A New Empirical Magnitude Scaling Relation for Switzerland

We estimate moment magnitudes M-w for earthquakes in Switzerland recorded between 1998 and 2009 using three different spectral methods. The M-w estimation in Switzerland is extended to lower magnitudes (local magnitude M-L 0.1), and scaling relations between M-L and M-w are investigated. Above M-L 4, the obtained M-w estimates are consistent with the previously obtained scaling relation of M-w = M-L -0.3 at the Swiss Seismological Service (SED). Below M-L 4, all three methods indicate that a 1:1-type relationship is inappropriate. Therefore, we propose a new piecewise empirical scaling relation for earthquakes in Switzerland. The scaling is linear below M-L 2 and above M-L 4. To obtain a smooth transition between the two linear scales we fit a quadratic relation in between (2 <= M-L <= 4). This scaling relation is also consistent with M-w estimates from moment-tensor (MT) solutions based on broadband waveform fitting of local earthquakes with M-L > 3.0. We have tested all three methods carefully to ensure that the observed break in scale at around M 3 cannot be attributed to bias in the M-w determination. However, we cannot determine with certainty from the dataset at hand whether the break in scaling is due to bias in the routine determination of M-L or to physical properties of the source.