Speed matters - seismicity and spreading processes of ultraslow spreading mid-ocean ridges

The creation of new ocean lithosphere at mid-ocean ridges is accompanied by characteristic earthquake activity. The analysis of mid-ocean ridge seismicity proved a powerful tool to study submarine accretion of lithosphere and the structure of mid-ocean ridges. During the last decade, the availability of large instrument pools of ocean bottom seismometers enabled surveys of mid-ocean ridge local seismicity that greatly added to our knowledge of active deformation and spreading processes at mid-ocean ridges, capturing magmatic events and mantle exhumation along detachment faults. At the slowest endmember of the global mid-ocean ridge system, however, acquisition of local seismicity data is hindered either by perennial sea-ice in the case of the Arctic Ridge System or by rough seas in the area of the Southwest Indian Ridge. Therefore, our knowledge of active deformation at the magma-poor ultraslow spreading ridges is still limited. Yet, the teleseismic earthquake activity of these ridges shows fundamental differences to any faster spreading ridges, with magma-poor areas being only weakly seismogenic, whereas strong and frequent earthquakes characterize volcanic centres. Long-lasting or repeated earthquake swarms with magnitudes above M>5 appear to precede spreading events, contrary to most faster spreading ridges, where magma intrusion happens unnoticed at land stations. With a comparative local seismicity study of 3 segments of ultraslow spreading ridges, we gained comprehensive insight into spreading processes and lithospheric structure of ultraslow spreading ridges. Maximum earthquake depths indicate strong variations in along-axis lithospheric thickness that may enable along axis melt flow towards the widely spaced volcanic centres, a concept postulated previously to explain the uneven melt distribution along ultraslow spreading ridges. Amagmatic ridge sections, in contrast, show more than 30 km deep earthquakes indicating a very cold lithosphere. Its upper 15 km are conspicuously aseismic down to depths where serpentinite becomes stable. We suggest that alteration of the mantle peridotite to serpentinite weakens the lithosphere such that it can deform aseismically either along deep reaching well-lubricated shear zones or due to pervasive serpentinisation. A consequence of this concept is that fluid flow must extend far deeper into the lithosphere than previously expected and lead to an enhanced exchange of energy and matter between ocean and lithosphere. We accidentally witnessed a rare spreading episode at the easternmost Southwest Indian Ridge. It started with teleseismic earthquake swarms 16 years ago. Currently a region of partial melt at about 8 km depth resides under the central volcano of the segment. It potentially fed two dike intrusion events at 35 km along axis distance below a neighbouring subordinate volcanic centre illustrating another aspect of along-axis melt distribution at ultraslow spreading ridges. Interestingly, the intrusions migrated downward and were accompanied by strong tidally modulated seismic tremor that we interpret to result from vigorous hydrothermal circulation fueled by the heat of the dike intrusion

Seismological in-situ observation of a submarine diking event at the Southwest Indian Ridge associated with tide-modulated tremor

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

Observing tidal effects on the dynamics of the Ekström Ice Shelf with focus on quarterdiurnal and terdiurnal periods

Antarctica's ice shelves stabilize the ice sheet and, therefore, understanding processes affecting the mass budgets of ice shelves is important for estimating grounded ice loss. To study the ice shelf dynamics, we analyzed seismological and GNSS data from the Ekström Ice Shelf in Dronning Maud Land. We extracted probabilistic power spectral densities (PPSD) in the frequency band 3.4-6.8 Hz, typical of icequakes, from seismological data and observed pronounced signals in the PPSD with near 3 and 4 cycles per day (cpd) corresponding to tidal overharmonics, in addition to the main tidal constituents near 1 and 2 cpd. GNSS data reveal the same components in ice flow speed but not in vertical displacements. Generally, tide-induced grounding line migration modulates the flow velocity of an entire ice shelf. We find that this velocity modulation causes the increased icequake activity in the tidal overharmonics with 3 and 4 cpd in an ice shear zone where the flow velocity drops to nearly zero

Event recognition in marine seismological data using Random Forest machine learning classifier

Automatic detection of seismic events in ocean bottom seismometer (OBS) data is difficult due to elevated levels of noise compared to the recordings from land. Popular deep-learning approaches that work well with earthquakes recorded on land perform poorly in a marine setting. Their adaptation to OBS data requires catalogues containing hundreds of thousands of labelled event examples that currently do not exist, especially for signals different than earthquakes. Therefore, the usual routine involves standard amplitude-based detection methods and manual processing to obtain events of interest. We present here the first attempt to utilize a Random Forest supervised machine learning classifier on marine seismological data to automate catalogue screening and event recognition among different signals [i.e. earthquakes, short duration events (SDE) and marine noise sources]. The detection approach uses the short-term average/long-term average method, enhanced by a kurtosis-based picker for a more precise recognition of the onset of events. The subsequent machine learning method uses a previously published set of signal features (waveform-, frequency- and spectrum-based), applied successfully in recognition of different classes of events in land seismological data. Our workflow uses a small subset of manually selected signals for the initial training procedure and we then iteratively evaluate and refine the model using subsequent OBS stations within one single deployment in the eastern Fram Strait, between Greenland and Svalbard. We find that the used set of features is well suited for the discrimination of different classes of events during the training step. During the manual verification of the automatic detection results, we find that the produced catalogue of earthquakes contains a large number of noise examples, but almost all events of interest are properly captured. By providing increasingly larger sets of noise examples we see an improvement in the quality of the obtained catalogues. Our final model reaches an average accuracy of 87 per cent in recognition between the classes, comparable to classification results for data from land. We find that, from the used set of features, the most important in separating the different classes of events are related to the kurtosis of the envelope of the signal in different frequencies, the frequency with the highest energy and overall signal duration. We illustrate the implementation of the approach by using the temporal and spatial distribution of SDEs as a case study. We used recordings from six OBSs deployed between 2019 and 2020 off the west-Svalbard coast to investigate the potential link of SDEs to fluid dynamics and discuss the robustness of the approach by analysing SDE intensity, periodicity and distance to seepage sites in relation to other published studies on SDEs

KNIPAS – exploring active seafloor spreading processes at segment-scale

Knipovich Ridge passive seismic experiment (KNIPAS) is a state-of-the-art seismological project that studies on segment scale the active spreading processes of an ultraslow mid-ocean ridge. The generation of new ocean floor is accompanied by characteristic seismicity that reflects ongoing spreading events and the physical state of the young lithosphere, and differs widely depending on spreading rate. While fast spreading ridges hardly show earthquakes that are large enough to be recorded on land, magmatic spreading events at the slowest spreading centres seem to be regularly preceded by earthquakes larger than M 5. The depth limit of earthquakes and their presence and absence reveal along-axis variations in the thermal and mechanical regime of the lithosphere. Therefore, it is necessary to record earthquakes locally with ocean bottom seismometers (OBS). Such surveys, however, typically have limited spatial extent and cannot reveal segment-scale spreading processes like along-axis melt flow, while spatially more extended data sets of hydro-acoustically recorded earthquakes yield no information on focal depth and can therefore not constrain lithospheric thickness or temperature. The project KNIPAS instrumented for the first time an entire ridge segment with OBS. During Polarstern cruise PS100 in July-September 2016 we deployed 23 OBS of the German Instrument Pool for Amphibian Seismology (DEPAS) along a 160 km long ridge section that covers Logachev Seamount and a neighbouring volcanic centre. An additional 3 OBS of the Institute of Geophysics, Polish Academy of Sciences, were deployed around Logachev Seamount. The instruments recorded seismicity until July-October 2017 depending on capacity. Cruise MSM67 of Maria S. Merian acquired wide-angle seismic profiles across Logachev Seamount and the subsequent cruise MSM68 successfully recovered all OBS. We now have a comprehensive seismological dataset at hand that will contain despite partly high noise levels in the vicinity of Logachev volcano an expected 9000 earthquakes M>1 and several dozens of well-recorded teleseismic events to study spatial variations of seismicity, thermal structure and lithospheric thickness of an ultraslow spreading ridge. In a joint project we will combine the expertise of our work groups to study seismicity pattern, analyse the large-scale lithospheric structure with modern passive seismic methods to be adapted for the special conditions of marine seismic surveys and to image at high resolution the structure of a volcanic centre

Local Seismicity and Sediment Deformation in the West Svalbard Margin: Implications of Neotectonics for Seafloor Seepage

In the Fram Strait, mid-ocean ridge spreading is represented by the ultra-slow system of the Molloy Ridge, the Molloy Transform Fault and the Knipovich Ridge. Sediments on oceanic and continental crust are gas charged and there are several locations with documented seafloor seepage. Sedimentary faulting shows recent stress release in the sub-surface, but the drivers of stress change and its influence on fluid flow are not entirely understood. We present here the results of an 11-month-long ocean bottom seismometer survey conducted over the highly faulted sediment drift northwards from the Knipovich Ridge to monitor seismicity and infer the regional state of stress. We obtain a detailed earthquake catalog that improves the spatial resolution of mid-ocean ridge seismicity compared with published data. Seismicity at the Molloy Transform Fault is occurring southwards from the bathymetric imprint of the fault, as supported by a seismic profile. Earthquakes in the northern termination of the Knipovich Ridge extend eastwards from the ridge valley, which together with syn-rift faulting identified in seismic reflection data, suggests that a portion of the currently active spreading center is buried under sediments away from the bathymetric expression of the rift valley. This hints at the direct link between crustal rifting processes and faulting in shallow sediments. Two earthquakes occur close to the seepage system of the Vestnesa Ridge further north from the network. We suggest that deeper rift structures, reactivated by gravity and/or post-glacial subsidence, may lead to accommodation of stress through shallow extensional faults, therefore impacting seepage dynamics