| What is Mj ? | ||||||||||||||||||||||||||||||||||||
|
Mj is the local magnitude defined and calculated by Japan Meteorological Agency (JMA).
It is one of the magnitude scales based on maximum amplitudes of seismograms.
There are some good points in Mj. First, Mj can be calculated faster than Mw. Second, Mj can be calculated for small earthquakes. And there is a large amount of earthquake magnitude data since August 1923. Procedure to calculate Mj is as follows. (See, for details, for example, JMA(2004) ). There are 5 magnitudes, MjJ, MjD, MjV, Mjd, and Mjv, as a candidate of Mj. Mj is adopted according to the priority order MjJ > MjD > MjV > Mjd > Mjv. (1) MjJ: The magnitude at the Local Meteorological Observatories It is calculated only for large and shallow (The depth is less than 60km.) earthquakes using acceleration data of the seismic intensity-meters at the local meteorological observatories of JMA. MjJ is given by the average of
using the maximum displacement amplitudes at the observatories (Tsuboi,1954).
Here the acceleration data are twice integrated to obtain the displacement, to which the high-pass (6s) filter is applied to simulate the mechanical strong motion seismogram. This method will be tentatively used while JMA can not confirm the independence of 
in the next eq.(2) on the magnitude for large earthquakes, especially of Mj7 class or above.
(2) MjD, Mjd : Displacement magnitude It is calculated as the average of
at stations in a range of the hypocentral distance more than 30km and the epicenter distance less than 700km, which record the maximum amplitudes of horizontal displacement (Katsumata,2004).
If the number of stations involved in the average is less than 3, the stations up to Δ=2000km are took into account. If the number of the stations used to obtain MjD is 2, it is denoted as Mjd. (3) MjV, Mjv: Velocity magnitude It is calculated as the average* of
at stations in a range of the hypocentral distance more than 5km and the epicenter distance less than 400km, which record the maximum amplitudes of velocity in vertical component (Funasaki et al.,2004).
If the number of stations involved in the average is less than 4, the stations up to Δ=1000km are took into account. If the number of the stations used to obtain MjV is 2 or 3, it is denoted as Mjv. (a) The averaging procedure in the above (1)-(3) is as follows. First JMA calculates the mean of magnitudes at all the stations. Then JMA recalculates the mean and the standard deviation of magnitude of stations, discarding those values deviating more than 0.5 from the initial mean. The mean values are to be adopted as magnitudes only if the standard deviations are less than 0.35. References: Japan Meteorological Agency (2004), The seismological and volcanological bulletin of Japan for January 2004, [2] - [4]. Tsuboi,C. (1954), Determination of the Gutenberg-Richter's magnitude of shallow earthquakes occurring in and near Japan (in Japanese), Zisin, 7, 185-193. Katsumata, A. (2004), Revision of the JMA Displacement Magnitude (in Japanese), Quart. J. Seis., 67, 1-10. Funasaki, J. and Earthquake Prediction Information Division (2004), Revision of the JMA Velocity Magnitude (in Japanese), Quart. J. Seis., 67, 11-20. | ||||||||||||||||||||||||||||||||||||
|
▶ Hi-net top page ▶ K-NET/KiK-net top page ▶ V-net top page (in Japanese) A question please click here. |
|
Broadband Seismic Network Laboratory,
Earthquake and Volcano Data Center,
Earthquake and Volcano Research Unit, Monitoring and
Forecast Research Department,
National Research Institute for Earth Science and Disaster Prevention. 3-1 Tennodai, Tsukuba City, Ibaraki Prefecture, 305-0006, JAPAN Copyright © National Research Institute for Earth Science and Disaster Prevention, All rights Reserved. | |