Description: 〈title xmlns:mml="http://www.w3.org/1998/Math/MathML"〉Abstract〈/title〉〈p xmlns:mml="http://www.w3.org/1998/Math/MathML" xml:lang="en"〉Coastal protection in the form of dike constructions has a long history at the German North Frisian coast dating back to the High Middle Ages. As the vast majority of the dikes built prior to the devastating storm surges of the Middle Ages have been irretrievably destroyed, mostly sparse remains and only a few well preserved of these medieval dikes are found along the German North Frisian coast and within the Wadden Sea. Not all details of their construction and dimensions are yet understood. In the present case study, we investigate the historical Schardeich on the island of Pellworm in the German North Sea in a noninvasive way using shear waves (SH‐waves). For the data interpretation, we applied a combination of seismic full waveform inversion and classical seismic reflection imaging to determine the interior structure of the dike and its underlying layers at the highest possible resolution. The results obtained on land are compared with dike remains found in the tidal flats. These remains show up in marine seismic sections as characteristic reflections, which probably represent a compaction layer caused by the load of the former dike. For ground truthing, we compare the seismic results with internal dike structures found in nearby excavations. The comparison highlights that FWI is a reliable tool for near‐surface archaeological prospecting. We find that SH‐wave FWI provides decimetre‐scale velocity and density models that allow, together with the seismic reflection section, to determine distinct construction phases of the dike. The investigated dike further shows a depression at base level of about 0.75 m, which is of the same order as observed for the dike base reflections in the tidal flats. Transferring these findings to the dike remains mapped in the tidal flats, we derive a height of the former dike from 2.2 to 4.4 m.〈/p〉

Description: Deutsche Forschungsgemeinschaft http://dx.doi.org/10.13039/501100001659

Description: Mid-ocean ridges spreading at ultraslow rates of less than 20 mm yr−1 can exhume serpentinized mantle to the seafloor, or they can produce magmatic crust. However, seismic imaging of ultraslow-spreading centres has not been able to resolve the abundance of serpentinized mantle exhumation, and instead supports 2 to 5 km of crust. Most seismic crustal thickness estimates reflect the depth at which the 7.1 km s−1 P-wave velocity is exceeded. Yet, the true nature of the oceanic lithosphere is more reliably deduced using the P- to S-wave velocity (Vp/Vs) ratio. Here we report on seismic data acquired along off-axis profiles of older oceanic lithosphere at the ultraslow-spreading Mid-Cayman Spreading Centre. We suggest that high Vp/Vs ratios greater than 1.9 and continuously increasing P-wave velocity, changing from 4 km s−1 at the seafloor to greater than 7.4 km s−1 at 2 to 4 km depth, indicate highly serpentinized peridotite exhumed to the seafloor. Elsewhere, either magmatic crust or serpentinized mantle deformed and uplifted at oceanic core complexes underlies areas of high bathymetry. The Cayman Trough therefore provides a window into mid-ocean ridge dynamics that switch between magma-rich and magma-poor oceanic crustal accretion, including exhumation of serpentinized mantle covering about 25% of the seafloor in this region.

Description: About 57% of the Earth’s surface is covered by oceanic crust and new ocean floor is continuously created along the ~60.000 km long mid-ocean ridge (MOR) system. About 25% of the MOR spread at an ultra-slow spreading rate of 〈20 mm/yr. At ultra-slow spreading rates the melt supply to the ridge is thought to dramatically decrease and crustal thickness decreases to a thickness of 〈6 km. Further, geological evidence suggests wide-spread un-roofing of mantle. Yet, seismic data provide little evidence for amagmatic lithospheric emplacement away from oceanic core complexes. Formation of crust from a magma chamber would suggest the creation of a well stratified crust, with an extrusive upper crust (layer 2) and a lower gabbroic crust (lower 3) and a well-defined crust-mantle boundary and hence a seismic Moho. In contrast, un-roofing of mantle would support a crustal structure where seismic velocities change gradually from about 4.5 km/s at the seabed to velocities of mantle rocks at depth. In addition, exposure of mantle to seawater would cause serpentinization. Serpentine, in turn, would support high Vp/Vs ratios of 〉1.9. Here, we report results from a seismic refraction survey from the ultra-slow spreading Cayman Spreading Centre in the Caribbean Sea, sampling mature crust along a flowline from both conjugated ridge flanks. The ocean-bottom-seismometer and hydrophones provide both P-wave and S-wave refracted arrivals. Travel time data were inverted using seismic tomography. Resulting Vp/Vs ratios suggest that up to 25% of the lithosphere have high ratios of 〉1.9, supporting serpentinization and exposure of hydrated mantle at the seafloor. Further, the mode of accretion has changed over time, supporting both areas of mantle exposure and magmatic crust. Magmatic crust has a typical layer 2 and layer 3 velocity structure and a thin crust of 3 to 5 km thickness. However, a well-defined Moho boundary was not observed. Thus, crustal rocks are characterized by typical crustal-velocities (〈7.2 km/s) and mantle has velocities of 〉7.6 km/s. Domains of un-roofed mantle have high Vp/Vs ratios and velocities gradually increasing to 7.4-7.6 km/s. In addition, we will use our results to re-assess the depth distribution of local earthquakes at ultra-slow spreading ridges, including the Cayman Trough and the Southwest Indian Ridge. Most importantly, the high Vp/Vs ratio of 〉1.9 characterizing serpentinized mantle causes earthquakes to focus at much shallower depth when compared to location procedures using a global average for Vp/Vs of 1.73; the bias in depth might be in the order of 10 km.