Hypocenters for the December 2024 -January 2025 sequence of events near Kerkrade

Between 2024-12-13 and 2025-01-17 six events occurred near Kerkrade in Limburg, the Netherlands. The origin time of the first event is 2024-12-13T00:08:26 UTC and the local magnitude is 1.1. For the second event about 5 min 31 s later on 2024-12-13T00:14:05 UTC, the local magnitude is 1.7. The next three events all occurred on 2025-01-03. The origin times and local magnitudes are respectively 06:22:13 UTC and 0.9, 07:07:02 UTC and 2.4, and 15:26:15 UTC and 1.1. The event on 2025-01-17T20:33:47 UTC has the local magnitude 0.5. The magnitude 2.4 was felt by many people. About 140 online questionnaire earthquake forms were submitted to the KNMI. All events were detected by the KNMI network (KNMI, 1993) and located near-real time with the hypocenter method by Lienert et al. (1986). This hypocenter method uses an average 1D model for the south of the Netherlands. The hypocenters calculated with this method are published by the Royal Netherlands Meteorological Institute (KNMI) in the earthquake catalogue for tectonic events (Aardbeving catalogus, www.knmi.nl/kennis-endatacentrum/dataset/aardbevingscatalogus). In this report, the hypocenters are relocated using a modified methodology and velocity model for Limburg. The P-wave and S-wave traveltime picks for the events are the same ones as used in the operational KNMI hypocenter analysis. 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 (Spetzler et al., 2024). 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.