Underestimation of Microearthquake Size by the Magnitude Scale of the Japan Meteorological Agency: Influence on Earthquake Statistics
Authors:Uchide, Takahiko Imanishi, Kazutoshi
Volume:123
Published:2018
Document Type:Article
Abstract: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. Plain Language Summary Defining and measuring the size of the phenomena of interest provide the basis of understanding. Magnitude scale (or Richter scale) is the measure for earthquake size. Many scales are used based on convenience, simplicity, physical meaning, and usability. The Japan Meteorological Agency magnitude (M-j), a convenient one, is based on the maximum amplitude of the seismic wave record, whereas the moment magnitude (M-w), a physics-based one, uses the seismic moment, a product of the fault area, the average fault slip, and the rigidity of rocks around the fault. They generally agree for moderate earthquakes. However, this paper reports that M-j is lower than M-w for microearthquakes (smaller than magnitude 3). M-j=1 is equivalent to M-w=2. This is of particular importance since a difference of 1 in M-w corresponds to a 32 times difference in the seismic moment. Though this paper carefully estimated M-w, usually only M-j or similar scales are usable for microearthquakes because of the ease of the calculation. One should be cautious using data analyses based on the magnitude of microearthquakes for this problem. The magnitude scales provide basic information on earthquakes and are known to the public; however, they can still be problematic.
Author Information
Corresponding Author:Uchide, T (通讯作者)，Geol Survey Japan, Natl Inst Adv Ind Sci & Technol AIST, Tsukuba, Ibaraki, Japan.
Reprint Address:Uchide, T (通讯作者)，Geol Survey Japan, Natl Inst Adv Ind Sci & Technol AIST, Tsukuba, Ibaraki, Japan.
Addresses:[Uchide, Takahiko; Imanishi, Kazutoshi] Geol Survey Japan, Natl Inst Adv Ind Sci & Technol AIST, Tsukuba, Ibaraki, Japan
E-mail Addresses:t.uchide@aist.go.jp