Calibration of local seismic arrays is crucial to quantify the magnitude of local microseismic events. This is traditionally done by performing a site survey consisting of calibration shots at known locations with known strength. However, when a calibration survey cannot be conducted, alternative calibration methods are required. We have demonstrated the feasibility of magnitude estimation of microseismic events recorded at the CO2 storage site at Ketzin (Germany), by analyzing earthquake signals that have been recorded on the noncalibrated geophone array and on standardized seismometers of a regional seismological network. We estimated a linear calibration relation by fitting three different signal attributes for 10 distant earthquakes. Through extrapolation of the linear fit toward lower magnitudes, we estimated the local magnitude of weak local seismic events. Reported magnitudes of the distant earthquakes show significant variation because there are different methods and constants used to calculate a magnitude estimate from recordings on multiple seismometers. As a consequence, there was a considerable spread in our magnitude estimates for the Ketzin events. We found magnitude values in the range between 2.2 and 0.3 for two local events recorded at the Ketzin site. We expect that by incorporating additional distant earthquake data, the uncertainty in this estimate can be further reduced.

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