Oceanic transform faults and fracture zones (FZs) represent major bathymetric features that keep the records of past and present strike-slip motion along conservative plate boundaries. Although they play an important role in ridge segmentation and evolution of the lithosphere, their structural characteristics, and their variation in space and time, are poorly understood. To address some of the unknowns, we conducted interdisciplinary geophysical studies in the equatorial Atlantic Ocean, the region where some of the most prominent transform discontinuities have been developing. Here we present the results of the data analysis in the vicinity of the Chain FZ, on the South American Plate. The crustal structure across the Chain FZ, at the contact between ∼10 and 24 Ma oceanic lithosphere, is sampled along seismic reflection and refraction profiles. We observe that the crustal thickness within and across the Chain FZ ranges from ∼4.6-5.9 km, which compares with the observations reported for slow-slipping transform discontinuities globally. We attribute this presence of close to normal oceanic crustal thickness within FZs to the mechanism of lateral dike propagation, previously considered to be valid only in fast-slipping environments. Furthermore, the combination of our results with other data sets enabled us to extend the observations to morphotectonic characteristics on a regional scale. Our broader view suggests that the formation of the transverse ridge is closely associated with a global plate reorientation that was also responsible for the propagation and for shaping lower-order Mid-Atlantic Ridge segmentation around the equator.

Plate tectonics separates Earth' surface into rigid plates (McKenzie & Parker, 1967;Morgan, 1968), and deformation or relative motion between plates reveals three different types of oceanic plate boundaries: (a) constructive plate boundaries at mid-ocean ridges (MOR) where new seafloor is created, (b) destructive plate boundaries at subduction zones where the oceanic lithosphere is transferred into the mantle and recycled, and (c) conservative plate boundaries and hence transform faults (TF) where the lithosphere is neither created nor destroyed as plates move past each other (Morgan, 1968). In ocean basins, transform faults

 Seismic structure along the St. Paul fracture zone reflects magmatically accreted oceanic crust  Oceanic crust across St. Paul shows only small thickness variations, lacking evidence for regional crustal thinning near fracture zones  Magmatic nature of crust supports a mechanism where transform crust is augmented before being turned into a fracture zone

 Seismic structure along the St. Paul fracture zone reflects magmatically accreted oceanic crust  Oceanic crust across St. Paul shows only small thickness variations, lacking evidence for regional crustal thinning near fracture zones  Magmatic nature of crust supports a mechanism where transform crust is augmented before being turned into a fracture zone

<p>Distributed Acoustic Sensing (DAS) is becoming increasingly popular due to its high spatial and temporal resolution. DAS holds great potential for geohazard applications as, in principle, anything affecting the strain on a fibre optic cable section can be measured. Examples are passing seismic surface waves and ambient temperature changes.  This presentation demonstrates the feasibility of DAS for quick clay monitoring, and presents data from a field trial in Rissa, Norway.</p><p>In Norway, almost all landslides in clays that have serious consequences are caused by the instability of quick clay. Examples include the landslides Trögstad (1967), Rissa (1978), and recently Gjerdrum (2020).</p><p>A research field site was established at Rissa by the Centre for Geophysical Forecasting (CGF). Long term monitoring with DAS over several months is carried out to monitor changes in the geophysical parameters of the soil before and after road construction work.</p><p>Due to the close relation between elastic parameters controlling seismic wave propagation and the petrophysical properties of the sediment, which determine the strength, DAS measurements from seismic waves, mainly Rayleigh waves, can be used to investigate the soil stability.</p><p>The Rayleigh waves of interest travel with a velocity that is approximately 0.9 times the shear wave velocity (Vs) and may have wavelengths of only a few meters. The shear modulus, which is the main geomechanical parameter controlling the stability and shear strength, can be approximately inferred from Vs. Therefore, observation of changes in Vs can be used to detect changes in shear strength of clay formations.</p><p>One of the main challenges for this application lies in the detection of seismic surface waves of shorter wavelengths. Commonly used methods for quick clay monitoring suffer either from lower spatial resolution or limited area coverage, and we also seek to address these challenges.</p><p>Alcatel Submarine Network Norway developed an interrogation technology (OptoDAS) enabling long-range measurement over 100km. Spatial sampling intervals as small as 1m can be chosen. It is, however, the gauge length and the spatial sampling that determines the spatial resolution. The gauge length varies from 40m to 2m, and is analogous to receiver (group or node) separation in conventional seismic methods. </p><p>Due to the inherent properties of DAS interrogation the SNR is lower for very small gauge lengths. Although the data quality is adequate, we strive to improve the SNR further to make DAS well suited for the analysis of seismic waves with wavelengths even shorter than 4m.</p><p>A cost-effective solution for increasing the data quality could be found by introducing fibre loops into the acquisition design. The gain of these optimization will be presented, and it will be demonstrated that data quality can be improved by stacking over multiple similar fibre optic pathways.</p><p>Results will be presented for seismic signals from passive sources – such as passing cars on the nearby road, and from an active source, a seismic hammer and plate shot.</p><p>The pros and cons of using long-range high-resolution DAS technology for soil monitoring will be discussed along with potential areas for future advances.</p>

Quick clay avalanche is one of the most devastating landslide types worldwide. Hence, an early warning system is in demand to mitigate the fatal consequences caused by such events. To address this, distributed acoustic sensing data are collected in an area containing quick clay deposits between July 2021 and February 2022 in Rissa, Norway, while a new road is constructed on the quick clay. Road construction can induce unwanted changes to the mass balance in the clay, and previous landslides have been triggered by such changes. For this purpose, passive and active data are collected to test and compare various analysis methods. Using extracted Rayleigh wave dispersion from active sledgehammer shots, shear-wave velocity depth profiles covering the first 15 m could be estimated and compared using a linearized and a nonlinear surface wave inversion method. Furthermore, ambient noise crosscorrelation is used to obtain the dispersion from the ambient noise and associated shear-wave velocity profiles, providing two possible data collection methods for the early warning system. The obtained dispersion curves and the estimated shear-wave velocity profiles show small time-laps variation during the acquisition period (up to approximately 23 m/s), where the variation is within one standard deviation. Such a small variation suggests that the construction work and the extra load added to the quick clay do not alter the quick clay’s properties. Nevertheless, the obtained results capture the nonrepeatability effects within the acquisition period and provide reference curves for the study area at undisturbed conditions and valuable information for future comparisons to refer to potential failure scenarios. This is the first step in exploring an early warning system for quick clay landslides using fiber-optic cables. Further work will investigate the possibility of automatizing the system and improving the accuracy of the sensing system.

 Seismic structure along the St. Paul fracture zone reflects magmatically accreted oceanic crust  Oceanic crust across St. Paul shows only small thickness variations, lacking evidence for regional crustal thinning near fracture zones  Magmatic nature of crust supports a mechanism where transform crust is augmented before being turned into a fracture zone