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Distributed Dynamic Strain Sensing (DDSS), a.k.a. Distributed Acoustic Sensing (DAS), is becoming a popular tool for volcano monitoring. The sensing method relies on measuring the phase-shift of Rayleigh back-scattered light throughout the fibre due to strain variations in the fibre glass. This provides distributed strain-rate measurements at fine temporal and spatial sampling intervals. During 3 months in 2019, we recorded signals from thousands of mild volcanic explosions from Mt. Etna using a multi-instrument network deployed in an area at ca. 2.5 km distance from the active craters. Infrasound sensors were laying at the surface with a dense array of broadband seismometers (BB). Two types of fibres were also buried ca. 30 cm depth in the non-consolidated scoria from the area. First fibre was a 1.5 km long standard fibre, interrogated with an iDAS unit. The second fibre was a 0.5 km long engineered fibre, interrogated with a Carina unit. Relation between infrasound and DDSS data suggests a ground response of the loose scoria due to the acoustic pressure waves from explosions. Further analysis suggests a non-linear relationship between acoustic pressure and strain-rate data. However, signal saturation is encounter in some of the strain-rate data, which affects the interpretation of the non-linear relation. Therefore, we present an algorithm to correct the signal artefacts, allowing us to restore the true strain-rate signal and exceed the dynamic range limited by the initial DDSS recording parameters. The outcome includes strategies in the selection of acquisition parameters prior to DDSS campaigns to avoid signal saturation., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Horizontal-to-vertical (H/V) spectral ratios have proven to be an inexpensive and easy tool to gain insight into local site effects. Here we attempt to extend the method to distributed acoustic sensing data by combined processing of records from an array of 26 broadband seismometers and a 1.5 km long fibre optic cable on the North-East flank of Etna volcano. Using the same processing steps as for conventional H/V computing on seismometers, we derive spectral ratios along the trajectory of the fibre.Results show, that this approach is suited to assess the site response in higher spatial resolution than with broadband seismometers only. The H/V peak frequency distribution generally agrees with results from the array, in some parts indicating otherwise not resolved geological structures. Localized strong amplitude changes in the spectral ratio, coincide with the cable crossing known fault zones., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

The Taiwan Milun fault zone located at the boundary between the Eurasian and Philippine Sea plates. This fault slips frequently and produced large earthquakes, as for example the Mw6.4 Hualien earthquake (6 February 2018). We map and observe the fault zone and its behavior at depth by high spatial resolution dynamic strain sensing with optical fiber. In 2021-2022, we drilled and cored the fault, and deployed a 3D multi-cross-fault fiber array comprising a borehole loop with a depth of 700 m (Hole-A, Hanging wall site, crossing the fault at depth), a surface array crossing the fault rupture zone using commercial fiber, and a second borehole loop of 500m fiber (Hole-B, Footwall site). The high spatial resolution from distributed acoustic sensing (DAS) and the retrieved core combined with geophysical logs allow us to characterize the structure on meter-scale. Within the Milun fault zone, we identified a 20-m wide fault core comprised of gray and black gouge in the core sample. DAS strain-rate records associated with the same depth as the fault core show a distinct amplification. The amplification ratio of 2.5-3 is constant as for all types of events (local, teleseismic ), when compared to DAS channels at larger depth, related to a consolidated rock material. Although the fault gouge is narrow, the nature of the amplification in strain is due to its strong material contrast from fault gouge. This result may shed the light on the understanding of fault-zone dynamics in terms of remote earthquake triggering and near-fault ground motion., The 28th IUGG General Assembly (IUGG2023) (Berlin 2023)

Volcano‐seismic signals such as long‐period events and tremor are important indicators for volcanic activity and unrest. However, their wavefield is complex and characterization and location using traditional seismological instrumentation is often difficult. In 2019 we recorded the full seismic wavefield using a newly developed 3C rotational sensor co‐located with a 3C traditional seismometer on Etna, Italy. We compare the performance of the rotational sensor, the seismometer and the Istituto Nazionale di Geofisica e Vulcanologia‐Osservatorio Etneo (INGV‐OE) seismic network with respect to the analysis of complex volcano‐seismic signals. We create event catalogs for volcano‐tectonic (VT) and long‐period (LP) events combining a STA/LTA algorithm and cross‐correlations. The event detection based on the rotational sensor is as reliable as the seismometer‐based detection. The LP events are dominated by SH‐type waves. Derived SH phase velocities range from 500 to 1,000 m/s for LP events and 300–400 m/s for volcanic tremor. SH‐waves compose the tremor during weak volcanic activity and SH‐ and SV‐waves during sustained strombolian activity. We derive back azimuths using (a) horizontal rotational components and (b) vertical rotation rate and transverse acceleration. The estimated back azimuths are consistent with the INGV‐OE event location for (a) VT events with an epicentral distance larger than 3 km and some closer events, (b) LP events and tremor in the main crater area. Measuring the full wavefield we can reliably analyze the back azimuths, phase velocities and wavefield composition for VT, LP events and tremor in regions that are difficult to access such as volcanoes., Plain Language Summary: Traditional seismographs usually include mass and spring systems which measure vibrations constrained to up‐down, north‐south and east‐west directions. We compare the traditional seismometer to a rotational sensor which measures ground rotation around the same three directions. We installed a rotational sensor on Etna volcano in 2019 to test these new sensors in a volcanic environment. We compare the performance of the rotational sensor, a traditional seismometer and the Istituto Nazionale di Geofisica e Vulcanologia‐Osservatorio Etneo (INGV‐OE) seismometer network. We detect two types of a few second long earthquakes and find that the rotational sensor performs as good as the seismometer. We use the rotational sensor to calculate directions of the earthquake locations and find that most directions agree with the INGV‐OE network location and the area of the active craters. We find that for some earthquakes the ground only moved horizontally while for others it also moved up and down. Using a rotational sensor on a volcano we can easily and reliably estimate the ground motion, the speed of the earthquake waves in the ground and understand better how these earthquakes are generated., Key Points: We tested the performance of a rotational sensor compared to a seismometer and a seismic network using long‐period (LP), volcano‐tectonic (VT) events and tremor on Etna. LP and VT events are dominated by SH‐ and SV‐waves, respectively. Tremor changed from SH‐ to a mixed wavefield during strombolian eruptions. LP event and tremor back azimuths point to the main craters consistent with the Istituto Nazionale di Geofisica e Vulcanologia location; VT event back azimuths are at times consistent., Eurovolc, Daimler Benz Foundation, https://doi.org/10.14470/ME7564062119