Assessment of the network operation of high-resolution Lippmann tiltmeters installed for the monitoring of the Mur-Mürz fault line (Austria)

Based on the decadal scientific cooperation between researchers of Geosphere Austria and the HUN-REN Institute of Earth Physics and Space Science, Hungary, a small polygonal network of Lippmann’s nanoradian-resolution 2D vertical pendulum type compact tiltmeters was established for the monitoring of ground tilts related to tectonic processes in the Mur-Mürz fault zone. Since the beginning of the year 2023, five stations have been operational at 5 Hz sampling rate, among them three stations are equipped also with Streckeisen STS2 or STS2.5 broadband seismometers. Although tiltmeters are applied usually for detecting low frequency deformations, in addition to long periodic signals (e.g. tidal tilt), waveforms of several local and plenty of distant seismic events also were recorded by them. The synchrony and consistency of tilt waveforms observed during local events stimulated the idea of testing the performance of the network by estimating epicentre locations of the events, independently from seismological data. This contributes to the knowledge of the performance, limitations, etc., of the tilt network as a whole system, and also of the high-frequency characteristics of the applied Lippmann-type tiltmeters. Since no standard procedure is available to process tilt data for seismological applications, two methods based on different simplifications of elastic wave propagation are provided for epicentre positioning. The geometric and the time delay inversion methods apply the plane and spherical wave front approaches, respectively. Both solve the problem of event localization in 2D based on the concept of apparent phase velocity of primary seismic waves. Generally the inner accuracy, i.e. the formal error estimates of the epicentre coordinates, of both methods are similar (~ ± 2 km). Although the network geometry was not optimized for such a task, the external positioning accuracy defined as the average deviation between the epicentres estimated by the applied time delay inversion method and the well-established seismological data processing is ~ 4.5 km (median: 2.3 km). The inner and the external positioning accuracy estimates suggest proper network operation and high consistency between recorded ground tilt and ground velocity data. It may give a chance to integrate the high-frequency tilt waveforms in further local seismo-tectonic investigations.