Optical fiber–based sensing technology can drastically improve Earth observations by enabling the use of existing submarine communication cables as seafloor sensors. Previous interferometric and polarization-based techniques demonstrated environmental sensing over cable lengths up to 10,500 kilometers. However, measurements were limited to the integrated changes over the entire length of the cable. We demonstrate the detection of earthquakes and ocean signals on individual spans between repeaters of a 5860-kilometer-long transatlantic cable rather than the whole cable. By applying this technique to the existing undersea communication cables, which have a repeater-to-repeater span length of 45 to 90 kilometers, the largely unmonitored ocean floor could be instrumented with thousands of permanent real-time environmental sensors without changes to the underwater infrastructure.
Seismic waves from earthquakes recorded on the seafloor are composed of complex multiple arrivals. Here, distributed acoustic sensing (DAS) observations along a cable located offshore the Sanriku Coast, Japan, show that the local earthquake wavefield is particularly rich in Scholte waves. We introduce a processing pipeline to extract these surface waves from DAS records. We then invert hundreds of dispersion curves along a section of the cable to form a shallow high‐resolution shear‐wave velocity model. Moreover, we focus on the possible generation mechanisms of Scholte waves through a series of 2D and 3D full‐wavefield numerical simulations. We show that water phase reverberations greatly contribute to the generation of Scholte waves on the ocean floor. This study demonstrates the potential of DAS to observe and better understand a poorly known marine wave phenomenon and image the offshore shallow seismic structure with an unprecedented spatial resolution.
Unraveling the influence of deep geodynamic processes on the Earth's surface stress field is critical for understanding the driving forces of tectonic deformation. To date, it is well-established that there are two main sources of stress in the lithosphere: (a) internal buoyancy forces arising from lateral density and thickness variations within the crust and lithospheric mantle (Lachenbruch & Morgan, 1990;Lachenbruch et al., 1985), and (b) vertical and horizontal basal tractions arising from buoyancy-driven mantle convection below the lithosphere (Hager et al., 1985;Steinberger et al., 2001). Stresses are continuous across plate boundaries and are not generated there. While substantial work has been done to define the kinematics of these two sources, their relative contribution on both the long-term stability of continents and their state of stress is largely unknown. Here, we investigate how mantle-based stresses affect the dynamics of the lithosphere through the analysis of crustal anisotropy.In general, the difficulty of elucidating the origin of lithospheric stresses stems from our imperfect knowledge of the physical properties of the crust and the lack of constrains on the degree of coupling between the tectonic plates and the convective flow of the mantle. Over the last few decades, numerous studies have aimed at constraining the mechanical structure of the crust. These efforts typically involve the modeling of the Earth's topographic response to tectonic loading (e.g., Kaufman & Royden, 1994;Wdowinski & Axen, 1992) or the use of seismic data (e.g., Schutt et al., 2018) to derive estimates of crustal viscosity and temperature. Findings show, for instance, that there can exist large compositional lateral variations across a single craton (e.g., Tesauro et al., 2014), and that certain regions around the world have the conditions for the lower crust to act as a weak viscous layer capable of accommodating the lateral pressure gradients within the lithosphere (e.g., Bird, 1991;Block & Royden, 1990). Methods
Seismic scattering originated from structural heterogeneity covers a wide range of scales within the Earth's interior. Conventionally, stochastic approaches are employed to study high-frequency scattering process from random heterogeneity (Sato et al., 2012). One typical example is the characterization of P and S coda waves from local and regional earthquakes (Aki, 1969). On the other hand, deterministic imaging of subsurface structures has long been undertaken using either backward or forward scattering. For example, in earthquake seismology, receiver functions rely on the forward P-to-S scattering to image seismic discontinuities in the crust and mantle (Langston, 1979). Seismic reflection surveys utilize backward reflected waves to characterize petroleum reservoirs in exploration seismology and the Earth's crust in controlled-source crustal seismology (Prodehl & Mooney, 2012; Sheriff & Geldart, 1995). Unlike subhorizontal structures, strong lateral heterogeneity associated with near-vertical structure poses a significant challenge in deterministic scattered-wave imaging. In exploration seismology, complex structures can be imaged using densely distributed sources and receivers, in combination with sophisticated imaging or inversion techniques, such as reverse time migration or full-waveform inversion (Sheriff & Geldart, 1995; Virieux & Operto, 2009). However, such frameworks do not generally apply to earthquake seismology. The sparse distribution of earthquake sources and seismic stations greatly limits the detection ability of subsurface lateral heterogeneities, such as basin edges, fault zones, and Moho offsets, despite them having strong effects on seismic waveforms. Furthermore, many seismic imaging methods, such as common conversion point stacking of conventional receiver functions, assume subhorizontal structures, which cannot readily be applied to image strong lateral heterogeneities. Body-to-surface wave conversion is a special case of seismic scattering originated from strong lateral heterogeneity. Compared to body-to-body wave scattering, scattered surface waves propagate horizontally along
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