Cross-validating Distributed Acoustic Sensing and Seismic Records for Shallow Ground Motion and Near-Surface Properties

Distributed Acoustic Sensing (DAS) offers dense spatial sampling of ground motion and has the potential to perform detailed seismic monitoring and constrain shallow velocity structure. In this study, we analyze ground motion recorded by broadband seismometers and a fiber-optic interrogator of two shallow tectonic earthquakes in the Roerdalen region (The Netherlands–Germany border) with local magnitudes M_L 2.2 (2025-09-09) and M_L 1.9 (2025-09-15) and hypocentral depths of ~15 km to quantify the differences in sensitivity and magnitude estimates from each type of instrumentation. The Distributed Acoustic Sensing (DAS) recordings consist of ground strain sampled at 250 Hz on a 30 km telecommunications dark-fiber with a channel spacing of 5 m and a gauge length of 50 m. Seismometer recordings consist of ground velocity sampled at 100 Hz on a Trillium Compact 20 s seismometer that has a flat frequency response up to ~100 Hz. Both types of sensors recorded the earthquakes with a minimum epicentral distance of ~20 and 10 km, respectively. We will present results showing the differences in frequency sensitivity, conversions to ground displacement, and estimated magnitudes, as well as an interpretation of differences based on the shallow ground velocity. 

We first convert DAS recordings that are initially measured in strain to ground displacement using a semblance-based approach, as well conventional seismic recordings initially recorded in velocity. We make a quantitative comparison of waveform characteristics, including amplitude-frequency dependence and its variability in space for point-wise seismic sensor measurements vs. DAS measurements. We will present an interpretation of the results based on the context of geological setting to identify spatial variations that cannot be resolved by the sparse seismic network alone. As DAS measurements reveal significant lateral variability in ground motion amplitudes that suggest a strong influence of near-surface conditions (density) and/or local coupling effects, we will also quantify the relative influence of each using a comparison of strain and converted ground displacement. In addition, we explore approaches to estimate earthquake magnitude from DAS data by relating observed strain amplitudes to ground-motion parameters derived from the co-located seismometer. Preliminary results suggest that DAS-based observations capture the relative scaling between the two events and show promise for magnitude estimation when calibrated against conventional seismic sensors. Our findings demonstrate the value of DAS for high-resolution observations of near surface properties and their influence on earthquake waveforms.  They also highlight the potential of DAS to complement existing seismic networks for monitoring small-magnitude earthquakes.