Non‐self‐similar source property for microforeshocks of the 2014 Mw 6.2 Northern Nagano, central Japan, earthquake

Abstract Foreshocks are considered as part of the preparation process for large earthquakes and, as such, can provide important constraints on earthquake generation. We examine the characteristics of the 4 day long foreshock sequence of the 2014 Northern Nagano earthquake (Mw 6.2) using seismograms recorded by a dense seismic observation network including data from a deep borehole closest to the foreshock region. The improved earthquake catalog shows a slow‐speed migration of the sequence toward the main shock hypocenter, implying possible slow‐slip transients which should have caused stress loading at the main shock hypocenter. An analysis of source spectra reveals that foreshocks for Mw < 1.5 display a weaker dependence of corner frequency on magnitude than expected for self‐similar scaling, suggesting that these microforeshocks can be regarded as small low‐frequency earthquakes. So far, we might have missed the expected weak low‐frequency preseismic signals, because they emerge only below a certain magnitude and are generally embedded in background noise.