From March to May 2023 four events occurred near Klimmen in Limburg. The origin time of the first event on 01-03-2023 is 17:58:49 UTC and the local magnitude is 2.2. For the second event on 19-03-2023, the origin time is 01:23:36 UTC and the local magnitude is 1.4. The third event on 23-02-2023 has the origin time at 05:18:54 and the local magnitude is 0.6. Lastly, the fourth event on 06-5-2023 has the low local magnitude of 0.6 and the origin time is 21:03:03 UTC. All events were detected by the KNMI network and located near-real time with the hypocenter method (Lienert et al., 1986). This hypocenter method uses an average 1D model for the south of the Netherlands. The hypocenter found by this method is published by KNMI in the earthquake catalogue for tectonic events.
In this report, the hypocenters are relocated using a modified hypocenter methodology and velocity model for Limburg. The P-wave and S-wave traveltime picks for the events are unchanged. The elastic velocity model for Limburg used in the improved hypocenter estimation was derived from a seismic tomographic experiment by Reamer and Hinzen (2004). The refined hypocenter methodology estimates the epicenter and depth of the event by combining the classical P-S phase time difference for single stations with the EDT principle for P-phase times for pairs of stations. The uncertainty of the hypocenter is estimated as well. The location error incorporates the local variations of the velocity field, modelling effects as well as picking errors. The final result is an estimate of the hypocenter of the events and the associated uncertainty in terms of the 95% confidence region. The uncertainty in epicenter and depth is the standard deviation which is the 3 dimensional 95% confidence region divided by 2.7 (Wang B., 2015).
J. Spetzler. Hypocenters for the events between March and May 2023 near Klimmen
KNMI number: TR-404, Year: 2023, Pages: 10