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Underestimation of Microearthquake Size by the Magnitude Scale of the Japan Meteorological Agency: Influence on Earthquake Statistics
Author(s) -
Uchide Takahiko,
Imanishi Kazutoshi
Publication year - 2018
Publication title -
journal of geophysical research: solid earth
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.983
H-Index - 232
eISSN - 2169-9356
pISSN - 2169-9313
DOI - 10.1002/2017jb014697
Subject(s) - magnitude (astronomy) , richter magnitude scale , microearthquake , moment magnitude scale , amplitude , seismology , attenuation , geology , physics , geodesy , mathematics , induced seismicity , geometry , astrophysics , scaling , quantum mechanics , optics
Magnitude scales based on the amplitude of seismic waves, including the Japan Meteorological Agency magnitude scale ( M j ), are commonly used in routine processes. The moment magnitude scale ( M w ), however, is more physics based and is able to evaluate any type and size of earthquake. This paper addresses the relation between M j and M w for microearthquakes. The relative moment magnitudes among earthquakes are well constrained by multiple spectral ratio analyses. The results for the events in the Fukushima Hamadori and northern Ibaraki prefecture areas of Japan imply that M j is significantly and systematically smaller than M w for microearthquakes. The M j ‐ M w curve has slopes of 1/2 and 1 for small and large values of M j , respectively; for example, M j = 1.0 corresponds to M w = 2.0. A simple numerical simulation implies that this is due to anelastic attenuation and the recording using a finite sampling interval. The underestimation affects earthquake statistics. The completeness magnitude, M c , for magnitudes lower than which the magnitude‐frequency distribution deviates from the Gutenberg‐Richter law, is effectively lower for M w than that for M j , by taking into account the systematic difference between M j and M w . The b values of the Gutenberg‐Richter law are larger for M w than for M j . As the b values for M j and M w are well correlated, qualitative argument using b values is not affected. While the estimated b values for M j are below 1.5, those for M w often exceed 1.5. This may affect the physical implication of the seismicity.