Relative Moment Tensor Analysis of the DEEP Heat Enhanced Geothermal System, Finland

A 40MWth geothermal district heating plant was planned in Espoo, Finland. Heat was to be mined from a granite reservoir at 6100m depth with a temperature of 120°C. To achieve sufficient flow rates, the reservoir was hydraulically fractured, but ultimately permeability was too low and the project was suspended. The wells have been donated to science and an ICDP proposal is in preparation. We here present relMT, a new method to estimate relative earthquake moment tensors (MT) of weak seismic events. Moment tensors yield a description of the size and orientation of the fracturing process at depth. They are important to know, because complexity and size of the activated fracture network limit the maximum magnitude of seismicity and contribute to the success of the stimulation. Relative MTs require less subsurface information than absolute MTs and can be computed small shaking events, allowing to characterize subsurface fracture networks at a finer scale than previously possible.