Premium
Non‐self‐similar source property for microforeshocks of the 2014 M w 6.2 Northern Nagano, central Japan, earthquake
Author(s) -
Imanishi Kazutoshi,
Uchide Takahiko
Publication year - 2017
Publication title -
geophysical research letters
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.007
H-Index - 273
eISSN - 1944-8007
pISSN - 0094-8276
DOI - 10.1002/2017gl073018
Subject(s) - foreshock , hypocenter , seismology , seismogram , geology , earthquake swarm , shock (circulatory) , magnitude (astronomy) , aftershock , geophysics , induced seismicity , physics , astronomy , medicine
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 ( M w 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 M w < 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.