Microearthquakes and seismic risk

Paper is presented in the nature of a review of the potential of microearthquake surveys in seismotectonic stc studies, particularly seismic-risk analyses. Miscroearthquakes are generally considered as shocks with magnitude less than 3. They radiate, in general, higher frequencies than larger earthquakes and therefore microearthquake seismographs must be sensitive to frequencies typically up to 30 or 40Hz. Mircroearthquakes are useful in seismotectonic studies because: (1) a greater number of small than large shocks occur in the same time, (2) extension to lower magnitudes of frequency-of-occurrence curves can yield more reliable seismic-risk statistics, (3) due to the reduced scale, both in space and time, very precise focal location is possible, often enabling mapping of fracture surfaces, (4) focal-mechanism solutions can further determine the fault surface and, in addition, the direction of slip, and (5) continuous monitoring of seismic activity is possible through the means of a permanent network of microearthquake systems. (author)