Minimum recording times for determining short-term seismicity from microearthquake activity

Abstract The minimum microearthquake recording time needed to determine short-term seismicity (10 years) has been established for regions having the same degree of activity as Socorro (400 shocks with M ≧ 0 each year). An analysis of more than five years of microearthquake data indicates that about six months of recording are required for a reliable estimate of the earthquake-frequency relation from which seismicity is calculated. If the slope b of the earthquake-frequency law is assumed, the short-term seismicity can be determined from four weeks of recording. The danger in the latter method is that a small error in b leads to relatively large errors in seismicity. Long-term seismicity (100 years) predicted from the present level of micro-earthquake activity in Socorro is much lower than the known seismicity of the region. The number of microearthquakes would have to increase by a factor of 20, or the value of b decrease approximately 20 per cent, before prediction would agree with the long-term seismic activity of the region.