| Abstract |
Thousands of microquakes are often associated with an injection at geothermal sites. When processed using novel geophysical techniques, this cloud of microquakes effectively illuminates the subsurface, and can be used to monitor plume growth, measure source properties and identify otherwise hidden structures. The virtual seismometer method (VSM) is a new technique of seismic interferometry that provides precise estimates of the Green function (GF) between earthquakes. It is very sensitive to the source parameters (location, mechanism and magnitude) and to the Earth structure in the source region. Using VSM, we are able to focus sharply on the cloud of microseismicity. We can monitor the evolution of seismicity over time, measure changes in the style of faulting and sort microseisms by location and magnitude. In the far-field, when most of the stations in a network fall along a line between the two events, the result is an estimate of the GF between the two, modified by the source terms. In this geometry each earthquake is effectively a virtual seismometer recording all the others. When applied to microquakes, this alignment is often not met, and we need to address the effects of the geometry between the two microquakes relative to each seismometer. Using data from the Salton Sea geothermal region, we demonstrate the power of the technique, illustrating our ability to scale from the far-field, where sources are well separated, to the near field where their locations fall within each other's uncertainty ellipse. We are able to separate closely spaced events into separate subclusters and to measure differences in faulting mechanism. |