Microseismicity Monitoring and Site Characterization With Distributed Acoustic Sensing (DAS): The Case of the Irpinia Fault System (Southern Italy)

Abstract

This paper reports the first results obtained thanks to the installation of a dedicated one‐km fiber‐optic cable, integrated within the Irpinia Near Fault Observatory regional network in Southern Italy. The cable was installed in a dry lake, located near the active faults responsible for the M6.9 earthquake that occurred in 1980. A distributed acoustic sensing (DAS) interrogator was deployed over a period of 4 months and a half and allowed to record tens of local events. To model the seismic phases observed in the recordings, simple seismic refraction experiments were conducted along with analyses of earthquakes and numerical simulations. Results show that in this peculiar geological context, DAS is mainly sensitive to waves guided horizontally by the subsurface low‐velocity layered structure of the site. This leads to considerable site amplification but also wavefront deformation and allows to detect local microearthquakes without any stacking or other array processing techniques. Magnitude estimation was performed using a dedicated amplitude correction process, along with detection threshold estimation. The benefit of using stacking of DAS channels for improving signal‐to‐noise ratio was also estimated.

From strain to displacement: using deformation to enhance distributed acoustic sensing applications

“…In and around sedimentary basins, distributed strain rate measurements are known to be sensitive to siteeffects and mainly record highly scattered/refracted phases (Trabattoni et al, 2022). We will show here that deformation measurements improve the sensitivity to waves with higher apparent speeds and allow one to observe phases that are otherwise indistinguishable.…”

“…Sedimentary basins produce complex wavefields because ballistic waves are distorted, amplified, trapped, and cause interference among each other. We used the geometry proposed in Trabattoni et al (2022) that corresponds to the case of a dedicated deployment located in the Irpinia Near-Fault Observatory (INFO, Southern Italy). It consists of a twolayer basin of 25 m depth resting on the bedrock (Fig.…”

“…First, strain measurements have a characteristic directional sensitivity (i.e., a dependence on the direction of propagation and on the particle motion of the recorded wave) with narrower directional sensitivity for P-waves and a clover-like response pattern for S-waves (Martin et al, 2021). Second, strain measurements are more sensitive to slow waves, amplifying highly scattered waves that are difficult to analyse (Trabattoni et al, 2022). And last, finite-gauge length DAS measurements exhibit spectral notches in their instrument response at integer multiples of the apparent phase velocity over gauge length (J. .…”

Sensing Optical Fibers for Earthquake Source Characterization Using Raw DAS Records

“…The data were acquired during a 5‐month experiment, involving a DAS interrogator (Febus A1‐r) connected to a 1.1 km‐long L‐shaped fiber optic cable, buried into a shallow trench (0.3–1.0 m) in a dry lake near the town of Colliano (Figure 4b). The instrument was set to work with a sampling rate of 200 Hz, further downsampled to 100 Hz, and a spatial sampling of 2.4 m, with a gauge length of 4.8 m (more details in Trabattoni et al., 2022).…”

“…Figure 5 shows the recordings of a M L 2.3 event occurred on 20 September 2021 at 13:07:55 and reveals the specific propagation pattern due to the shallow sedimentary layering, as described in Trabattoni et al (2022).…”