Tuning the GRID MT method for the moment tensor inversion of small to moderate size earthquakes

In order to rapidly detect, locate and characterize seismic events at regional scale, we are currently improving the parametrization of a continuous grid-search moment tensor inversion tool, called GRiD MT (Grid-based Real-Time Determination of Moment Tensor, Kawakatsu, 1998, Tsuruoka, 2009, Guilhem et al., 2011 and 2013, Menager et al., 2021). Here, we first design the procedure for small to moderate size earthquakes in south-eastern France, but we target its expansion to any potential regions of interest. This rapid and continuous moment tensor approach strongly depends on an adequate pre-selection of inversion parameters such as velocity models, station set, frequency band, grid spacing. In a proof of concept for France, we find source solutions (mechanism and magnitude) for two recent and moderate earthquakes (Mw 4.8 2019 Le Teil and M4.9 2014 Barcelonnette) very similar to those found by other institutes (CEA, GEOAZUR, USGS, INGV, …).

Moreover, instead of using a fixed time window in the inversion, we show that the quality of the solution can be improved by modifying each station’s time window depending on P and S wave travel times. This appears to be particularly true for small magnitude earthquake and potentially noisy data.

However, the previous selected parameters for Le Teil and Barcelonnette events are not the best ones to characterize the lower size events (3.4 < Mw < 4.0). We also propose an approach for automatically defining the best set of stations and frequency band depending on the targeted magnitudes in a region of interest. Based on signal/noise ratio (SNR) analysis to determine the signal quality for each station of a possibly dense seismic network, it helps the analyst in its GRiD MT parametrization. Moreover, here, azimuthal coverage is also taken into account in the station selection.

We illustrate the relevance of these proposed improvements with a selection of events. The ultimate aim is to facilitate the implementation of the GRiD MT method for other regions of interest and for different magnitude ranges.