  With the present study we introduce a fast and robust method to calculate the
source displacement spectra of small earthquakes on a local to regional scale.
The work is based on the publicly available Qopen method of full envelope
inversion which is further tuned for the given purpose. Important source
parameters -- seismic moment, moment magnitude, corner frequency and
high-frequency fall-off -- are determined from the source spectra by fitting a
simple earthquake source model. The method is demonstrated by means of a data
set comprising the 2018 West Bohemia earthquake swarm. We report moment
magnitudes, corner frequencies, and centroid moment tensors inverted from short
period body waves with the Grond package for all earthquakes with a local
magnitude larger than 1.8. Moment magnitudes calculated by envelope inversion
show a very good agreement to moment magnitudes resulting from the
probabilisitc moment tensor inversion. Furthermore, source displacement spectra
from envelope inversion show a good agreement with spectra obtained by multiple
taper analysis of the direct onsets of body waves, but are not affected by the
large scatter of the second. The seismic moments obtained with the envelope
inversion scale with corner frequencies according to $M_0 \propto
f_{\mathrm{c}}^{-4.7}$. Earthquakes of the present data set result in a smaller
stress drop for smaller magnitudes. Self-similarity of earthquake rupture is
not observed. Additionally, we report frequency-dependent site amplification at
the used stations.
