Non‐self‐similar source property for microforeshocks of the 2014 M_w 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.

Investigating microearthquake finite source attributes with IRIS Community Wavefield Demonstration Experiment in Oklahoma

“…Earthquake source spectra are often modelled to investigate the finite source attributes of earthquakes (e.g. Houston & Kanamori 1986;Houston 1990;Prieto et al 2004;Baltay et al 2010;Chen & Shearer 2011Baltay et al 2013;Uchide et al 2014;Goebel et al 2015;Imanishi & Uchide 2017). Two key parameters describing an earthquake spectrum are the average corner frequency (f c ) and fall-off exponent (n).…”

“…The geometric mean is performed here to have the arithmetic mean of the spectra in the log space. Finally, we apply a nonlinear least-squares algorithm to fit the spectra from ∼5 to ∼35 Hz with the Boatwright spectral model assuming γ = 2 (Boatwright 1980;Huang et al 2017;Imanishi & Uchide 2017). The spectra fitting range is suggested in Huang et al (2017) for M2-induced earthquakes in Arkansas.…”

A source model for earthquakes near the nucleation dimension

“…In contrast, observations suggesting a break in self-similarity have been reported in local studies (e.g. Harrington & Brodsky, 2009;Bouchon et al, 2011;Lin et al, 2016;Imanishi & Uchide, 2017;Trugman & Shearer, 2017;H. Wang, Ren, Wen, & Xu, 2019;Mayeda, Malagnini, & Walter, 2007;Bindi, Spallarossa, Picozzi, & Morasca, 2020), even though their interpretation is hampered by well known artifacts due to trade-offs between path and source effects and attenuation of high-frequencies (Abercrombie, 1995;Ide, Beroza, Prejean, & Ellsworth, 2003;Abercrombie, 2021).…”

Spatiotemporal Variability of Earthquake Source Parameters at Parkfield, California, and Their Relationship With the 2004 M6 Earthquake

“…Stress drop has been found to be independent of magnitude by multiple studies, and compilations of studies, covering different magnitude ranges, from small and moderate (Allmann & Shearer, 2007;Goebel et al, 2015;Imanishi & Ellsworth, 2006;Uchide et al, 2014) to large ones (Allmann & Shearer, 2009). In contrast, other studies have reported non-self-similarity, typically over smaller magnitude ranges (Bindi, Spallarossa, et al, 2020;Imanishi & Uchide, 2017;Mayeda et al, 2005;Oye et al, 2005). Spatial and temporal variations of stress drop have the potential to reveal heterogeneities and changes in stress distribution within fault zones (e.g., Allmann & Shearer, 2007;Chaves et al, 2020;Chen & Shearer, 2013;Moyer et al, 2018;Ruhl et al, 2017;Shearer et al, 2006;Uchide et al, 2014).…”