Estimation of the moment magnitude and its uncertainty for small-to-moderate earthquakes in Sweden using a generalised inversion approach.

Probabilistic seismic hazard assessment (PSHA) is challenging in cratonic regions such as Sweden, where the characteristics of strong motion (detrimental for structures) are uncertain due to a lack of data. The current approach is therefore to use available data sets of small-to-moderate earthquakes to identify seismogenic areas and to adapt models from more seismically active regions. One issue encountered in this process is estimating the moment magnitude (Mw) of these earthquakes. In fact, evaluation of local magnitude (Ml) is preferred for magnitude <4 earthquakes due to the difficulty of deriving Mw using standard methods.

In this work, we propose to estimate the Mw of earthquakes in Sweden using a generalised inversion technique (GIT), and to investigate the uncertainty of this measurement in this seismic context. To do so, a ground-motion data set of Ml=0-4.1 earthquakes recorded by the Swedish National Seismological Network (SNSN) is compiled, and the non-parametric generalised inversion of Oth et al. (2011) is applied. This approach identifies the source spectrum of earthquakes, the apparent attenuation of the region and the site amplification of stations by performing a spectral decomposition of the Fourier amplitude spectrum (FAS). The parameters of the Brune (1970) source model and the frequency-dependent elastic and anelastic attenuation models are derived from these terms in post-inversion.

We present here the resulting source parameters (Mw and corner frequency fc) that are analysed and compared with the moment based Ml computed by the SNSN. The consistency of these estimates is then evaluated using higher-sample-rate recordings from a temporary network that was deployed for four years in Sweden’s most seismic active region. The 100 Hz sampling rate of the permanent stations limits the estimation of Mw and fc for earthquakes of magnitude <2, as their fc is expected to exceed the Nyquist frequency (~45 Hz). The 200 Hz sample rate set for the temporary network enables the source spectrum to be derived up to 90 Hz and therefore allows the veracity of the Brune (1970) model (which has been derived from a shorter frequency band) to be analysed.

Applying this inversion algorithm also provides new insights into anelastic attenuation in cratonic regions and into site amplification observed at hard rock/bedrock sites (which constitute the majority of SNSN installations). Knowing the apparent attenuation and site conditions is useful for future studies, especially for correcting the FAS at stations to measure Mw using other approaches or models, and for achieving a quasi real time estimate.