ALBERT

Description: The Lau basin is an active back arc system with several spreading centres and microplates rapidly evolving in time. In the northeast basin, we observe a complex setting of a back arc rifting system and a volcanic arc. The Fonualei Rift and Spreading Center (FRSC) is a part of this complex setting, striking northwest and accommodating eastwest extension between the Niuafo'ou microplate and the Tonga plate. The volcanic arc strikes northeast southwest, in order to this striking direction the Fonualei Rift offset to the volcanic arc decreases to the south. This results in an minimal offset off 10-15km to the active volcanic arc. The interplay of the FRSC and the Tofua volcanic arc are not fully understood. Therefor we want to use the microseismicity as a tool to understand these active tectonics. One goal was to record seismicity data and create an event catalogue. We deployed a network of 16 ocean bottom seismometers (OBS) across the FRSC which recorded approximately 750 local events within a time span of 31 days. The events were located with the NonLinLoc (Non-Linear Location) software package. Further more we determined local and moment magnitudes. The seismicity catalogue shows an average event rate of 24 events per day. The events are focused beneath the network and in a region south of the network. Especially in the southern region we recognised a highly increased event rate depending on time and locality. In a time span of 6 days we observed 600 events resulting in a event rate of up to 100 events per day. This high event rate is an indicator for an earthquake swarm. For the magnitudes frequency distribution we determined a b-value for the entire catalogue of 1.05 with a magnitude of completeness of 1.0. With this microseismicity we were able to create an event catalogue which can be used as a starting point for more investigation within this region. For example we will analyse the focal mechanisms of selected events which will allow insights to the tectonics of the region.

Description: Young extension centers in the proximity of back-rolling subduction zones constitute an ideal natural laboratory to investigate the early evolutional stages of back-arc basins. We present a catalog of 711 microearthquakes recorded during a 32-days deployment of ocean bottom seismometers at the southern part of the Fonualei Rift and Spreading Center in the Lau Basin. The majority of epicenters are concentrated along the central region of the axial valley and about 450 events are associated with an earthquake swarm that is likely linked to magmatic diking in the crust. The earthquake swarm location coincides with several volcanic mounds on the seafloor, covered by fresh lava flows. The spatial distribution of microearthquakes and the maximum depth of brittle faulting is more similar to oceanic spreading centers than to continental rift zones. We interpret our results in conjunction with refraction seismic data and magnetic anomaly data from the same region. In combing all findings, we conclude that the southern Fonualei Rift and Spreading Center is currently in transition from a rifting dominated mode of extension to a seafloor spreading dominated mode of extension. This transition may be associated with fundamental changes in the underlying mechanisms of melt generation leading to a diminishing influence of hydrous flux melting and the growing influence of decompression melting at this back-arc extension zone.

Description: The multibeam echo sounder (MBES) data was collected from the 28.04.2019 (23:19 UTC) to the 07.05.2019 (16:42 UTC) with the Kongsberg EM122 on RV Maria S. Merian during the transit from Montevideo (Uruguay) to Las Palmas (Canary Islands, Spain) in the Southern Atlantic Ocean (MSM82/2). The deep-water MBES EM122 operates with an acoustic frequency of 12 kHz and a beam opening angle of 2° x 2°. The time, motion and position data (WGS84, geographic) was measured by the Kongsberg Seapath system on board. During the acquisition the echo sounder was monitored as well as settings and filter adjusted according to the environment. The swath opening angle was set between 65° and 70°. As the data was collected during transit, no specific survey was performed. Sound velocity profiles (SVP) were measured on the 01.05.2019 (XSV probe) and 04.05.2019 (with CTD) and applied in the MBES acquisition software SIS. To improve the bathymetric data before the first SVP measurement, an synthetic SVP extracted from the World Ocean Atlas 2009 (Levitus, 2013) using the software Sound Speed Manager (developed by the UNH Center for Coastal and Ocean Mapping and NOAA Coast Survey Development Laboratory (CSDL)). The SVP data is not part of this submission. The data was post-processed using the software QPS Qimera. No further SVP re-computation, navigation correction or tide correction was applied. Erroneous depth measurements were deleted manually by using the swath and slice editor. The vertical datum is mean sea level (MSL). The cleaned data was exported bathymetric grids in a resolution of 100 m (geotiff format, projected coordinates WGS84/UTM zone 23S (EPSG: 32723) to 26S (EPSG: 32726) and 26N (EPSG: 32626)). The coordinate system is indicated in the file name.

Description: The multibeam echo sounder (MBES) data was collected from the 28.04.2019 (23:19 UTC) to the 07.05.2019 (16:42 UTC) with the Kongsberg EM122 on RV Maria S. Merian during the transit from Montevideo (Uruguay) to Las Palmas (Canary Islands, Spain) in the Southern Atlantic Ocean (MSM82/2). The deep-water MBES EM122 operates with an acoustic frequency of 12 kHz and a beam opening angle of 2° x 2°. The time, motion and position data (WGS84, geographic) was measured by the Kongsberg Seapath system on board. During the acquisition the echo sounder was monitored as well as settings and filter adjusted according to the environment. The swath opening angle was set between 65° and 70°. As the data was collected during transit, no specific survey was performed. Sound velocity profiles (SVP) were measured on the 01.05.2019 (XSV probe) and 04.05.2019 (with CTD) and applied in the MBES acquisition software SIS. To improve the bathymetric data before the first SVP measurement, an synthetic SVP extracted from the World Ocean Atlas 2009 (Levitus, 2013) using the software Sound Speed Manager (developed by the UNH Center for Coastal and Ocean Mapping and NOAA Coast Survey Development Laboratory (CSDL)). The SVP data is not part of this submission. The data was post-processed using the software QPS Qimera. No further SVP re-computation, navigation correction or tide correction was applied. Erroneous depth measurements were deleted manually by using the swath and slice editor. The vertical datum is mean sea level (MSL). The cleaned data was exported in gsf format.

Description: The multibeam echo sounder (MBES) data was collected from the 28.04.2019 (23:19 UTC) to the 07.05.2019 (16:42 UTC) with a Kongsberg EM122 during the transit from Montevideo (Uruguay) to Las Palmas (Canary Islands, Spain) in the Southern Atlantic Ocean. The deep-water MBES EM122 operates with an acoustic frequency of 12 kHz and a beam opening angle of 2° x 2° and up to 432 beams. The raw and unprocessed bathymetric and backscatter data is stored in Kongsberg format (*.all), each containing up to 30 min of data. The therein included time, motion and position data (WGS84, geographic) was measured by the Kongsberg Seapath system on board. During the acquisition the echo sounder was monitored as well as settings and filter adjusted according to the environment. The swath opening angle was set between 65° and 70°. As the data was collected during transit, no specific survey was performed. Sound velocity profiles (SVP) were measured on the 01.05.2019 (XSV probe) and 04.05.2019 (with CTD) and applied in the MBES acquisition software SIS. The SVP data is not part of this submission but can be extracted from the .all files. To improve the bathymetric data before the first SVP measurement, an synthetic SVP extracted from the World Ocean Atlas 2009 (Levitus, 2013) using the software Sound Speed Manager (developed by the UNH Center for Coastal and Ocean Mapping and NOAA Coast Survey Development Laboratory (CSDL)).