Conversion between the local magnitude (ML) and the moment magnitude (Mw) for earthquakes in the Croatian Earthquake Catalogue

Based on 153 earthquakes (1959–2020) listed in the Croatian Earthquake Catalogue, a conversion relation was obtained between the local magnitude ML,CR and the corresponding moment magnitude Mw as reported by the global and regional agencies. As errors were present in both variables the York regression was used. The best fit line is given by: MwL = (–0.106 ± 0.122) + (1.002 ± 0.027) ML,CR (coefficient of determination R2 = 0.90). The earthquakes considered occurred in Croatia and the neighbouring regions, and their local magnitudes ML,CR ranged between 3.5 and 6.5. Residual analysis suggests that an artificial positive magnitude shift of up to 0.3 magnitude units may have occurred in the early 1980s, when Wiechert mechanical seismographs were replaced by the instruments with velocity proportional recordings without proper recalibration of the magnitude formula. The slope of the regression close to 1.0 indicates that on the average the faults’ aspect ratio (width/length) is about 1/2.

A seismic investigation of the Witwatersrand earth tremors*

The results are presented of an investigation of the foci of Witwatersrand earth tremors by means of 5 small mechanical seismographs. Timing accuracy of about 0.05 sec. was obtained by the use of synchronous electric clocks at each station, the time marks from which were correlated daily by means of radio time signals. The instruments are described, and an account of the locating procedure is given. The average velocities for the area were found to be Vp = 5.40 ± .07 km/sec., Vs = 3.19 ± .03 km/sec. The energy in a large tremor was estimated to be of the order of 1018 ergs.

TRUE GROUND MOTION FROM MECHANICAL SEISMOGRAPH RECORDS

Records of several types of ground motion are obtained with two vertical mechanical seismographs of widely different characteristics, mounted on the same base and recorded on the same moving photographic paper. Companion records obtained are shown in one case to yield substantially the same calculated ground motion although the original records are very dissimilar.