JMA magnitude scale

The JMA magnitude scale ( Japanese 気 象 庁 マ グ ニ チ ュ ー ド , Kishō-chō magunichūdo ), M _j or M _{jma for short} or with a capitalized index, is the measurement method used in Japan by the Japan Meteorological Agency (JMA, Kishō-chō ) of earthquakes - magnitudes .

The method was changed on September 25, 2003 and consists of 5 metrics that are used in the following priority: M _yy , M _yD , M _jV , M _jd and M _jv . It thus combines ground amplitudes for large earthquakes with ground velocity amplitudes for small earthquakes and is suitable for earthquakes of any depth.

Old method until September 24, 2003

The calculation rule was originally defined by Chūji Tsuboi in 1954 as a dimensionless number as follows:

${\displaystyle M_{\mathrm {j} }=\log _{10}\underbrace {\sqrt {A_{N}^{2}+A_{E}^{2}}} _{A}\,+1{,}73\log _{10}\left(\Delta \right)-0{,}83}$

where A denotes the movement of the surface as the maximum soil amplitude, measured by means of the horizontal north-south or east-west components in micrometers with a period of less than 5 seconds (in practice about 3 seconds) and the distance to the epicenter in Kilometre. ${\displaystyle A_{N}}$ ${\displaystyle A_{E}}$ ${\displaystyle \Delta }$

In principle, it corresponds linearly to the surface wave magnitude scale M _S (which, however, is measured at a period of 20 seconds). The deviation M _j - M _S for earthquakes of M _S > 6.5 is only −0.14. For M _S <5.0, however, this increases to +0.55 and the JMA magnitude scale then corresponds more to the short-period sky wave magnitude scale m _b .

For smaller earthquakes (M ≤ 5.5), the velocity amplitudes are still included and used according to Kanbayashi and Ichikawa (1977) or Takeuchi (1983):

${\displaystyle M_{\mathrm {j} }=\log _{10}\left(A_{Z}\right)+1{,}64\log _{10}\left(\Delta \right)+\alpha }$

where the maximum velocity amplitude is in cm ⁻³ / s and a seismometer- specific correction value. ${\displaystyle A_{Z}}$ ${\displaystyle \alpha }$

Both formulas are only used for near-surface earthquakes, i.e. H. those with a focus depth ≤ 60 km are used. Instead, the following formula according to Katsumata (1964) is used for greater focus depths: ${\displaystyle H}$

${\displaystyle M_{\mathrm {j} }=\log _{10}\left(A\right)+K\left(\Delta ,H\right)}$

the coefficient being taken from a list. ${\displaystyle K\left(\Delta ,H\right)}$

An attempt is made to use as many measuring stations as possible from whose individual magnitudes the mean value is then calculated.

New method from September 25, 2003

The new method uses the five metrics , , , and which are used in descending priority. ${\displaystyle M_{\mathrm {jJ} }}$ ${\displaystyle M_{\mathrm {jD} }}$ ${\displaystyle M_{\mathrm {jV} }}$ ${\displaystyle M_{\mathrm {jd} }}$ ${\displaystyle M_{\mathrm {jv} }}$

${\displaystyle M_{\mathrm {jJ} }}$is used as with the old method for large and near-surface earthquakes (H <60 km) and corresponds to Tsuboi's formula, based on the acceleration data from the local meteorological observatories .

If the JMA cannot (usually ) confirm the independence of a seismometer-specific correction value (= 0.2 for D93 seisometers) on the magnitude , the following formula from Katsumata (2004) is used: ${\displaystyle C_{D}}$ ${\displaystyle M_{\mathrm {j} }>=7}$

${\displaystyle M_{\mathrm {jD} }={\frac {1}{2}}\log _{10}\left(A_{N}^{2}+A_{E}^{2}\right)+\beta _{D}\left(\Delta ,H\right)+C_{D}}$

where describes the interdependence of both parameters. ${\displaystyle \beta _{D}\left(\Delta ,H\right)}$

For the measuring station, > 30 km and <700 km must apply. If fewer than three stations meet this condition, others up to <2000 km are used, the scale then being referred to as. ${\displaystyle H}$ ${\displaystyle \Delta }$ ${\displaystyle \Delta }$ ${\displaystyle M_{\mathrm {jd} }}$

For stations that meet > 5 km and <400 km, a scale based on speed magnitude according to Funasaki (2004) is used: ${\displaystyle H}$ ${\displaystyle \Delta }$

${\displaystyle M_{\mathrm {jV} }=\alpha \log _{10}\left(A_{Z}\right)+\beta _{V}\left(\Delta ,H\right)+C_{V}}$

${\displaystyle \alpha }$is a constant with the value 1 / 0.85, is the maximum velocity amplitude of the vertical components of an EMT, EMT76 or E93 seismometer in 10 ⁻⁵ m / s and analogous to . If fewer than 4 stations meet this condition, further up to <1000 km are used, the scale then being referred to as. ${\displaystyle A_{Z}}$ ${\displaystyle \beta _{D}\left(\Delta ,H\right)}$ ${\displaystyle C_{V}}$ ${\displaystyle M_{\mathrm {jD} }}$ ${\displaystyle \Delta }$ ${\displaystyle M_{\mathrm {jv} }}$

All these magnitudes are individually calculated and averaged for the measuring stations in question, then the respective station magnitudes that deviate by more than 0.5 from the respective mean value are removed and averaged again. , , , Or with a standard deviation of ≥ 0.35 are disregarded. ${\displaystyle M_{\mathrm {jJ} }}$ ${\displaystyle M_{\mathrm {jD} }}$ ${\displaystyle M_{\mathrm {jV} }}$ ${\displaystyle M_{\mathrm {jd} }}$ ${\displaystyle M_{\mathrm {jv} }}$

See also

• JMA scale for measuring intensity

Individual evidence

1. ↑ 気 象 庁 マ グ ニ チ ュ ー ド 算出 方法 の 改 訂 に つ い て . (PDF; 233 kB) Japan Meteorological Agency, September 17, 2003, accessed August 14, 2011 (Japanese). 
2. ↑ ^{a b c d e f} What is Mj? Broadband Seismic Network Laboratory, Earthquake and Volcano Data Center, National Research Institute for Earth Science and Disaster Prevention, accessed August 14, 2011 .  
3. ↑ ^{a b} Shin'ichi Noguchi: On the Relation between Surface-Wave Magnitude and JMA Magnitude . In: Journal of the Faculty of Science, Hokkaido University. Series 7, Geophysics . Volume 6, No. 1 , 1979, p. 213-224 ( handle.net ).
4. ↑ ^{a b c d} マ グ ニ チ ュ ー ド (M) と 震 度 に つ い て . (No longer available online.) Archived from the original on August 23, 2011 ; Retrieved August 14, 2011 (Japanese). Info: The archive link was inserted automatically and has not yet been checked. Please check the original and archive link according to the instructions and then remove this notice.