Seismicity of Victoria to 1980

Since 1976 a number of analogue and digital seismographs, which record high frequency seismic waves in the range 4 to 20 Hz, have been installed in Victoria. These enable the detection and location of microearthquakes with Richter magnitude Ml down to less than zero. Because there are many more smaller than larger earthquakes, about 250 events are now being located in Victoria each year, compared with about 25 per year between 1960 and 1975, and an average of two per year between 1840 and 1959. Despite an uneven statewide seismograph coverage, the new instruments and new lithospheric models have led to substantial improvements in earthquake location accuracy. All Victorian earthquakes with reliable depth determinations have been found to lie within the crust, most at a depth of less than 18 km. Some very small but shallow microearthquakes have been felt or heard. The isoseismal map for such an event (of magnitude Ml 1.3) which occurred at Preston during July 1976, is given.

The greater range of magnitudes now recorded permits more reliable estimates of seismicity. On average, the recurrence of earthquakes in Victoria is represented by: log₁₀ (P) = 0.92M_L —2.40, where P is the return period of an earthquake with magnitude greater than M_l in an area of 100 km square. It is suggested that attenuation of seismic wave amplitudes with distance is quite high in Victoria.