Oceanic lithospheric S wave velocities from the analysis of P wave polarization at the ocean floor

Our knowledge of the absolute S wave velocities of the oceanic lithosphere is mainly based on global surface wave tomography, local active seismic or compliance measurements using oceanic infragravity waves. The results of tomography give a rather smooth picture of the actual S wave velocity structure and local measurements have limitations regarding the range of elastic parameters or the geometry of the measurement. Here, we use the P wave polarization (apparent P wave incidence angle) of teleseismic events to investigate the S wave velocity structure of the oceanic crust and the upper tens of kilometres of the mantle beneath single stations. In this study, we present an up to our knowledge new relation of the apparent P wave incidence angle at the ocean bottom dependent on the half space S wave velocity. We analyse the angle in different period ranges at ocean bottom stations (OBS) to derive apparent S wave velocity profiles. These profiles are dependent on the S wave velocity as well as on the thickness of the layers in the subsurface. Consequently, their interpretation results in a set of equally valid models. We analyse the apparent P wave incidence angles of an OBS data set which was collected in the Eastern Mid Atlantic. We are able to determine reasonable S wave velocity-depth models by a three step quantitative modelling after a manual data quality control, although layer resonance sometimes influences the estimated apparent S wave velocities. The apparent S wave velocity profiles are well explained by an oceanic PREM model in which the upper part is replaced by four layers consisting of a water column, a sediment, a crust and a layer representing the uppermost mantle. The obtained sediment has a thickness between 0.3 km and 0.9 km with S wave velocities between 0.7 km s−1 and 1.4 km s−1. The estimated total crustal thickness varies between 4 km and 10 km with S wave velocities between 3.5 km s−1 and 4.3 km s−1. We find a slight increase of the total crustal thickness from ∼5 km to ∼8 km towards the South in the direction of a major plate boundary, the Gloria Fault. The observed crustal thickening can be related with the known dominant compression in the vicinity of the fault. Furthermore, the resulting mantle S wave velocities decrease from values around 5.5 km s−1 to 4.5 km s−1 towards the fault. This decrease is probably caused by serpentinization and indicates that the oceanic transform fault affects a broad region in the uppermost mantle. Conclusively, the presented method is useful for the estimation of the local S wave velocity structure beneath ocean bottom seismic stations. It is easy to implement and consists of two main steps: (1) measurement of apparent P wave incidence angles in different period ranges for real and synthetic data, and (2) comparison of the determined apparent S wave velocities for real and synthetic data to estimate S wave velocity-depth models

Monitoring submarine fault deformation using direct-path ranging

The seafloor stores crucial information on sub-seafloor processes, including stress, elastic strain, and earthquakes. This information may be extracted through the nascent scientific field of seafloor geodesy. The GeoSEA (Geodetic Earthquake Observatory on the SEAfloor) array uses acoustic signals for direct-path ranging and relative positioning at mm-scale resolution for a period of up to 3.5 years. The transponders also include high-precision pressure sensors to monitor vertical movements and dual-axis inclinometers in order to measure their altitude as well as any change in submarine fault zones and characterizing their behavior (locked or aseismically creeping). A further component of the network is GeoSURF, a self-steering autonomous surface vehicle (Wave Glider), which monitors system health and is able to upload the seafloor data to the sea surface and to transfer it via satellite. Seafloor transponders are currently installed across a dextral strike-slip fault to measure the instability of the eastern flank of Mt Etna in Sicily, along the North Anatolian Fault offshore Istanbul to measure the strain build-up along the fault in a seismic gap. In addition, three arrays are currently deployed on the marine forearc and outer rise of the South American subduction system around 21°S. This segment of the Nazca-South American plate boundary has last ruptured in an earthquake in 1877 and was identified as a seismic gap prior to the 2014 Iquique earthquake (Mw 8.1). The southern portion of the segment remains unbroken by a recent earthquake. The first 12 month of all geodetic installations were analyzed and we discuss baselines with precision less 5 mm for ranges up to 2000 m of distance and compare them to synthetics baselines. The North Anatolian across-fault baseline changes remains within the resolution and preclude fault-displacement rates larger a few millimeters-per-year, which suggests a locked fault zone

Offshore-aftershock sequence of the Mw 8.1 2014 Iquique earthquake

On 1 April 2014, a Mw 8.1 earthquake ruptured a portion of the subduction zone in northern Chile offshore Iquique between 19.5◦S to 21◦S. A large earthquake had been expected in the subduction zone off northern Chile, because it had not ruptured in a megathrust earthquake since a M∼8.8 event in 1877. The 2014 earthquake did only affect the northern region of the 1877 rupture and left an unbroken segment to the South. In December 2014 we deployed an offshore network of 15 ocean-bottom-seismometers (OBS) between 19◦S and 22◦S using the Chilean Navy ship OPV Toro, covering the aftershock zone of the 2014 Iquique event and the un-ruptured offshore domain in the South. The network was recovered in November 2015 with RV SONNE

Patterns of seafloor morphology as a response to tectonic- and sedimentary processes south of the Messina Strait, Italy

The Ionian Sea between Sicily and Calabria is known for its complex geological setting, as it is located at the convergence zone of the African and Eurasian plates. The seismogenic potential in this region is manifested by several high magnitude and disastrous earthquakes like the 1908 Messina Earthquake. Furthermore, the area is affected by intense volcanism like the Aeolian Island volcanos in the Tyrrhenian Sea and Europe’s largest active volcano, Mt Etna, sitting directly at the eastern coast of Sicily. During the last years, the possible presence of Subduction Tear Edge Propagator faults (STEP-faults) has been heavily debated. The main candidates for these faults are the Ionian Fault in the Northeast and the Alfeo-Etna Fault in the Southwest of the working area between Sicily and Calabria. Nevertheless, only little is known about near seafloor deformation zones and sedimentary processes in the Ionian Sea directly south of the Messina Strait.peer-reviewe

Word assistant app with speech recognition

This thesis deals with the development of a mobile app to assist people during conversations and writing. The app provides a subset of alternative words and is controllable with voice recognition. This should help to search the words without long interruptions during the conversation

Word assistant app with speech recognition

This thesis deals with the development of a mobile app to assist people during conversations and writing. The app provides a subset of alternative words and is controllable with voice recognition. This should help to search the words without long interruptions during the conversation