| Abstract |
We deployed a 6 station borehole microearthquake network around an Engineered Geothermal System stimulation-site beneath the city of Basel Switzerland. The borehole stations ranged in depth between 400 m to 2,700 m. During the stimulation several thousand accurately-locatable microearthquakes were generated, culminating in an M~3.4 event late in this man-made tremor sequence. This event exceeded the license limits for the stimulation, shutting down the associated Hot/Dry Rock power venture. Using standard source parameter analysis procedures and theory we have plotted the radiated energy versus seismic moment of the EGS sequence. The results suggest that the bulk of the stimulated events do not follow standard source scaling relations, with only a small fraction of small events and event larger than about M~1, falling along a constant stress drop line. Among the possible explanations for this break down are continued near source attenuation and scattering effects, which could be hiding the critical high frequency portions of the borehole seismic data. It could also be that the stimulation events actually follow different source physics than standard double couple events, owing to their origin in hydraulic fracturing. If so, then it might be possible to use this type of analysis to forecast the likely distribution of induced event magnitudes prior to licence exceeding induced earthquakes. |