抄録
Relations among several earthquake magnitude determination formulas were investigated by using data obtained by high resolution digital accelerometers. Digital accelerometer, JMA-87 type electromagnetic strong motion seismograph (ESM) which was recently deployed at 74 stations over the Japanese Islands by the Japan Meteorological Agency (JMA), is one of the wide-frequency band and large-dynamic range seismograph. Digital records enabled us to simulate any type of seismograms from the same data and to measure several maximum amplitude values for corresponding magnitude determination formulas. To evaluate consistency among the magnitude formulas, magnitude values from the same station were compared with each other in the present study. One of the advantageous points of the method is its independency from the influence of site condition and inhomogeneous attenuation along ray path on the observed amplitude.
As a first step, magnitudes which are derived from the Tsuboi's formula (MT) and the JMA-EMT formula (M67 or M76), both of them are employed as standard JMA magnitudes, are compared with each other. MT is calculated from the maximum amplitude of horizontal ground displacements recorded by medium period seismograph (JMA-59 type and JMA-S type, T=5s) and M67 and M76 are from the maximum amplitude of vertical ground velocity recorded by short period seismograph. JMA's magnitude is given as a arithmetic mean of both formulas. Although JMA-EMT formula was designed as a substitute of Tsuboi's formula for smaller earthquakes which can be recorded only by sensitive short period instruments, as several studies have already pointed out, it is consistent with Tsuboi's formula only in a narrow magnitude range. Compared to the Tsuboi's formula, while the JMA-EMT formula provides smaller magnitude values for larger earthquakes, it gives larger values for earthquakes which magnitudes are less than 3 and discrepancy increases as the size of earthquake becomes smaller. Introduction of a coefficient to the formula which adjusts the relation between magnitude value and logarithm of maximum amplitude value seems to be indispensable to correct the discrepancy.
Beside the coefficient, constant in the EMT formula depends largely on conditions of installation sites. Otherwise, borehole installation in a hard basement gives smaller magnitude value and surface installation on an alluvial soil basement gives large magnitude value. Classification of site effect is important to determine appropriate constant for EMT formula.
As a second step of this study, it is indicated that amplitude data within 50km epicentral distance provide smaller magnitude values for both Tsuboi's formula and EMT formula. The EMT formula also gives smaller magnitude value for data beyond 200km epicentral distance. There by, the study shows importance of appropriate selection of epicentral range for applying the formula.
Finally relation between JMA's magnitude and Richter's local magnitude (ML), which is the oldest magnitude scale and of the international scale, was investigated by using simulated Anderson-Wood seismograms. Results show that ML from simulated seismograms is generally larger than MT for smaller earthquakes. However, difference becomes smaller as earthquake magnitude becomes larger.
