Abstract
Empirical formulas for the determination of the earthquake magnitude M from the duration of oscillation F-P have been derived from the observational data obtained by the Wakayama Micro-earthquake Observatory, Earthquake Research Institute.
The instruments used by the Observatory and its substations are the short period vertical seismographs of the same characteristics (T1=1.0sec, T2=0.025sec, velocity sensibility=40-120uμ kine/mm at 1 to 10c/s).
The formula obtained from the comparison of the F-P (sec) observed by the network with the M determined by the Japan Meteorological Agency is:
M=-2.53+2.85log(F-P)+0.0014Δ
where Δ is the epicentral distance in km. This formula is applicable to the earthquakes with Δ<1000km and focal depth ≤60km.
Since the coethclent for Δ is very small, the correction term of Δ can be neglected for the earthquakes with Δ<200km, and in this case the following simpler formula is more conveniently applied:
M=-2.36+2.85log(F-P).
The magnitudes obtained by the former and the latter formulas are designated, in the figures and table, MF-P and M′F-P respectively.
The largest merits of this method are:
1) Wide range of magnitude can be covered with a single seismograph, since there is no difficulty in reading F-P such as ‘saturation’ or ‘under exposure’ in the case of maximum amplitude.
2) The treatment is very simple, especially for the earthquakes of short epicentral distances.
