Spatiotemporal variations in the b-value of earthquake magnitude–frequency distributions: Classification and causes

• Published in: Tectonophysics 2014-03, Vol.615-616, p.1-11
• Main Authors: El-Isa, Z.H., Eaton, David W.
• Format: Article
• Language: English
• Subjects: b-Value; Classification; Earth; Frequency–magnitude; Geophysics; Heterogeneity; Regression; Seismic phenomena; Seismicity; Spatiotemporal; Stresses
• ISSN: 0040-1951; 1879-3266
• DOI: 10.1016/j.tecto.2013.12.001

Interpretation of the b-value of earthquake frequency–magnitude distributions has received considerable attention in recent decades. This paper provides a comprehensive review of previous investigations of spatial and temporal variations in b-value, including their classification and possible causes. Based on least-squares regression of seismicity data compiled from the NEIC, IRIS and ISC catalogs, we find an average value of 1.02±0.03 for the whole Earth and its two hemispheres, consistent with the general view that in seismically active regions the long-term average value is close to unity. Nevertheless, wide-ranging b-variations (0.3≤b≤2.5) have been reported in the literature. This variability has been interpreted to arise from one or more of the following factors: prevailing stress state, crustal heterogeneity, focal depth, pore pressure, geothermal gradient, tectonic setting, petrological/environmental/geophysical characteristics, clustering of events, incomplete catalog data, and/or method of calculation. Excluding the latter, all of these factors appear to be linked, directly or indirectly, with the effective state of stress. Although time-dependent changes in b-value are well documented, conflicting observations reveal either a precursory increase or decrease in b value before major earthquakes. Our compilation of published analyses suggests that statistically significant b-variations occur globally on various timescales, including annual, monthly and perhaps diurnal. Taken together, our review suggests that b-variations are most plausibly linked with changes in effective stress. •Temporal and spatial b-variations express in different ways prior to and after major earthquakes.•Factors including tectonic setting, focal depth and incomplete catalog data affect b value.•The primary factor is effective state of stress on a fault and surrounding rockmass.