A New Empirical Magnitude Scaling Relation for Switzerland

Abstract 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.