The magnitude scale
MJ, defined by TSUBOI (1954), is widely used for local earthquakes in Japan. JMA (Japan Meteorological Agency) has officially applied it to amplitude data obtained by the JMA network of seismographic stations. The average period of the waves used is usually about 3sec. The
MJ scale agrees approximately with the 20sec surface-wave magnitude
MS around
MS=7. However,
MJ deviates very systematically from
MS as
MS decreases, and
MJ is overestimated by as much as 0.6 at
MS=4. For determining the equivalent of
MS from JMA data, the new formula is defined to be
MS(JMA)=1.32log
A+2.61log Δ-4.18, where
A(μm) is the combined maximum horizontal amplitudes available from JMA and Δ(km) is the epicentral distance. The new formula is regressed against reliable
MS values. The
MS(JMAA) and
MS values from the observed data are essentially equivalent between
MS=4 and 8.5. The coefficient of log
A in the new formula is explained from the scaling law of seismic spectra, if the difference of the wave period used between the present study and the original definition of
MS is considered.
抄録全体を表示