Microearthquake activity recorded by portable seismographs of high sensitivity

Abstract

The increase in frequency of occurrence of earthquakes with decreasing magnitude is well known. In a few cases observations have shown that this relation holds for extremely small events, including those with magnitudes well below zero, and that the energy of the smaller shocks is confined largely to the higher seismic frequencies. These facts suggest that portable seismographs with ultra-high sensitivity might record a sufficient number of nearby microearthquakes in a short interval of time, say one day, so that some measure of the seismic activity of a very local area might be obtained very quickly. This idea was tested in west central Nevada where ten sites were occupied for short intervals of time. Microearthquakes were recorded at rates ranging from several per day to over two hundred per day. Generally, consistently high microseismicity was observed in areas of recent faulting. A lower level of activity, well above that of aseismic areas however, was observed at other sites in Nevada. Some indication of variation of microearthquake activity with time was obtained. Activity averaged over an interval of about ten hours usually differed by less than a factor of two from the overall mean for that site. In one case, however, swarms of events persisting for a total interval of a little more than one day were observed. The method appears very promising as a technique for monitoring current tectonic activity.