ALBERT

Description: Spreading events at mid-ocean ridges are rarely observed in-situ. Especially at the slowest spreading mid-ocean ridges, spreading episodes occur seldom and these ridges are situated in remote areas with difficult working conditions precluding for example rapid response missions when a spreading event is detected. During an ocean bottom seismometer (OBS) experiment at the eastern Southwest Indian Ridge we accidentally recorded two earthquake swarms at a subordinate volcanic segment called Segment 7. The earthquake swarms occurred in January and April 2013 and lasted for few days. They originate at the same location at a depth of about 8 km bsf. In January, seismicity clearly migrated downward during the early hours of the swarm while in April earthquakes immediately spread over the entire area activated by the swarm. With local earthquake tomography, we imaged a region of partial melt beneath the neighbouring Segment 8 volcano extending to about 8 km depth beneath the seafloor. We propose that the earthquake swarms indicate stress release during two dike intrusion events that are potentially fed by the melt reservoir underneath Segment 8 volcano at about 35 km along axis distance. At the same time, seismic tremor was recorded at the seismic station situated above the intrusion area. Tremor signals already precede the seismic swarm in January and are clearly harmonic with a fundamental frequency of about 0.8 Hz and several harmonics. Spectra of the long lasting tremor throughout late April and May are more complex. Tremor particle motion is almost linear indicating body waves. The direction of the polarisation is pointing to a source SSW of the station. During the main phase of the tremor in April the polarization turns to a more southerly direction and the incidence angle steepens from 65° to about 35° from the vertical, indicating possibly a different tremor excitation area at deeper levels matching the deeper intrusion in April. Interestingly, the tremor is strongly modulated by the tides with higher fundamental frequencies and higher tremor amplitudes coinciding with low tides. With more than 6 weeks duration, the tremor lasts considerably longer than the swarm activity in April. We thus speculate that the tremor signal is not caused by magma movement during dike intrusion but is generated by vigorous hydrothermal flow in the crust or upper mantle that is fuelled by the heat of the dike intrusions. Semidiurnal variations in tremor frequency and amplitude may result from higher outflow rates in the hydrothermal system during low tides. We assume that or OBS was accidentally located near the conjectured hydrothermal system as the neighbouring stations at about 15 km distance do not record the tremor signal. However, no measurements of hydrothermal plume in the water column exist that would confirm the existence of a hydrothermal vent field.

Description: At mid-ocean spreading ridges, tectonic plates drift apart and upwelling magma builds new seafloor and tectonic plates. Spreading ridges are grouped according to their spreading velocity. The slowest ridges among them work entirely different than their faster counterparts. They do not receive enough melt to close the gap between the diverging plates. Instead, there are widely spaced volcanic centers among magma-poor areas that exhibit rocks from the Earth’s mantle. In seismicity studies earthquakes give insights into the subsurface properties, for example temperature, and accompany any active spreading processes such as volcanic eruptions and tectonic extension. Because such studies require many ocean bottom seismometers, local seismicity studies of ultraslow spreading ridges until now have only covered either volcanoes or magma-poor areas. Our study collected for the first time earthquakes from several spreading segments to study the interplay between magma-rich and poor segments. The network of 26 ocean bottom seismometers covered around 160 km along axis of the ultraslow spreading Knipovich Ridge in the Greenland Sea and recorded earthquakes for about one year. We find seismicity varying distinctly along-axis. The maximum depth of earthquakes marking the thickness of the mechanically strong lithosphere shallows over distances of 70 km towards the Logachev volcanic center. Underneath the volcano, earthquake swarms and a seismicity gap indicate recent magmatic activity. Melts may thus be channeled towards major volcanic centers explaining the uneven along-axis melt distribution typical for ultraslow ridges. Presumably magma-poor regions exhibit deep earthquakes and a lack of shallow earthquakes. We think that the alteration of mantle rocks in these areas makes the lithosphere too soft to break in earthquakes.

Description: Ultraslow spreading ridges form the slowest divergent plate boundaries on Earth. Their distinct spreading processes build volcanically active magmatic segments in between amagmatic segments that exhibit mantle rocks at the seafloor. Local seismicity studies along ultraslow spreading ridges are up to now limited in their extend and can only give insights into spreading processes of segment parts. With our new microseismicity dataset we extend the coverage to multiple segments allowing us to study spreading processes on the scale of entire segments. The network of ocean bottom seismometers consisted of 26 stations deployed for one year approximately 160 km along the Knipovich Ridge in the Greenland Sea. More than 8000 events were reliably located with HYPOSAT and they exhibit a varying seismicity pattern along the rift axis. Maximum earthquake hypocentres shallow over distances of 70 km towards the Logachev volcanic centre, where swarm activity occurs in an otherwise aseismic zone. The undulating brittle-ductile boundary might map the focusing of melt towards major volcanic centres along the parallel lithosphere-asthenosphere boundary. Numerous earthquake swarms close by the volcanic centre indicate its current activity. The absence of shallow seismicity in the upper 8 km underlain by a band of seismicity characterizes presumably melt-poor regions. Both boundaries of the seismicity band are supposedly temperature controlled. Aseismic zones may mark areas, where mantle rocks are altered and too weak to exhibit seismicity recorded by our network. One of the studied segments cannot be identified as magmatic or amagmatic, rather the reorientation of the ridge axis in this area and related changes in the stress regime might lead to a more complex seismicity pattern. Although detachment faults are expected along amagmatic spreading segments, we do not observe clear indication on this type of faulting. We observe a fine-scale segmentation of seismic activity similar to a stripe and gap pattern, where the seismicity band narrows and only little activity is observed within an otherwise broad seismicity band. This can possibly indicate transform motion on short obliquely oriented faults producing small magnitude events not recorded by our network.

Description: Ultraslow spreading ridges form the slowest divergent plate boundaries and exhibit distinct spreading processes in volcanically active magmatic sections and intervening amagmatic sections. Local seismicity studies of ultraslow spreading ridges until now cover only parts of segments and give insight into spreading processes at confined locations. Here we present a microseismicity dataset that allows to study spreading processes on the scale of entire segments. Our network of 26 ocean bottom seismometers covered around 160 km along axis of the ultraslow spreading Knipovich Ridge in the Greenland Sea and recorded earthquakes for a period of about one year. We find seismicity varying distinctly along‐axis. The maximum earthquake depths shallow over distances of 70 km towards the Logachev volcanic center. Here, swarm activity occurs in an otherwise aseismic zone. Melts may thus be guided along the subparallel topography of the lithosphere‐asthenosphere boundary towards major volcanic centers explaining the uneven along‐axis melt distribution typical for ultraslow ridges. Absence of shallow seismicity in the upper 8 km of the lithosphere with a band of deep seismicity underneath offsets presumably melt‐poor regions from magma richer sections. Aseismic deformation in these regions may indicate weakening of mantle rocks by alteration. We do not find obvious indications for major detachment faulting that characterizes magma‐poor spreading at some ultraslow spreading segments. The highly oblique spreading of Knipovich Ridge may be the reason for a fine‐scale segmentation of the seismic activity with zones of weak seismicity possibly indicating transform motion on short obliquely oriented faults.

Description: Ultraslow spreading mid-ocean ridges with full spreading rates up to 20 mm/yr are described as the melt poor endmember of the entire mid-ocean ridge system. The melt supply along ultraslow spreading ridges is uneven resulting in the formation of volcanic centres and amagmatic segments. Amagmatic segments show thicker brittle lithosphere of up to 30 km, whereas magmatic segments have much thinner lithosphere of less than 15 km. It is supposed that melt travels along the lithosphere - asthenosphere boundary from amagmatic segments to magmatic events, where it can reach the seafloor and erupt. These spreading events are rare at ultraslow spreading ridges compared to faster spreading ridges and in situ observations hardly exist. During an ocean bottom seismometer (OBS) experiment at the eastern Southwest Indian Ridge two earthquake swarms were accidentally recorded. The swarms occurred in January and April 2013 and both lasted for a few days. In this thesis, the events of the earthquake swarms were relatively located with HypoDD for better spatial resolution. I created earthquake catalogues to study the characteristics of the earthquake swarms. The results allowed for studying active spreading processes at an ultraslow spreading ridge. The earthquakes occurred in depths, where the magma chamber of the nearby Segment 8 volcano is located. This magma chamber potentially fed a sill intrusion, which was recorded as earthquake swarms. During the first hours of the earlier earthquake swarm a migration pattern was identified. The hypocentres migrated away from the Segment 8 volcanic centre and slightly downwards. Later events occurred more randomly in the active area. Horizontal intrusion can be favoured due to rotation of stresses in extensional settings and rigidity contrast between peridotite and serpentinised peridotite. Simultaneously with the swarms seismic tremor was recorded at the station closest to the swarm locations. The tremor lasted longer for the shorter earthquake swarm in April. Amplitude and frequency of the tremor signal are modulated with a 12 hour period. I therefore speculate that the heat of the intrusion drove hydrothermal activity and fluid flow modulated by the tides produced the tremor signal.

Description: Ultraslow spreading mid-ocean ridges (〈15 mm/y full spreading rate) differ from faster spreading ridges by their uneven melt distribution. Crustal thickness varies along axis from zero to more than 8 km at volcanic centers. These volcanic centers receive more melt than the regional average and may remain spatially located for millions of years. The segmentation pattern and active volcanism at ultraslow spreading ridges therefore differs from faster spreading ridges where elongate axial volcanic ridges typically erupt magma. Using networks of ocean bottom seismometers with an along-axis extent of about 60 km at three differing ridge segments, we could show that the maximum depth of brittle faulting, equivalent approximately to temperatures of 600-700°C, varies drastically along axis. Ridge sections that lack an igneous crust exhibit a thick lithosphere as evidenced by the deepest mid-ocean ridge earthquakes observed so far at more than 30 km depth. Beneath areas of basalt exposure, in particular beneath pronounced volcanic centers, the axial lithosphere may be more than 15 km thinner allowing for melt flow at the base of the lithosphere towards the volcanoes, a process that has been postulated to explain the anomalous melt distribution at the slowest spreading ridges. Spreading events at ultraslow spreading ridges are unusual as we found from two spreading episodes at 85°E Gakkel Ridge and Segment 8 volcano on the Southwest Indian Ridge. These eruptions were preceded or accompanied by large (M〉5) and long-lasting earthquakes swarms and active magmatism lasted over 3-16 years. A massive hydrothermal event plume and sounds from deep submarine explosive volcanism were observed at Gakkel Ridge. At the Segment 8 volcano, we imaged a melt reservoir extending to about 8 km depth below the volcano that potentially fed a sill intrusion recorded by an ocean bottom seismometers about 30 km away at a neighboring subordinate volcanic center. To better understand the segmentation and melt transport at ultraslow spreading rigdes, we recently conducted a segment-scale seismicity survey of Knipovich Ridge in the Norwegian-Greenland Sea, instrumenting the ridge along 160 km of its axis with 28 ocean bottom seismometers for a period of a year, the currently largest mid-ocean ridge microseismicity experiment.

Description: Ultraslow spreading mid-ocean ridges with full spreading rates up to 15 mm/yr are described as the melt poor endmember of the entire ocean ridge system. The melt supply along ultraslow spreading ridges is uneven resulting in the formation of volcanic centres and amagmatic segments. Amagmatic segments show thicker brittle lithosphere of up to 30 km, whereas magmatic segments have much thinner lithosphere of up to less than 15 km. It is supposed that melt travels along the lithosphere asthenosphere boundary from amagmatic segments to magmatic events, where it can reach the seafloor and erupt. These spreading events are rare at ultraslow spreading ridges compared to faster spreading ridges. During an ocean bottom seismometer (OBS) experiment at the eastern Southwest Indian Ridge two earthquake swarms were recorded. The swarms occurred in January and April 2013 and both lasted for a few days. The events of the earthquake swarms were relative located with HypoDD for better spatial resolution. This unique dataset allowed for studying active spreading processes at an ultraslow spreading ridge. The earthquakes occurred in depths, where the magma chamber of the nearby Segment-8 volcano is located. This magma chamber potentially fed a sill intrusion, which was recorded as earthquake swarms. During the first hours of the first earthquake swarm a migration pattern was identified. The hypocentres migrated away from the Segment-8 volcanic centre and slightly downwards. Later events occurred more randomly in the active area. Simultaneously seismic tremor was recorded at the station closest to the swarm locations. The tremor lasted longer for the shorter earthquake swarm in April. During both tremor phases the signal was modulated with a 12 hour period, suggesting a hydrothermal system affected by the intrusion and modulated by the tides.