Fast and robust earthquake source spectra and moment magnitudes from envelope inversion

TL;DR

This paper presents a fast, robust envelope inversion method to determine earthquake source spectra and magnitudes, validated on the 2018 West Bohemia swarm, showing good agreement with traditional methods and revealing insights into earthquake source properties.

Contribution

The study introduces a tuned envelope inversion technique for rapid, reliable estimation of earthquake source parameters from local to regional scales, improving upon existing methods.

Findings

Envelope inversion yields moment magnitudes consistent with probabilistic moment tensor results.

Source spectra from envelope inversion align well with multiple taper analysis.

Seismic moments scale with corner frequency as M0 ∝ fc^{-4.7}.

Abstract

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.