelectric stations in Taiwan and found at least two stations had recorded geoelectric signals significantly correlated with known seismic events. These two stations are installed on large reverse faults, which are probably characterized by high stress concentrations before earthquakes. In addition, they share the same resistivity variation pattern, namely, a conductor layer sandwiched between two high-resistivity layers. Moreover, both stations are above the electrical resistivity boundaries, which means they have an increased probability of recording the abnormal disturbance once EM signals have been emitted from EQs. Our analysis results support the viewpoint that lateral and vertical electrical resistivity variation and conductors in the subsurface may be able to amplify seismic electric signals.
- seismic electric signals
- shifting correlation method
- Statistical seismology
- tectonics and deep structures in Taiwan