Abstract
Focal mechanisms of shallow crustal earthquakes with magnitude 4 or less remain unresolved in many regions of the world due to the sparseness of seismic networks. Relatively distant stations must be used, but modeling those waveforms is difficult due to imprecision of seismic-velocity models.
The waveform inversions also suffer from instrumental problems, such as flipped channel sign and errors in gain or timing. In such situations, waveforms should therefore be substituted by a robust characteristic which still carries information about focal mechanism and moment magnitude.
We propose inversion of waveform envelopes. Three application examples demonstrate the performance of the method for earthquakes in various ranges of epicentral distance and frequency.
In the examples, the focal mechanisms of the sample earthquakes are known from the waveform inversion approach based on near-source stations. The near-source stations are then ignored completely.
Use of the remaining stations fails to provide the true mechanism through waveform inversion, but a correct estimate of the solution is found using envelopes. This method may be of interest to seismologists who need to compute focal mechanisms for stress-field studies in sparse networks, where traditional waveform inversion and amplitude inversion methods often fail.