Shear wave structure of a transect of the Los Angeles basin from multimode surface waves and H/V spectral ratio analysis

We use broad-band stations of the ‘Los Angeles Syncline Seismic Interferometry Experiment’ (LASSIE) to perform a joint inversion of the Horizontal to Vertical spectral ratios (H/V) and multimode dispersion curves (phase and group velocity) for both Rayleigh and Love waves at each station of a dense line of sensors. The H/V of the autocorrelated signal at a seismic station is proportional to the ratio of the imaginary parts of the Green’s function. The presence of low-frequency peaks (∼0.2 Hz) in H/V allows us to constrain the structure of the basin with high confidence to a depth of 6 km. The velocity models we obtain are broadly consistent with the SCEC CVM-H community model and agree well with known geological features. Because our approach differs substantially from previous modelling of crustal velocities in southern California, this research validates both the utility of the diffuse field H/V measurements for deep structural characterization and the predictive value of the CVM-H community velocity model in the Los Angeles region. We also analyse a lower frequency peak (∼0.03 Hz) in H/V and suggest it could be the signature of the Moho. Finally, we show that the independent comparison of the H and V components with their corresponding theoretical counterparts gives information about the degree of diffusivity of the ambient seismic field

Urban Seismic Site Characterization by Fiber‐Optic Seismology

Accurate ground motion prediction requires detailed site effect assessment, but in urban areas where such assessments are most important, geotechnical surveys are difficult to perform, limiting their availability. Distributed acoustic sensing (DAS) offers an appealing alternative by repurposing existing fiber‐optic cables, normally employed for telecommunication, as an array of seismic sensors. We present a proof‐of‐concept demonstration by using DAS to produce high‐resolution maps of the shallow subsurface with the Stanford DAS array, California. We describe new methods and their assumptions to assess H/V spectral ratio—a technique widely used to estimate the natural frequency of the soil—and to extract Rayleigh wave dispersion curves from ambient seismic field. These measurements are jointly inverted to provide models of shallow seismic velocities and sediment thicknesses above bedrock in central campus. The good agreement with an independent survey validates the methodology and demonstrates the power of DAS for microzonation.Key PointsWe demonstrate the potential of DAS for site effect analysisDAS recordings are used to compute dispersion curves and horizontal‐to‐vertical spectral ratio (HVSR)Joint inversions suggest that the crystalline bedrock lies 115 m beneath Stanford University central campusPeer Reviewedhttps://deepblue.lib.umich.edu/bitstream/2027.42/154310/1/jgrb54043.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154310/2/jgrb54043-sup-0001-Text_SI-S01.pdfhttps://deepblue.lib.umich.edu/bitstream/2027.42/154310/3/jgrb54043_am.pd

Marine Sediment Characterized by Ocean‐Bottom Fiber‐Optic Seismology

The Sanriku ocean‐bottom seismometer system uses an optical fiber cable to guarantee real‐time observations at the seafloor. A dark fiber connected to a Distributed Acoustic Sensing (DAS) interrogator converted the cable in an array of 19,000 seismic sensors. We use these measurements to constrain the velocity structure under a section of the cable. Our analysis relies on 24 hr of ambient seismic field recordings. We obtain a high‐resolution 2‐D shear‐wave velocity profile by inverting multimode dispersion curves extracted from frequency‐wave number analysis. We also produce a reflection image from autocorrelations of ambient seismic field, highlighting strong impedance contrasts at the interface between the sedimentary layers and the basement. In addition, earthquake wavefield analysis and modeling help to further constrain the sediment properties under the cable. Our results show for the first time that ocean‐bottom DAS can produce detailed images of the subsurface, opening new opportunities for cost‐effective ocean‐bottom imaging in the future.Plain Language SummaryDistributed Acoustic Sensing (DAS) is a relatively new measurement method that has the potential to convert existing fiber optic communication infrastructure into arrays of thousands of seismic sensors. In this research, we connected a DAS to a cable that was originally installed at the bottom of the ocean to sustain a seismic and tsunami observatory in the Sanriku Region. We show that this new type of measurement can provide reliable information to image and explore the shallow subsurface under this fiber cable. This is the first time such analysis is performed in an oceanic environment, and our methods could be readily exportable to other fiber‐optic cables that are the backbones of our modern telecommunication.Key PointsOcean‐bottom Distributed Acoustic Sensing is used to image shallow VS structureRayleigh wave phase velocity dispersion curves are extracted from frequency‐wave number analysisReflection image is obtained from autocorrelations of ambient seismic fieldPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/156494/3/grl61098_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/156494/2/grl61098.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/156494/1/grl61098-sup-0001-2020GL088360-Text_SI-S01.pd

Marine Sediment Characterized by Ocean‐Bottom Fiber‐Optic Seismology

The Sanriku ocean‐bottom seismometer system uses an optical fiber cable to guarantee real‐time observations at the seafloor. A dark fiber connected to a Distributed Acoustic Sensing (DAS) interrogator converted the cable in an array of 19,000 seismic sensors. We use these measurements to constrain the velocity structure under a section of the cable. Our analysis relies on 24 hr of ambient seismic field recordings. We obtain a high‐resolution 2‐D shear‐wave velocity profile by inverting multimode dispersion curves extracted from frequency‐wave number analysis. We also produce a reflection image from autocorrelations of ambient seismic field, highlighting strong impedance contrasts at the interface between the sedimentary layers and the basement. In addition, earthquake wavefield analysis and modeling help to further constrain the sediment properties under the cable. Our results show for the first time that ocean‐bottom DAS can produce detailed images of the subsurface, opening new opportunities for cost‐effective ocean‐bottom imaging in the future.Plain Language SummaryDistributed Acoustic Sensing (DAS) is a relatively new measurement method that has the potential to convert existing fiber optic communication infrastructure into arrays of thousands of seismic sensors. In this research, we connected a DAS to a cable that was originally installed at the bottom of the ocean to sustain a seismic and tsunami observatory in the Sanriku Region. We show that this new type of measurement can provide reliable information to image and explore the shallow subsurface under this fiber cable. This is the first time such analysis is performed in an oceanic environment, and our methods could be readily exportable to other fiber‐optic cables that are the backbones of our modern telecommunication.Key PointsOcean‐bottom Distributed Acoustic Sensing is used to image shallow VS structureRayleigh wave phase velocity dispersion curves are extracted from frequency‐wave number analysisReflection image is obtained from autocorrelations of ambient seismic fieldPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/156494/3/grl61098_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/156494/2/grl61098.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/156494/1/grl61098-sup-0001-2020GL088360-Text_SI-S01.pd

Nonlinear Earthquake Response of Marine Sediments With Distributed Acoustic Sensing

Soft sediment layers can significantly amplify seismic waves from earthquakes. Large dynamic strains can trigger a nonlinear response of shallow soils with low strength, which is characterized by a shift of resonance frequencies, ground motion deamplification, and in some cases, soil liquefaction. We investigate the response of marine sediments during earthquake ground motions recorded along a fiber-optic cable offshore the Tohoku region, Japan, with distributed acoustic sensing (DAS). We compute AutoCorrelation Functions (ACFs) of the ground motions from 105 earthquakes in different frequency bands. We detect time delays in the ACF waveforms that are converted to relative velocity changes (dv/v). dv/v drops, which characterize soil nonlinearity, are observed during the strongest ground motions and exhibit a large variability along the cable. This study demonstrates that DAS can be used to infer the dynamic properties of the shallow Earth with an unprecedented spatial resolution.Plain Language SummarySeismic waves from earthquakes are amplified by shallow and soft sediment layers of the Earth. This amplification is linear for weak seismic waves, but can become highly nonlinear during strong ground motions. Nonlinear soil response, which can lead to a complete failure of the ground through soil liquefaction, threatens the safety of human-made constructions and needs to be accurately characterized. We study the response of marine sediments offshore the Tohoku region in Japan using earthquake data recorded along a fiber-optic cable with distributed acoustic sensing (DAS). We use an autocorrelation approach to analyze the ground motions from 105 earthquakes recorded by thousands of DAS channels. We detect a clear nonlinear behavior of shallow sediments during the strongest ground motions. Moreover, we show that soil nonlinearity significantly varies along the cable. Our methodology could easily be applied to earthquake DAS data recorded in populated and seismically active regions to help understand better the dynamic behavior of shallow soils.Key PointsAutoCorrelation Functions (ACFs) of earthquakes recorded by a distributed acoustic sensing (DAS) experiment exhibit phase delays with increasing ground motionsACF phase delays are converted to relative velocity drops in the medium that characterize soil nonlinearityDAS is used to infer the nonlinear behavior of soils with an unprecedented spatial resolutionPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/175187/1/grl65056.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/175187/2/2022GL100122-sup-0001-Supporting_Information_SI-S01.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/175187/3/grl65056_am.pd

Subsurface Imaging With Ocean‐Bottom Distributed Acoustic Sensing and Water Phases Reverberations

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.Plain Language SummaryDistributed acoustic sensing (DAS) is a measurement technique that has recently demonstrated its utility for marine geophysics. DAS offers the possibility to observe the seismic wave at a scale and an extent previously unattainable with traditional passive seismic surveys. Here, we use a linear DAS array located offshore the Sanriku Coast, Japan. We propose a processing pipeline to extract surface waves from local earthquake DAS records and obtain hundreds of measurements along the cable. These measurements are used to infer a high‐resolution model of the near‐shore subsurface. Supported by a series of numerical simulations, we suggest that acoustic reverberations in the water column greatly contribute to the generation of surface waves on the ocean floor. This study further demonstrates that DAS can be used to understand marine wave phenomena better and image offshore seismic structures.Key PointsPassive high‐resolution Vs imaging of shallow sedimentary layers with ocean‐bottom distributed acoustic sensing and low‐magnitude earthquakesGridded slant‐stack method to extract Scholte waves from local earthquake wavefieldsFull‐wavefield simulations suggest that water phase reverberations generate Scholte waves at the ocean floorPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/171579/1/grl63590_am.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/171579/2/grl63590.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/171579/3/2021GL095287-sup-0001-Supporting_Information_SI-S01.pd

Locating the Precise Sources of High-Frequency Microseisms Using Distributed Acoustic Sensing

Although microseisms have been observed for more than 100 years, the precise locations of their excitation sources in the oceans are still elusive. Underwater Distributed Acoustic Sensing (DAS) brings new opportunities to study microseism generation mechanisms. Using DAS data off the coast of Valencia, Spain, and applying a cross-correlation approach, we show that the sources of high-frequency microseisms (0.5–2 Hz) are confined between 7 and 27 km from the shore, where the water depth varies from 25 to 100 m. Over time, we observe that these sources move quickly along narrow areas, sometimes within a few kilometers. Our methodology applied to DAS data allows us to characterize microseisms with a high spatiotemporal resolution, providing a new way of understanding these global and complex seismic phenomena happening in the oceans.Plain Language SummaryMicroseisms are a type of seismic noise that is ubiquitous on Earth and has been studied for over 100 years. However, we still have no way of knowing exactly where it is generated in the ocean. Recent advances in underwater fiber-optic sensing bring a tremendous opportunity to better understand the source mechanism of microseisms. We use seafloor Distributed Acoustic Sensing data to achieve for the first time a precise localization of the noise sources of high-frequency microseisms. We found that the sources of high-frequency microseisms are very narrow, often only a few kilometers. Moreover, the noise source area is constantly changing with the wind direction.Key PointsA fiber-optic cable on the seafloor is used to locate the sources of high-frequency microseisms with an unprecedented precisionThe sources of high-frequency microseisms quickly move within narrow areas of a few kilometersThe constantly changing source locations are most likely related to the ephemeral behaviors of windPeer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/174919/1/2022GL099292-sup-0001-Supporting_Information_SI-S01.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/174919/2/grl64775.pdfhttp://deepblue.lib.umich.edu/bitstream/2027.42/174919/3/grl64775_am.pd